DIY focal reducer for C11

bojan · #1 23-06-2022, 01:39 PM

Well, because my Meade f/0.63 FR doesn't perform as it should, I decided to make my own, just for comparison.
I had couple of binocular achromats of adequate diameter (ø42 mm) and FL (~180 mm), I machined the holder (black Delrin) that fits into 2" adapter barrel (in place of 2" filter).

I don't expect much of it, but still hope for clear skies next weekend...

bojan · #2 26-06-2022, 02:04 PM

As expected, not better than Meade (after re-assembly), but it has excellent price-performance ratio

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Could be good for visual.

bojan · #3 27-06-2022, 03:29 PM

Also, I will try with another lens (ø37 mm, ~150 mm FL, objective from theodolite), just for fun. It fits nicely and tight inside M48x0.75 filter ring.

Sunfish (Ray) · #4 01-07-2022, 08:09 AM

Well done. I am surprised it works as well as it does. I suppose with a reflector the CA is not so much of a problem.

I have often wondered if I could repurpose some old 400mm lense reducers.

Quote:

Originally Posted by bojan

As expected, not better than Meade (after re-assembly), but it has excellent price-performance ratio

.
Could be good for visual.

bojan · #5 02-03-2023, 09:43 AM

My Meade 4000 series is now OK with C11 after re-assembly (not ideal but acceptable and probably within design specifications..), but I am still thinking about that binocular objective lens.. (the other, objective from theodolite was very bad)
I decided to do some more tests with better setup.
Using this M48x0.75 to M42x1 adapter (after machining it a bit so the lens fits inside nicely), I will try to play with working distance to optimize the coma in corners of the APS sensor. Hopefully the weather will cooperate..

bojan · #6 05-03-2023, 07:30 PM

Attached below are results from more comprehensive & comparative tests.
(Left image is Meade, right is ø42mm 180 mm FL doublet (from Russian binos).

Meade has big problem with field flatness at corners of APS sensor, while doublet behaves better.. This is not a big problem because of heavy vignetting at those distances from frame centre... and cropping helps a lot.

However, Meade is quite flat in the central part (inside distances 1/2 of the shorter side of the frame), while doublet has a bit of coma, visible closer to the centre of the frame (where Meade is OK).

Also, I noticed a slight CA with Meade (easily removable with PtLens), and no CA with doublet (I was expecting the other way around)..

All in all, doublet is definitely worth the money I paid for it

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