Today I checked the zero positions of all the mechanisms (apart from the flip mirror, which I do not want to mess with). Those were ok, but in the course of the tests I began to suspect that enabling/disabling the motors may be causing inconsistencies in slit positioning....will need to look at it more to be sure.
I spent way too much time trying to figure out why I couldn't keep the FCLS 100µm source on slit 4 while I was scanning though all the slits saving images. It was quite inconsistent.
Eventually, to determine where the motion issue was, I did a series of measurements of slit center position going back and forth between the slit wheel open position, slit 4, and slit 7. It was rock-solid and the slit position on-screen did not move at all.
But when I tried again to image the FCLS through the series of slits, the source immediately fell off the slit when I started taking and saving frames through the 'observing sequence' tab. I moved from slit 7 to slit 6, measured and recorded the pot reading, then collected a frame. The source (which should have been close to centered) was not on the slit, and when I rechecked the pot with an 'update all', I got an error dialog stating that the voltage and degree settings did not match within the volt_err limit, even though no move had been executed.
My hunch is that when the motors are disabled during the 'observing sequence' operation, the mechanism relaxes a bit and the slit shifts in the FOV. The back-and-forth moves I had done did not involve the 'observing sequence' pane so the motors were continuously enabled, so the fact that they were very repeatable in that test may not translate well to actual observing.
If this hunch turns out to be true, we might find ourselves setting up on a target and then having it fall off the slit when we actually take data.
Curtis has said that he's experienced dropping steps with the motors, even when operating through the LabView GUI. I need to look at this further, and hopefully we'll find it's something else (that's easy to fix!).
In other news, it turns out that the s/d/f cassette wheels zero positions were all very close, but the lens wheel was obscuring our view of the s/d/f aperture when set to 'open' from the pulldown menu. I shifted the volt_zero from 7.003V to 7.028V, and I believe it is good now.
I next moved each of the slits into position and noted the pixel positions of each slit end, and then inferred the position of each slit center. here is what I got for each of the selections in the pull-down slit wheel menu:
slit top bot center Approx. width
7 (396,896) (542,86) (469,491) 48 px.
6 (394,895) (539,85) (467,490) 20
5 (394,895) (539,84) (467,490) 16
4 (391,895) (536,86) (464,490) 12
3 (391,893) (535,85) (463,489) 9
2 (387,893) (529,86) (458,489) 5
1 (386,892) (528,87) (457,489) 5
So the slits are in close but not perfect registration, and there is no point in the field of view that fits in all seven slits. I decided to center the FCLS on slit 4, which would get at least some light through in slits 3-7. I had meant to collect images through these slits at this FCLS position, and that's when I ran into the problem with the source falling off the slit :(
The center of slit 4 measured above is about 1.11mm of FCLS travel from the HIPO boresight (~4.5" of arc).
The 100µm pot is defocussed mostly in the spectral dimension. It's about 13.5 x 3.6 pixels, but that might not mean anything since we're in camera mode and we care most about hi-res focus.
Tomorrow we'll look more at the motor issue, and hopefully come off the TAAS for some gas-cell tests.
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