From: Andrew C. Phillips
Subject: Slit images seen in internal flats
Date: Tue, 15 Oct 2002 14:25:29 -0700
Some of us have noted features in some internal flats that look like "arcs" of light and vary in number and position depending on which slitmask is in place. These have been seen with the 900-line grating. They appear only with internal flats; identical-setup dome flats are clean. I spent some time after the last DEIMOS run investigating these. Here's what I've found:
Another check on this has to do with angles: the path between slitlet and camera edge is at an angle of 14 deg wrt DEIMOS Z-axis. If I'm doing all my arithmetic right, this means it enters the camera at an angle of about 2-deg wrt the camera axis, clearly within the acceptance angle of the camera. Assuming (correctly, I believe) that the grating is on-axis wrt the camera axis, this light should strike the grating about 3.8-in off center; since the grating is 8-in total length, this means the light does (barely) hit the grating as well. Thus, this also supports my hypothesis, since the proposed path will indeed throw light into the camera.
I suspect the "curved" appearance has to do with the fact that the reflecting surface is a ring, and since the images are far from focus, their location will depend on what area of the pupil is illuminated. For most straight slitlets, the portion of the pupil illuminated will follow an arc (since the reflecting ring is narrow and circular) and thus the out-ot-focus image produced will also be an arc.
The beams will be slow -- as I recall, the ring is about 1/4" wide, so at 10" distance this is f/20. I'm not sure if this is slow enough to explain the images we see, though. However, the focus will be much better across the slit than along it, and this is indeed observed.
I'm not sure about the other, fainter images seen -- perhaps someone can check if there are other surfaces near the camera mouth that could reflect light along a similar path?
In summary, I believe the "ghosts" seen in internal flats with the 900-line grating are due to reflected light off the steel ring around camera mouth.