Page 1 of 25 A Digital Astrophotography Primer - The Sequel -
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Table of Contents
A Digital Astrophotography Primer...........................................................................................................................................................1 Table of Contents.......................................................................................................................................................................................2
Introduction............................................................................................................................................................................................3 AESTHETICS .......................................................................................................................................................................................4
General...............................................................................................................................................................................................4 Focus..................................................................................................................................................................................................4 Composition.......................................................................................................................................................................................4 Standards............................................................................................................................................................................................5 Look 4 a Hook ...................................................................................................................................................................................6 Location, location, location................................................................................................................................................................7
EQUIPMENT ........................................................................................................................................................................................9 Tracking mounts ................................................................................................................................................................................9 Telescopes..........................................................................................................................................................................................9 Cameras .............................................................................................................................................................................................9 Lenses ..............................................................................................................................................................................................10 Filters ...............................................................................................................................................................................................10 Intervalometer ..................................................................................................................................................................................10
TECHNIQUES ....................................................................................................................................................................................11 Polar Alignment ...............................................................................................................................................................................12 Guiding ............................................................................................................................................................................................12
LET’S GET STARTED.......................................................................................................................................................................13 KISS.................................................................................................................................................................................................13 What to expect from your first images.............................................................................................................................................14
Some New Terminology ......................................................................................................................................................................15 Signal ...............................................................................................................................................................................................15 Noise ................................................................................................................................................................................................15 Signal to Noise Ratio .......................................................................................................................................................................16 Integration time................................................................................................................................................................................17 Signal to Noise Ratio .......................................................................................................................................................................18 THE GOOD NEWS:........................................................................................................................................................................19 THE BAD NEWS:...........................................................................................................................................................................19
SHOOTING DARK FRAMES............................................................................................................................................................20 How to take a dark frame.................................................................................................................................................................20 What a dark frame does ...................................................................................................................................................................20
SHOOTING BIAS FRAMES ..............................................................................................................................................................21 How to take bias frames...................................................................................................................................................................21 What a bias frame does ....................................................................................................................................................................21
SHOOTING FLAT FIELDS................................................................................................................................................................22 How to take a flat field.....................................................................................................................................................................22 What a flat field does .......................................................................................................................................................................22
Combining/stacking image files...........................................................................................................................................................23 Processing image files..........................................................................................................................................................................23
Calibrate Your Monitor....................................................................................................................................................................23 Find some software ..........................................................................................................................................................................24
Summary..............................................................................................................................................................................................25 Sources & Suggested Reading .............................................................................................................................................................25
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Introduction
While I realize that sequels are never as good as the original, I was prompted to write a Part Two by necessity.
This paper is a logical step or progression in my learning curve. When I wrote Part One I thought I had reached
the end of my astro-photographic journey. It didn’t take me long to realize that the items I wrote about in Part
One were merely the tip of the iceberg, so to speak. I hadn’t written a definitive work, I had merely brushed on
the basics of astrophotography and digital slr’s.
One evening, not so very long ago, fellow astrophotographer, Dave G and I were surfing the web and looking at
some of the truly stunning Astrophotos that were posted on the www. I’m talking about posts like you see on
APOD, (Astronomy Photo Of the Day) or Nasa’s Space Image of the day. Stunning photos like you see in Sky
& Telescope & Astronomy magazine. You know the kind of images I mean. You look at these photos and
exclaim, WOW!
Dave appropriately called it the “WOW FACTOR”.
“What”, Dave asked, “do we have to do to get images like that? Not images that people will look at and say,
“Oh, that’s nice’. Images that cause that little rush of excitement in folks, and create the little “WOW” that
involuntarily escapes their lips. You know what I’m talking about. You’ve heard it a thousand times. That
WOW you hear when a child sees the moon through your telescope for the first time, or an adult see the rings of
Saturn through a telescope for the first time.
This paper is the result of my personal search for WOW! Some
of the topics we discuss will be technical in nature. Some topics
will be simply common sense, nonsensical guidelines, and some
topics will deal with intangible things like esthetics.
To achieve WOW everything has to click.
We have to master the technical aspects of our equipment. We
have to know our gear intimately. We have to know the range
and the limits of our tools.
We have to follow common sense guidelines and techniques to assure repeatability and achieve the ultimate
effects of our gear.
And, lastly, we need to have a true love of the beauty of astronomy. We need an artist’s eye. To capture WOW
we have to be able to see it in our viewfinders. We need to recognize it when we see it on our PC’s and we need
to be able to feel it in our guts. We need to remember back to the days of olde when we used to say WOW.
Then we need to shoot accordingly…..
This paper will not teach you to see or feel beauty. You already know how to do that, or you wouldn’t be here.
What it will strive to do is to bring together some of the skills and techniques you’ll need to create an image of
beauty. My goal is to teach myself (and hopefully, you, too) how to point our cameras and telescopes
heavenward and capture a little piece of that heaven. To save images to our screens that will prompt others to
look upon them and say WOW !!!
