Some Aspects of Light Pollution in the Near Infrared

Lastovo 2010: Some Aspects of Light Pollution in the Near Infrared 1


Some Aspects of Light Pollution in the Near Infrared

Željko Andreić1 and Doroteja Andreić2

1Faculty of Mining, Geology and Petroleum Eng., University of Zagreb, Pierottijeva 6, 10 000 Zagreb, Croatia, E-mail: [email protected]

2Faculty of Veterinary Medicine, University of Zagreb, Heinzelova 55, 10000 Zagreb, Croatia, E-mail: [email protected]



Overview

1. Introduction

2. monitoring methods and results

3. conclusions



Introduction

Light pollution (LP) is usually conected to human vision, so only

visible part of EM radiation (light) is considered in studies of LP.

Light is per definition the part of electromagnetic spectrum to which our

eyes are sensitive, or, in other words, which we can see. However, it is a

long tradition that the neighbouring regions of the electromagnetic spectrum

are also called “light”, i.e. ultraviolet and infrared light. This terminology is

based on the fact that the same type of optical instruments can be used to

investigate all three regions. The regions that lie outside the grasp of

classical optical instruments are often called “far” or “extreme” radiation,

i.e. Far infrared radiation and extreme ultraviolet radiation.

In this presentation, we will adhere to this traditional terminology, even if it

is not completely correct.



CCD cameras are sensitive to infra-red (IR) up to 1000 nm!

IR

and there is "LP" in this spectral region too!

The reason that we expand discussion of light pollution to the infrared part

of the spectrum is that today most astronomical “observations” are not made

by our eyes anymore, but with some sort of electronic camera, in most cases

based on a silicon CCD or CMOS deector. This is equally true for amateurs

and proffesionals, the only difference lying in the quality (and price) of the

devices used.

All silicon based cameras have roughly similar spectral sensitivities, that

extend into infrared to about 1100 nn, and also into the ultraviolet to about

300 nm. However, sensitivity in the ultraviolet is very low, and is often

limited by the filtering properties of the glass window in front of the

detector itself, that is a part of the detector chip and can not be removed.

Also, many optical systems in astronomical use (telescope lenses or camera

lenses) do not transmit ultraviolet at all.

The graph above illustrates typical spectral sensitivites of silicon-based

CCD detectors, the pink region on the left side corresponds to the infrared

radiation that we can not see. The transition from red into the infrared is not

strictly defined and depends on the intensity of light and individual

differences of observers, but 750 nm can be taken as a good practical limit.



C-MOS detectors (used in most DSLR cameras!) are simmilar.

IR

Spectral sensitivity of CMOS silicon detectors is quite similar to CCDs.

CMOS detectors are cheaper to produce than CCDs, thus most digital

cameras have CMOS image sensors inside.

In normal use, to make the sensitivity of the camera similar to the sensitivity

of the human eye, the infrared light is filtered out by the so called infrared

blocking filter, which is usually placed in front of the CMOS sensor.

If this filter is removed, or some other filter is used instead, the camera is

called “modified” and becomes sensitive to the infrared light, but less usable

for everyday photography, unless a separate filter is used to block the

infrared radiation.



DSLR cameras have UV-IR rejection filter in front of the sensor!

Standard infrared blocking filters even block a large part of the red light, to

produce better color reproduction in the photographs (red line), which

reduces camera sensitivity to the hydrogen-alpha spectral line (656 nm)

dramatically.

For astrophotography, comercially modified cameras are available with

better red sensitivity (blue line), but even such cameras do not record

infrared radiation.

For infrared photography, the blocking filter is completely removed and

usually replaced by a piece of clear glass, to preserve the autofocus

capability of the camera. A modified camera can be used even without this

glass cover, but then the focus will shift and normal photographic lenses will

not be able to focus the image of faraway objects anymore. In both cases,

the spectral sensitivity becomes similar to the one shown on the previous

slide.



Monitoring methods: modified DSLR + fish-eye lens + filter

blue visible Nd and -IR

25A R72 RG830

Light sources at the horizon hiden by a circular lens hood covering horizon up to 10o altitude.

To monitor LP in the infrared we used a modified DSLR (Canon EOS300D

with a glass plate instead of the blocking filter), 8mm F/3,5 fish-eye lens

(Peleng) and a custom set of filters to isolate visible or infrared light we

wanted to study.

The blue filter transmits only blue light, visible is similar to the blocking

filter in camera, Neodymium filter transmits most of the visible light, but

blocks most of the yellow light produced by all sorts of natrium lamps, 25A

transmitts red and infrared radiation above 620 nm, R72 infrared above 720

nm and RG830 radiation above about 830 nm.

