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How to evaluate a Celestron SCT?

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#1 TailsMPH


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Posted 17 January 2020 - 12:42 AM

Hey guys!


I've been using a big dobs for a couple years now and know my way around them somewhat, but I'm in a new place now and can't manage bringing that wonderful giant with me. It turns out to my great delight someone in the area is selling a used C-8 but I have virtually no experience getting an idea the quality of an SCT's optics.


Any advice for an SCT newbie looking at picking up a used scope? Anything in particular I should be looking for? 


Thanks a lot guys 

#2 charlesgeiger



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Posted 17 January 2020 - 01:41 AM

There are many things to look for and since you have used large Dobsonians you will have an idea about collimation and basic star testing.  I would look to see the visual condition, what vintage it is (fork mount orange tube, black tube) or modern versions using german equatorial mounting with Vixen or Losmandy type dovetail mounting.  You want to check clarity of optics (mirrors both primary and secondary and corrector plate and whether it is coated)...earlier models were either not coated or had special coatings.  Later models had starbright coatings, and the latest have XLT coatings and the tubes should have a label on them near the top of the tube which identifies type of coatings.  Older models have a ribbed style mirror casting and more modern units have a flat or triangular shaped back to the main mirror casting.  Does it have a finder, Telrad of some sort and what eyepieces, diagonal and accessories are included...does it have a tripod and wedge (for a fork mounted type).  How does the focusser work (is it smooth).  You want to know what is the approximate age as you do not probably want a Halley era scope as they were not the best optically. (Halley era is early to mid 1980's vintage.  You can buy an 8" optical tube used for $200.00 on up to $1000.00 for a HD variety.  The old fork mounts go for about $50.00 to $100.00.

If you can star test the scope you can find out if it is collimated and also look for good in and out focus patterns (similar on both sides of focus and with focus just slightly off in either direction).  With all the units you want to see if they have been disassembled ...this is more difficult to tell if you are not familiar with SCT's.  Rod Mollise has many articles and books regarding SCT's and what to look for in a used specimen.  More recent scopes have goto mountings either 1 arm or two arm forks or german equatorial goto mountings.  So there is much to choose from and it really depends on what era.


#3 Littlegreenman



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Posted 17 January 2020 - 01:53 AM

Here is a link to an article by Rod Mollise, author of a couple of books on SCTs'




You can download his Used SCT Buyer's guide here, (it is a large pdf)



The first thing to do is to identify what model or version of the C8 it is. Although they all share the basic optical design, some made in the 1980's during the Haley's Comet craze are subpar, some have complicated electronics which can be good or bad. The guide tells you what to look for in specific models, and in general.


#4 bobhen



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Posted 17 January 2020 - 07:58 AM

1. Check the cosmetics


2. Check the corrector for slicks and scratches


3. If it comes with a mount, check the electronics and tracking


Test under the stars…
4. Make sure the scope is acclimated. Check collimation: If it isn’t on then collimate the scope.


5. If the scope is on a GEM, swing the scope from east to west: does the collimation hold or slip. If it slips, pass on the scope, as that indicates mirror flop.


6. Check mirror shift at high power: Can you live with it.  Do you need to add an aftermarket focuser or is there too much for your liking?


7. Star test the scope: Does the shadow breakout when you defocus look about the same size inside and outside of focus? If so, the scope should have decent optics


8. Check some objects that you are familiar with and compare them to what you remember with your current scope. Is the quality of the image to your liking?



#5 TailsMPH


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Posted 20 January 2020 - 04:50 PM

Awesome, thanks a lot guys!

#6 michaeldurban


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Posted 10 February 2020 - 02:08 PM

Awesome, thanks a lot guys!

check the price too...waytogo.gif

#7 luxo II

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Posted 10 February 2020 - 03:41 PM

In addition to Bobhen:

9. Inspect the screws/bolts - any missing, or rusted,
10. Signs of wear and tear are expected but look for dings or scratches implying it has been roughly handled;
11. Focuser shaft - rotates smoothly and true or bent ?
12. If it’s on a dovetail, and it’s in bad shape can it be replaced ? Screwed on is fine but pop-rivets are a pain to deal with as it means opening the OTA to remove debris after drilling them out.

Edited by luxo II, 10 February 2020 - 03:42 PM.

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