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How to check SCT Secondary Mirror is central


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Hi,

It’s my first post as an astronomy newbie but hopefully not too daft a question.

What is the best way to check that the secondary mirror on an SCT is centred (laterally) with respect to the primary mirror and optical axis?

It’s a non-Fastar SCT so the secondary can’t be as easily removed and replaced.

I’ve read conflicting statements as to whether it’s even important or not (at least to within a few mm). What are peoples thoughts on that?

I can see how you might still be able to achieve collimation via the secondary tilt angle even if it’s off centre, but are there any visible side effects compared to it being centred e.g. increased coma elsewhere in the FOV, some of the light reflected from the primary missing the secondary?

Thanks in advance,

Tim

 

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Hi Tim, welcome to SGL 👍

If it were me I’d measure carefully from the edge of the secondary holder to the edge of the corrector plate on opposing sides.   If the measurements are close I’d be happy with that.   A rigid rule would be better than a flexible tape measure.   Take care not to contact the optical surface of the corrector plate.

Have you noticed any issues that lead  you to suspect a problem with secondary position ?

Ed.

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Hi Ed,

Thanks for the reply.

I haven’t noticed an issue as such, but don’t seem to get as sharp a view as others have described with similar scopes, or the same level of magnification without it starting to blur. To date I’ve assumed / hoped that was just due to poorer seeing conditions.

However, I made my first attempt at collimating the scope recently, as a star test had showed it to be slightly out. When I tried to turn one of the collimation screws the whole secondary moved very slightly sideways. It did move easily back to it’s original position and upon further examination I noticed that the secondary holder locking ring wasn’t fully tightened, so nipped it back up tight.

This did then make me wonder whether the secondary had been off centre to start with and maybe that was why I’m not getting sharp views, hence why I wanted to check it’s position, or indeed whether it being exactly central mattered. Previous star tests did show the scope to be in rough collimation at least.

I wasn’t sure if measuring to the tube edge would be sufficient as that assumes that the primary is also mounted dead centre. I know the corrector plate is shimmed to align it with the primary.

I was able to complete the collimation just before the clouds came over and haven’t had a clear night since to check things out on a familiar target.

Tim

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I think you are worrying about something that 99.99% of SCT owners don't worry about.  The manufacturing tolerances of the corrector and secondary supports should ensure that it is well enough centred for the purpose.  There has been some discussion here about whether the angular position of the secondary mirror is critical, should it come loose, but the general opinion was that the recent secondaries are well enough figured so that the angular position should not matter. 

Some SCTs are said to perform better than others, with the finger of blame being pointed mainly at some older production runs (of a particular make). But I have found that at an 8" size the main limiting factor is the atmosphere. 

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Hi Geoff, thanks for the reply.

I also suspect that there is little to worry about, but thought it worth checking. I’ve also read that angular rotation is less of a concern nowadays. I think in the more distant past they used to sometimes mount the secondary off centre within its mount in order to counter any error in the centering of the drilled corrector hole. Rotating the secondary would have more of an impact in that scenario but I’d hope that modern manufacturing processes can drill a hole centrally with much better precision.

The only thing I was thinking is that if the secondary was too far off centre then despite being in collimation tilt wise, some of the light cone reflected from the primary would miss it. Using a slightly oversized secondary would give you some margin for error here but at the cost of increasing the central obstruction, so wasn’t sure if they would have done that.

Hopefully I’ll get a clear night soon to check things out and put my concerns to bed.

Tim

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Hi Tim.   If it were myself I’d just do the best I could with securing the secondary as close as you can to the central position, then carefully star collimating at 150-200x with the star central in field of view.  As the SCT primary is factory set this makes collimation simpler for users.

One thing not mentioned so far is cooldown, you may already be aware but worth a mention.

Ed.

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Hi Ed,

Thanks, while I do often start observing while it’s still cooling down, just in case the clouds roll in, I left it outside for a couple of hours before checking and correcting the collimation. 

Tim

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