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SCT collimation question: different methods give different results?

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#1 LukaszLu

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Posted 02 November 2021 - 12:36 PM

Dear Friends,
as a sct-newbie who has played mostly with refractors all my life, I am asking more experienced colleagues for advice. I decided to experiment a bit with larger apertures and bought the 8 '' Meade LX200 ACF. The telescope is almost new, originally bought in the store in April this year. According to the previous owner, it has never even been cleaned, not to mention taking it apart or opening it.

 

I have had it for several days during which I am not able to get a satisfactory image quality. Perhaps it is a matter of special weather conditions and seeing - the image in my refractors has also been very poor lately. However, in such a situation, I started looking for possibilities to improve the image by improving the collimation.

 

And here I ran into a problem. If I collimate a telescope on a star, obtaining a regular, even ring of a defocused image, the collimation assessment method already mentioned here on the forum and described here: http://www.robincasa...ro/collimation/ shows considerable deviations from the norm. Having positioned myself so that I can see the shadow of the central obstruction is more or less centered around it, the outer rings proof to be not only non-centric, but not even circular. When I was drawing the image below from the photo, I discovered that the outer ring was in the shape of an ellipse.

 

Conversely, when I look at these rings from the outside, I position the secondary to make them more or less centered, and then make a star-test, the ring becomes slightly uneven in the defocused image.

 

What do you think about this situation? Does this mean that some elements such as the corrector plate or the primary mirror, are not mounted centrally?

Attached Thumbnails

  • kolimacja.jpg

Edited by LukaszLu, 02 November 2021 - 12:43 PM.

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#2 rgsalinger

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Posted 02 November 2021 - 01:46 PM

You can't accurately collimate an SCT the way it's described in that article, IMHO. I think that you can use that technique to get a rough collimation but for accurate collimation you need to use a star on a night with good seeing and this method. Basically, until you are very close to focus you can't really see if the collimation is really good. The "big donut" is fine to get going but you need to be very close to best focus to get an excellent collimation.

Rgrds-Ross


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#3 LukaszLu

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Posted 02 November 2021 - 02:00 PM

A valuable hint, thank you. My donut had a diameter of about 20% of my field of view. At this size it seemed to be perfectly regular, but you are right - I should check what happens when it gets 2 times smaller.


Edited by LukaszLu, 02 November 2021 - 02:00 PM.


#4 jesco_t

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Posted 02 November 2021 - 03:15 PM

The donut is just the first step when collimating on a star (real or artificial). The next step would be symmetrizing the diffraction pattern close to focus. And the last step is to symmetrize the Airy disc pattern in focus - although that step needs really good seeing. The last 1/16th of a turn of the screw/knob really can make a difference.

 

All this needs a reasonable magnification to make sense. It doesn't help to collimate at 50x. Visually, I do it at 150x or higher, depending on seeing. When I intend to image Luna or a planet, I use my planetary camera. With Barlow and ADC it has less than 4 arcmin field of view. Just to give you an idea of the level of magnification that can be useful.

 

Jesco


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#5 LukaszLu

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Posted 02 November 2021 - 04:07 PM

Thanks, I'm aware that magnification does matter. With 11 mm eyepiece I was around 185x while collimating on the star (or rather Jupiter...).


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#6 Bean614

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Posted 02 November 2021 - 07:55 PM

A Star is much preferred over a Planet!


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#7 Jeffmar

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Posted 02 November 2021 - 10:48 PM

I do pretty well collimating with a 10mm eyepiece with my SCT’s. I also try to center the donut to check symmetry after each adjustment, just in case being off center creates some distortion. That might not be an issue with your AFC model. 


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#8 LukaszLu

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Posted 03 November 2021 - 12:05 PM

This model has RC design - I have the impression that the susceptibility to distortion is much lower than in a typical SCT, but still noticeable, so of course I try to keep the donut precisely in the center of the field of view. :-)

 

The weather is consistently bad, as soon as it gets better I'll repeat the collimation on the star, reducing the donut diameter even more ...

