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SCT Collimation Frustration

Collimation SCT
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#1 Bituminousbear

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Posted 21 February 2021 - 06:26 PM

Hello.  I recently picked up a used Celestron  C8, and been trying to get collimation on it.  I can get collimation on the center start pretty well, but stars outside the center are a bit all over it seems. I am not sure if this is user error, equipment error, or something else.  

 

Gear list is below, but I'm using the Celestron f6.3 corrector/reducer and and ASI385.  The attached screenshot is Betelgeuse that was collimated (using much lower exposer), but as you can see all the other stars are out of collimation.  I've tried chasing collimation on different stars and in different areas of the image to no luck.  I've also watched a ton videos and tuts and read through many different forum posts, but still seem to be at a loss. Any thoughts or suggestions would be wonderful.

 

Gear:

Celestron C8 (used)

Bob's Knobs

Celestron 6.3 reducer/corrector

ASI385 camera

106mm back focus (cannot get spot on 105mm with current setup)

EQ6r-pro 

 

Thanks!

 

Attached Thumbnails

  • feb19 collimate.JPG


#2 slepage

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Posted 21 February 2021 - 06:43 PM

When Colimating an SCT you only concern yourself with the center star and you use a high power eyepiece.  You should not collimate using a reducer.  Once colimated and you are adding your reducer for your imaging it will be perfectly normal not to have stars on the edge or for that mater any star away from the center to not look collimated.  The reason is due to one or more factors.  1) Your reducer is not providing you with a true flat field and the field curvature is creating that effect. 2) you have induced tilt in your image train, 3) optical alignment is off ( could be due to placement of primary or secondary mirrors or a host of over issues with alignment).  The most common reason is not have a true flat field.  There are better reduces then the Celestron reducer if you are unhappy with your stars away from the center.

 

Steve


Edited by slepage, 21 February 2021 - 06:44 PM.

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#3 MitchAlsup

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Posted 21 February 2021 - 07:07 PM

Hello.  I recently picked up a used Celestron  C8, and been trying to get collimation on it.  I can get collimation on the center start pretty well, but stars outside the center are a bit all over it seems. I am not sure if this is user error, equipment error, or something else.  

 

Gear list is below, but I'm using the Celestron f6.3 corrector/reducer and and ASI385.  <snip>

 

Gear:

Celestron C8 (used)

Bob's Knobs

Celestron 6.3 reducer/corrector

ASI385 camera

106mm back focus (cannot get spot on 105mm with current setup)

EQ6r-pro 

Take off everything:: and use the telescope straight through (not even a diagonal, and for sure not a reducer.)

 

Use a high power EP, something like 3.5mm (7mm in a Barlow/PowerMate.) Which for most applications would be considered stupidly high magnification. Even better if the EP has a reticule so you can properly center the star near the reticule. You ARE, after all, looking at diffraction patterns not fields of view.

 

Use a star high in the sky to minimize atmospheric effects. Do not attempt to collimate if the sky has rivers of air running through them (or use an artificial star from a long way away in daylight.)

 

First order of business: As you get close to focus, there will be a ring of light around a dark circle (as seen in your posted image). You want this ring of light to be evenly illuminated around the whole ring--AND--you want this ring of light to collapse into focus, and then spring back into a ring of light just beyond focus.

 

Based on your photo it does not look terrible as we see stars with a nice disk (with a hole in it). I can see that the collimation is off because the disks to the left shows significantly more coma than the disks to the right. But I can also see the center bright star is WAY TOO BRIGHT. you should be using a 4M star bright enough to see the disks and patterns, not so bright as to over illuminate the whole view.

 

ETA:: after you get it collimated for visual, add back in the diagonal. If collimation changes, collimate the diagonal with paper shims if necessary so that the collimation does not change as you rotate the diagonal around the holder.

 

After the diagonal is collimated, then you can try the reducer. If it alters collimation--I don't know what to advise.


Edited by MitchAlsup, 21 February 2021 - 07:09 PM.

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#4 Bituminousbear

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Posted 21 February 2021 - 07:48 PM

When Colimating an SCT you only concern yourself with the center star and you use a high power eyepiece.  You should not collimate using a reducer.  Once colimated and you are adding your reducer for your imaging it will be perfectly normal not to have stars on the edge or for that mater any star away from the center to not look collimated.  The reason is due to one or more factors.  1) Your reducer is not providing you with a true flat field and the field curvature is creating that effect. 2) you have induced tilt in your image train, 3) optical alignment is off ( could be due to placement of primary or secondary mirrors or a host of over issues with alignment).  The most common reason is not have a true flat field.  There are better reduces then the Celestron reducer if you are unhappy with your stars away from the center.

 

Steve

Thanks for your thoughts.

 

I've never thought to collimate without the reducer on and with an eyepiece.  I can definitely try that and see if I can dial in a bit more.  My concern about stars outside the center are the star shapes. They always have little flares to one side or another.

 

I've tried changing my image train a couple different times, so don't think that is it.  I have heard the celestron f6.3 are not the best, but have seen a ton of great photos with them inline so was hoping it'd fare a bit better. :)

 

3 is definitely possible as this is used, but I'll try your first suggestion and see how it looks.



