Casey Long Long 1 Observational Astronomy Using Skynet Introduction to Observational Astronomy Everybody has been an observational astronomer at some point in their lives. For most people this consists of simply observing the brightest stars and planets in the night sky. Professionals have access to cutting edge technology allowing them to probe deeper and deeper into the sky, unlocking a world of galaxies and nebulae. With such a vast number of observable objects, effective communication between fellow astronomers is necessary. The two most important properties for identifying an object are name and location. To standardize these properties, astronomers have developed coordinate systems and naming schemes to promote easy communication. Celestial Coordinate Systems For casual stargazers, it is convenient to use the horizontal coordinate system for defining the location of celestial objects. In this system, an object’s location is described by its Altitude and Azimuth. Altitude is a degree measure from 0° (the horizon) to 90° (directly overhead) of the “height” of the object relative to the ground. Azimuth is a degree measure of direction from North (0°), to East (90°), to South (180°), to West (270°). This system is useful because the observer needs no special equipment to make a fairly good estimate at where an object is. The problem with this system, however, is coordinates differ for different observers as well as over time. That is, horizontal coordinates are only valid in one location and at one time for a given celestial object. To overcome this obstacle, the equatorial coordinate system can be used. In this system, Earth’s equator and poles
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Casey Long
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Observational Astronomy Using Skynet
Introduction to Observational Astronomy Everybody has been an observational astronomer at some point in their lives.
For most people this consists of simply observing the brightest stars and planets in
the night sky. Professionals have access to cutting edge technology allowing them to
probe deeper and deeper into the sky, unlocking a world of galaxies and nebulae.
With such a vast number of observable objects, effective communication between
fellow astronomers is necessary. The two most important properties for identifying
an object are name and location. To standardize these properties, astronomers have
developed coordinate systems and naming schemes to promote easy
communication.
Celestial Coordinate Systems
For casual stargazers, it is convenient to use the horizontal coordinate
system for defining the location of celestial objects. In this system, an object’s
location is described by its Altitude and Azimuth. Altitude is a degree measure from
0° (the horizon) to 90° (directly overhead) of the “height” of the object relative to
the ground. Azimuth is a degree measure of direction from North (0°), to East (90°),
to South (180°), to West (270°). This system is useful because the observer needs
no special equipment to make a fairly good estimate at where an object is.
The problem with this system, however, is coordinates differ for different
observers as well as over time. That is, horizontal coordinates are only valid in one
location and at one time for a given celestial object. To overcome this obstacle, the
equatorial coordinate system can be used. In this system, Earth’s equator and poles
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are projected onto the sky. A measure called Declination (DEC) (from ‐90° (S) to
+90° (N)) measures an object’s position relative to the celestial equator, much like
lines of latitude on Earth’s surface. Due to Earth’s rotation, longitudinal coordinates
cannot be projected as simply. An arbitrary starting point synonymous with Earth’s
Prime Meridian must be defined as a constant reference point. This point was
chosen to be the location of the Sun during the vernal equinox. From this point,
Right Ascension (RA) is measured from 0° to 360°, although this is often reported as
sidereal time from 0 to 24 hours (approximate rotation of the Earth in one day).
The advantage of this coordinate system lies in its consistency for all observers at all
times. Although star locations will slightly change due to the procession of the
equinoxes, this effect is extremely subtle. Recalibrations are necessary only once
every 50 or so years, even for distant objects.
Astronomical Catalogues
While many major stars and celestial objects have common names, there are
simply too many to make this system practical for naming everything in the night
sky. Most visible stars, even those with common names, are referred to by their
constellation preceded by a Greek character indicating its brightness relative to
other stars in the constellation. For example, the star commonly known as Antares
is also known as α Scorpii because it is the brightest star in the constellation
Scorpius. For deep sky objects, much less are known by common names, making
catalogues even more necessary. The famous Messier Objects (M1 – M110) were
catalogued by comet hunter Charles Messier in 1781 to help differentiate potential
comets from fixed objects. The list contains a collection of galaxies, star clusters,
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and nebulae that are among the brightest and easiest to see deep sky objects.
