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Edge HD Collimation

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#1 mehdymo

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Posted 04 April 2020 - 02:51 PM

I have an 8" Edge HD and use it with 0.7X reducer for astrophotography. I removed the corrector and cleaned it and tried to collimate it again. Here is the result without reducer. I am wondering if it is good enough. I did this test on Dubhe star of the big dipper. The exposure time was 100 ms, so the star shape was changing very fast (expect a little more dofermation on the star shape as taking the right frame is hard at this speed). I took an image of the slightly defocused star at every edge/corner of the ASI1600 sensor and combined all in one image and did some post processing to make it more clear. I checked the moon with it and the view was really amazing (APO Quality), but couldn't take any image as the clouds rolled in(will try another night). Please let me know what you think. Hope you are all doing well these days and not affected by this virus.

Thanks

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Edited by mehdymo, 04 April 2020 - 02:58 PM.


#2 John Miele

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Posted 04 April 2020 - 03:00 PM

I think you are pretty close but still off just a little. See how the lower half portion of the outer ring is just a little brighter on most of the images? You need to not only try to make the rings concentric but to make them evenly bright all around the circumference. But like I said, you are very close already...smile.gif


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#3 aeroman4907

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Posted 04 April 2020 - 03:23 PM

I would say you are off on collimation a bit.  It is important to note this is only one of the first steps of collimation.  Using a highly defocused start just gets you in the general vicinity of proper collimation.  With the offset you have with a highly defocused star, I would not attempt high resolution imaging on the moon quite yet.  You'll need to get a much more tightly focused star to really be able to determine if you are properly collimated.

 

Check this post from Tom Glenn:  https://www.cloudyni...-a-photo-guide/


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#4 Migwan

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Posted 04 April 2020 - 03:40 PM

Top is a bit long at the top in the center image.   If you can catch the airy disk at focus, that would be best.    jd


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#5 markb

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Posted 04 April 2020 - 04:01 PM

Aeroman4907, thanks for a great link I never found in a search before your post!

 

I will add one (to me, major) link to review, the Robin Casady DAYTIME collimation method. It gets one within 95% of the way collimated in daytime, with no risk of screwdrivers or Allen keys slipping, and reserves your first good night for the final tweak, in focus, never out of focus for the donut, another 'almost there' method of starting collimation but a poor method of finalizing collimation.

 

http://www.robincasa...ro/collimation/

 

I modify it somewhat by moving back and forth to check concentricity of the different reflections and shadows.

 

A real time and frustration saver!


Edited by markb, 04 April 2020 - 04:02 PM.

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#6 mehdymo

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Posted 04 April 2020 - 04:03 PM

Thanks for your good point. I will try it again, but should I expect perfect circles at the edges without reducer?

I think you are pretty close but still off just a little. See how the lower half portion of the outer ring is just a little brighter on most of the images? You need to not only try to make the rings concentric but to make them evenly bright all around the circumference. But like I said, you are very close already...smile.gif



#7 mehdymo

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Posted 04 April 2020 - 04:05 PM

Thanks, I checked the airy disk and it was not bad (saw a 3-4 circles around the star). But, do you think I should get perfect circles at the edges?

I would say you are off on collimation a bit.  It is important to note this is only one of the first steps of collimation.  Using a highly defocused start just gets you in the general vicinity of proper collimation.  With the offset you have with a highly defocused star, I would not attempt high resolution imaging on the moon quite yet.  You'll need to get a much more tightly focused star to really be able to determine if you are properly collimated.

 

Check this post from Tom Glenn:  https://www.cloudyni...-a-photo-guide/



#8 mehdymo

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Posted 04 April 2020 - 04:06 PM

Thanks, that would be great, I will give it another try.

Aeroman4907, thanks for a great link I never found in a search before your post!

 

I will add one (to me, major) link to review, the Robin Casady DAYTIME collimation method. It gets one within 95% of the way collimated in daytime, with no risk of screwdrivers or Allen keys slipping, and reserves your first good night for the final tweak, in focus, never out of focus for the donut, another 'almost there' method of starting collimation but a poor method of finalizing collimation.

 

http://www.robincasa...ro/collimation/

 

I modify it somewhat by moving back and forth to check concentricity of the different reflections and shadows.

 

A real time and frustration saver!



#9 mehdymo

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Posted 04 April 2020 - 04:10 PM

Thanks for your reply. It is not much defocused. I have zoomed it 3 times (digital zoom). But, I will try it again and defocus just a bit this time. 

