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Why do focal reducers work like this???

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#1 Orion64

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Posted 18 January 2013 - 02:40 AM

Hi am using the ASA 2Korr reducer on my f/4 newtonian system. This is what I have observed. When focus on a star in the center of the fov is achieved, it seems that in all three corners of the field, focus is good, but in one corner it is way out.
When I adjust the distance between ccd and reducer to be closer to the correct distance, the same effect happens, but now the focus in the one corner is less out of focus, with the other corners still in focus. I am guessing, when I achieve the correct distance between ccd and reducer, all four corners will be in focus.

My question is therefore simpel: shouldnt all 4 corners come closer to being corrected and in focus together? Why is one corner different from the others?

I just want to add that I think my collimation and offset is pretty spot on, so this shouldnt be causing the effect.

#2 siovene

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Posted 18 January 2013 - 08:51 AM

I guess that might be because something is not perfectly perpendicular to the optical axis.

#3 jmasin

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Posted 18 January 2013 - 02:06 PM

It seems something to me is not perpendicular, as Salvatore mentions. You can imagine a slightly curved focal plane and only one corner of the sensor being *noticeably* out of focus... noticeably is key as there's a threshold that you see and only one corner is past that threshold, but the others probably aren't all exactly the same...

How is your camera attached? Threaded or compression? Is there a way to shim near that corner to see if it gets better/worse?

How have you collimated?

Do you have CCD Inspector?

#4 Orion64

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Posted 19 January 2013 - 10:21 AM

My camera is attached by a t thread to my atik electronic filter wheel. When I rotate my camera, the image gets better in this corner and worsens agin in other corners.
I used ccd inspector, but my license expired a while ago.

What goes for collimation, I really think my scope is collimated well. I calculated my exact offset and collimated with an holographic attachment and howie glatter tools.
A question that I was wondering to ask, should one collimate then with or without the offset of the secondary?
I know this might seem like a stupid question, but I am getting out of ideas on what to do.

#5 shams42

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Posted 21 January 2013 - 12:47 AM

With your laser collimator in the focuser, if you put some weight on the drawtube (simulating the weight of your camera) in different directions, does the collimation shift?

f/2.8 is horrifically unforgiving. Maybe grab a Paracorr and try f/4.6?

#6 nytecam

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Posted 21 January 2013 - 04:59 AM

I guess that might be because something is not perfectly perpendicular to the optical axis.

More specifically with the CCD alignment - perhaps ;) As quoted fast optical systems are very sensitive to alignment problems :o I run my 12" f/10 SCT @ f/3.6 + tiny CCD and understand your concerns :p






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