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AESTHETICS
To get a WOW your image must be aesthetically pleasing. There are several aspects involved in producing a
pleasing image. Of course, the obvious rules apply.
General
Obviously, we need to capture photons to make our image. In astrophography this usually involves an exposure
that is built up over time. Which means, most likely, there will be tracking issues. Trailing stars and bouncing
stars and skipping stars are all OK for run of the mill images. When we’re talking WOW Factor, though, we’re
talking ROUND stars. People know stars are round and appreciate it when you show them images of round
stars. Later we’ll learn how to assure that we have round stars in our images.
Focus
The image should be in focus. Yea, so you
captured the comet that only comes around once
every 86 years. For you it may be a lifetime
achievement. But, nobody cares, if it’s out of
focus. Nobody ever says, WOW, really nice out of
focus image, buddy. Good job!
There are some really neat programs available that
do a remarkable job of focusing using Full Width
Half Medium routines. These programs measure
the diameter and brightness of a selected star. As
we focus and the star becomes brighter and smaller
in diameter the program records these changes and
aids us in determining when the best possible
focus exists
Composition
Next is composition. If your shooting an image of the moon, compose the image so the
moon is centered. If half the moon is missing from the image, or the moon is off to one
side of the image people will be confused. They’ll be too busy questioning why the
moon wasn’t centered to say WOW. Similarly, notice the orientation of the moon. It’s
easy to rotate a telescope around and have your camera positioned at an odd angle.
People will wonder why Tycho is in the top of your lunar image instead of where they
are used to seeing it.
Just like you compose an image when shooting terrestrial scenes, you need to take some
time and learn to “see,” compose and frame your astrophotos.
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Standards
Stick with the standards. Everybody knows that the Orion Nebula is red and the Pleiades are blue. Just because
we CAN manipulate images in Photoshop, doesn’t mean we should. Modifying your image of the Orion Nebula
so the red hues match the red in the neon Budweiser sign might make you and your drinking buddies happy, but
probably won’t get a WOW from other folks.
Likewise, people know that the sky is dark. Our camera chips sometimes produce red, brown and even pink
skies, depending on stray light and pollution levels. Now we need Photoshop to modify our images to achieve
standards or results that people expect.
Some folks like to see a jet black sky, while others prefer a deeper blue cast. I use both to good effect depending
on the circumstances. The general rule of thumb I use is that stellar objects that have no worldly association get
a black background. These otherwordly objects exist ONLY in the blackness of space, so I prefer to give them a
black sky background. Worldly objects, sometimes seen in twilight, like comets, planets, ISS, satellites, etc; can
look good with a blue sky or a black sky depending on the circumstances of the image. Use your judgement in
these cases.
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Look 4 a Hook
Every image needs a hook. There needs to be something in an image to invoke a WOW. There needs to be
something that makes the image unique and/or wondrous. Lunar images are a dime a dozen. You’ll rarely get a
WOW from a straight Full Moon photo. But, take a shot of the full moon eclipsing half of Saturn’s rings, and
the WOW’s are numerous. Or, do a closeup of the moon. You know the kind, one of those where it feels like
you’re hovering above the surface looking down. Or, better yet, the ones where it feels like you’re hanging in
space and the lunar surface is gliding overhead. These images always illicit at least a slight WOW.
And sometimes the hook can be subtle. Recently, while focusing on Comet Holmes I noticed an M object off to
the side of the frame. “Wow”, I thought, “that’s neat.”
Then it occurred to me. Duh, compose the image to include the M object. I got a lot of comments from that
photo. It escalated the image from plain to WOW! Evidently, what I thought was neat pleased other folks, too.
Go figure!
Another example is the photo to
the left. I was awaiting Comet
Holmes to rise out of the treeline
in my yard. I was focusing on
the comet while it was still in
the trees, so I would be ready to
shoot as soon as it climbed
above the treetops.
While focusing I noticed how
the trees framed the comet and
M34. The addition of the
“earthly” trees gave the image
an eerie, other worldy effect,
while grounding the image at the
same time.
“WOW,” I said, “that’s neat!”
CLICK…
Some hooks are impromptu, like when a jet flies in front of the eclipsed sun or moon, or a meteor streaks
through a timed exposure, but some hooks can be planned. Look for transient events like iridium flares. While
iridium flares are nice, after a while, they even become blasé. Watch for an iridium flare that will cross the
surface of the moon, or zip by Saturn. Recently, Comet Tuttle passed by M33 and Comet Holmes passed by the
California Nebula. What stunning images. WOW!
And sometimes, it pays to think outside the box. A friend once handed me an odd photo of the full moon. I
immediately recognized that it was framed upside down, so I flipped it over to view it in the proper
configuration. I noticed a strange ripple in the center of the moon, almost as if he had bent the negative while
printing the image. Then I noticed an upside down caption on the mask, so I flipped the image back around. It
read, “A reflection of the Moon in Grampa’s Pond.” Now I understood why the moon was upside down and
what had caused the ripple in the image. WOW! Nice image. Nice hook.
