We could do away with some of our woes if we used a DSLR CMOS camera instead of the current system. There are several benefits, and a few drawbacks:
PROs: With a DSLR camera we would get RGB colours at the same time, at the same sky conditions, and at the same focus setting. Alignment issues would go away. Shutter precision issues would go away.
With the modifications described in this paper
we would even get the NDVI index which is what we wanted to get by using VE1 and VE2 filters.
CONs:
a) The shutter would be at the focal plane, instead of in the pupil. This could mean that fast exposures were not possible – insertion of a fixed ND filter woudl extend exposure times.
b) With CMOS chips at best being 14 bit (16 bit exist but cost a lot) we could probably not use the ‘direct imaging mode with BS and DS in same frame’.
c) Dark Frame issues appear murky for CMOS cameras. And what about cooling to get stability?
Neutral issues – i.e. same problem as before: We would still need an SKE to block the BS.
Suggestions:
1) Use an unmodified DSLR to check what the halo structure around the Moon – or Jupiter – looks like in R G and B. If these are the same – which is not the case with Johnson B V and not at all with VE2 – then we might be on to a system to get ‘same-halo images’ with known subtractive benefits! We could then go on and modify a DSLR camera and get the NDVI Index and see if halo issues are reduced.