#5 Bituminousbear

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Posted 21 February 2021 - 07:52 PM

Take off everything:: and use the telescope straight through (not even a diagonal, and for sure not a reducer.)

 

Use a high power EP, something like 3.5mm (7mm in a Barlow/PowerMate.) Which for most applications would be considered stupidly high magnification. Even better if the EP has a reticule so you can properly center the star near the reticule. You ARE, after all, looking at diffraction patterns not fields of view.

 

Use a star high in the sky to minimize atmospheric effects. Do not attempt to collimate if the sky has rivers of air running through them (or use an artificial star from a long way away in daylight.)

 

First order of business: As you get close to focus, there will be a ring of light around a dark circle (as seen in your posted image). You want this ring of light to be evenly illuminated around the whole ring--AND--you want this ring of light to collapse into focus, and then spring back into a ring of light just beyond focus.

 

Based on your photo it does not look terrible as we see stars with a nice disk (with a hole in it). I can see that the collimation is off because the disks to the left shows significantly more coma than the disks to the right. But I can also see the center bright star is WAY TOO BRIGHT. you should be using a 4M star bright enough to see the disks and patterns, not so bright as to over illuminate the whole view.

 

ETA:: after you get it collimated for visual, add back in the diagonal. If collimation changes, collimate the diagonal with paper shims if necessary so that the collimation does not change as you rotate the diagonal around the holder.

 

After the diagonal is collimated, then you can try the reducer. If it alters collimation--I don't know what to advise.

I'm going to try your (and the commenter's above) idea of collimating without anything and on an eye piece.

 

Totally understood that I should be collimating on a lower magnitude star.  This one was just a great example that help to illustrate some of the issues I was talking about.

 

Thanks for your feedback!



#6 MitchAlsup

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Posted 21 February 2021 - 09:41 PM

COllimation.jpg

 

I put a yellow circle where I think the center of your collimated field happens to be.


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#7 osbourne one-nil

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Posted 22 February 2021 - 02:38 AM

Moved from reflectors. 


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#8 Bituminousbear

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Posted 22 February 2021 - 05:53 PM

Ok, had some clear skies last night with pretty good seeing. Taking the Celestron f6.3 reducer off definitely made a difference. 

 

I think the reducer/corrector I have may be bad.  I know there are much better ones (Starizona for instance), but I've heard the Celestron should be at least better than I'm experiencing.



#9 MitchAlsup

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Posted 22 February 2021 - 06:34 PM

Ok, had some clear skies last night with pretty good seeing. Taking the Celestron f6.3 reducer off definitely made a difference. 

 

I think the reducer/corrector I have may be bad.  I know there are much better ones (Starizona for instance), but I've heard the Celestron should be at least better than I'm experiencing.

The lenses might just be jarred out of alignment.

 

If you an find the retaining ring, and loosen it 1/2 turn, jiggle the whole assembly, then firm it up, it might come around all by itself.


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#10 bips3453

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Posted 22 February 2021 - 09:00 PM

The lenses might just be jarred out of alignment.

 

If you an find the retaining ring, and loosen it 1/2 turn, jiggle the whole assembly, then firm it up, it might come around all by itself.

smile.gif Love this advice!!! Any recommendations on how much should the body move and which song to use? wink.gif


Edited by bips3453, 22 February 2021 - 09:13 PM.


#11 KTAZ

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Posted 22 February 2021 - 09:19 PM

Temper your expectations with a non-EdgeHD SCT. As mentioned, the reducer/corrector should help with the coma, but the curved mirrors that are inherent to SCT’s introduce spherical aberration and that is all there is to it. 

 

My Celestron .63 reducer does a better job at correcting it than running without it, but it is clearly still there. Unfortunately, the cost of the Starizona offering is pretty darn steep.



#12 starman876

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Posted 23 February 2021 - 09:05 AM

Diagonals can introduce a lot of error if they are not properly aligned.  Heaven knows the troubles I have had with improperly aligned diagonals.  Always check alignment without anything else in the scope.  Just the scope and a good eyepiece.  Then one by one put the other pieces into the light path.  The minute you see a diference you have found the culprit.  



#13 Chris Johnson

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Posted 23 February 2021 - 10:50 AM

I follow the guide in the first post of this link https://www.cloudyni...ation-epiphany/


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#14 Bituminousbear

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Posted 23 February 2021 - 11:45 AM

...

My Celestron .63 reducer does a better job at correcting it than running without it, but it is clearly still there. Unfortunately, the cost of the Starizona offering is pretty darn steep.

Does the Starizona flattener do a pretty decent job?  I've debated getting one, but worried about the 27mm image circle.  Seems to limit camera sensor size to below APS-C, especially if using with a OAG.  

 

For visual I definitely don't think it would be worth the price. The f6.3 is fine, even if slightly annoying.



#15 Bituminousbear

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Posted 23 February 2021 - 11:47 AM

The lenses might just be jarred out of alignment.

 

If you an find the retaining ring, and loosen it 1/2 turn, jiggle the whole assembly, then firm it up, it might come around all by itself.

Interesting idea.  I'm going to try this if I can find the retaining ring.  May end up worse than I am now, but here to hoping. :)




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