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I remembered that image and the hook, and many years later shot an image of Comet Hale Bopp, reflected in
silvery waters of Lake Erie.
Location, location, location
One of the most frequently overlooked yet most
important considerations in astrophotography is
location. Probably the most significant
contribution you can make to your images is to
improve your signal to noise ratio. By signal I
mean starlight and by noise I mean light pollution,
stray light, etc. Basically, noise is unwanted
signal. (More on this later.)
We, as astrophotographers recognize this truism,
but we tend to disregard it for the sake of
convenience. We build our observatories in less
than optimal conditions because it’s convenient.
But convenience doesn’t always lend itself to
WOW images. More often than not, it detracts
from them.
Our professional counterparts, of course, realize this necessity and build their observatories on remote
mountaintops, high above weather patterns and city lights.
Not to despair, we can take steps to minimize
the effects of our locations. Many of us build
observatories. While they may not be in the
best location, they are used as much to store our
equipment and provide us a place to take
photos as well as providing shields from
obtrusive stray lighting.
We can use special filters to block unwanted
wavelengths or pass desirable wavelengths. We
can modify the way we use our equipment in
light polluted areas to achieve optimal results.
We can learn to shoot small, narrow sections of the sky
from our city locations, and save those wide panoramic
vistas for our visits to the countryside. We can learn to
shoot lunar and planetary images from our light polluted
downtown locations and save those dim emission
nebulas for our trips to dark skies.
There are numerous ways to combat the negatives of
shooting in a less than optimal environment, and we will
address most of them in upcoming chapters, but
recognizing the fact that we need to increase the signal
to noise ratio is the first step in taking care of the
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problem. After all, we can’t all move to the mountaintops. If we did, there would be light pollution there…
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EQUIPMENT
Tracking mounts
This is my portable mount, a Celestron CG5-GT. I
selected it because of it’s portability, light weight
and strength.
As you can see in the photo I have a 120mm short
tube refractor AND an 80mm short tube refractor
mounted atop this mount. Even with all this weight I
can repeatedly take up to 3 minute exposures.
This type of mount is also capable of being auto
guided by a computer.
Owning a small mount like this is an excellent way
to access dark sky sites for those really important
shots that you just can’t get from suburbia.
Telescopes
The best telescopes for astrophotography are fast telescopes. Today
there are some really nice systems on the market, but most of the fast
telescopes are VERY expensive. You can find astrographs and
Schmidt cameras out there in the many thousands of dollars range.
For a few thousand dollars you can purchase Newtonian reflectors
optimized for astrophotography, and an impressive line of
Apochromatic refractors. For about one thousand dollars you can buy
some small apo’s and some decent camera lenses that serve the same
purpose as a telescope. If your budget is under a thousand dollars you
can still purchase some nice equipment like medium fast reflectors,
semi apo and ED refractors.
Cameras
Digital SLR cameras are abundant and offer many features. I don’t feel qualified to discuss particular cameras
that I haven’t owned, so will only speak in generic terms. My experiences are primarily with Canon digital slr’s.
I’ve owned a digital Rebel and currently own a 30D. I’ve learned that it is extremely hard to focus dslr’s. Before
you purchase a camera, look closely at how they focus and what the viewfinder looks like to you. The ability to
actually see stars in the viewfinder was the selling point that convinced me to trade in my Rebel for the 30D.
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Lenses
Like telescopes you want fast lenses.
Most cameras come with 2 kit lenses and these are usually
the lowest quality lenses made by the manufacturer of your
camera. It’s basically a hook to make you think you’re
getting more than you really are. For really tack sharp
images you will probably wish to purchase fast lenses and
prime lenses. Prime lenses and usually higher quality lenses,
they have a lot less glass inside than a zoom and are
therefore inherently faster, lighter and all around better made
lenses.
Filters
Filters is sort of a no brainer. If you want WOW and a filter is needed, get it. If you have an achromatic
refractor and get purple halos around bright stars, by all means, buy a minus violet filter. If you live downtown
and the Merc-vapor lights turn the sky, in even your shortest images, into brown murky goo, buy a light
pollution filter.
Filters are designed to enhance positive attributes in our images, or to block unwanted
attributes. Either way, the bottom line remains the same. If you want WOW and a
filter will help you, buy it. I know, filters are expensive.
You’re not rich and neither am I. I’ve learned to prioritize my filter needs, set goals,
place them in my budget and then acquire the filters as the priorities & budget
dictates. Remember that the journey of a thousand miles begins with the first step….
Intervalometer
I’ve never been much of an observer. In my mind
observing is not the goal, it’s a task performed in
preparation of capturing an image. I have observed
and I know how to, but I never really enjoyed
observing. That is, until a simple little electronic
device called an intervalometer came along and
changed all that.
The new computer controlled intervalometers
allow us to practically automate our imaging
sessions. I can set the unit up to take, let’s say, 30
three minute exposures with a 5 second gap in
between each image. Then I depress the start
button and find myself with 90 minutes on my
hands. During these 90 minutes I can’t touch the
camera or telescope as it busily captures images
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for me. I can’t turn on the lights. So, what is there to do, standing outside ‘neath the starry skies?
Recently I’ve been setting up my portable scope on the deck outside my observatory. While the main instrument
is working away, I’ve started observing. I mean real observing. Like finding a dim fuzzy, or a blinking
planetary, or a nice globular and just observing them. No hurry to get the camera out. It’s already busy. Just take
my time and look, and look, and look some more. And stare at objects. Stare for a long, long time, like they
used to do back in the olden days. I’ve even started getting out my sketch pad and drawing images like I used to
do back in the sixties and early seventies before I owned a camera.
I’m impressed with how much detail I can see if I actually sit down, get comfortable, stare for a while, relax,
stare some more, and then some more. I think over the years I’ve become acclimatized to the “star party”
method of observing. That’s where someone finds an object, you run to his scope, take a quick peek and then
move on to the next telescope/object.
When I’m doing this, I always want to look just a little bit longer. But, hey, you know the feeling. You’re
standing at the eyepiece and the guy behind you is staring a hole in the back of your neck and fidgeting around,
wondering how much gawdawfully longer you’re going to stand there, semmingly frozen to the eyepiece. So
you look quickly and then reluctantly pull your eye away from that wondrous little jewel in the eyepiece. You
don’t want to get a rep as an eyepiece hog, now, do you?
My intervalometer has changed all that. Now I can stand and look at M33 for a full twenty minutes or so, if I
dare. I can remain at the eyepiece until that rare little window of seeing opens up and offers me a visual view of
dust lanes and motes and knots of dark & light matter just too faint to be seen except under those fleeting
seconds of transparency we never seem to see during those brief glimpses in the eyepiece at a star party.
So, get your self an intervalometer. Let it do the work for you while you enjoy your hobby of choice.
TECHNIQUES
There are two important processes you will need to familiarize your self with before doing any serious
astrophotography. They are polar alignment and guiding. (Not to be confused with tracking!) Of course you are
familiar with these terms, but I can’t stress how very important they are to successful astrophotography. You
can have the very best equipment money can buy and have the darkest skies on the planet, but if you don’t
properly polar align and guide your equipment you seriously limit what you can do with your equipment.
Take some time to learn an alignment technique that works for you. Find a program or technique for guiding
that works best for you. Don’t scrimp, and don’t worry about how long it takes. It’s a pain, but the rewards for
doing it right will be the “WOW” you hear when you show folks your images.
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Polar Alignment
As we discussed earlier, imaging astronomical objects using
exposures greater than a few seconds makes it necessary to
carefully align the telescope mount so that the primary axis of
rotation ( the polar axis ) is oriented so as to be "exactly"
parallel to the Earth's axis of rotation. This can be a long
process for those who are not experienced at it, but the
following guidelines will make the task a bit shorter and a lot
easier.
A POLAR SCOPE can be useful for approximate polar
alignment IF it is well designed and well aligned with the polar
axis of the telescope. However, due to the mechanical
imperfections inherent in most telescope mountings AND the
effects of refraction, a POLAR SCOPE can only provide an
approximate alignment setting for the telescope. The main
function of using the polar scope is to provide for a first order
approximation to polar alignment which then translates directly
into a savings of time when performing the accurate polar
alignment using the DRIFT PROCESS.
The DRIFT PROCESS is the most accurate method available for portable mountings. If you use the drift
method properly you will be able to do long exposures with virtually any lens or telescope and NEVER see the
image ruining effects of FIELD ROTATION ( Photos taken near the horizon are still subject to the effects of
differential refraction which is altogether a separate and different effect!! )
Drift alignment can be accomplished by eye, using your camera or using some very accurate and worthwhile
astronomy programs available on the www.
Guiding
Many people, myself included, get confused on this one. Guiding is not the same as tracking. Tracking is what
your RA motor does when you turn the telescope on. Your tracking motor slowly slews the telescope westward
at the same speed as the earth rotates eastward. Guiding, wether done by hand, or with computer aided devices
is a totally different beast. Guiding involves setting up on a stellar object and guiding the telescope to follow
that object and keep it motionless while shooting your image.
There are many guiding programs and different methods of guiding out there today. Meade, with their DSI and
Orion with their Starshoot are two of the most popular commercial units. High end users with deep pockets will
frequently go with units like SBIG. If you want to take the inexpensive route, there’s PHD guiding and the
GPUSB adaptors.
