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Collimation frustration

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#1 lipman


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Posted 17 November 2013 - 10:56 AM

My CPC1100 has held good collimation (or so I thought) for a long time. For the last few nights I have been observing with the addition of an f/6.3 reducer/corrector. Racking in the focus I noticed that the secondary shadow looked far off center, so tried to collimate. I finally got it looking OK, although then realized that on the other side of focus the secondary again looked off center so adjusted again until the secondary was centered. However when I then went back to the other side of focus the secondary was off center again. Long story short, I couldn't get the diffraction rings consistent inside and outside of focus. I must be doing something wrong - or perhaps this is something to do with the fact that I am collimating with a reducer? For context, I have a CPC1100 and was using a 6.5 eyepiece to collimate (lowest I have - actually a 13mm with 2x barlow). I was carefully following the instructions here: http://www.astromart...?article_id=548

I have read numerous guides online that say that collimation is easy. It wasn't for me! Any advice?



#2 herrointment


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Posted 18 November 2013 - 09:26 PM

Pull the reducer and see what happens.

#3 Larry F

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Posted 20 November 2013 - 01:42 PM

Every time you add more optics to the light path, you run the risk of a component not being orthogonal to the light path. Maybe the reducer optics are slightly tilted. Maybe the visual back isn't seated exactly parallel, or the eyepiece is slightly askew. Since collimation is most important at high power, where you wouldn't be using a focal reducer anyway, you should collimate without the FR and accept whatever defects are present when you do use it.

Also, those FR's aren't made by Zeiss, you know.

#4 rboe



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Posted 22 November 2013 - 05:02 PM

I'll second Larry here.

#5 zuben122


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Posted 22 November 2013 - 09:23 PM

There was a focal,reducer article posted here back in April and a sentence in it says...

For best results get your scope in critical collimation (I don't mean close; I mean dead on) before screwing on the reducer/corrector. My personal preference is not to collimate with the reducer/corrector installed.

There are additional threads here on CN dating back to 2010 and on a couple other forums and they all recommend collimating without a reducer installed.

Bottom line... Follow Larry.

#6 zuben122


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Posted 22 November 2013 - 09:29 PM

And I found this from 2004 and it seems to describe your situation pretty well

Remember collimation, will change on an SCT (though it should not be by
much), as you adjust the focus, unless the baffle tube is absolutely
perfectly aligned (unlikely...). When you add a reducer, it is normal to
have to shift the focus by a significant amount. Hence it is quite common to
find that previously 'perfect' collimation, is no longer quite correct. This
is why for truly 'critical' collimation, it is best to align with the scope
in it's final optical configuration.
Second, if the collimated optical axis of the scope, does not pass through
the central axis of the reducer, collimation will again show problems.
It sounds as though your reducer has the second problem (since another
reducer doesn't display the same fault), but the comment 'different
F-ratio', does imply that the focus distance was probably changed between
the units, so the first possibility should not be ruled out. Also certain
types of fault will get worse as the focal ratio decreases (if for instance
the CCD is not perfectly perpendicular to it's mechanical mount, the edges
will show both coma, and misfocus, when using a reducer, and the amounts
will increase with the reducing ratio, perhaps being invisible without the
The probability is that the optics are not properly aligned in the body.
Hence 'assembly' (or even a significant knock, that has shifted the optics
to one side of their adjustment), could well be the problem. However it is
also worth being aware, that a scope which is perfectly collimated with a
diagonal, which is itself not perpendicular, will have the collimated
optical axis 'off centre' to the rear baffle tube. If a reducer is then
fitted at this point, this will be off-axis, and will show problems.
Generally, the F/6.3 units are normally pretty good, and it may well be that

#7 lipman


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Posted 25 November 2013 - 12:59 AM

Great - thanks all. I will re-collimate at f/10 and then leave well enough alone whenever I need to use the reducer in future.



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