I am forwarding the attached message from Jim Gunn to apo35-dis. -Ed Turner -------------------------------------------------------------------------- The following table is calibration data taken with DIS on June 13 1994, on the AB79 standard +26 2606. The exposure was taken at an airmass of 1.01, slitless, with the low-dispersion gratings. The following table gives the wavelength in Angstroms, lam, the counts AVERAGED over a 7 column swath in the blue and an 11-column swath in the red, cnt, the AB79 monochromatic magnitude of the star ABnu, the assumed atmospheric extinction, a, (magnitudes), and the computed quantum efficiency on the sky with the atmosphere corrected for--i.e., the quantum efficiency referred to the entrance of the telescope, qe. The assumed dispersion in the blue was 6.25 A/pixel, in the red 7.00 A/pix, and the gains 0.97 e/DN and 1.47e/DN, respectively. The exposure time was 5 seconds. BLUE SIDE (multiply cnt by 42.4 get electrons/A ) lam cnt ABnu a qe 3500 10 10.79 0.71 0.001 3600 50 10.72 0.58 0.003 3700 500 10.65 0.53 0.025 3800 2000 10.39 0.48 0.077 3900 3200 10.31 0.44 0.114 4000 4300 10.14 0.38 0.127 4500 6600 9.93 0.24 0.159 5000 7300 9.77 0.18 0.159 5250 6200 9.72 0.16 0.133 5500 2800 9.67 0.15 0.060 5750 1100 9.63 0.14 0.023 6000 600 9.60 0.13 0.013 3650 140 10.61 0.55 0.0068 4046 4600 10.12 0.37 0.134 RED SIDE (multiply count by 113.2 to get electrons/A ) lam cnt ABnu a qe 5000 20 9.77 0.18 0.001 5250 350 9.72 0.16 0.016 5500 1420 9.67 0.15 0.064 5750 2250 9.63 0.14 0.102 6000 2450 9.60 0.13 0.112 7000 2660 9.51 0.08 0.124 8000 1650 9.47 0.06 0.083 9000 780 9.46 0.14 0.047 9500 350 9.46 0.43 0.029 10000 150 9.46 0.04 0.009 Thus the peak qe in the blue is about 16 percent, and in the red a bit better than 12 percent. What do we expect? The measured reflectivities of the mirrors are 0.88, 0.88, and 0.80 for the tertiary, secondary, and primary, respectively (at about 6400, but these numbers should serve well except below 4000 and in the infrared dip around 8000; the throughput of the telescope is thus 0.61. The spectrograph throughput is compounded of the following approximate numbers, which are roughly the same in the blue and in the red: 0.90 dichroic 0.97 field optics 0.98 collimator 0.60 grating efficiency 0.92 camera (mostly secondary obscuration in the Schmidt) The spectrograph throughput is thus approximately 0.47. The blue CCD has peak qe around 0.60, that of the red about 0.50. Thus we expect qe's of about 17 percent and 14 percent, respectively, at peak. The performance is therefore roughly as expected except for the precipitous drop in sensitivity shortward of 4000 A, the half-sensitivity point occurring at about 3800 instead of the design number of 3400. The CCD qe was measured to decline from about 60 percent at 4000 to about 45 percent at 3500 in the spring of 1994. A series of tests to measure the ratio of the throughput at 3650 A and 4046 A (the two strong Mercury arc lines) of the blue-side optical elements was performed in June 1995, and it was determined that serious losses were occurring in both the main camera block and the field optics group; those elements showed ratios of transmission of about 4 and 3, respectively. For all the elements, the transmission ratio is about 12:1, the other elements contributing negligibly. Both of those elements have internal reflective surfaces. All of the purely transmissive optical elements including the dichroic had ratios near unity. Correcting for the known slight decline in CCD qe, the ratio of the system qe at 3650 and 4046 should be about 1:16, .062, with an uncertainty of perhaps 20 percent. It was measured the year before with the above data at .0068/.134 = .051, well within the measuring error of the rather crude technique to measure the optical transmissions. It thus does not appear that the condition has worsened over the past year, and that it does stem from bad reflective coatings. Don Loomis, the optician who constructed the optics for DIS, has reported that it is possible to disassemble the cemented quartz assemblies involved, and could do so with a turnaround of about two weeks. The coaters, ZC&R, have not yet been contacted concerning quotes to recoat, nor for what might have gone wrong or might go wrong again, nor for how much responsibility they might we willing to take for the condition. Coating the camera, in particular, is complex, and one could expect a turnaround time of at least a month with another week back at Loomis's for reassembly, and a week at APO for refocussing and collimation. Thus the spectrograph would be out of service for about two months. I would suggest that the fix be done during the summer monsoon of 1996, with exchanging the detectors for more modern CCDs and electronics to follow within a year. With more modern detectors, 'super' coatings on the secondary and tertiary and a good coat on the primary, one could expect peak qes of about 24 percent in the blue and 29 percent in the red (assuming 94 percent and 88 percent, respectively, for the secondary and tertiary, and primary, respectively, and 65 and 80 percent for the CCD qes. APO APO APO APO APO Apache Point Observatory 3.5m APO APO APO APO APO This is message 7 in the apo35-dis archive. You can find APO the archive on http://astro.princeton.edu:82/apo35-dis/INDEX.html APO To join/leave the list, send mail to apo35-request@astro.princeton.edu APO To post a message, mail it to apo35-dis@astro.princeton.edu APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO