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Star test results and questions

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#1 Mitrovarr

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Posted 28 December 2017 - 04:37 PM

So, I recently picked up an old Meade 10" SCT (I know I've been talking about refractors a lot but a larger SCT was planned for when one popped up at a decent price, which finally happened). I think it's the model that immediately pre-dated the LX200, the Premiere. Anyways, conditions prevented a really good star test at the time I was buying it (20 degrees outside, scope at room temp, scope was miscollimated, and it was cloudy), but it seemed ok. So I went for it.

 

I've been star testing it indoors to fix the collimation (which was badly off - why anyone tolerates such bad collimation in an SCT is beyond me, but it's really common) and check out the optical quality. And I notice that outside of focus (I tried to go for the 4.4mm distance recommended in posts here), the star images have a bright outer ring and the secondary shadow is huge, and inside of focus the star pattern is kind of fuzzy and indistinct. I think this would indicate overcorrection. Is this correct?

 

Also, is there anything in an SCT that can cause overcorrection or other optical problems similar to this, that can be fixed? I accidentally detached the secondary mirror while trying to collimate (I thought it had a collimation screw cover cap - it did not). Fortunately I had the scope pointing downward (since I knew I could be wrong...) and the mirror didn't go anywhere, and was easy to re-attach. It's collimated now and everything seems ok, but I thought I should mention it as a potential source of problems.

 

I did get to star test it a little last night on an actual star (it was hazy, but the test seemed to still work) and it looked similar.



#2 junomike

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Posted 28 December 2017 - 04:45 PM

Sounds like you performed the "Breakout" test as first mentioned by Eddgie.

Also, It's common to have a little over/under correction. 

Adding spacing in the optical train can compensate for one (not sure).

If that's all you noticed I say It's fine.  Enjoy the scope!



#3 Mitrovarr

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Posted 28 December 2017 - 04:54 PM

Hopefully a longer optical train compensates for overcorrection because I'm definitely going to extend it! I'm going to attach a 2" mirror diagonal to it. It came with a weird old focal reducer (an orion rich field adapter) but since I already have a 2" SCT diagonal and good 2" eyepieces that seems like the way to go.

 

Hopefully the scope will be fun. It's going to get deforked and stuck on an AVX as a large-aperture goto telescope. The existing mount is pretty nice, though. Is there any demand for old manual-style SCT forks? I might try to rehome it after setting up the AVX and making absolutely sure it works well.



#4 fcathell

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Posted 28 December 2017 - 04:55 PM

The scope will definitely show over/under correction (not sure which) when using a artificial star placed 20 meters or less from the scope.  The only way to really test for correction is with a star at infinity, steady atmospheric conditions, and the eyepiece the correct distance from the rear of the scope, typically through a 90* star diagonal.

 

Frank


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#5 Mitrovarr

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Posted 28 December 2017 - 05:18 PM

The scope will definitely show over/under correction (not sure which) when using a artificial star placed 20 meters or less from the scope.  The only way to really test for correction is with a star at infinity, steady atmospheric conditions, and the eyepiece the correct distance from the rear of the scope, typically through a 90* star diagonal.

 

Frank

 

I suspected this would be true, but I got the same general impression last night using an actual star.



#6 PETER DREW

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Posted 28 December 2017 - 05:39 PM

Only time will tell if a SCT is a good one. I have an excellent 16" which can give stunning lunar and planetary images but on many a night it would be unsaleable. Bare SCT forks have a certain attraction for DIY enthusiasts for mounting cameras or binoculars so don't bin it.  smile.gif



#7 jjack's

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Posted 28 December 2017 - 05:58 PM

I have read somewere that adding more optical train on a SCT (reducing the space between the two mirrors)  add also more overcorrection. It is the reverse for  a true cassegrainian.



#8 rmollise

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Posted 29 December 2017 - 04:35 PM

Your indoor tests are meaningless unless you've got a really large indoors. Unless the distance you your artificial star is well over 100-meters, false spherical aberration is being introduced. If you want to star test, star test on a star. ;)


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#9 jjack's

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Posted 29 December 2017 - 05:14 PM

rmollise : youre perfectly right ! i must put my glasses on the next time i have to read something !

effectively an indoor test add a lot of overcorrection on this scope.



#10 Mitrovarr

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Posted 29 December 2017 - 05:32 PM

I'll try to star test on a real star when I can. I swear it looked similar on a real star last time, but conditions were pretty bad and shockingly a 10" SCT takes a while to cool off.

#11 Darren Drake

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Posted 30 December 2017 - 09:28 AM

You can also do an indoor test if you happen to have a larger scope known to have good optics and place an artificial star in the focuser. This creates a star test at infinity in your living room. I've been doing this type of testing for many years.
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#12 Mitrovarr

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Posted 30 December 2017 - 04:13 PM

You can also do an indoor test if you happen to have a larger scope known to have good optics and place an artificial star in the focuser. This creates a star test at infinity in your living room. I've been doing this type of testing for many years.

A larger scope? Does it have to be larger than the test scope, or just larger than a certain size? The only thing I have larger than the 10" SCT is a 12" lightbridge and that's rather a pain to set up.



#13 Eddgie

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Posted 30 December 2017 - 05:02 PM

As Jon says, artificial star needs to be many times the distance of the focal length of the scope and in the case of the scope in question, we are talking hundreds of feet.

 

The reason is that you have to be far enough distant so that the aperture cannot resolve the blur of the artificial star being used.   

 

I recommend a minimum of 50 focal lengths.  Suiter I think says 20, but if you read carefully, he says that this is the absolute minimum to not get more than 1/4th wave error from spacing.