However, this list is extremely limited and more extensive catalogues were needed
to keep track of all the observable deep sky objects. The New General Catalogue of
Nebulae and Clusters of Stars includes 7,840 objects designated by the initials NGC
followed by a four‐digit number. The even larger Catalogue of Principal Galaxies
contains 73,197 galaxies designated by the initials PGC followed by a five‐digit
number. Many objects are included in multiple of these catalogues and can be
referenced with many names. For example, the famous Andromeda Galaxy is also
known as M31, NGC 224, and PGC 2557. While many other astronomical catalogues
exist, these three were sufficient for referencing all of the objects I was looking for.
Observing
Once you know what an objects name is, and where it is located in the sky,
you can observe it using appropriate equipment. For bright stars and planets,
horizontal coordinates are generally enough to find objects with the naked eye. For
deep sky objects, telescopes are usually necessary. Most advanced telescopes are
computer‐controlled to improve accuracy and easy use. For these systems,
equatorial coordinates are preferred for their consistency. When taking images, a
computerized system has additional benefits. Due to Earth’s rotation, an object’s
position in the sky is always changing slightly. Telescope mounts with computer‐
controlled motors keep the telescope focused on the object during long exposure
shots. This eliminates streaks and blurs that would distort stationary photographs
over long exposures.
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Skynet System
The Skynet system is composed of an extensive collection of computer‐
operated telescopes from around the world. Users log on to the Skynet website
where they electronically request images to be taken. The program is maintained by
staff at UNC and generously allows students from other universities and high
schools to utilize the service. Thanks to some inside connections, I was lucky
enough to get to try the program out myself. The following is an overview of the
regular procedure I used to collect images.
Image Taking Process
The first step for getting images was picking a target in the night sky. Using
the free software Stellarium, I was able to see what the night sky looked like in Chile
and could target specific objects visible to the telescopes. After finding an object of
interest that was visible this time of year, I logged onto the Skynet website. Using
the Observation Manager, finding objects’ coordinates was very easy. The following
screenshot shows what a typical search would return. In this example the
Andromeda Galaxy (M31) was searched.
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Screen Shot of SKYNET Observation Manager
After searching for an object, the computer does much of the heavy lifting. It
searches its archives and finds accurate coordinates using the equatorial coordinate
system. It defaults to a maximum airmass of 3. Airmass is a relative measure of
how much atmosphere light has to penetrate. By definition, the airmass at the
zenith (straight up) is equal to 1. Basically this restricts the telescopes from taking
images of objects too close to the horizon. At these low points in the sky, light must
travel through a significantly greater amount of atmosphere distorting images.
Another default is the maximum sun elevation, set at ‐18°. In short, this ensures the
picture is taken at night. There are a variety of filter options, but for galaxies, an
open filter usually suffices, which is what I chose for all observations.
After searching for an object, the following graph appears:
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Observation Manager M31 Visibility Graph
This graph shows the visibility of the given object from the locations of
various telescopes. Most of my images were taken from the Prompt telescopes
labeled CTIO in the above graph (red line). For this example, the Andromeda Galaxy
is not visible for any of the telescopes. Lines are only plotted for nighttime hours,
and if they do not go above the 20° elevation/3.0 airmass barrier, they will not
return good images. So for this time of year, M31 simply does not get high enough
in the sky during dark hours to allow image taking. The following is a graph of the
Sombrero Galaxy’s (M104’s) visibility, showing what a visible object’s graph looks
like.
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Observation Manager M104 Visibility Graph
As you can see, this galaxy is visible for most of the night, especially for the
Prompt telescopes. If an object is sufficiently visible and all the parameters are set,
pressing “Next” yields the following screen.
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Telescope Selection Screen
On this screen, you can choose which of the available telescopes you want to
use to take images. My first choice was always the Prompt Telescopes, as they
reportedly return the best images. If multiple telescopes are chosen, the first to
become available during your specific visibility window is used to capture your
desired images. After selecting your telescopes, pressing “Next” brings you to the
exposure page.
