>Below are some answers to some general questions about the status of >DIS II which were asked by Dan Zucker recently. The answers to >these questions might be of general interest to many users of the >3.5-m. jeff morgan UW >Hi Dan, I hope you don't mind me posting my responses to your questions to the general observing community. Several of the question that you have asked may be of interest to all who are observing with DIS. jeff >Hi, > >We're planning our observations for tomorrow night (UW05, June 5 A + B) >and were wondering about the current status of DIS II. Our understanding >is that there is a new CCD in the blue side as well as a new blue-coated lens. Yes. This is correct. > Does anyone have a sensitivity curve for the new setup? No this hasn't been done yet. The data are in but the analysis is pending. My guess is that we will have this information and make it available within a couple of weeks. >Is there >still a problem with vignetting, and is it a function of wavelength (i.e. >worst at blue and red ends)? Of course, the new chips are much larger than the old. Plus, they are Schmidt cameras, you always have vignetting from the secondary, its just a matter of how much and how fast it changes over the field of view. The vignetting over the central portion of the chips has not changed from the original DIS because the lenses themselves have not changed. Inside the central 800 pixels the camera throughput changes by less than 5%. Outside the original bandpass of the system you are now able to see the vignetting fall off. This vignetting "problem" will not be solved until the camera lenses themselves get replaced. This will require a change in the pixel resolution from the current configuration. The DIS instrument web page put together by Cameron H. has a table which shows what the current and projected instrument resolutions will be once the new camera optics are in place. That will not happen for at least another 3 months. By the way...when you are trying to compare the new DIS performance with data which you may have taken with the old DIS, please remember that the pixel resolutions are quite different with the new chips! You have to take this into account when comparing the results of both configurations. >Is there still an electronic noise problem >with simultaneous readout of both cameras? Yes, but the chips don't read out simultaneously so you won't see this noise. We are currently building new electronics which we hope will allow us to read out the chips simultaneously. This will probably be done within a month. >Finally, are there any other >issues we should be aware of in planning DIS II observations? The red side currently has a 120 Hz noise pattern which shows up in the form of single pixel spikes of a mean of about 10 ADU above the bias separated by about 470 pixels (along a row). The origin of this noise is not understood. We continue in our efforts to track it down, but have no clue as to when this problem will get solved. These spikes do not come in a fixed pattern. To avoid them in sensitive measurements you are advised to take multiple exposures of your object and then use a median filter. These chips have rather significant fringing. Clearly, this also only affects measurements made with the red side. If you are observing anything red-ward of about 7350 A, then you should plan on using only a single grating setting for your entire night. You should also make efforts to minimize your use of the turret. We have done repeatability tests of each of these motions and have found that turret movements degrade the fringe corrections, but probably do not destroy your observations. Grating tilts will make fringe corrections hopeless because the grating motions on DIS are not accurate enough to reposition yourself well enough to make corrective flat fields after the grating has been moved. Rotations of the whole instrument will also affect the fringe corrections, but probably not in ways which will destroy the value of your observations. Instrument rotations (when taking flats) tend to induce slow spatial variations in the fringe corrections. This is true when taking lamp flats and is due to small changes in the illumination of the slit. It is not yet clear how strongly instrument rotations affect sky flats. If you have measurements of this, we would like to hear about them. > >We are particularly concerned with response at 3727 A ([O II]), which we >need for our science. If you had asked about this earlier, you would have found that even with the old flat field lens with its improper coating you would have not been strongly affected at this wavelength. The old coating affected the spectral region between 4900 and 3800 A. And in this region the old throughput measurements indicated that even with the improperly coated lens the old blue detector was more sensitive that the old detector. The replacement blue chip (I apologize once again for the damage of the old one!) has even better blue QE than the blue chip which was damaged and lab measurements seem to indicate the the new coating has solved the problem of absorption near 4300 A. But, there are difficulties with the lab measurements that make us uncertain as to how well we have done with this fix. Real proof awaits the analysis of the through-put measurements. jeff APO APO APO APO APO Apache Point Observatory 3.5m APO APO APO APO APO This is message 582 in the apo35-general archive. You can find APO the archive on http://www.astro.princeton.edu/APO/apo35-general/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-general@astro.princeton.edu APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO