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C90 Maksutov collimation error

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11 replies to this topic

#1 MikeTelescope

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Posted 05 August 2019 - 03:00 PM

I bought a used C90.  Star testing at a 0.6mm exit pupil (138x) shows fairly good diffraction pattern.  Rings look concentric.  As it is brought to focus, when there are only one or two diffraction rings visible, the light collects at one point on one of the rings.  Bright spot on one ring gets brighter and brighter until it reaches focus.  Going through focus, the light stays collected at that point, and the diffraction rings open up again, away from that point of light. 

 

Is this a collimation error, or a different aberration?  There are 6 collimation screws on the back mirror. 

 

Planetary views at 138x are OK.  I can see the Cassini division and some cloud bands on Saturn, and several cloud bands on Jupiter.  Going up to 208x, the image significantly breaks down.  Images from an 8" Newtonian at the same time are fairly good, so it is not poor seeing.  I understand the resolution of a a 90mm Maksutov should be less than that of an 8" Newtonian, but I have read others get sharp views up to 200x and beyond on a C90, and wonder whether there is a collimation error that could be improved. 

 

I have tried the star test without the diagonal and see something similar. 

 

Assuming good seeing, what would be a reasonable maximum magnification for a 90mm Maksutov for planetary detail?  



#2 Saturnalia

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Posted 05 August 2019 - 04:07 PM

Generally 50X per inch of aperture is a rule of thumb 'maximum' magnification so around 175x for your Mak.

 

Try somewhere between 150 to 180x and see if it helps. The quality (rigidity) of your mount will also play a big part in the image quality at high magnifications.

 

What eyepieces are you using...?


Edited by Saturnalia, 05 August 2019 - 04:07 PM.

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#3 MikeTelescope

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Posted 05 August 2019 - 04:25 PM

Generally 50X per inch of aperture is a rule of thumb 'maximum' magnification so around 175x for your Mak.

 

Try somewhere between 150 to 180x and see if it helps. The quality (rigidity) of your mount will also play a big part in the image quality at high magnifications.

 

What eyepieces are you using...?

Orion Expanse 9mm and 6mm.  Also tried Celestron Plossls at 13mm, 8mm, and 6mm.  Similar trend seen.  Mount is a fairly heavy Manfrotto that dampens quickly.  

 

The diffraction rings look pretty good when far out of focus, and come in nicely, until the very end when the rings are very small just before focus, and the light concentrates on the edge of one of the rings, rather than equally around the ring to a single point at focus.  This is what makes me think there could be a small collimation error that could be cleaned up.


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#4 kathyastro

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Posted 05 August 2019 - 04:55 PM

Which C90 do you have?  The original orange tube C90 was f/11, and is a different item than the modern f/13 C90.

 

There was a problem with the orange tube C90s made in the 1980s.  Celestron was cranking them out in the lead-up to Comet Halley, and quality control suffered.  Poor collimation was common.  Unfortunately, collimation was permanently set at the factory and can not be adjusted: the mirror is glued in place. 

 

I don't know the construction details of the modern C90s.  Perhaps they can be collimated.

 

It sounds like you may be pushing the magnification too hard.  On the old f/11 units, 11mm is the highest magnification eyepiece you should be using.  On the newer f/13 units, 13mm is the maximum magnification.  On my f/11 C90, quality deteriorates rapidly with eyepieces shorter than 11mm.  Even 9mm is too much.


Edited by kathyastro, 05 August 2019 - 04:57 PM.

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#5 MikeTelescope

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Posted 05 August 2019 - 05:27 PM

Which C90 do you have?  The original orange tube C90 was f/11, and is a different item than the modern f/13 C90.

 

There was a problem with the orange tube C90s made in the 1980s.  Celestron was cranking them out in the lead-up to Comet Halley, and quality control suffered.  Poor collimation was common.  Unfortunately, collimation was permanently set at the factory and can not be adjusted: the mirror is glued in place. 

 

I don't know the construction details of the modern C90s.  Perhaps they can be collimated.

 

It sounds like you may be pushing the magnification too hard.  On the old f/11 units, 11mm is the highest magnification eyepiece you should be using.  On the newer f/13 units, 13mm is the maximum magnification.  On my f/11 C90, quality deteriorates rapidly with eyepieces shorter than 11mm.  Even 9mm is too much.

It is one of the modern f/13.8 C90's.  There are 6 collimation screws on the back plate.  I'm tempted to tweak them a bit to see if I can get that last bit of error out.  But I'm a little wary of making it worse too.  

 

I have tried a 10mm Kellner and got good planetary detail, and a 9mm Expanse looked decent as well.  8mm Plossl started getting softer, and 6mm Plossl was unusable (as was the 6mm Expanse).  

 

The above-described asymmetric diffraction ring brightening was more pronounced with the 6mm.  



#6 Jim1804

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Posted 05 August 2019 - 08:04 PM

I collimated mine - I still might have a tweak or two to go, but it definitely helped. Just try REALLY small movements - lots of instructions in different posts on CN.
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#7 Jim1804

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Posted 05 August 2019 - 08:07 PM

See these instructions- https://www.cloudyni...0/#entry6217519
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#8 MikeTelescope

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Posted 12 August 2019 - 01:40 PM

Just to close this thread out.. I did follow the instructions in the link (thank you Jim), and the short story is that the C90 is now better collimated than before.  Star test on Polaris now shows a symmetric pattern, with almost no hint of head or tail.  Formerly, there was a pronounced head/tail at 139x, moreso at 208x.  The effect in the eyepiece is much sharper and more detailed views of Saturn and Jupiter all the way to 208x.  Moon detail at 208x is also increased.   Epsilon Lyrae split at 139x very cleanly (probably could split at a lower mag but didn't try.)  At 208x, the brightest ring of the Airy disk of each of the 4 components did not overlap with its neighbor's ring. 

 

Long story is that I had to do the opposite of what the link said.  I did have a mirror diagonal in place, as mentioned in the link.  When I followed the instructions, the situation got worse.  So I swapped counterclockwise and clockwise, and I had it collimated in about 15 minutes.

 

As Jim mentioned, use small movements and check after each one.  It was worth the 15 minutes of work to get noticeably better high-magnification views from this scope.     


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#9 Asbytec

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Posted 12 August 2019 - 06:17 PM

But I'm a little wary of making it worse too.  

 

Sorry I am late to the party reading your thread. It does sound like collimation as you seem to have figured out. Two things come to mind. First, you have to be a little wary of making it worse before you can make it better. In other words, you have to collimate it and figure out which way to turn the knobs. Second, if you can make it worse by adjusting collimation, that's a good thing. It means you can likely make it better by collimating the other way. You did exactly that, well done. Now, get it 'perfect' at high power close to focus, then you're on it. 


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#10 MikeTelescope

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Posted 13 August 2019 - 05:10 PM

Sorry I am late to the party reading your thread. It does sound like collimation as you seem to have figured out. Two things come to mind. First, you have to be a little wary of making it worse before you can make it better. In other words, you have to collimate it and figure out which way to turn the knobs. Second, if you can make it worse by adjusting collimation, that's a good thing. It means you can likely make it better by collimating the other way. You did exactly that, well done. Now, get it 'perfect' at high power close to focus, then you're on it. 

I have a knack for taking things that are at 85% and making them 75%.  I use almost exclusively second-hand Craigslist gear, to keep costs down and so when I break things, it doesn't hurt as much.  I like to make all my mistakes on inexpensive used things first and learn in the process! 


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#11 Jim1804

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Posted 14 August 2019 - 12:03 AM

Mike - glad it worked out - those instructions by Eric63 are really clear.

And I hear you - when I first tried to collimate mine, I used the instructions on the Orion site for their Mak (also a Synta identical to the C90). They recommend collimating from the back during the day - you stick a collimating eyepiece in the visual back, and just get everything lined up and concentric. I got so turned around with the hall of mirrors, that I made it much, much worse. I ended up having to loosen all the screws, which reset the primary to more or less the right position. From there it wasn’t too bad to collimate on a star - but man was that a frustrating experience. My kids heard some new words that afternoon!

I haven’t had mine over 150x or so (8mm EP). I’ll have to go higher and see how it looks. I suspect I have a small turn or two to make on a couple of screws, but the seeing is so bad here it’s hard to get Polaris stable enough to check!
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#12 MikeTelescope

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Posted 14 August 2019 - 01:04 PM

Jim, thanks again for sharing the instructions.  I printed them out and had them in hand while collimating in Polaris.  I got lucky and had a night with good seeing.  The good seeing also allowed me to enjoy some fruits of the labor by viewing a crisp Saturn at 208x and resolving Epsilon Lyrae following the collimation.  

 

I'm still confused why I had to swap clockwise and counterclockwise.  I did have a mirror diagonal in place, as the directions specified.  Could be that the C90 screw orientation is different from the 127mm Mak for which the instructions were written.  In any case, it was easy to detect that I had gone the wrong way and then change course.  I think I practiced some of those words at that moment as well.

 

I think I'm about 90-95% there and still want to get it to that elusive 100%.  This little scope seems to be able to be pushed to 60x per inch with good seeing, or perhaps a bit beyond that.  Great for grab-and-go planetary and moon viewing and double star resolving.  




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