From: jegunn [mailto:jeg@astro.princeton.edu] Sent: Friday, February 20, 2004 8:45 AM To: french Subject: filter specs Attached (plain text files) are a verbal description of the filters as well as response curve data. The curves are the FINAL response expressed as a quantum efficiency, and include the detector, telescope, and all camera optics (in column 2) and the atmosphere looking through various airmasses in the next 4 columns. It is the final response you want to mimic; the verbiage says roughly what you need to do with the filters to achieve this, but much depends on the characteristics of the CCDs you guys use. Give this to Brenna (do you have her email?). If she has some response curves for the chips you are likely to use, I can check them. --jim ************************************************************************ *** THE SDSS FILTERS AND RESPONSE FUNCTIONS I. Introduction There has been and we anticipate will continue to be interest in the community in building photometric hardware systems which allow replication as well as possible of the SDSS photometric system. The original `theoretical' definition of the SDSS system as defined in Fukugita et al and implemented with fair accuracy in the USNO filters/CCD combination which was used to set up the original u' g' r' i' z' standards is, unfortunately, not very close to the natural system of the SDSS camera, upon which the survey magnitudes are based. This paper presents the MEASURED response functions of that system, and gives some guidance to those who would like to specify filters and detectors which will allow simple transformations to it. The system is not easy to reproduce in the u and z bands, but should be relatively simple to replicate in the g, r, and i bands. In the u band, the ccd should have relatively flat response in the range 3200-3800 \AA. Normally coated CCDs, which may have higher quantum efficiency at the u effective wavelength of about 3500, have steeply falling quantum efficiencies in this wavelength region, and will produce a system with considerably larger effective wavelengths with the filter we will specify; unfortunately, alternative glasses which would correct this are not available. In the z band, the cutoff to long wavelengths is determined by the response of the detector, so the z measurements cannot be well reproduced without choosing a detector which fairly closely matches ours. Even in the SDSS camera with SITe detectors of the same type and produced relatively closely in time, there is a quite large variation in z response; the measured effective wavelengths of the z bands in the camera range from 8884 to 8974 \AA, resulting in a 3 percent RMS variation in i-z color from one column of the camera to another for the same object. This is for objects of neutral color; for very red objects the situation is worse. For the other bands the variation of detector response makes less difference, but even here for given filters the effective wavelengths can move a few tens of \AAngstroms with changing slopes of the detector across the bands, because the bands are quite wide. II. The Detectors There are in the SDSS camera three kinds of detectors. In the UV we use thinned CCDs with a special UV coating which, as mentioned above, results in relatively flat QE across the u filter band. For reference, the model CCD response used to derive the filter characteristics discussed below is given in table 1a. In the middle bands (g, r, and i) thinned CCDs with a visual antireflection coating is used, and the response in the relevant wavelength region is given in table 1b. Finally, in z, thick (frontside illuminated) chips are used; the average response in the z region is given in table 1c. The g, r, i, and z chips operate at -80C; the u chips at -85, and the QE tables are values at the operating temperatures. III. The Filter Prescriptions The short wavelength cutoffs of the g, r, i, and z filters are set by colloidal longpass filters of the usual Schott GG,OG,RG series. The long wavelength cutoffs of g, r, and i are set by a single-surface interference coating. The difficulties incurred in understanding the behavior of these films with humidity and vacuum and temperature are perhaps interesting in themselves and in any case were the cause of much headache in the SDSS project. Suffice it to say here that the survey filters in the camera, which define our photometric system, are in the CCD vacuum dewars and as a result have characteristics which are much different from the same filters as manufactured and measured in ambient air. They appear to be extremely STABLE in vacuum, much more so than in air, but the red edges are considerably bluer than those of the same filters measured in air at ordinary temperatures. The factor by which the wavelength is reduced is 0.9730 +/- .0009 for all the filters and is the same within the scatter for the three kinds of filters. The phenomenon almost certainly reflects the change in refractive index of the relatively porous thermal-evaporation deposited films with hydration in air. Asahi has developed ion-beam deposition techniques which result in much denser films and which do not display this phenomenon, but we did not know of its existence when the filters were manufactured. It is this change which is primarily responsible for the large differences from the filter responses in the Fukugita et al paper, but the system described there also did not take account of the differences in the detectors in the camera, and was defined for a single UV-coated CCD with rather inaccurate predictions about the infrared response. 1. The g Filter The g filter is a combination of 2.00 mm of Schott GG400, and 3.00 mm of Schott BG38 cemented and coated with a shortpass coating with a measured 50 percent transmission point at 5327 \AA. The standard deviation of this point in the camera is 3 \AA. The glass varies much more than this; the blue 50 percent cutoffs of the GG400 vary from 3953 to 3997 (3980 +/- 18(sd)); the effective wavelength is 4664+/-7(sd). 2. The r Filter The r filter is a cemented combination of 4.0 mm OG570 and 1.0 mm BK7 (to keep the optical thickness of all filters at 5.0 mm) coated with a shortpass coating with a measured 50 percent transmission point at 6746+/-6(sd)\AA. The blue 50 percent cutoff of the glass filters is 5586+/-16 \AA, again with much worse variation than the interference films. The effective wavelength is 6160+/-7\AA. 3. The i Filter The i filter is a cemented combination of 4.0 mm RG695 and 1.0 mm BK7, coated with a shortpass coating with a measured 50 percent transmission point of 8212+/-5 \AA; the glass 50 percent cutoff is 6915+/-9. The effective wavelegth is 7496+/-4. 4. The u Filter The u filter is a cemented combination of 1.00 mm UG11, 1.00mm BG38, and 3.0mm UBK7. There is a coating which acts as a fair antireflection coating in the 3200-3800 \AA region but whose primary purpose is to reduce the redleak of the combination of glasses in the 6600-8200 \AA region; the transmission of the assembly should be less than 1.e-4 in that region. The vacuum properties of this film in the SDSS filters is similar to that of the shortpass coatings. In the case of the u filters the changes result in movement of small interference features in the u passband with negligible effects on the photometric properties of the filters, except that the redleak blocking is quite severely compromised. The passband is largely determined by the UG11, but lengthened somewhat by the BG38. The effective wavelength is 3507+/-6 \AA. 5. The z Filter The z filter is a cemented combination of 4mm of RG830 and 1mm of BK7, with an antireflection coating efficient between 7800 and 11000 \AA. The 50 percent transmission point of the glass is 8266+/-12\AA, and the effective wavelength is 8929+/-40 \AA, the large variation again because of the variation of the infrared response of the CCDs and the fact that it is the detectors which determine the long-wave cutoff. ************************************************************************ \ res fun for sdss u0 created Sun Dec 02 16:59:19 2001 \ lam vac air1.0 air1.3 air1.6 air1.9 2980 0.0000 0.0000 0.0000 0.0000 0.0000 3005 0.0019 0.0001 0.0001 0.0000 0.0000 3030 0.0097 0.0009 0.0005 0.0003 0.0001 3055 0.0174 0.0021 0.0012 0.0007 0.0004 3080 0.0270 0.0041 0.0026 0.0016 0.0010 3105 0.0427 0.0077 0.0051 0.0033 0.0022 3130 0.0630 0.0133 0.0091 0.0063 0.0043 3155 0.0853 0.0217 0.0157 0.0114 0.0083 3180 0.1074 0.0312 0.0235 0.0178 0.0134 3205 0.1281 0.0408 0.0317 0.0246 0.0191 3230 0.1474 0.0501 0.0397 0.0314 0.0248 3255 0.1654 0.0592 0.0476 0.0382 0.0307 3280 0.1817 0.0677 0.0550 0.0447 0.0363 3305 0.1956 0.0756 0.0621 0.0510 0.0419 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