Some Aspects of Light Pollution in the Near Infrared

Transcription

1 Some Aspects of Light Pollution in the Near Infrared Željko Andreić 1 and Doroteja Andreić 2 1 Faculty of Mining, Geology and Petroleum Eng., University of Zagreb, Pierottijeva 6, Zagreb, Croatia, zandreic@rgn.hr 2 Faculty of Veterinary Medicine, University of Zagreb, Heinzelova 55, Zagreb, Croatia, dandreic@vef.hr Near Infrared 1 Near Infrared 1

2 Overview 1. Introduction 2. monitoring methods and results 3. conclusions Near Infrared 2 Near Infrared 2

3 Introduction Light pollution (LP) is usually conected to human vision, so only visible part of EM radiation (light) is considered in studies of LP. Near Infrared 3 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. Near Infrared 3

4 CCD cameras are sensitive to infra-red (IR) up to 1000 nm! IR and there is "LP" in this spectral region too! Near Infrared 4 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. Near Infrared 4

5 C-MOS detectors (used in most DSLR cameras!) are simmilar. IR Near Infrared 5 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. Near Infrared 5

6 DSLR cameras have UV-IR rejection filter in front of the sensor! Near Infrared 6 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. Near Infrared 6

7 Monitoring methods: modified DSLR + fish-eye lens + filter Light sources at the horizon hiden by a circular lens hood covering horizon up to 10 o altitude. blue visible Nd and -IR 25A R72 RG830 Near Infrared 7 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. Near Infrared 7

8 semi-rural sky, SQM-L: 20,2 at zenith, VIS (normal DSLR) Near Infrared 8 This is an all-sky photograph of the semi-rural sky with moderate lightpollution, which is quite strong near the horizon. Near Infrared 8

9 same sky, IR (mod. DSLR+RG830) Near Infrared 9 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. Near Infrared 9

10 Results VIS (normal DSLR) IR (mod. DSLR+RG830) isophote each 10% % Near Infrared 10 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. Near Infrared 10

11 Monitoring methods: modified DSLR + spectrograph Near Infrared 11 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 degrees from the optical axis of the instrument) are recorded. Near Infrared 11

12 Fast prismatic spectrograph nm Near Infrared 12 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 mikrometers 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. Near Infrared 12

13 LP spectrum, same place Ne comparison Sky spectrum arround zenith Near Infrared 13 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. Near Infrared 13

14 Results 1 0,9 0,8 0,7 HP Na lamps Na 818,3 nm intensity (a.u.) 0,6 0,5 0,4 0,3 0,2 0, wavelength (nm) Near Infrared 14 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). Near Infrared 14

15 HP-Na bulbs are the culprit! HP-Na sky MH Near Infrared 15 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. Near Infrared 15

16 possible solutions 1 custom interference filter? 0,9 0,8 RG 850 0,7 intensity (a.u.) 0,6 0,5 0,4 0,3 0,2 0, H α wavelength (nm) Near Infrared 16 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 hydrogenalpha 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. Near Infrared 16

17 A better sky in Korenica (SQM 21,2), VIS (Nd-IR) Near Infrared 17 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. Near Infrared 17

18 A better sky in Korenica (SQM 21,2), IR (RG830) Near Infrared 18 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. Near Infrared 18

19 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. Near Infrared 19 Near Infrared 19

20 Thank you for your attention! Questions? Near Infrared 20 Near Infrared 20