Hello all, The responses to my earlier solicitation for opinions on DIS grating availability after the upgrade were many and varied (surprise!). Most respondents seemed to feel that the suggestions I made sounded good, but the gratings they eventually ended up using would depend upon the throughput and S:N with the new detectors. We have now completed much of the characterization of the new DIS, and in some senses the throughput is not yet as good as we could wish. The blue field flattening lens has a red coating on it which causes a sharp dip in response starting at about 4600 Angstroms. The red chip has some additional read noise in the form of periodic spikes every few hundred pixels. Both of these issues are expected to be resolved in April, along with an improvement of the focus/tilt of the blue detector, and at that time further characterization will be needed. A third problem, that the optics designed for the old detectors cause some vignetting on the new larger-format detectors, will not be addressed until the summer shutdown (and then FURTHER characterization will be needed!) You can read some of the initial results of the characterization at http://ganymede.nmsu.edu/holtz/3.5m/dis/ . Further work is still ongoing. Also, check the plot of vignetting at the top of http://galileo.apo.nmsu.edu/~mcmillan/displots.html . The significance of all this for the choice of which gratings to use is that there may still be reasons for users to want any of the six available gratings. Although the medium resolution gratings permit a full spectral range to fall on the chip, only the central portion of the chip has the best throughput. For 80% of maximum throughput or better, the relevant portions of the spectrum should go between columns 400-1600 (on either chip), and for 90% or better the spectrum should fit between 700-1300. This means that some observers pursuing the very faintest objects might still want to use the low resolution gratings. Note that the sensitivity and S:N are still an improvement over the old detectors, and there are other options for changing resolution without changing gratings; the chips can be easily binned in either X or Y to consolidate light along the slit or in the dispersion direction. Given all these considerations, we will not be making any gratings unavailable during the next few months, but we do hope to encourage users to stick to a limited set. The default sets we have arrived at are the high resolution pair and another pair including the low blue and the medium red (both these gratings are 300 lines/mm, providing a dispersion of 3.1 Angstroms/pixel on the detectors). One pair of gratings provides the best resolution we can offer, while the other provides a compromise between full spectral range, good S:N, and reasonable resolution (all better than with the old detectors and the low OR medium gratings). S:N can be further improved by binning, if needed. For the time being, we are continuing to use the old low, medium, and high names for the gratings, even though the two low gratings, for instance, do not have the same number of lines/mm. We are discussing new terminology now and would welcome suggestions; simple names for pairs (e.g., Pair 1), or specific names for individual gratings, (e.g., Red 300 l/mm?) Here's the procedure that will be used for DIS observations during the next quarter: 1) Regardless of the grating requests included in your Q2 proposals, we will assume that one of the default grating pairs will meet your needs. 2) If you are happy with either of the defaults, you don't need to send us advance notice. We would still appreciate a note one day before your observations to let us know you're aware that the observing time is coming up, and to tell us what grating centers you will likely use; if you change your mind at startup, those numbers can be altered. 3) If you are NOT happy with the defaults and want another grating pair, you must inform us at least 3 days in advance, with confirmation 1 day in advance of your observations. a) Please tell us why you need a non-default setup, and specify at MOST one red grating and one blue grating for your program. Check the list of available gratings at http://galileo.apo.nmsu.edu/~hastings/Manual/instrumentinfo/dis/main.htm to be sure you're requesting gratings that we actually have, and that they will provide the resolution you need. Gratings may be mixed and matched (as with the default BL+RM combination), but red gratings cannot be placed on the blue side or vice versa. b) If your request arrives late, or if you request non-existent gratings, it's possible the setup will not be ready at your scheduled start time. Grating change and re-calibration requires approximately half an hour per grating, one hour per red/blue pair. c) If your non-default grating request conflicts with the default request of another observer on the same night, you will have to give up time to allow the grating change to occur without detracting time from the other program. If you have a non-default grating request which DOESN'T result in any conflict, you shouldn't lose any time. If both the first and second-half observers have non-default grating requests which result in a conflict, the time required for the change will be split between both programs. This is similar to the policy for planned vs. unplanned instrument changes, and should be your incentive to try the default gratings if they can work for you. At the end of the second quarter (or perhaps when the optics are upgraded), we will review this policy. At that time it may be possible to retire some gratings from use or make some other simplifying change. For now, all the gratings are still available but we encourage users to try the default sets. Thanks to everyone who gave us their input on this question, and we hope you'll all be happy with the new detectors! Russet McMillan. APO APO APO APO APO Apache Point Observatory 3.5m APO APO APO APO APO This is message 563 in the apo35-general archive. 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