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SCT vs Mak Collimation

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9 replies to this topic

#1 vidrazor

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Posted 24 June 2021 - 02:44 PM

I've been looking at possibly picking a Celestron C5, and I noticed that, unlike my Celestron C90, the collimation adjustments are on the front, with apparently none in the rear. So is collimation of a scope like this strictly from the front, or do you have to take the rear apart to have access to adjustment screws in the rear, and deal with having to adjust both mirrors in collimation?

 

Thanks.


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#2 james7ca

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Posted 24 June 2021 - 03:01 PM

SCTs are usually collimated by adjustment on the front mirror (the secondary mirror only).


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#3 Asbytec

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Posted 24 June 2021 - 03:25 PM

I find collimating an SCT (tilting the secondary mirror) is a bit easier than tilting the primary mirror in a MCT. Mostly this is due to the arrangement of the collimation screws. The SCT has three and the MCT has six screws in either a push pull arrangement or locking screws. The latter can affect the collimation. I think I prefer to tilt the primary, though, by tilting the primary to point at the secondary rather than tilting the secondary to compensate for (lack of) primary tilt. If this is a decider then I'd prefer a MCT, but between the two options in the OP I'd go with the larger aperture SCT.  


Edited by Asbytec, 24 June 2021 - 03:30 PM.

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#4 Redbetter

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Posted 24 June 2021 - 04:35 PM

As Norme says, I found the locking screw arrangement of the 127 Mak to be a little trickier to dial in than the SCT, because the lock screws could alter the final collimation unless one took extra care--very light touch after collimation, getting everything in contact before tightening the locks.  The first couple of passes I found the collimation was moving slightly when I tightened the lock screws.

 

Hmm...maybe that is why the factory collimation was so far off as I received it?  Perhaps they collimated it with very loose lock screws, then just zipped down the lock screws without a final check?  



#5 luxo II

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Posted 24 June 2021 - 05:26 PM

Depends on the scope. SCT's are collimated via the secondary, not the primary. Maks can have adjustments for either, or both.

 

If yours has push-pull screws note these only need to be firm, barely more than finger tight; for two reasons - first you're securing a mirror that only weighs a few ounces - not the Titanic - and second, in the cheap scopes the metal is a soft die-cast alloy ("pot metal") and the threads will easily strip if you are too zealous.


Edited by luxo II, 24 June 2021 - 05:28 PM.


#6 Redbetter

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Posted 24 June 2021 - 06:31 PM

I am pretty gentle about not over tightening screws, but "barely more than finger tight" on a push-pull/lock mechanism runs the risk of collimation flopping about during transport when the lock screws vibrate loose.  Little chance it would make it through shipping without ending up loosened that way.    

 

The problem I saw with the Mak was that the initial tightening seemed to throw it out. From what I recall, the solution was iterative: backing off less each time and getting it right at contact, but tightening the same at the end once this was achieved.   


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#7 vidrazor

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Posted 24 June 2021 - 07:03 PM

Thanks for your replies. I'm glad there's only one mirror to adjust in the SCTs, as videos I've seen of Newtonian collimation put hives on my back. :)

I've been pretty confused about trying to collimate the Mak, because in some info they talk of push and pull screws, and others they are lock and adjustment. I have to assume that "push" is lock and "pull" is adjustment. The SCTs certainly look easier to adjust. Once adjusted, Maks are known to hold their alignment, is it similar with SCTs?

Thanks again for your replies.

#8 Redbetter

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Posted 24 June 2021 - 07:48 PM

My SCT has held its collimation well over the decades, despite being driven around a lot, including on very rough ranch roads in the Davis mountains.  About the only time it has gotten truly out was from shipping it back and forth to Hawaii for the Venus transit.  It was handled so roughly that they cracked a fork in its original shipping packaging, and bent the DEC shaft over 90 degrees.  It is a wonder that the optics survived, but it was enough to throw the collimation out noticeably.

 

Back in the 90's/2000's Meade SCT's were bad about not holding collimation from what their owners said.  I don't know if that is still true or not.  And I have read some examples of folks having to collimate frequently after adding Bob's Knobs to SCT's...making the "need" for the knobs a self-fulfilling prophecy for some.  I won a free set at TSP many years ago, but never collected them as I had no need for them.

 

There are other things that can happen with Maks and SCT's that can appear as collimation problems.  So I have wondered how many of the "needs frequent collimation" claims for any particular scope are actually manifestations of other alignment issues that masquerade as collimation.  Wonky diagonals, loose/decentered secondaries, secondary baffle tubes sliding (on some Maks), loose nuts on the primary baffle to primary, even a loosely threaded backplate (on a Mak) come to mind.  



#9 Bill Barlow

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Posted 25 June 2021 - 10:20 AM

The SCT with its three collimation screws for adjusting the secondary mirror seem much easier to me than the six collimation screws for adjusting the primary mirror on a MAK.  I’ve owned both types of scopes and prefer the SCT.

 

Bill



#10 rmollise

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Posted 25 June 2021 - 01:40 PM

I've been looking at possibly picking a Celestron C5, and I noticed that, unlike my Celestron C90, the collimation adjustments are on the front, with apparently none in the rear. So is collimation of a scope like this strictly from the front, or do you have to take the rear apart to have access to adjustment screws in the rear, and deal with having to adjust both mirrors in collimation?

 

Thanks.

Only on the front (secondary assembly). In the early days, in the 60s, Celestron's original (Blue and White) scopes had rear cell and secondary collimation, but the company quickly determined that collimating the secondary is good enough, and having adjustments for both just muddied the waters for users. 




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