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Mak Collimated?

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#1 vidrazor

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Posted 08 June 2021 - 06:41 AM

I recently picked up a Celestron C90 here, and I ran a pseudo star test on it using pinprick foil on a  flashlight about 25 feet away. It seems SLIGHLY off to me, should I worry about this and try to adjust it, or should I leave well enough alone?

 

Thanks.

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  • mak test.jpg


#2 GaryShaw

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Posted 08 June 2021 - 07:14 AM

FWIW - I’d be thrilled with that…

Gary


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#3 Asbytec

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Posted 08 June 2021 - 09:06 AM

You are way to far from focus to accurately judge collimation. You want to be as close to focus as possible with only a few rings visible, as close to focus as seeing will allow you to discern the rings and the small Poisson spot in the center. Actually the most accurate collimation and the finishing step is to collimate in focus with the first bright diffraction ring as evenly illuminated as you can tell. The in focus image should not "flare" more rings off to one side than any other. Seeing can make that difficult, but you can watch the in focus image over time and not it's tendency to "flare" one way or another. The image above is really the shadow of the obstruction more so than it is an image of diffraction. Of course, it is composed of diffraction rings, but that far from focus we lose sensitivity. We collimate on diffraction, not shadows of the secondary. Check it closer to focus if you want to know whether you should leave it as is. 


Edited by Asbytec, 08 June 2021 - 09:08 AM.

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#4 cuzimthedad

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Posted 08 June 2021 - 10:51 AM

Moving to Cats & Casses



#5 Bill Barlow

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Posted 08 June 2021 - 10:59 AM

Your collimation looks very good, but as others have said, way too much defocused to be sure.  
 

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#6 vidrazor

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Posted 08 June 2021 - 12:36 PM

Moving to Cats & Casses

Thanks, I realized after I posted it that I put it inthe wrong place, and didn't know how to delete it.

 

You are way to far from focus to accurately judge collimation. You want to be as close to focus as possible with only a few rings visible, as close to focus as seeing will allow you to discern the rings and the small Poisson spot in the center. Actually the most accurate collimation and the finishing step is to collimate in focus with the first bright diffraction ring as evenly illuminated as you can tell. The in focus image should not "flare" more rings off to one side than any other. Seeing can make that difficult, but you can watch the in focus image over time and not it's tendency to "flare" one way or another. The image above is really the shadow of the obstruction more so than it is an image of diffraction. Of course, it is composed of diffraction rings, but that far from focus we lose sensitivity. We collimate on diffraction, not shadows of the secondary. Check it closer to focus if you want to know whether you should leave it as is. 

Thanks for your reply. Sorry, I'm new to mirror scopes, and don't know what's a right setting for this. This is closer to focus, is this good enough? It still looks a bit off to the two o'clock position, but I'm wondering if I should mess with this or not.

 

Thank again.
 

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  • mak test 3.jpg

Edited by vidrazor, 08 June 2021 - 12:38 PM.


#7 dweller25

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Posted 08 June 2021 - 01:06 PM

Yes it’s slightly off as you said.

I would improve it as I like perfection.

You still need to go closer to focus



#8 KTAZ

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Posted 08 June 2021 - 01:31 PM

How old is your c90? Can it even BE collimated? The older ones had the primary mirrors glued in place.


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#9 vidrazor

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Posted 08 June 2021 - 02:03 PM

How old is your c90? Can it even BE collimated? The older ones had the primary mirrors glued in place.

Yes, see below.

 

Yes it’s slightly off as you said.

I would improve it as I like perfection.

You still need to go closer to focus

Hmm, OK, well this time I defocused it "before" focus, as opposed to "past" focus towards infinity. Is this better?

I think perhaps I should just leave this alone, but if I'm to adjust this, would I turn the screw at roughly 8 o'clock counter-clockwise, say 1/8-1/16 turn, and the screw at 6 o'clock about 1/16-1/32, or turn them equally, or just the 8 o'clock screw, or is there a different approach to do this? That is a right-reading image from my camera, should I adjust the opposite screws, or adjust the 6 and 8 o'clock screws in the opposite direction?

 

Thanks for your replies.
 

Attached Thumbnails

  • mak test 5.jpg

Edited by vidrazor, 08 June 2021 - 02:09 PM.

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#10 luxo II

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Posted 08 June 2021 - 08:08 PM

The pic of the left again shows collimation is slightly off, left-right. To correct this:

 

a) do not touch the bottom screw,

 

b) adjust the top left screw, literally just "crack" the screw;

c) adjust the top right screw the same amount but in the OPPOSITE direction;

 

Retest. This should either correct it, or make it worse; if the opposite way turn the screws the opposite way and repeat.


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#11 sevenofnine

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Posted 08 June 2021 - 08:19 PM

How does a real star test look? If that's pretty good, I vote for leaving well enough alone. It's just too easy to make things worse. The push/pull collimation system is somewhat counter intuitive  waytogo.gif


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#12 KTAZ

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Posted 08 June 2021 - 08:21 PM

This has been discussed many times on these forums. That donut is far to refocused to give you anything but an approximation. The more refocused you get, the less sensitive the test is.


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#13 vidrazor

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Posted 08 June 2021 - 08:38 PM

The pic of the left again shows collimation is slightly off, left-right. To correct this:

a) do not touch the bottom screw,

b) adjust the top left screw, literally just "crack" the screw;

c) adjust the top right screw the same amount but in the OPPOSITE direction;

Retest. This should either correct it, or make it worse; if the opposite way turn the screws the opposite way and repeat.

Thanks, I wasn't sure which screws to work with. Gonna do some real-world tests first, then decide if I want to mess with it.

 

How does a real star test look? If that's pretty good, I vote for leaving well enough alone. It's just too easy to make things worse. The push/pull collimation system is somewhat counter intuitive  waytogo.gif

I think I'm gonna go with that for now. Just shoot and see what I get. Yes the push/pull thing is weird, not sure why you can't just have one screw to push or pull. Looking at some Newtonian videos on collimation however, I don't feel so bad. grin.gif

 

Thanks for everyone's replies.
 


Edited by vidrazor, 08 June 2021 - 08:39 PM.


#14 luxo II

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Posted 08 June 2021 - 09:44 PM

Please note do not over-tighten the push-pull screws - they are only holding a mirror weighing a few ounces - and do not need to be any more than finger-firm to stop any movement.


Edited by luxo II, 08 June 2021 - 09:44 PM.

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#15 maroubra_boy

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Posted 08 June 2021 - 11:31 PM

vidrazor,

 

One thing I have learned from Luxo ii is you start the collimation process with a large doughnut like you have pictured in post No. 6, but then to fine tune you make the doughnut small.

 

The thing with a large doughnut is whatever collimation error remains the large doughnut will be evening out any error.  By then making the doughnut small, these wee errors are then much easier to spot and fix.

 

For those of you not too familiar with the collimation process, regardless of the type of reflector you have (Newt, SCT, Mak, whatever) this collimation process is done using high magnification.  And when it is done using a star, the final check is known as star testing and should be done ALL the time if you use a laser to collimate with as only this process will truly verify of your work with a laser.  We should take a leaf out of professional observatories where initial collimation is done using lasers, but verified by star testing, which after all it is stars we are looking at, not a laser dot.

 

Alex.


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#16 jesco_t

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Posted 09 June 2021 - 01:23 AM

I find this description of the collimation process very helpful: http://www.astrophoto.fr/collim.html

Collimation is a multi-step procedure. The out of focus is only the beginning. The link has some nice simulated star patterns and images to illustrate this.

To add on to what other said, I really like MetaGuide as a collimation aid. It uses your camera to display the Airy disc pattern. Since it uses high framerate my and stacks the disc pattern it works even in not so perfect seeing. Steep learning curve, but very comfortable afterwards.

Maybe this help you.

Cheers,
Jesco
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#17 Redbetter

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Posted 13 June 2021 - 04:41 AM

Thanks, I realized after I posted it that I put it inthe wrong place, and didn't know how to delete it.

 

Thanks for your reply. Sorry, I'm new to mirror scopes, and don't know what's a right setting for this. This is closer to focus, is this good enough? It still looks a bit off to the two o'clock position, but I'm wondering if I should mess with this or not.

 

Thank again.
 

Looks substantially out of collimation to me.  I would expect the actual airy disk pattern to show substantial coma like this.  Needs to be collimated if you want the best visual images out of it.  



#18 vidrazor

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Posted 26 July 2021 - 12:36 AM

OK, so I went out to find some stars and try to collimate the C90, and found a sky full of clouds. So I went back home and set up a pinhole flashlight again and decided to try to collimate with that. I just slightly knocked the lens out of focus until I had a some rings. for some reason I could not figure out APT's controls for their collimation tool, probably because I was a bit apprehensive about doing this. Fortunately I had run across Al's Collimation Aid and used that instead.

 

I found the process a bit nerve wracking, because I kept thinking I was going to screw this thing up. I kept doing micro turns and it seemed like it never moved, I must have spent two hours trying to do this! I had no idea how much to push a turn of a screw, so I erred on the side of caution and it took forever. At first I wasn't sure which way to turn these things too, and not see any significant change had me never knowing if I was going in the right direction or not. Looking through APT's low res screen wasn't helping.

 

Anyway, it SEEMS to me as though I got this pretty decent, have a look at these screens and let me know what you think.

 

Thanks.

Attached Thumbnails

  • col6.jpg
  • col 5.jpg


#19 luxo II

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Posted 26 July 2021 - 12:43 AM

Looks pretty good, but you're a bit too far off focus to tell. 



#20 jesco_t

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Posted 26 July 2021 - 05:49 AM

It’s not bad, but it looks a bit asymmetrical. The right hand side is thinner than the left hand side.

And as luxo said, you can go more into focus for the final adjustments.

#21 luxo II

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Posted 26 July 2021 - 05:55 AM

It’s off a tad between 10 o’clock (fatter) and 4 o’clock (thinner). The difference between “near enough” and nailing it is significant in image quality.

#22 vidrazor

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Posted 26 July 2021 - 08:27 AM

It’s off a tad between 10 o’clock (fatter) and 4 o’clock (thinner). The difference between “near enough” and nailing it is significant in image quality.

Yeah that's where I've been coming in from, trying to expand the 2 o'clock point. Hopefully I still have enough resolution off the APT preview screen (which I have to zoom in a lot to see) when I turn focus closer.

 

I was loosening the 2 o'clock pull screw and tightening the 10 o'clock pull screw, but after a while it felt like the 2 o'clock screw was beginning to feel loose, so I started to tighten the 4 o'clock push screw and loosened the 7 o'clock push screw while continuing to tighten the 10 o'clock pull screw. I don't know how much more I can loosen the 2 o'clock pull screw, but it seems that's the one screw I should be loosening more. I'm not sure how much more I can tightened the 10 o'clock pull screw either. I'm hesitant to adjust the push screws any further too.

 

Thanks for everyone's replies.
 

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  • c90 screws.jpg

Edited by vidrazor, 26 July 2021 - 08:42 AM.


#23 vidrazor

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Posted 26 July 2021 - 03:35 PM

I have to say this is like play whack-a-mole. I've gone through so many iterations of this I don't know where I am. smile.gif I focused, then brought the defocus as minimal as I could to see rings. I have to say I had to turn the adjustment screws more aggressively than most people seem to recommend to see any kind of difference. Anyway, this is where I am. These images are at 200% enlargement (although the collimation overlay is at 100%). Again, it seems decent to me. What do you folks think?

 

Thanks.

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  • col 200 percent.jpg

Edited by vidrazor, 26 July 2021 - 03:38 PM.


#24 vidrazor

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Posted 27 July 2021 - 01:13 PM

Well apparently I have no idea what I'm doing. grin.gif  After all that adjustment, this is what Jupiter and a pair of moons came out looking like. So I'm not sure what to do with this, other than toss it. bombdrop.gif

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Edited by vidrazor, 27 July 2021 - 01:13 PM.


#25 Bill Barlow

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Posted 27 July 2021 - 03:54 PM

Your collimation images don't look bad, pretty close.  Maybe you got a C90 with not so good optics, no matter how good the collimation is.




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