 

This is what I've managed to get so far - it is far, far below expectations. However, I cannot assess the influence of the exceptionally bad seeing:

Attached Thumbnails

  • JUPITER-2021-11-01.jpg
  • ARISTARCHUS-2021-10-31m.jpg

Edited by LukaszLu, 03 November 2021 - 12:41 PM.

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#9 Jeffmar

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Posted 03 November 2021 - 02:33 PM

Seeing has to be considered. If seeing isn’t great it doesn’t matter how well you collimate your scope. It may be that your scope is well collimated, but you haven’t had a calm enough atmosphere to get sharp images. In my area I have seen crystal clear images of planets and round moons going around Jupiter and I have seen Jupiter looking like a water balloon bouncing down a hill. We all need personal space telescopes!grin.gif


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#10 LukaszLu

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Posted 03 November 2021 - 04:27 PM

This is how I console myself, although I have to admit that I have never had a situation where poor seeing lasted so long. The quality of the images shown here is second to those that in the same location can be obtained from a small, old achromatic refractor from 1962, 76 mm in diameter.

For comparison, Jupiter and the Moon photographed by the Royal Astro 76/1200 mm retro-refractor with the same camera (but NOT a the same time):

 

JUPITER-2021-07-30.jpg

 

THE-MOON-2021-10-25-2.jpg


Edited by LukaszLu, 03 November 2021 - 04:50 PM.

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#11 Kokatha man

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Posted 03 November 2021 - 07:04 PM

...let's not talk about "donuts" too much - that is usually confined to a very de-focused star-image, which can only be seen as a very rudimentary step in the processing routine!

 

I collimate "in camera" with an SCT at the f/l we mainly use (circa 6,000mm) because I am almost exclusively a planetary imager, but whichever way (camera or ep) you really need to be down to about 6-10 diffraction rings outside of focus to start enabling proper collimation, concentrating upon the central Poisson Point's position as well as the uniformity of the DR's around said PP .....once this is done to the best of the actual seeing's possibilities, the objective is to move within 2 or 3 rings of diffraction & observe/adjust once more.

 

At this "very-close-to-focus" position the centrality (is that a word? lol.gif ) - or not - of the PP becomes much more noticeable: noting that a true Airy Disk is very often not able to be seen "in camera" although at focus the central PP ought to collapse into a small point with uniform "flares" surrounding it. (further explanation is on our website here: http://momilika.net/...3Processing.htm )

 

Note also that the state of thermal equilibrium plays quite a profound effect in this process, with SCT's it can induce heat flares as well as deformity of the circular nature of the DR's etc...& seeing (especially as f/l or magnification becomes high) introduces all sorts of other effects - why many imagers use Metaguide - although "old hands" like us prefer eye-adjustments...but it takes a bit of experience to determine whether a wandering PP or varying DR's are where you want them in poorer seeing! wink.gif


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#12 gnowellsct

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Posted 03 November 2021 - 07:36 PM

You can't accurately collimate an SCT the way it's described in that article, IMHO. I think that you can use that technique to get a rough collimation but for accurate collimation you need to use a star on a night with good seeing and this method. Basically, until you are very close to focus you can't really see if the collimation is really good. The "big donut" is fine to get going but you need to be very close to best focus to get an excellent collimation.

Rgrds-Ross

This.  Plan your first collimation as an exercise all by itself.  Use a 3rd or 4th magnitude star.  

 

This may help  

 

https://astromart.co...e-circumstances



#13 LukaszLu

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Posted 03 November 2021 - 08:31 PM

Great links and resources - thanks a lot! I can't wait for the weather to start my experiments :-)



#14 LukaszLu

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Posted 04 November 2021 - 04:19 PM

Today the sky was clear enough for a while, to twist the screws a bit. What do you think? What about the fairly pronounced disturbance of the circular ring shape that is visible between 6 o'clock and 9 o'clock?