I would say you are off on collimation a bit.  It is important to note this is only one of the first steps of collimation.  Using a highly defocused start just gets you in the general vicinity of proper collimation.  With the offset you have with a highly defocused star, I would not attempt high resolution imaging on the moon quite yet.  You'll need to get a much more tightly focused star to really be able to determine if you are properly collimated.

 

Check this post from Tom Glenn:  https://www.cloudyni...-a-photo-guide/



#10 mehdymo

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Posted 04 April 2020 - 04:22 PM

I did this a few days ago with my canon camera on a tripod instead of my eyes to have a fixed image and avoid shaking or moving. The boundary between the black and white disks in camera was not clear enough to judge the collimation. I found it very useful for rough collimation. I used a Hotech advanced laser collimation, but it has its own problems as well.

Aeroman4907, thanks for a great link I never found in a search before your post!

 

I will add one (to me, major) link to review, the Robin Casady DAYTIME collimation method. It gets one within 95% of the way collimated in daytime, with no risk of screwdrivers or Allen keys slipping, and reserves your first good night for the final tweak, in focus, never out of focus for the donut, another 'almost there' method of starting collimation but a poor method of finalizing collimation.

 

http://www.robincasa...ro/collimation/

 

I modify it somewhat by moving back and forth to check concentricity of the different reflections and shadows.

 

A real time and frustration saver!



#11 aeroman4907

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Posted 04 April 2020 - 04:33 PM

Thanks, I checked the airy disk and it was not bad (saw a 3-4 circles around the star). But, do you think I should get perfect circles at the edges?

I am not aware of collimation procedures where you don't have the star located as close to dead center of the sensor as possible.  When I collimate, I will typically have to realign the star to center as I get closer and closer to collimation.  I don't know of a particular reason you would evaluate collimation off center.  Perhaps their is another aspect that I am not aware of why one would do this.

 

Best regards,

 

Steve


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#12 mehdymo

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Posted 04 April 2020 - 04:38 PM

I am asking this because one may have a perfect collimation at center but alongated/deformed stars in the edges due off-center secondary, astigmatism, and spherical aberration. An Edge HD is supposed to be free of these errors to a good degree. I wanted to know if this is acceptable for an Edge HD.

Thanks

I am not aware of collimation procedures where you don't have the star located as close to dead center of the sensor as possible.  When I collimate, I will typically have to realign the star to center as I get closer and closer to collimation.  I don't know of a particular reason you would evaluate collimation off center.  Perhaps their is another aspect that I am not aware of why one would do this.

 

Best regards,

 

Steve


Edited by mehdymo, 04 April 2020 - 04:42 PM.


#13 aeroman4907

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Posted 04 April 2020 - 05:13 PM

I am asking this because one may have a perfect collimation at center but alongated/deformed stars in the edges due off-center secondary, astigmatism, and spherical aberration. An Edge HD is supposed to be free of these errors to a good degree. I wanted to know if this is acceptable for an Edge HD.

Thanks

For that purpose, I think you would still want to have tightly focused stars.  I hope I am being accurate, but I believe you would be trying to create a spot diagram that can indicate how your scope is performing.  You can find a link at Celestron' s website here: https://www.celestro...s/edgehd-optics

 

If you go to the bottom of the webpage, you can download a pdf of their whitepaper on Edge HD optics.  While it has a marketing aspect to it, it does appear to be quite informative.  While they have less aberrations than some other optical designs, they do not create perfect stars in the corners of most sensors, but the stars are 'acceptable', unless of course you try and use a 35mm "full frame" sensor.


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#14 Stelios

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Posted 04 April 2020 - 05:26 PM

You *always* collimate WITH THE STAR CENTERED. Whenever you make an adjustment, you must re-center the star in order to evaluate. 

 

So the quality of the images outside of center is irrelevant. EVERY scope has some distortion far from center. Look at the Edge HD white paper (go to page 4). Even though the Edge HD is far superior to other designs for SCT's, it still has aberrations off axis. Collimation is SO VERY EXACTING that you *need* the perfection of the on-axis image. 

 

Your collimation is definitely not bad, but to improve on it you need a night of steady seeing and to defocus even less. Eventually, for perfect collimation, you have to find a great seeing night and collimate in focus with extremely high close up. 

 

A program like Metaguide (free, if a bit hard to understand) helps you by stacking a lot of images, thus getting rid of some of the effects of seeing. It also shows you what part needs adjustment. 


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#15 markb

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Posted 04 April 2020 - 05:29 PM

+1 on Steve's post, collimation is always done in the center of the field. 

 

No idea on an Edge but on a regular SCT or refractors, stars towards the edge will appear to be slightly out of collimation. I assume this is a physics thing. One trick for knowing which screw to turn is to move the scope so the star away from the center looks better, closer to collimated, and then turn the screw needed to move that star towards the center, repeating as needed. I haven't done this since I started doing the Casady method as it gets me close enough to tweak on an In focus star.


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#16 mehdymo

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Posted 04 April 2020 - 07:06 PM

Yes, you are right, that is what I am trying to do. I will check the whitepaper. Thanks.

For that purpose, I think you would still want to have tightly focused stars.  I hope I am being accurate, but I believe you would be trying to create a spot diagram that can indicate how your scope is performing.  You can find a link at Celestron' s website here: https://www.celestro...s/edgehd-optics

 

If you go to the bottom of the webpage, you can download a pdf of their whitepaper on Edge HD optics.  While it has a marketing aspect to it, it does appear to be quite informative.  While they have less aberrations than some other optical designs, they do not create perfect stars in the corners of most sensors, but the stars are 'acceptable', unless of course you try and use a 35mm "full frame" sensor.



#17 mehdymo

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Posted 04 April 2020 - 07:15 PM

Sorry, I had to explain how I have done it. I collimated the star at the center of the field like you said. After each try, I recentered the star until it got perfect, then I moved the star to different corners by hand controller to see how is the star shape there and didn't touch the collimation. 

Thank for the the whitepaper. I got my answer on page 5 where it shows the star shapes off-center at different distances. I have to check Metaguide as well.

Thanks again for your help.

You *always* collimate WITH THE STAR CENTERED. Whenever you make an adjustment, you must re-center the star in order to evaluate. 

 

So the quality of the images outside of center is irrelevant. EVERY scope has some distortion far from center. Look at the Edge HD white paper (go to page 4). Even though the Edge HD is far superior to other designs for SCT's, it still has aberrations off axis. Collimation is SO VERY EXACTING that you *need* the perfection of the on-axis image. 

 

Your collimation is definitely not bad, but to improve on it you need a night of steady seeing and to defocus even less. Eventually, for perfect collimation, you have to find a great seeing night and collimate in focus with extremely high close up. 

 

A program like Metaguide (free, if a bit hard to understand) helps you by stacking a lot of images, thus getting rid of some of the effects of seeing. It also shows you what part needs adjustment. 


Edited by mehdymo, 04 April 2020 - 07:17 PM.

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#18 mehdymo

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Posted 08 April 2020 - 10:13 PM

I made the collimation more accurate (see the star shape at center that is very close to focus. Please note that the exposure was 100ms and it was hard to catch a perfect frame,but is was very symmetric to me) and checked the star shapes on the corners. Attached are two images including stars in the corners with/without the reducer. The one with reducer has a tail\glare on the corners that looks like elongated in long exposure. Without reducer star looks very good even on the edges. Please note that the scale is not correct and stars are magnified significantly for better visualization. 

Like the previous results, the collimation is done in center, then different star images were taken at the corners without changing the collimation. I guess Edge HD is excellent without reducer, but it is not as good with reducer and it needs cropping.

Attached Thumbnails

  • combined-perfect.jpg
  • Reducer.jpg

Edited by mehdymo, 08 April 2020 - 10:15 PM.

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#19 mehdymo

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Posted 13 April 2020 - 08:55 PM

I have a misunderstanding of spacing. It seems like I should measure the spacing from the tip of the reducer not the end of threads where the spacer sits. I fixed this problem and noticed that the glare\tail on the corner is gone. but, I am still not satisfied with the results. I am wondering how you evaluate these two pictures. Here is without reducer and the next one is with reducer.

 

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  • No Reducer-.PNG


#20 mehdymo

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Posted 13 April 2020 - 08:55 PM

With Reducer:

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  • With Reducer-.PNG

Edited by mehdymo, 13 April 2020 - 08:56 PM.


#21 mehdymo

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Posted 12 May 2020 - 04:06 AM

Now, I am happy with the results.Here are two examples made by a few 1 minute exposure frames. Edgehd 8" +No reducer+ASI1600 2x2 binned.

- M27 (fix focus point for all filters i.e LRGB).

- Open Cluster with refocusing for R, G, and B (No L)

 

 

Attached Thumbnails

  • photo_2020-05-12_04-41-15.jpg
  • photo_2020-05-12_04-55-39.jpg



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