Subject: Questions about the current status of DIS II

From: Jeff Morgan

Submitted: Wed, 5 Jun 2002 10:35:34 -0800

Message number: 582 (previous: 581, next: 583 up: Index)

>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 
jeff morgan

>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.

>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

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 

>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.


APO APO APO APO APO  Apache Point Observatory 3.5m  APO APO APO
APO  This is message 582 in the apo35-general archive. You can find
APO  the archive on
APO  To join/leave the list, send mail to
APO  To post a message, mail it to