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  • kolimacja.jpg


#15 gonnessia

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Posted 04 November 2021 - 05:27 PM

I just made a tri-bahtinov mask. The difference with a regular bahtinov mask, is that the tri-version can also be used for collimating a SCT !

Of course, I have had bad weather ever since, but it may  be an idea when trying to collimate.

(See image)

Attached Thumbnails

  • Screenshot_20211104-232147_Gallery.jpg

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#16 freestar8n

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Posted 04 November 2021 - 06:29 PM

"old hands" like us prefer eye-adjustments...but it takes a bit of experience to determine whether a wandering PP or varying DR's are where you want them in poorer seeing! wink.gif

Based on the results I constantly see in the planetary imaging forum - I would never question the skill of the old hands I see manually tweaking collimation.  Experience and skill clearly get the job done well manually.

 

But no matter what - you gotta use an in-focus star for the final tweaking.  All the other methods can get you close - but you need a real star, overhead, and in-focus at high mag, whether visual or video, for the final tweaks.

 

If you do that, there should be no "different results" by different methods.  They all end with final tweaks on an in-focus star.

 

If you are doing it visually, then yes you should not use too bright a star.  3rd or 4th mag - choose what works best.  But if using video you can use as bright as you want - just turn down the exposure.  It should be nearly overhead to avoid atmospheric dispersion, which gives the star a stretched, coma-like appearance.

 

Alternatively if you are imaging jupiter, there may be benefit in collimating directly on the target itself - but not jupiter itself.  Some study the fuzz around the moons as a way to tweak collimation.  Again - at high power and in-focus.

 

Frank


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#17 jesco_t

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Posted 05 November 2021 - 03:20 AM

Today the sky was clear enough for a while, to twist the screws a bit. What do you think? What about the fairly pronounced disturbance of the circular ring shape that is visible between 6 o'clock and 9 o'clock?

I am not an expert, but to me this looks like your dewshield (or something else) partially obstructs the aperture.

 

From the more zoomed-in views (last and second-to-last row) it's evident that the scope is a bit out of collimation. 12 o'clock is compressed compared to 6 o'clock. This is actually a good example of something that is not visible in the big out-of-focus donut.


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#18 LukaszLu

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Posted 05 November 2021 - 05:41 AM

Thanks, I've noticed right away that the dewshield had an effect on the picture. So I collimate WITHOUT the dewshield. If the donut is not round despite this, it probably means something - I wonder what exactly does it mean...?

 

What's more - when I tried to align collimation at high magnification, the large donut became less regular. Is it normal?



#19 dcaponeii

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Posted 05 November 2021 - 08:37 AM

Thanks, I've noticed right away that the dewshield had an effect on the picture. So I collimate WITHOUT the dewshield. If the donut is not round despite this, it probably means something - I wonder what exactly does it mean...?

 

What's more - when I tried to align collimation at high magnification, the large donut became less regular. Is it normal?

It has always been the case for my scope.  Collimation at near focus yields a less than concentric large donut.  But since you observe/image at focus that's what matters.  You might try Metaguide if you are an imager.  Use of an IR Pass or Red filter can help with collimation as well.



#20 LukaszLu

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Posted 05 November 2021 - 08:52 PM

Today I could only take advantage of the brief moment when the sky was clear. I did not have time to play with collimation, but did manage to take a test photo of Jupiter to see if my previous collimation attempts (the telescope is in the state shown in the image above) had any effect.

 

They didn't bring any. The image is still out of focus, imprecise and clearly inferior to the quality of images from a simple achromatic refractor with a diameter of a few centimeters. I have doubts whether the clolimation shown in the pictures may be responsible for such a dramatic result. Collimation - although not perfect - seems to be within the standards provided by equipment manufacturers and presented by them in their manuals.

 

What do you think?

Attached Thumbnails

  • JUPITER-2021-11-05.jpg

Edited by LukaszLu, 05 November 2021 - 08:55 PM.

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#21 Kokatha man

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Posted 05 November 2021 - 10:01 PM

Lukas, forget those large donut images as I said earlier...do some more as in the last line of images in your Post #14, but lower the exposure by dropping gain &/or increasing the fps: also make sure it is defocused to only about 6 individual rings of diffraction for your first images...& then with only a couple of DR's - then finally at focus or as close as you can deem it.

 

It might appear as though the images you capture are too dim but you can stretch/brighten these in processing - run short captures of about a 1,000 frames for each if possible.

 

What I don't know from your images is whether these are at or near the actual capture scale you took...as I state in my tute commentary, one indicator of approaching good collimation - even in poorer seeing as per this example - is that the inner DR's will begin to light up/appear.

 

Untitled-2.png

 

Remember to leave the scope out for several hours beforehand if you can - I won't suggest you follow my extreme (but highly effective) method of bringing the scope to ambient, but it does need to be very well near ambient/air temperature equilibrium to get a good appraisal, & decent seeing also. (these can be tall orders!)


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#22 LukaszLu

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Posted 06 November 2021 - 07:07 AM

Hi, as soon as the weather improves I will of course start "carving" the collimation according to your advice.



#23 LukaszLu

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Posted 06 November 2021 - 03:23 PM

I haven't been able to get such a precise image near the focus to distinguish the individual DRs, I need to work on a better quality of these photos - sorry ...

 

Today I've tried to start with the eyepiece collimation. I finished the process at 9 and 6 mm focal lengths, trying to get a maximally regular image near the focus.

 

To my surprise, after connecting the camera, the image of the donut turned out to be very asymmetrical - completely different from what I observed through the eyepiece. But when I moved back behind the focus, the situation turned around and the image of the donut returned to what I had seen through the eyepiece.

 

Is such a difference between intra and extra focal rings normal?

 

 

I also took another series of photos of Jupiter to see if these subsequent changes in collimation had some effect on the precision of the image. I will show the effects as soon as my laptop ends up bothering with it.

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  • kolimacja1.jpg


#24 carolinaskies

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Posted 06 November 2021 - 03:50 PM

I haven't been able to get such a precise image near the focus to distinguish the individual DRs, I need to work on a better quality of these photos - sorry ...

 

Today I've tried to start with the eyepiece collimation. I finished the process at 9 and 6 mm focal lengths, trying to get a maximally regular image near the focus.

 

To my surprise, after connecting the camera, the image of the donut turned out to be very asymmetrical - completely different from what I observed through the eyepiece. But when I moved back behind the focus, the situation turned around and the image of the donut returned to what I had seen through the eyepiece.

 

Is such a difference between intra and extra focal rings normal?

 

 

I also took another series of photos of Jupiter to see if these subsequent changes in collimation had some effect on the precision of the image. I will show the effects as soon as my laptop ends up bothering with it.

To check collimation don't use the stock focuser to check inside/outside focus because on an SCT that is moving the primary mirror which can slightly pitch. 

Instead it's better to use an external focusing method. Then you get a true representation of inside/outside focus.  

If you don't have an external focuser option available, use an extension tube setup to create an inside and outside focus point. If you have a long and short barlow... remove the optics from both. Put the shorty in w/eyepiece and focus your star. Remove the shorty and put the eyepiece in place... that is 'inside focus'. Now remove the eyepiece and install the long tube/eyepiece in place... that's outside focus.  This will eliminate mirror movement as a source.  


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#25 LukaszLu

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Posted 06 November 2021 - 04:36 PM

Thanks - that's a valuable advice. I use an external focuser but it only has a range of approx. 2 cm - maybe that's enough. However, what you write means that the collimation results will be different for each point I hold the mirror - it's quite a scary prospect... :-)

 

The telescope is new, bought in April and has not yet had any problems with the mirror shift - the image is perfectly stable during sharpening.




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