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Focal reducer drawing
Hi,
Here’s some more images and basic layout of my focal reducer. Originally I started out using a camera lens reducer, fine for every day photography but no good no good for astro imaging! Way too much coma.
Then I tried various fast lenses, but to find a single one that didn’t produce coma proved difficult. I’m sure you could get one made at great expense. So I toyed with the idea of using slower lenses in line, this seemed to work. But spacing them was the bit that took time.
I eventually came up with an arrangement of two lenses, which gave me a reasonably coma free field of 22 arc minutes on the long axis. I used this setup for a long time, but wanted to see if I could get more with a third lens.
I did, 29 arc minutes, but the coma was quite bad. Then I replaced the third lens with a 0.5 focal reducer which gives approximately the same field, with less coma. But I do think I’ll need a coma corrector, or just back off on the field slightly.
Anyway, I’ll keep you informed of my progress.
Note, spacing depends on, focuser height - lens housing material thickness - where the CCD chip is placed in the CCD housing and the focal length of lenses used.
Cheers,
Justin
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