The UFTI shutter is currently in an open state and is unplugged. Please make sure that UFTI is set “blank” while closing down at the end of the observations or during long periods at night when the instrument is not in use.
Anomalous structures/patterns in images
Very occasionally, unusual rectangular bias structures or patches of “warm” pixels may appear in images. In all cases, the fix seems to be to take a sequence of short dark exposures (use the “flush array” sequence in the ORAC OT template library). These rare structures usually result from leaving the array exposed to background signal (i.e. leaving the shutter open with a filter in place) or from observing very bright sources. Example images are given here. Arcs or “fringes” probably result from reflections when observing sources near to (within a few arcminutes of) bright stars. Again, see example images are available.
Even with multiple resets before every image we are seeing persistence at JHK of 0.3-0.4%, and at shorter wavelengths the effect is worse – 0.7% at Z and 0.9% at I. This acts like an enhanced dark current so that it gets worse with long exposures. For example after exposing standard stars to ~4000 counts, even three frames later, with an 80 second blanked off exposure, blobs could be seen at the standard star jitter positions which had ~2 counts above the background. An example of the effect on a dark frame after a jitter on a standard star is shown in this postscript plot .
This persistence makes flatfielding difficult and as the UFTI flatfield is very stable you may want to take a sky flat with a clean array at the start of the night and use that throughout the night (as opposed to creating flats from median filtered jittered sets). Take clean dark frames at the start or end of the night also. If possible put any bright objects in a different part of the array than your faint targets, and keep your counts at the couple of thousand level, as opposed to 5000 or greater. Finally and most importantly, blank off between exposures so that while you’re acquiring the next object, or preparing the next observation, you are not illuminating the array. If you do get a bright object onto the chip you will want to take a couple of short, 4 second darks (note: not long as the latency acts like an enhanced dark current) before taking more science data.
Negative Count Downs
Although this is very rarely seen, the data acquisition may hang with an integration count down going to less than -30 secs (depending on readout mode; the overhead for full-array, normal NDSTARE is about 5 seconds). When this happens your data are normally still successfully written to disk and displayed. To recover you need to reboot the UFTI VxWorks crate . Your T.S.S. or Support Astronomer can do this in a minute or two. They must go to the computer room and reboot the UFTI VxWorks from the instrument control console; instructions are attached to the keyboard (n.b. try control-x if you can’t get a prompt). In the control room you’ll see UFTI’s Epics display go white for a few seconds and then recover. IMPORTANT: From the UFTI epics window the T.S.S. must then initialise the two filter wheels and the shutter, before setting these to the chosen filters and opening the shutter. You can easily reset the filters – after initialisation – by reloading your sequence and running through the “Configuration” step: however, the shutter can only be opened from the Epics screen. Check also on the Epics screen that the data is not being written to “Eng” (on the epics window).
On occasion you may see your image develop rectangular block-like structure, which we believe is associated with a jump in the bias level due probably to a charge trapping anomaly. Unfortunately the only cure currently is to repeat your exposure. The effect seems to be random and occurs very infrequently. It is usually associated with leaving the array idle for a time without reading out, or exposing on a bright object. If you do either of these then blank off the array and perform a couple of short reads (short darks). Example images are given here
Right-Hand Quadrant Dropout
On occasion you may see the right two quadrants contain several “black” pixels with zero counts. This is probably due to a reset voltage anomaly (these quadrants have a lower bias level than the left-hand quadrants). This is more noticeable with the HiGain readout mode. In Spring 2000 the blacklevel was increased with UFTI to counteract this affect and it is seen far less frequently now. Nevertheless, if it does occur the only cure currently is to repeat your exposure.
Filter Motors or Shutter
Although this has not been seen for a long time now, we did have occasional problems with the filter wheel motor control, and the shutter has stuck half-open in the past. If you suspect there’s a problem, check the counts on background sky in an image; see exposures for a table showing what you should get.
If you do see uneven illumination or low counts, initialise the filters and shutter from the Epics display. Check that the datum status is reported as “OK” with a step number running at either a few tens (e.g. <~30) or close to a full revolution (125000). If it looks OK select a filter and test sky counts again.
If things still look odd, or if you get an error on datuming, try power-cycling the motor controllers. Go into the computer room. At the back of the room you’ll find UFTI’s VxWorks crate. Go to the back of the crate and close to the ground you’ll find an on/off switch for the bottom rack which contains the motor controllers. Power these off and on. Go to the instrument control console in the middle of the computer rack and reboot UFTI (instructions are attached to the keyboard). In the control room you’ll see UFTI’s Epics display go white then recover. Initialise and then reset the filters and shutter from the Epics display, then repeat the sky counts test. Please report any large datum step errors to the instrument scientist, but wait till you’ve datumed a couple of times since after power cycling the motor controller the datum step numbers may look odd (even negative).
Can’t REBOOT UFTI!?
If you’re having trouble rebooting the UFTI crate from the console switch (epics screens go “white” but don’t recover) and you get errors on the console switch, check that the UFTI crate and the motor controller (under the UFTI crate) in the computer room are both switched on (big red lights on the former and a small green LED on the latter should be lit). Also, check that the UFTI “Astrocam” array controller in the dome is powered on. This is the bottom box under the UFTI lakeshore in the UFTI electronics cabinet on ISU2 (its installed back-to-front, so you only see the “back” of the box when you open the cabinet; the switch is on the back). During engineering, ETS may have switched the controllers and crate off and forgotten to switch them back on…