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LET’S GET STARTED
KISS
Have you ever heard of the KISS (KEEP IT SIMPLE, SILLY) principle? Have you come to the conclusion that
this astrophotography stuff is just too much work, way too expensive and requires too much equipment? If so I
must apologize for leading you down the wrong path. In fact, let me show you how very easy astrophotography
can be. The only equipment you need is a dslr, the kit lenses that came with it, a tripod and your computer. Most
cameras these days come with 2 lenses, a wide to medium and a medium to tele lens. The former is somewhere
in the range of 15 to 70mm and the latter somewhere between 75 to 300mm or so. We’ll use these lens
configurations for this experiment.
Take your camera out under the night sky, place it on the tripod and point it heavenward. Find a bright star and
focus. Slowly rock the lens in and out of focus. Notice the point where the stars seem to “POP” into sharpest
focus. Do this several times until you are sure you’ve reached the finest point.
For this exercise set your ISO level to 800. Further experimentation will illustrate what iso level your particular
sky can support.
Since you don’t have a fancy intervalometer yet, you need to shoot your image without introducing vibrations
into the equation. Go to your cameras’ settings and set it to take a delayed image. (Some cameras use a clock
icon to mark this feature.) A 2 second delay or longer is fine.
Use the exposure chart below to set your cameras’ exposure time without getting star trails.
Lens Aperture Exposure Length
in millimeters in seconds
14 30
28 15
50 08
75 06
100 04
150 03
200 02
OK, that’s all done. Compose your image and start having some fun. Take some pictures. Take a couple dozen,
or even more if you’d like, at 14 millimeters for 30 seconds. Then go to your computer, upload the image files
and stack them using Deepsky Stacker. You can download it for free on the web.
If you took 20 exposures at 30 seconds, you shot the equivalent of a 600 second, or 10 minute (20x30=600
seconds) exposure. If your final image is noisy, reduce the iso level and try again. If there’s no noise, consider
yourself quite lucky and increase your iso level.
As your profiency level grows, move down the chart. Do your own experimentation. Have fun. You’ll know if
and when it’s time to move up to the next level. Additionally, commit this chart to memory. If you ever find
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yourself out with a camera and no other gear, you can use the guidelines above to get that once in a lifetime
shot.
What to expect from your first images
If you’re anything like me, when you view your first astrophoto, you will be a little disappointed. Digital
cameras are, after all, setup to take daylight photos. That’s where the majority of the buyers want the cameras to
be set up to, so that’s how it is. As soon as you release the shutter button, the camera starts running your image
through different algorithms to make it friendly to the mass buyer. This program does things to an image like
adjust color parameters, adjusts brightness and contrast and a whole slew of other things it’s probably best we
don’t concern ourselves with.
But, you say, I’m doing astrophotography! I don’t want the camera doing all
this “stuff” to my image. And that’s where the raw image comes into the
picture. (Excuse the pun.) When you record a raw image, the camera collects
light while the shutter is open, saves the pixel counts and displays the image
(well almost) EXACTLY the way it was shot.
Also, raw images are beginning to become accepted as the “negative” of the
digital world. The Digital Negative Group has done a lot of work developing
standards and methods of foolproofing the raw image information.
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Some New Terminology
Signal
The goal of astrophotography is to capture those faint little photons that have travelled all those hundreds and
thousands and millions of light years through the murky depths of interstellar space. We want to capture them in
our meager little telescopes and focus them onto our camera chips. For the purposes of this article we will call
this starlight SIGNAL.
Noise
However, trying to capture that faint signal streaming in from the far reaches of the universe is no easy task.
When we point our instruments heavenward, we are catching more than just starlight. We’re also capturing
unwanted light, or NOISE. Ice crystals in the upper atmosphere sometimes light up the sky with what we call
skyglow. For our intents, skyglow is noise. Light pollution is also noise. Stray houselights, nearby streetlights,
traffic, etc can cause flare and reflections in our optics. This is another form of noise. Yet another form of noise
comes from the electronics in the cameras we use to capture our signals from space.
Skyfog Light Pollution
Chip Noise (Upper Right) Vignetting, dust on lens, noisy chip
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Signal to Noise Ratio
Basically, then, our ultimate goal is to capture as much signal as possible while limiting or eliminating noise.
The higher the signal to noise ratio, the better our photos.
In this example exposing past 20 seconds will result in
the sky pollution being as bright as the starlight.
Exposing for more than 22 seconds will result in an
image with light pollution, camera read noise and skyfog.
Here, in a heavily light polluted area we see that exposing
for more than 12 seconds will result in the sky pollution
being as bright as the starlight. Exposing for under 12
seconds will yield us nice images.
By keeping our images below the noise thresholds and
through the usage of flat frames, dark frames and bias
frames we can greatly reduce and/or eliminate the effects
of skyfog, light pollution, poor optics and electronic
noise from our cameras.
There are two sure fire methods of increasing signal to noise ratio.
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Integration time
Integration time means exactly what it implies. The more time we expose our chips to starlight the more signal
we acquire. However, by increasing integration time we are also increasing the possibility of acquiring noise.
