Scattered Light in Telescope

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I've begun to investigate scattered light in the telescope in the imaging train by following the work of Grundahl and Sorensen (Astron. Astrophys. suppl. ser. 116, 367-371, 1996).

Here are three images taken with a camera obscura (~0.5mm pinhole mask in front of the CCD camera) of a flat screen. The pinhole mask was placed in an empty slot of the CCD camera filter wheel. All exposures are 5 seconds. Images were not calibrated. The images are log-scaled to make the light scatter more obvious.

I have not totally accounted for the causes of the scattered light, so this is a work in progress.

This first image is with the telescope in its original state. The white disk with the darkened central spot is an image of the secondary mirror. The next mottled ring (Ring1) is caused by reflections off of the inside surface of the hole at the back of the OTA. The mottling is probably caused by "hot spots" off the surface were there is more intense reflection. The next ring (Ring 2) is caused by reflections off the inside of the focuser. The cause of the outer most ring (Ring 3), which is more pronounced on the left, is unknown.

I installed a wide ring of Proto-star flocking paper to the inside surface of the hole at the back of the OTA.

The next image is shows the mottling eliminated but the ring  (Ring 1) is still present, albeit more uniform. Rings 2 and 3 are still present. I was disappointed that the flocking paper was not more effective.

I then tried to install two (2) field stops made from thin, black construction paper; each having an inside diameter of 2.125". One stop was placed where there light path exits the OTA and immediately in front of the focuser. The second field stop was placed between the focuser and the CCD Camera. 

The next image a slightly less intense Ring 1, a significantly diminished Ring 2, but a persistent Ring 3.


I am beginning to see the results that Grundahl and Sorenson reported, but I was expecting a cleaner image. Grundahl and Sorenson only added baffling inside the primary mirror baffle. The Meade 16" SCT-ACF already has that baffling, Given the existing baffling of the Meade, the addition of flocking paper and two field stops, it is somewhat perplexing that I cannot achieve the same improvements as Grundahl and Sorenson.

Perhaps field stops with smaller inside diameters are required. The telescope FOV is 16 x 11 arc-minutes. The CCD chip is the Kodak KAF-1603ME having dimensions of 13.8 x 9.2 mm. Can a filed stop with a much smaller diameter be used, and if so, how small without introducing vignetting?

Comments welcome.