Troubleshooting guide

VIII.Troubleshooting
1) Problem: The image looks funny. You can see all 32 amplifiers, and the signal level is ~58,000 ADU;
the preceding or following image(s) has 0 counts.
Solution: This is a hardware problem related to electrical noise causing an early readout of the detector. In
imaging mode, MCE4 reads the array once, and then lets it integrate before reading it again (cf. Appendix 2:
Readout Schematic). The final image written to disk is the difference of these two images. This problem occurs
when MCE4 spits out the image before making the second read and difference. There is nothing you can do to
resolve the problem.
Typically, we get bad reads for about 2-5% of the images taken for exposure times ≤ 60 seconds. If you are
getting a much higher failure rate you should call for help. You should be cautious with the image immediately
following a bad read frame, because the ADU level in the image is frequently lower than it should be.
We have found that if you get more than two zero frames in a row, then the MCE4 array controller needs to be
rebooted and reinitialized. See the solution to item 4a of the Troubleshooting Guide for how to do this. NOTE:
the first image taken immediately after rebooting MCE4 also looks like a bad read.
Important Note: If the detector is saturated due to excess radiation, one will get an image which looks very
much like a bad read. If rebooting the MCE4 does not solve the problem, move the grism to the true_dark
position and take another image. If the problem persists, call for help. A saturated image will have a level ~
49000 ADU, and the dead pixels in the lower left will be evident; a bad read has a level ~ 58000 ADU and the
dead pixels are not seen (Fig. 5).
Figure 5: Images resulting from flux saturation (left) and a bad read (right).
2) Problem: The image looks funny. Various bits of it are scrambled or not in the correct position, or some
bits of it are completely black (0 ADU).
Solution: See if the next image looks normal, if it does keep taking data. If the next image is similarly messed
up, then the MCE4 array controller needs to be rebooted. See the solution to item 4a.
3) Problem: Excess Background
Possible solutions:
a) Image a very bright star (V ~ 2) in the Ks band, with the telescope way out of focus, say 2000 units
lower. You should see a donut with a central obscuration and spider vanes. If it's not circular, then there could
be something obscuring the beam such as the dome, the mirror covers, or a mispositioned filter, lyot stop, or
Grism wheel.
b) Check that the ion gauge on the Granville-Phillips pressure monitor is off. When left on, the filament
FLAMINGOS@2.1-m, Ver. 2.39, 2011 May 13 Page 34 of 46