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Just back from the Messier star party - the driest and warmest night that I remember at the LMDSS.
The bad news: The astrograph is almost unusable as it is, due to internal reflections.
The good news: It produces perfect stars, corner to corner, and the internal reflection is easy to rectify - so I'm completely happy with it.
I noticed that on 5min exposures, that I stretched severely, faint reflection arcs were visible, that appeared to be produced by off the field bright stars.
In the end I used Jupiter, as the brightest light in the sky, to characterise the problem. With Jupiter in the field I was getting only the expected halo circles, due to faint multipath reflections. When Jupiter was just off the field, I was getting no spurious reflections at all, but as I kept increasing the offset, at some point, perhaps 5 degrees off the field, a bright arc appeared across the chip.
It is quite clear now where I went wrong. When I designed the primary baffle, I concentrated only on cutting out stray light that can enter through the front lens and reflect off the inside of the baffle at a grazing angle.
It turns out that this optical arrangement is trying to produce a much larger image circle, that what can actually fit through the aperture of the small corrector lenses.
So, these partially focused stars that cannot fit through, end up hitting the inside of the baffle tube at an incidence angle that will allow them to reflect onto the chip.
The fix should be as simple as machining out the small ridges from the inside of the baffle tube and applying a strip of self adhesive synthetic velvet to absorb that unwanted photons.
I will post some test images in a few days, when I had time to do some processing.
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