Add Exposures Page
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On this page, you can select various exposure times as well as the number of
exposures. For particularly bright objects, or if a bright star was in an objects field
of view, a message appears setting a maximum exposure time. This is mainly to
protect light sensitive instruments from overexposure to bright objects. Even for
faint objects, the absolute maximum exposure time allowed is 80 seconds for the
Prompt telescopes. In my earlier observations, I often selected a wide range of
exposure times until I got a feel for appropriate exposure times for a given object.
For example, my first target was the Sombrero Galaxy. It is roughly 30 Mly away
and has an apparent magnitude of about 8.98. I decided to take images at exposure
times of 20, 40, and 60 seconds. These times yielded the following images.
The Sombrero Galaxy at 3 different exposure times
As you can see, there is not a dramatic difference, however the 20‐second
exposure definitely has the lowest contrast. The halo around the galaxy is most well
defined with 60 seconds of exposure. For most objects I simply chose 60 seconds,
since this gave the best results. For extremely bright objects, like the Orion Nebula, I
was restricted to shorter exposure times. Conversely, for the very faint Hoag’s
Object, I chose to use closer to the maximum allowable 80‐second exposure.
M104 – 40s M104 – 20s M104 – 60s
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After this page, you are brought to a confirmation page where you can either
send your request for observation, or cancel if you realize a mistake in your inputs.
Once a request is submitted, your observation will automatically happen at the
earliest possible time. Most requests were processed the night after submission,
unless cloud cover or high telescope use delayed them.
Telescopes
Most of my observations were completed on Prompt telescopes (Prompt1 –
Prompt5). This set of smaller telescopes is part of the Cerro Tololo Interamerican
Observatory (CTIO) in central Chile. Nestled in the Andes at over 2,000 m, the site is
an ideal location for telescopes. Its elevation minimizes the amount of distorting
atmosphere, while its isolation removes unwanted light pollution.
I have always been very interested in observational astronomy but have
never gotten the chance to look deeper than my eyes or handheld binoculars had to
offer. This project was a great opportunity to delve into some of the amazing objects
that lie out of sight. It has only increased my interest in the field. Before obtaining
my first image (M104), I was highly skeptical of how well the pictures would turn
out. On the whole, I was blown away with the results. A lot of variables contributed
to the quality of the images. First and foremost, the distance to an object along with
its apparent magnitude played a big part. Additionally the time of year plays a role.
Best images will come from objects directly overhead during the darkest hours in
the middle of the night. These objects have the least interference from the sun, and
have minimal atmospheric distortion. While conditions were not always ideal, most
of the images were at least comparable to professional photos in terms of shape and
light distribution.
Experiencing the image taking process first hand has given me a whole new
appreciation of the field of observational astronomy. Its amazing to think that just
plugging some numbers into a website can control a telescope to take an image of
any celestial object, let alone a galaxy over 500 million light‐years away. The Skynet
system is very user friendly and I am very grateful to have been given a chance to