The good news is that we can take steps to subtract noise from our images.
A common mistake of novices and old time film photographers alike is to expose their chip to the sky until it is
totally fogged over with noise, and then to try to process the signal out of the noise. I’ve done this myself. When
I take an image and look at the back of the camera and see a purely black image, it’s plain to see that the
exposure time is too short. When I shoot longer images, I can see stars, nebulae, etc. Yea, so the sky is pink. I
can fix that with processing. WRONG! What I really need to do is find the exposure time where the sky is still
black, (NO skyfog) and the stars are white (Maximum signal to noise ratio) and then take enough of those
images to stack (integration time) and achieve the desired image.
Properly exposed image Overexposed image Image after processing
Properly exposed image Over exposed image Under exposed image
Actually, an image that has been adequately exposed should in fact NOT have a black background. The SNR
(Signal to Noise Ratio) in your final, stacked and processed images has a direct relationship to how much
skyfog you have.
Total integration times used in stacks of frames need to be in direct proportion to how much skyfog you have at
your site. Six times more skyfog (eg semi-rural suburbia), six times longer integration time. So to reproduce
those beautiful DSO images that took 3-hours integration at a dark site will require 18-hours integration at the
light polluted site; for the same SNR. Basically, extend integration times to the limit of your stamina, at any
site.
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Signal to Noise Ratio
With proper processing we can remove most sources of camera noise, but not all. With the new offerings of
low-noise DSLRs and the use of darks, flats and bias frames we can actually remove most noise very
effectively. But not the Read-Noise of the camera and noise in the skyfog. Longer and ever longer integration
times will reduce the skyfog statistical noise by a factor proportional to the square root of the integration time.
But when we stack frames, each with a short
exposure, we are also stacking the camera's
Read Noise in each frame! We need to
separate this Read Noise from the skyfog
statistics. Fortunately there is a way to do this.
Expose each frame so that the skyfog peak is
well removed from the origin of the histogram
display on the LCD screen on the back of the
camera. Expose so that the skyfog peak is at a
quarter to a third of the way from the left side
of horizontal axis of the back-of-camera-
histogram.
In the illustration to the left, You can see an
image of Mars on the viewfinder LCD screen.
To the right of the image is the Histogram
window. You can toggle the histogram chart
on/off with the info button
My own rule-of-thumb at a dark site is to use the highest ISO (usually 1600) your camera offers, but not one
where you have to invoke some special function, like the H setting on most of the newer cameras. ISO 1600
actually seems to have lower Read Noise than lower ISO’s. Beware, though, high ISO in heavily polluted areas
increases skyfog, thus reducing integration time. (More on this later.)
Expose each frame long enough that you can see a distinct gap
between the TRAILING edge of that skyfog mountain and the
origin of the histogram on the back of the camera. Tracking is
usually the main limitation to how long each exposure can be.
(More on tracking in another chapter.)
In the illustration to the left you can see the histogram skyfog
mountain is about 1/3 of the way in from the left side of the
chart. This is right where we want it to be. This gives us
latitude to stretch the histogram to keep the sky dark and
enhance details in the lighter elements of the image.
You will find that from a dark site, the use of fast optics, eg f4, and ISO 1600 allows you to to shoot individual
exposures no longer than a couple of minutes to do excellent DSO imaging. Of course, in less than pristine
conditions, shorter individual exposures means you’ll have a larger stack of images to achieve the same
integration time. Most DSOs need 2 to 3 hours integration time at a dark site to come out nicely, so shooting for
a whole night on just one DSO is not uncommon. When morning comes and you begin processing, you’ll
probably wish you had shot more frames.
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THE GOOD NEWS:
Modern DSLRs have low enough thermal noise at reasonable ambient temperatures (25deg C and cooler) that
with proper procedures (darks, flats and bias frames used in calibration) they are capable of beautiful astro
images of very dim DSOs, and even narrowband imaging using modified DSLRs. They also have low Read-
Noise compared to many astroCCDs. This enables their use in the desirable, so-called skyfog-limited mode with
quite short exposures using relatively fast optics; a couple of minutes per frame with f4 optics, even at very dark
sites. Much shorter exposures may be used at light polluted sites and still remain in the skyfog-limited regime.
Many people with excellent tracking prefer to shoot longer exposures per frame, perhaps even at a lower ISO.
What really matters is that the skyfog statistics in each frame are not interfered with by your camera's Read-
Noise and that you have ample integration time.