 

It is also critical to have the scope at infinity focus!!!! If you focus on a very close object you have to push the primary mirror spacing quite far, and this by itself induces spherical aberration because the corrector can only perfectly correct it at the optimal spacing, which is usually the spacing that the mirrors  will be set at when the scope is using the factory diagonal and factory visual back, and the scope is focused at infinity. 

 

And now for my traditional closing statement when this kind of topic comes up.   For less than the price of a decent  Plossl, you can own the book that tells you more about telescopes than you are likely go get hanging around CN for a year. Why people insist on trying to star test their telescopes without knowing how to do it properly is one of the great mysteries of life and anyone can learn to do it easily, but much much more importantly, they can learn to estimate the quality of their optics for the rest of their lives.

 

Test on a real star.   Test with the scope set to factory mirror spacing using factory diagonal and visual back.  

(And of course the secondary shadow breakout is only one of the many tests that are required to fully evaluate the optics You still have to check for surface roughness, zones, turned edge, and astigmatism..  Again, all in the book.   I am done begging people to buy it, but I just don't get why people won't.)


Edited by Eddgie, 30 December 2017 - 05:04 PM.

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#14 Magnus Ahrling

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Posted 30 December 2017 - 06:06 PM

Hey, I bought the book  second edition for christmas -09 after reading about first edition in S&T -94 I believe it was. So you are not totally unheardsmile.gif Very good an informative and so is CN.

 

Happy New Year all CN:rs

 

Magnus 57N.



#15 Mitrovarr

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Posted 30 December 2017 - 06:24 PM

I was under the impression that book is really hard to get. But I mean... reading a whole textbook is a lot to ask. It's not like you can just buy it and absorb the knowledge.

 

That being said, if I ever have a good opportunity to get the book I probably will.



#16 Magnus Ahrling

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Posted 30 December 2017 - 06:58 PM

I have not read it directly from page 1 to page to page 413 and understaning everything. I use it as a sort of "referencebook". When issues comes up in any of my scopes I have a look in Suiters book. Many things one can find by googling but having this book is a must in my astronomy bookshelf. I have had it for 9 years and I have not even yet read everything. Or undertsood everything. But I know everything (well almost) I need to know for understanding optics is there. I dont regard this book as novel more like a manual/reference. I admire Suiter who has put so much efforts in wring these boks. Edition 1 and 2.

 

Magnus 57N.



#17 Mitrovarr

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Posted 30 December 2017 - 09:22 PM

It's not looking good.

 

Testing on a real star tonight. With the 4.4mm distance (a bit fudged, but it's roughly that far) the star images have a huge secondary inside of focus (to the point of being basically a very bright ring) and the secondary is almost invisible that far outside of focus. Of course if you defocus more you get more normal images the further away you get, but this isn't a good sign.

 

I think the scope is cooled down. Sky is ok, as far as I can tell, not great. Transparancy is awful and there's a moon so it's pretty hard to find a lot of the fainter doubles to test on. I did find Iota cass, and it split that, so that's something.



#18 Peter Besenbruch

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Posted 30 December 2017 - 10:52 PM

That's a shame. If the spherical aberration is that bad, not amount of futzing with the optical train length is going to help. Can you return it?



#19 Mitrovarr

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Posted 30 December 2017 - 11:31 PM

Nope! Used sale, paid cash, etc.

 

Is there anything maladjusted that could make the correction worse? The scope is pretty old, it could have been through anything.

 

On a positive note it split Eta Orionis so it's probably not horrendously bad.



#20 jjack's

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Posted 31 December 2017 - 07:58 AM

Don't worry a lot about SCT's star test. I have seen thru a lot of them and has two of them. The last one i keep is a recent made with xlt coatings. It have the worst in an out of focus test of all i have seen in the past (undercorrected). But it give me the best image i ever see !

The most important with SCTs is collimation (poisson'dot centered with a 4 or 5mm eyepiece and just a little defocus) and cooldown.



#21 Mitrovarr

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Posted 31 December 2017 - 01:49 PM

Well, I'll have to put it through its paces this summer. So far when actually using it, not testing it, it seems ok (if not great). It has the big scope brightness one expects from a 10", which is great. I'm quickly finding that I hate the mount, which is fine, because it's getting deforked and stuck on a AVX when the dovetail bar arrives. Surprisingly the fork mount is just not that stable despite looking pretty impressive (this one even came with a motofocuser and dec motor), but it might just need some work. Some of the bolts are a little loose and such. No point when I'm deforking it, though.



#22 junomike

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Posted 31 December 2017 - 06:57 PM

Nope! Used sale, paid cash, etc.

 

Is there anything maladjusted that could make the correction worse? The scope is pretty old, it could have been through anything.

 

On a positive note it split Eta Orionis so it's probably not horrendously bad.

IME for DSO's it's not an issue unless there's a serious problem.

Try it out under the Stars (looking at DSO's, Luna, Planets when avail) and see how it is.



#23 Mitrovarr

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Posted 31 December 2017 - 07:42 PM

I will when I can. Boise, in the winter, suffers from universally bad transparency and is nearly always overcast. Plus, my neighbor has Christmas lights up 10 ft. from where I observe. It will be a while before it is transparent and dark here again. Frankly, it's very lucky I even got to star test it at all.

#24 Mitrovarr

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Posted 31 December 2017 - 07:43 PM

I will say the moon looked nice, but I never look at the moon seriously so I have no idea what to expect.

#25 Mitrovarr

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Posted 02 January 2018 - 03:23 PM

The dovetail rail arrived and the scope has been deforked! Let's see if an AVX can handle a 10" Meade SCT.




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