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Field illumination vs contrast

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Hello everyone

I'm in the process of optimizing my Skywatcher 130pds (650mm focal length, 130mm aperture, 220mm from secondary to focal plane-i think i measured it correctly) for imaging. I will never put an eyepiece on it so i think it's a good idea...

The first thing i've already done is shortening the focuser's tube by 2cms to protrude less in the ota when focused, i will no longer have a problem with D-shaped stars and there will be more light hitting the primary-less obstruction.

The next thing i wanna do is replacing the secondary with a bigger one. The scope has a 47mm secondary and i think a 58mm one (from my 200pds) will illuminate better the apsc Canon sensor of my 600d.

Of course the central obstruction will be bigger and this means less contrast...

What's your thoughts?

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