THE BAD NEWS:
Integration time! There is no obvious way around having to achieve proportionately longer integration times at
light polluted sites compared to a dark site. Shorter exposures per frame simply implies having to take even
more frames. Let’s work out some numbers: Assume that your mount can deliver satisfactory tracking for 6-
minute exposures at our desired focal length. At a very dark site we can often achieve a satisfactory (not great,
but satisfactory) image of a faint fuzzy with about an hour integration time. A "great" image will typically
require 3 to 5x as much integration time, even at a dark site. On the other hand, star clusters such as globulars
require much less integration time. Depending on the focal ratio of our OTA we may find that at ISO 1600 there
is a nice gap between our skyfog mountain and the origin, on the back-of-camera histogram using 3-minute
exposures. So we decide to use 20x3minute exposures to deliver the one-hour integration time. Back at our
home base in semi-rural suburbia we find that the skyfog is 6x brighter than at the dark site.
This is not atypical and we need to shoot 6-hours integration time to achieve the same final SNR. We will also
likely find that at ISO 1600 there is already a nice gap between the origin and the skyfog mountain with
exposures as short as one-minute. So we have a choice, shoot 360x1min frames at ISO 1600 or use longer
exposures at a lower ISO? We wish to achieve 6-hours integration time in all cases. Perhaps we decide to settle
on 60x6min frames at ISO 400. Either route is valid and the resultant SNR after stacking should be similar,
20x3min at ISO 1600 at the dark site compared to either 360x1min at ISO 1600 or 60x6min at ISO 400 at the
light polluted home base. There is no getting around having to increase the integration time enormously.
To keep the total number of frames reasonable it is often wise to use a lower ISO at a light polluted site. In all
cases you also wish to have a clear gap between the trailing edge of the skyfog mountain and the origin, so that
Read-Noise does not mess up your skyfog photon statistics. The greater the number of frames in your planned
stack, the clearer the gap has to be and the more dependent you become to the absolute linearity in the response
of your camera. I often try to use exposure length and ISO that keeps the total number of frames in a stack
under a hundred.
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SHOOTING DARK FRAMES
I normally take my dark frames at the end of each session. Some camera operating manuals say you can re-use
dark frames and re-calibrate them before subtracting them from your image. I have found that although it might
work in principle, in practice it does not work, or if it does, it does not work as well as taking one prior to,
during or immediately after each session. The key consideration here seems to involve temperature. Here in
northern Ohio we can have some very wide temperature swings throughout the year and matching darks to
lights taken at different temperatures is critical to achieving good calibration.
On a night where the temperature will only fluctuate several degrees I will wait until the end of the session to
take darks. However on a night where we will have very large temperature fluctuations I will shoot darks
throughout the night. I try to keep my eye on the thermometer and take darks every time the temperature moves
more than a few degrees from the prior reading.
How to take a dark frame
The dark frame is taken by covering the CCD chip, usually by covering the end of the telescope so no light can
reach the chip. The thing to remember is to take the dark frame at the same temperature and for the same time
period and at the same ISO setting as the astronomical images you shoot during the session. I use a very
lightweight strip of black cloth that I swing overtop of the dewshield. Then I shoot just like I’m taking lights.
An intervalometer helps with this needed but mundane task and allows me the freedom to do other chores while
the darks are being taken. I frequently take my darks while closing down for the night. I throw the scarf over the
front of the tube, and start taking darks. While the darks are being shot, I’m closing up my observatory, packing
away my eyepieces, lenses, filters, adapters, etc.
What a dark frame does
At a given temperature and for a given exposure time a CCD chip will accumulate a certain amount of noise
that is not related to an image. This noise is not random. That is if you repeat the same exposure under the same
circumstances (temperature & time) the noise can be repeated. This means that if we take a dark frame and then
take an image at the same temperature and for the same time as the dark frame the unwanted noise that is
accumulated and recorded during the exposure on the dark frame will be on the image as well. So if we subtract
the dark frame from the image we can remove all the unwanted noise.
IMAGE (including unwanted noise) - DARK FRAME (the image of the unwanted noise) = IMAGE (without the
unwanted noise).
Note that although I said the noise was repeatable it is not quite. There is a very small amount of variation
between exposures, most likely caused by very slight temperature variations. This can best be accounted for by
taking several dark frames (I usually let the amount of temperature change dictate how many darks I take.) and
average them together before subtracting them from the image. Note that DSS does this for us.
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SHOOTING BIAS FRAMES
I take the bias frames just before the dark. As with the dark frames, some camera manuals say the bias frames
can be taken and then saved to be recallibrated later to match the temperature of the dark frames and image
frames. I take fresh bias frames during each session as I think I get better results.
How to take bias frames
Cover the telescope just like you would for the dark frame to stop any light from getting on the CCD chip. Then
simply take an exposure for as short a time period as your camera will allow and at the same temperature as the
image exposure will be taken at. I usually take a minimum of six exposures and then average them all together
before applying them to the dark frames and the final image. It is important to take the bias frames at the same
temperature as the dark frames and the final image frames are taken.
