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Collminating an iOptron RC6

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#1 galacticinsomnia

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Posted 20 October 2020 - 11:41 PM

I used a combination of method from the iOptron manual and this website, and followed it to the best of my ability.

https://britastro.org/node/16167

 

Things look pretty good to me, but I couldn't get much time to collimate under the stars.  It took a long time and was horribily out of alignment, even though I seemed pretty happy with my images, looking back I could see the fishbowl effects.

Now the frame is looking flat again but time will tell.

Follow the manual, use a simple cheshire, and utilize the target provided by the website, it works pretty good.

This is a very inexpensive scope as far as scopes go, but here is my test image, let me know if you think I can do better. 

Single exposure from

Canon m6ii
iOptron RC6
15s, ISO6400
 

Developed IMG 1298

 


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#2 MapleEve

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Posted 21 October 2020 - 01:27 AM

You can always use CCD Inspect to test the collimation.

 

Anyway the iOptron RC is GSO OEM product. You can search how GSO users collimate their RC.


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#3 galacticinsomnia

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Posted 21 October 2020 - 02:41 AM

You can always use CCD Inspect to test the collimation.

 

Anyway the iOptron RC is GSO OEM product. You can search how GSO users collimate their RC.

Are there any free alternatives for that ? 200.00 is a bit steep.



#4 galacticinsomnia

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Posted 23 October 2020 - 04:31 PM

After another two outings to collimate my scope..  Can't seem to be able to get rid of the Edge pointy stars.  Center field looks looks satisfying, but I'm not sure how much more I should expect to get out of this scope.  Suggestions ??

Vega

Canon m6ii
iOptron RC6 1370mm
20s, ISO1600, on cem25p, mount tracked.

 

JPeG IMG 2020 10 22 19 17 09


#5 dan_hm

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Posted 23 October 2020 - 07:56 PM

Are you sure your spacing is correct? Bad stars at the edges could be a spacing/tilt problem rather than a collimation problem. I can only see a low-res version of your image so I can't be sure. Also CCD Inspector has a free trial with full features. You can install it, figure out your issue, and just not buy it.


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#6 galacticinsomnia

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Posted 24 October 2020 - 01:42 AM

Are you sure your spacing is correct? Bad stars at the edges could be a spacing/tilt problem rather than a collimation problem. I can only see a low-res version of your image so I can't be sure. Also CCD Inspector has a free trial with full features. You can install it, figure out your issue, and just not buy it.

I've done I think 3 or 4 different collimation processes, and this last one has been the best, and while only making small adjustments, all the outside stars point to the center which from what I have read does seem like a spacing issue.  I'll try and and post a high res one, I'd rather trust someones eyes than the software I don't know how to use.  Thank you for the pointers :)



#7 galacticinsomnia

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Posted 24 October 2020 - 03:05 AM

Are you sure your spacing is correct? Bad stars at the edges could be a spacing/tilt problem rather than a collimation problem. I can only see a low-res version of your image so I can't be sure. Also CCD Inspector has a free trial with full features. You can install it, figure out your issue, and just not buy it.

 

here are links to full size

https://drive.google...iew?usp=sharing

https://drive.google...iew?usp=sharing

https://drive.google...iew?usp=sharing



#8 dan_hm

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Posted 24 October 2020 - 09:57 AM

Yeah it looks like you are out of collimation, although not by a lot. Stars look slightly distorted across the field, whereas if collimation was perfect but spacing was still an issue, stars would only be distorted at the edges. Possibly both are in play here because the edge stars do look more distorted than the others.

 

Have you tried Madratter's method? I would maybe PM him and see what he thinks. He is quite knowledgeable about RCs.


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#9 galacticinsomnia

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Posted 24 October 2020 - 03:21 PM

Yeah it looks like you are out of collimation, although not by a lot. Stars look slightly distorted across the field, whereas if collimation was perfect but spacing was still an issue, stars would only be distorted at the edges. Possibly both are in play here because the edge stars do look more distorted than the others.

 

Have you tried Madratter's method? I would maybe PM him and see what he thinks. He is quite knowledgeable about RCs.

Thank you. I will check out the method, my primary seems to be really spot on along the optical tube and focuser and the secondary is bullseyed perfect in the cheshire, so I'm really confused.  Thank you for giving it a look and pointing me in a direction.  I've not collimated a lot of optics, but have done a lot of laser stuff back in the day.

Thank you



#10 the Elf

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Posted 25 October 2020 - 08:47 AM

Don't expect too much. This is a low cost scope. The rear cell is a poor construction. While the mount rotates the scope over the course of the night gravity pulls so strong that you see several pixels movement between 5min subs when guiding with a guide scope. So what you call a perfect collimation on the table will look completely different if you point the scope up or turn it upside down.

Unless you use a flattener the stars in the corner will always be oval. The field is curved. The best you can get is either the short or the long axis of the elliptical stars point to the center. For a field you need a flattener.

I'd also like to point out that the engraving on the secondary is not exactly the center of the curvature. The procedure you use relies on that. The procedure described in the pdf by DSI does not.

After learning the basics with the RC6 I upgraded to the RC8 carbon which is a bit better mechanically.

If it is about testing the collimation only you might want to use PI's FWHM eccentricity tool. If the plot is somewhat symmetric and best in the center you are in the ballpark.

Here is what my RC8 looks like. Scale is 0.733 arcsec/pixel using the TS 0.66 reducer.

 

MasterHa17x15min_FWHM.jpg

 

MasterHa17x15min_eccentricity.jpg

 

Note that 0.5 is the limit of what appears round. So I always crop the image. If I have a larger target I use a flat field refractor with a shorter focal lenght. The RC is really a small target scope unless you use a flattener.