work with it. Overall, collecting images was an informative, rewarding process that
increased my interest and understanding of observational astronomy.
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Observation Log
Date Object Type RA DEC Filter Exposure 4/6 M104 (Sombrero Galaxy) Galaxy 12:39:59.4 ‐11:37:23.0 Open 20 4/6 M104 (Sombrero Galaxy) Galaxy 12:39:59.4 ‐11:37:23.0 Open 40 4/6 M104 (Sombrero Galaxy) Galaxy 12:39:59.4 ‐11:37:23.0 Open 60 4/8 NGC 6744 Galaxy 19:09:46.1 ‐63:51:27.1 Open 10 4/8 NGC 6744 Galaxy 19:09:46.1 ‐63:51:27.1 Open 20 4/8 NGC 6744 Galaxy 19:09:46.1 ‐63:51:27.1 Open 30 4/8 NGC 6744 Galaxy 19:09:46.1 ‐63:51:27.1 Open 60 4/8 NGC 5128 (Centaurus A) Galaxy 13:25:27.6 ‐43:01:08.8 Open 5 4/8 NGC 5128 (Centaurus A) Galaxy 13:25:27.6 ‐43:01:08.8 Open 15 4/8 NGC 5128 (Centaurus A) Galaxy 13:25:27.6 ‐43:01:08.8 Open 25 4/8 NGC 5128 (Centaurus A) Galaxy 13:25:27.6 ‐43:01:08.8 Open 60 4/8 M64 (Black Eye Galaxy) Galaxy 12:56:43.7 +21:40:57.6 Open 10 4/8 M64 (Black Eye Galaxy) Galaxy 12:56:43.7 +21:40:57.6 Open 20 4/8 M64 (Black Eye Galaxy) Galaxy 12:56:43.7 +21:40:57.6 Open 30 4/8 M64 (Black Eye Galaxy) Galaxy 12:56:43.7 +21:40:57.6 Open 60 4/8 PGC 54559 (Hoag's Object) Galaxy 15:17:12.8 +21:35:03.1 Open 25 4/8 PGC 54559 (Hoag's Object) Galaxy 15:17:12.8 +21:35:03.1 Open 50 4/8 PGC 54559 (Hoag's Object) Galaxy 15:17:12.8 +21:35:03.1 Open 75
4/9 NGC 5139 (Omega Centauri) Globular Cluster 13:26:47.3 ‐47:28:46.1 Open 30
4/9 NGC 5139 (Omega Centauri) Globular Cluster 13:26:47.3 ‐47:28:46.1 Open 60
4/12 M42 (Orion Nebula) Nebula 05:35:17.3 ‐05:23:28.0 Open 1 4/12 M42 (Orion Nebula) Nebula 05:35:17.3 ‐05:23:28.0 Open 5 4/12 M42 (Orion Nebula) Nebula 05:35:17.3 ‐05:23:28.0 Open 15 4/13 M51 (Whirlpool Galaxy) Galaxy 13:29:52.7 +47:11:42.9 Open 30 4/13 M51 (Whirlpool Galaxy) Galaxy 13:29:52.7 +47:11:42.9 Open 60 4/13 M51 (Whirlpool Galaxy) Galaxy 13:29:52.7 +47:11:42.9 Open 30 4/13 M51 (Whirlpool Galaxy) Galaxy 13:29:52.7 +47:11:42.9 Open 60
4/13 M83 (Southern Pinwheel Galaxy) Galaxy 13:37:00.9 ‐29:51:56.7 Open 70
4/13 M100 Galaxy 12:22:54.9 +15:49:20.6 Open 70 4/13 M99 Galaxy 12:18:49.6 +14:24:59.4 Open 30 4/13 M99 Galaxy 12:18:49.6 +14:24:59.4 Open 60 4/19 M59 Galaxy 12:42:02.3 +11:38:49.0 Open 60 4/19 M89 Galaxy 12:35:39.9 +12:33:21.7 Open 60 4/19 M87 Galaxy 12:30:49.4 +12:23:28.0 Open 60 4/19 M49 Galaxy 12:29:46.8 +08:00:01.5 Open 60
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Works Cited
Celestial Object Information
http://en.wikipedia.org/wiki/Sombrero_Galaxy
http://en.wikipedia.org/wiki/NGC_6744
http://en.wikipedia.org/wiki/Black_Eye_Galaxy
http://en.wikipedia.org/wiki/Hoag's_Object
http://en.wikipedia.org/wiki/Messier_83
http://en.wikipedia.org/wiki/Messier_99
http://en.wikipedia.org/wiki/Orion_Nebula
http://en.wikipedia.org/wiki/Centaurus_A
http://en.wikipedia.org/wiki/Messier_100
http://en.wikipedia.org/wiki/Omega_Centauri
http://en.wikipedia.org/wiki/Whirlpool_Galaxy
http://en.wikipedia.org/wiki/Messier_59
http://en.wikipedia.org/wiki/Messier_89
http://en.wikipedia.org/wiki/Messier_87
http://en.wikipedia.org/wiki/Messier_49
SKYNET System
Various links and pages from: http://skynet.unc.edu/index.php
Skynet Authorship Policy Images and data obtained from images taken by a user with Skynet may only be used by that user or by others designated by that user. However, at least the first three people from the Skynet builders list and at least the first two people from each used telescope's builders list must be included as authors on any publications, unless waived by the Director of the Skynet Robotic Telescope Network (currently Reichart) in writing.
Builders Lists
Skynet: Daniel E. Reichart, Kevin M. Ivarsen, and Joshua B. Haislip