What a bias frame does
A CCD chip has thousands of separate collectors called pixels on it that measure the amount of light that falls
on them. When you download an image you are downloading the value of each individual pixel, then you put
them all together on the monitor screen in the right order to make an image. If you zoom in on an image you can
see the individual pixels. So a pixel with a low number is darker than a pixel with a large number. The problem
with this is that the pixels do not all start with a value of zero. This means that if two pixels close together
receive the same amount of light they will have different values. The value of the light received plus the the
value they started with.
Pixel 1. Start-up value (50) + light received value (1000) = total value (1050)
Pixel 2. Start-up value (20) + light received value (1000) = total value (1020)
Pixel 3. Start-up value (0) + light received value (1000) = total value (1000)
To remove these unwanted values we simply subtract the bias frames from the dark frames and the image
frames. This means that every pixel starts with the same value.
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SHOOTING FLAT FIELDS
Dark frames & bias frames are both designed for correcting unwanted noise in the CCD chip. Flat fields are
designed to correct problems in the optics i.e. dust, dirt, internal reflections & even vignetting. Because of this
every time you change the optical system you should take another flat field. This means even if you just refocus
or change a filter.
How to take a flat field
The flat field must be taken at the same temperature as the dark, bias & image frames. The time should be as
short as possible while at the same time getting the correct saturation level. For the correct saturation level the
flat field’s average value should be approximately one third of the maximum saturation level of your camera.
To take a flat field you need an evenly illuminated surface. There are several ways of doing this. The simplest
method is to get a piece of white card & position it parallel to the telescope. Then shine a light onto the card.
Take several images with different exposure times to get the right average saturation level. The light can be a
porch light or any low powered light. The card can be exchanged for a blanket over the washing line or the
inside of your observing dome or a wall of a roll-on roll-off observatory. Paint a section (Larger that the
diameter of your telescope) of the wall white, and surround it with small, low wattage light bulbs. Place a baffle
in front of these bulbs to stop the light from shining directly into the telescope. The light from the bulbs is
reflected off the baffles and the observatory walls, providing the white illuminated surface needed for flats.
What a flat field does
When you take an exposure any dust or dirt in the optical system will show to some extent on the image. A
speck of dust on the mirror will be out of focus and appear on the image as a doughnut shaped smudge.
Generally the nearer the dirt is to the camera the darker the smudge will be. Vignetting (a gradual darkening
towards the outside of the image) may also show up if the optics have this problem. (Many zoom and kit lenses
and fast optical systems exhibit vignetting.)
Because the image you are taking is even (that means the image reflects the same amount of light across the
whole of the image.) any imperfections in the optical system will be recorded on the flat field image. These
imperfections will also be on the final astronomical image. To remove these imperfections we divide the final
astronomical image by the flat field. This is different to dark & bias frame processing because we are trying to
flatten the background not remove hot & cold pixels.
Take 2 pixels that receive the same value of light.
Pixel 1.
Image (1000) + extra value caused by light reflecting inside the telescope (200) = 1200
1200 \ flat field value (400) = 300
Pixel 2.
Image (1000) - light reduction caused by dirt on mirror (400) = 600
600 \ flat field value (200) = 300
The internal workings of the camera can also create localized hot spots, which will alter the sensitivity of the
pixels in that area.
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Combining/stacking image files
Once you’ve finished the session and have all the lights, darks, flats and bias
frames, you need a program to do all the dividing and adding and subtracting
and multiplying that needs to be done before during and while stacking your
images. Doing all this work yourself would take countless hours. Using the
new software available today only takes minutes of your time. Two programs
I recommend are Deep Sky Stacker (Free!) and Images Plus. Both are quick,
easy, clean and efficient. There are many other stacking programs and you
would be best suited to try some for yourself and see which you prefer.
Processing image files
Once you’ve finished stacking your image, you are almost ready to process it.
Calibrate Your Monitor
Before processing, however, you need to perform one step. You will only need to do this once. Go to your
monitor settings and calibrate your monitor. You’ll see a series of blocks, aligned in steps, black on the left,
white on the right, with shades of gray in between. Make sure you can see every shade of block. If you can’t
follow the on screen instructions until you can.
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Find some software
There’s a lot of really good software out there today. Probably the premium suite is Adobe’s Photoshop CS2.
It is a HUGE program with a steep learning curve and is quite expensive. It’s also worth every penny.
In the middle ground of expense there are quite a few programs. Images Plus, Corel Paint Shop Pro X and Iris,
to name a few.
To learn more about software or any of the topics discussed in
this paper, check out Yahoo Groups. There are groups and
forums for each topic we discussed here and all are manned
with intelligent and caring people who are ready to help
novices at a moment’s notice.
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Summary
This paper is not intended in any way to be an inclusive treatise on digital cameras and/or astrophotography.
Each item I’ve touched on in this article could probably have a book written on it. (And probably has!) This
article is intended more as a beginner’s guide to provide a starting point for your personal exploration of digital
astrophotography.
Information for this article came from my personal experiences and numerous sources, some of which I’ll list
below.
Sources & Suggested Reading
Your Camera’s Users Manual