 

I tried your vega image but it does not contain enough stars to generate a map.


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#11 galacticinsomnia

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Posted 25 October 2020 - 03:09 PM

Don't expect too much. This is a low cost scope. The rear cell is a poor construction. While the mount rotates the scope over the course of the night gravity pulls so strong that you see several pixels movement between 5min subs when guiding with a guide scope. So what you call a perfect collimation on the table will look completely different if you point the scope up or turn it upside down.

Unless you use a flattener the stars in the corner will always be oval. The field is curved. The best you can get is either the short or the long axis of the elliptical stars point to the center. For a field you need a flattener.

I'd also like to point out that the engraving on the secondary is not exactly the center of the curvature. The procedure you use relies on that. The procedure described in the pdf by DSI does not.

After learning the basics with the RC6 I upgraded to the RC8 carbon which is a bit better mechanically.

If it is about testing the collimation only you might want to use PI's FWHM eccentricity tool. If the plot is somewhat symmetric and best in the center you are in the ballpark.

Here is what my RC8 looks like. Scale is 0.733 arcsec/pixel using the TS 0.66 reducer.

 

attachicon.gifMasterHa17x15min_FWHM.jpg

 

attachicon.gifMasterHa17x15min_eccentricity.jpg

 

Note that 0.5 is the limit of what appears round. So I always crop the image. If I have a larger target I use a flat field refractor with a shorter focal lenght. The RC is really a small target scope unless you use a flattener.

 

I tried your vega image but it does not contain enough stars to generate a map.

Thank you.  I love my scope, but between the focuser, and as you mentioned the rear cell, it was feeling like I am swimming up stream.  Capturing decent images, IMO, and was just trying to improve a few things because my center and corner stars were outrageously deformed. smile.gif  For a mass produced 400 scope, I think it performs well, I just like to try tune it the best I can.  I used the table top method because that one seemed to be the most logical that meshed with my past optic experiences. 

Thanks for having a look.  I have found slop in the focuser, very minimal movement, in the mirror, and it was hard to tell when rotating the scope if it was the focuser or the rear cell until I noticed the way the coma was moving. 

Getting a reducer in the chain is my next step.

 

So do you think I have it about as good as it is gonna get ?


Edited by galacticinsomnia, 25 October 2020 - 03:15 PM.


#12 the Elf

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Posted 25 October 2020 - 04:23 PM

 

So do you think I have it about as good as it is gonna get ?

I'd need a final stack with plenty of stars to answer this. The reducer is the best step to improve image quality. It was a breakthrough for me.


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#13 galacticinsomnia

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Posted 25 October 2020 - 04:39 PM

I'd need a final stack with plenty of stars to answer this. The reducer is the best step to improve image quality. It was a breakthrough for me.

Thank you for your help. Appreciate that, I'll try and grab a stack tonight if weather allows.. should be clear but its going to be a cold one. ..



#14 galacticinsomnia

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Posted 26 October 2020 - 09:15 AM

I'd need a final stack with plenty of stars to answer this. The reducer is the best step to improve image quality. It was a breakthrough for me.

Do you think you could make a map for this one?  This is from my lastest collimation.

https://drive.google...iew?usp=sharing

It looks pretty good to me..



#15 the Elf

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Posted 26 October 2020 - 10:45 AM

Sorry, the tool is still refusing the maps. But what it tells us is a median eccentricity of 0.35 and that is excellent!

 

JPeG_IMG_2020_10_25_19_26_27_L_stars_mosaic.jpg

 

Here is a mosaic of corners, sides and centers (PI AberrationInspector). Really not many stars. But from the few that are there I see the coma point away from the center so I'd say yes Sir! You are very close to the optimum. Stop tuning the scope but go imaging.

Hint: If you image a small object so that you will crop the final image focus the scope to the center and put the object in the center. If you image a large object that almost fills the field of view move the focus star to a position half way to the long edge and focus there. This results in the center being slightly out of focus but the corner a bit closer to focus while you have a ring of optimal focus that covers a large part of the image. This is a good compromise for larger objects when field curvature cannot be corrected.


Edited by the Elf, 26 October 2020 - 10:46 AM.

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#16 galacticinsomnia

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Posted 26 October 2020 - 10:52 AM

Sorry, the tool is still refusing the maps. But what it tells us is a median eccentricity of 0.35 and that is excellent!

 

attachicon.gifJPeG_IMG_2020_10_25_19_26_27_L_stars_mosaic.jpg

 

Here is a mosaic of corners, sides and centers (PI AberrationInspector). Really not many stars. But from the few that are there I see the coma point away from the center so I'd say yes Sir! You are very close to the optimum. Stop tuning the scope but go imaging.

Hint: If you image a small object so that you will crop the final image focus the scope to the center and put the object in the center. If you image a large object that almost fills the field of view move the focus star to a position half way to the long edge and focus there. This results in the center being slightly out of focus but the corner a bit closer to focus while you have a ring of optimal focus that covers a large part of the image. This is a good compromise for larger objects when field curvature cannot be corrected.

Hey thank you so much.  It looked pretty good to me, but sometimes I just don't trust my eyes, okay, a lot of times I don't.  Good to know that its pretty much good to go, I really don't think I can get it any better, but at least I found a method that I know works for the scope. 
Now I'm going to put it away and try to image the Iris nebula, or something similar, with a new svbony 503 and do some write up for that.  Looking pretty good so far but have to figure out back focus etc for my gear. 

Again, thank you for your time and your assistance, much appreciated.




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