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How to Quickly Tell Poor Collimation From Bad Optics?


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It seems there are a few questions about collimation, but unfortunately none answer my question, so here is yet another.

I'm going to be looking an 8" Meade SCT from the mid 1990's later this week, it's an appropriate price considering it comes with a fair few accessories, and the seller seems pretty trust worthy. However the concern I have is if the scope isn't perfectly uncollimated when I try it out, will I be able to tell that it is poorly collimated, or will it appear as bad optics, and is there a way to tell?

Thanks in advance for any advice, I don't want to too buy a scope with bad optics, or let a good scope go because it just isn't collimated properly.

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I would read in advance how to do a collimation on an sct. Then you can do a daytime startest post collimation with a christmas bulb you hang in the distance(as far away as possible and preferably in the sun, but flashlight on it will do if that is all you have. Another option is a ronchi eyepiece to look at mirror with knife testing.

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I would ask the current owner to ensure the scope is well-collimated before your arrival. You can use star testing to evaluate optics; however, star testing does require both knowledge and experience. I do not know how verse you are with star testing but do as much homework as you can beforehand.

Evaluating optics using star testing can still be done with a slightly miscollimated scope.

Even with little experience, you should be able to spot "major" issues with optics – should they exist.

Of course, conducting star testing does require a steady atmosphere. You can use an artificial star providing you know how to use it. If the artificial star is placed relatively close then you will see false spherical aberrations.

Jason

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Meade are a pretty good make so chances are the optics are going to be reasonable, so a visual inspection for damage, fungus, etc will probably be enough on the physical side. Collimation is something you can play with at your leasure afterwards.

I'd be as much concerned with the mount side of things with a scope of that age, especially if its seen significant use. Motors, gears etc are replaceable but they're not cheap so pay as much or more attention to the mechanics...

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I agree with all the above but would add that if the guy is respectable and knowledgeable then he'd be a bit of a muppet if he didn't ensure it was collimated before you arrived (and cooled) as if he doesn't then it's likely to put a buyer off with soft fuzzy views. if it's in an observatory then it will be likely to be well collimated as it's when scopes get moved about that collimation can shift drastically.

I also agree that with a scope of this nature, replacing the mount would be expensive (possibly more expensive than the scope itself) so pay attention to how well this works. I'd wager that given the brand, the mount is more likely to have issues than the scope but both might be fine.

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sorry, I realised that in my mumblings, I didn't answer the question. the answer is that you eliminate the collimation problem by doing a collimation or asking him to as suggested above. then assuming the scope is cooled, use it on some challenging targets (high power moon/Jupiter or some tight doubles) and if it performs well then it is likely to be fine. doing this will also put the mount through its paces. once problem if Canada is anything like the UK at the minute is that I have not used my scope in March and have had one night where I could use binoculars for about an hour due to could cover.

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SCTs do not lose collimation easily (they are very different to Newts in that respect). I have transported my SCT to many places, and every time I check collimation it is OK. A scope in and observatory should be properly collimated. There are simple masks you can put in front of the scope to quickly check collimation on a bright star. The owner might have such a mask. You could also simply ask how he collimates the scope, and if he can give you a quick demo.

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I agree, you should ask him?

I suppose, to a first approximation, does an out-of-focus (both sides) star look circularly symmetrical?

With mirror systems and secondaries, you will get some guidance from the "black blob" at the centre. ;)

Of my budget scopes, it's only the Skywatcher Maks that have produced "text book" diffraction rings.

My ST102 produces them on ONE side of focus (with chromatic aberration). My TS F4 Newt doesn't

look THAT great, but... :p It doesn't mean that these scopes aren't "fit for purpose", at the price tho'! :)

But I would expect a CLEAR diffraction ring pattern with a Catadiopteric system. If that is there, you have

something to work with, e.g. in comparing this to some template optimal / non-optimal star test images...

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A visual check will tell you a lot.

Is the glass at the front marked or scratched?

Do the screws holding the front glass in place look like they have been removed and put back in?

Does the main mirror appear free from marks and blemishes?

Looking down the focuser does the secondary mirror look clean and without dinks or scratches?

Does the secondary holder rattle or rotate easily within the corrector lens?

Are other any signs that the scope may have been dropped at any point?

Is the focuser smooth and freely moving through a long range?

If the scope seems to have been looked after, chances are it will be OK. The mount is a separate issue.

Hope it works out for you

Tim

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Thanks for all the advice, I have been doing a bit of research as suggested and I think that I'm pretty well prepared to give it a look over and determine if there is anything wrong, also I spoke to the owner and asked him to ensure that the scope would be collimated and cooled by the time I get there so I should be able to use the star test (assuming clear skies tomorrow). Thanks again.

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If you get time, have a look through the guide to used SCT's by Rod Mollise (click the cat to download it !):

http://skywatch.brainiac.com/used/index.htm

This covers older models by the main manufacturers and can give some pointers to look for. With Meade I believe it's the mounts that can have issues so be sure to ask lots of questions regarding the mount functionality.

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If you get time, have a look through the guide to used SCT's by Rod Mollise (click the cat to download it !):

http://skywatch.brai.../used/index.htm

This covers older models by the main manufacturers and can give some pointers to look for. With Meade I believe it's the mounts that can have issues so be sure to ask lots of questions regarding the mount functionality.

Thanks for the link, this came in really handy, the scope in question turned out to be a Meade 2080 B 8" SCT (the one with "muli-coatings" and and a 50mm finder), the guy selling wasn't super clear about things, I think he just didn't know much about it, and the scope was not collimated properly, though it was good enough to tell the optics were fine. In the end I was able to get it for a steal due to his lack of clarity etc, which is good since I'm a university student with a tight budget. I did a bit of observing through the scope last night, and collimated it as well as I could given my current lack of eyepieces (only came with a 26mm), the sights even with imperfect collimation were extraordinary and way better than my previous 60mm refractor. Additionally the mount and old school quartz control box work well and should be sufficient for beginning astrophotography some day.

All told this may be a bit more scope than I need given the fact that I have only been observing for a few weeks, but considering it cost less than an expensive date I think it will do for now. Once again thanks for all the advice it came in really handy when making the deal and understanding the scope.

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