July 2000 - Report from R. Barkhouser
Robert Barkhouser obtained pictures looking into the 20-inch telescope from its focal plane. The camera is a commercial pinhole system (described below) modified to accept either a 1.25 inch eyepiece or a C-mount device such as a CCD camera. It's installed in a copy of the PT CCD dewar lid that includes an uncoated version of the dewar window and the CCD light shields. An X-Y stage allows exploration of the CCD area.The pinhole camera was mounted in place of the CCD dewar and the dome was opened to the daytime sky. No filters were installed on the camera but the yellow fabric cover was kept on the telescope opening.
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This is the view from the center of the CCD field.
The central disk is the sky; anything else is unwanted.
The field of view for all pictures here is 54x42 degrees.
The worst item is the glint off the C1 lens cell. There is a 1/8 inch long cylinder wall that in retrospect should've been beveled out of the beam. Farther out is the fainter outline of the filterbox top baffle. This hole can be beveled to eliminate this light. The square pattern is a reflection off the inside of the filter holders. These would be blocked by filters and not seen during regular observing (we will check this). |
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This view is from the edge of the CCD field.
The top baffle reflection is faint and not marked
but the inside of the C1 mounting flange is now
visible. This is a black anodized aluminum part
about 1/2 inch thick that can be rifled and painted
to eliminate this light.
Also visible is what we think is the secondary retaining ring (it shows the three tabs that hold the secondary in place). This is an anodized aluminum part that might be painted to reduce the reflected light. |
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This is the view from the CCD corner. New here is the smudge of light off the inside edge of the dewar lid. While this isn't the actual CCD dewar, we expect a similar result with the CCD system. This particular problem would be difficult to eliminate but it only affects the CCD corners. |
These pictures were taken after the permanent primary baffling was installed. Before and after pinhole inspections showed no changes due to this work. Robert Barkhouser reports that these pictures capture fully what is visible to the eye, that is, no features besides these were seen when an eyepiece was used. Unfortunately, we do not have calibrations to measure the quantity of stray light.
The light off the C1 cell was not anticipated and while we don't know if it's causing problems it's easy to fix if we decide to do so. Except for the light off the dewar lid, the other reflections are also easy to eliminate. It's probably not worth the effort or risk to modify the dewar lid.
July 2000 - Report from J. Briggs
John Briggs sent this report on July 26, 2000, describing another small source of scattered light, the outside surface of the secondary mirror baffle:Eliminating this scattered light probably means replacing the secondary baffle. This would be a moderate effort....
I just completed some scattered light observations this morning, using the crescent Moon. Without the potential bias of studying your scattering report on the WWW beforehand, I independently discovered most of the issues mentioned there, and evidently one new one. Using the Moon, what was especially noticeable to me were the filterholder reflections, the filterbox top baffle, and the C1 cell. I did not notice any other problems, except one possibly related to what you described as the "secondary retainer" reflection.
What I saw, in about the same location as the secondary retainer reflection, was a striking glint of moonlight off the outside surface of the secondary mirror baffle. One notices this problem only if two conditions are met: 1) if you are looking from a position corresponding to a corner of the CCD; 2) if the Moon is positioned just right, off-axis. In these circumstances, this glint is brighter than anything else!
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