The camera was put onto a horizontal patform (the fish-eye lens camera

always points to the zenith) mounted on an EQ-2 mount with its original RA

motor. This is accurate enough for such short focal lengths. The light

sources at the horizon were masked by a circular lens hood (not seen on this

picture) to avoid strong reflections in the lens system.



semi-rural sky, SQM-L: 20,2 at zenith, VIS (normal DSLR)

This is an all-sky photograph of the semi-rural sky with moderate light-

pollution, which is quite strong near the horizon.



same sky, IR (mod. DSLR+RG830)

Same sky photographed in the infrared. The LP is still present, but milky

way is more pronounced. Note a very bright tree. Leaves reflect infrared

light very strongly, thus the vegetation appears very bright on infrared

images.



Results

10%

20%

VIS (normal DSLR) IR (mod. DSLR+RG830)

0 30 60 900 30 60 90

iso-

phote

each

Isophotes (curves connecting points of the same light intensity) of the

previous two images. Upper row: isophotes are for each 10% of the

brightness increase, relative to the sky brightness in the zenith). Lower row:

isophotes are for each 20% .

The isophotes show that in the infrared LP increases a little slower towards

the horizon than in the visible. The horizontal scale on lower row images is

zenith distance, and the vertical red line is at 100% increase of sky

brightness, relative to the zenith.



Monitoring methods: modified DSLR + spectrograph

The second method used was spectroscopy. A specially designed

spectrograph, capable of recording very dim spectra was used. In all cases

spectra of the sky arround zenith (the sensitivity cone is about +- 10 degrees

from the optical axis of the instrument) are recorded.



Fast prismatic spectrograph 420-1100 nm

The spectrograph revealed: At the right side is the slit cover and protector,

into which a gas-discharge calibrating lamp is built in. The large knob

controls the slit width, which is not essential. A fixed slit of 20-25 mikro-

meters would be as good as the variable slit we used.

In the middle of the construction are thee glas prisms, cemented together,

that disperse the light without changing the direction of the optical axis (the

so-called direct vision prism). Such prism makes constructing and use of the

spectrograph much simpler. In the black plastic ring to the right of the

prisms is the collimating lens, in this case an achromatic objective lens from

an old binocular. At the right side is the so-called camera lens that produces

the spectrum image on the camera detector. This is a fast 50 mm F/1,8

photographic lens. The camera itself is attached to the black ring at the left

end of the device, which is slightly tilted to compensate for the focal shift of

the infrared part of the spectrum.



LP spectrum, same place

Ne comparison

Sky spectrum arround zenith

The sky spectrum of the moderatelly poluted sky (same night as image on

slide 8). At the top is much brighter comparison spectrum of the neon,

produced by the gas discharge lamp mentioned before. It is used for

wavelength calibration of recorded spectrum. Long vertical lines are spectral

lines of the night sky, in this case all produced by the artifical light sources.



Results

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

400 500 600 700 800 900 1000 1100

wavelength (nm)

inte

nsi

ty (

a.u

.)Na 818,3 nm

HP Nalamps

The final result is intensity-calibrated spectrum of the light-polluted night

sky. All features seen are due to high pressure natrium lamps. Note that

intensity calibration is poor at the ends of the spectrum (below 500 and

above 100 nm).



HP-Na bulbs are the culprit!

HP-Na

sky

MH

Direct comparison of high pressure natrium lamp spectrum (upper

spectrum), night sky spectrum (in the middle) and metal halide lamp

spectrum which proves that all lines seen in the night sky spectrum are due

to the light pollution. Lamp spectra are taken from street lamps near the

observing site. Note that metal halide lamps can have quite different spectra,

depending on the model, but a lot of blue light is typical for all types of such

lamps.



possible solutions

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

400 500 600 700 800 900 1000 1100

wavelength (nm)

inte

nsi

ty (

a.u

.)RG 850

custom interference filter?

Hαααα

To improve contrast in images of the night sky, a custom filter that transmits

only light with wavelengths between 620 and 800 nm(blue rigion) could be

used for gaseous nebula photography, as it passes through the hydrogen-

alpha line. Alternatively, an RG 850 glass filter that lets only infrared

radiation above the sodium 820 nm line can be used for infrared

photography, but note that CMOS sensitivity (thick blue line) is quite low at

such long wavelengths.



A better sky in Korenica (SQM 21,2), VIS (Nd-IR)

Just for comparison, much better night sky (korenica in Lika, central

Croatia) in visible and in infrered (next slide). Some presence of light

pollution from nearby street lamps can still be detected near the horizon,

here formed by the future observatory wall.



A better sky in Korenica (SQM 21,2), IR (RG830)

In IR image a lot of faint cirrus can be detected. They are not an LP-related

effect, but thin cirrus clouds which are visible near the horizon due to faint

water vapour emissions in the infrared.



Conclusions

1. There is strong "LP" in the near infrared too.

2. It is produced by the same sources responsible for the LP in

the visible. Natrium bulbs produce very strong IR LP.

3. IR is not so crowded with LP spectral lines as visible, good

filtering still possible.

4. scattering of the IR light is not so effective as for the visible

light, so sky quality is little better.

5. Light cirrus clouds often prominent in the IR, invisible in the

visible light.



Thank you for your attention!

Questions?

Some Aspects of Light Pollution in the Near Infrared

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