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Coma? Pinched optics? Bad collimation? All?

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#1 carlosm

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Posted 06 August 2020 - 03:09 PM

Hello Dear forum,

 

I'm using a Newtonian GSO 6" F/5 (and i'm very happy with it) with no Coma Corrector but I realized that my stars are not really pinpoint even in the center of the frame.

 

I realized this in this picture : (30s exposure)

before.PNG

 

I think stars look "triangular" I did some research about this and I thought that I had pinched optics so I took the primary mirror holder out and just untightened the screws that hold the mirror (they were very tight and I think they still are as I have no real reference of how tight they should be) .

That picture was before getting my bahtinov mask so I guess that my focus was not good also.

 

Yesterday I tried to see if my "pinched optics" problem was solved and I think stars in the center of the frame do not look that triangular anymore but still not pinpoint.

centro.PNG

 

Then I took a look in the outter part of the frame and this is how stars look (I know that being a "fast" newtonian I may have coma).

orillas.PNG

 

Is it just coma? Or a mix of coma + pinched optics, or coma + bad focus,....

I'm using a laser collimator and these is how vega looks out of focus I think that my collimation is not that bad.

 

What do you think guys?

Attached Thumbnails

  • colimacion.jpg

Edited by carlosm, 06 August 2020 - 04:17 PM.


#2 drew2578

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Posted 06 August 2020 - 03:25 PM

Collimation is definitely off.  The second pic shows the secondary mirror is mot centered.


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#3 drew2578

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Posted 06 August 2020 - 03:29 PM

See pic

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  • 20200806_132830.jpg


#4 carlosm

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Posted 06 August 2020 - 03:34 PM

Ok I see, and what about the edges? Is it just coma + bad collimation? 

 

See pic



#5 GIFTED1570

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Posted 06 August 2020 - 03:53 PM

first, is it a newtonian or an rc? by the description i think you have a newtonian, first make sure you have the right spacing from coma corrector to the sensor back focus distance, then using a secondary alignment tool make sure the secondary is perfectly displaying a circle and the rotation of the secondary is perfectly looking towards the focuser, if not the laser will give you an absolutely bad collimation. Make sure there is no tilt, typical of compression screws. Make sure all collimation steps are done after you tighten the rings mounted on the GEM and after the scope reaches thermal equilibrium, for me after 1-2 hours, even more when weather is cold.


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#6 carlosm

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Posted 06 August 2020 - 04:19 PM

first, is it a newtonian or an rc? by the description i think you have a newtonian, first make sure you have the right spacing from coma corrector to the sensor back focus distance, then using a secondary alignment tool make sure the secondary is perfectly displaying a circle and the rotation of the secondary is perfectly looking towards the focuser, if not the laser will give you an absolutely bad collimation. Make sure there is no tilt, typical of compression screws. Make sure all collimation steps are done after you tighten the rings mounted on the GEM and after the scope reaches thermal equilibrium, for me after 1-2 hours, even more when weather is cold.

Yes it is a Newtonian, i'm not using coma corrector.

 

Thanks I will try these steps today.



#7 Asbytec

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Posted 07 August 2020 - 08:28 AM

"Yesterday I tried to see if my "pinched optics" problem was solved and I think stars in the center of the frame do not look that triangular anymore but still not pinpoint."

That looks like coma at the field center due to miscollimation.

"Then I took a look in the outter part of the frame and this is how stars look (I know that being a "fast" newtonian I may have coma).

Is it just coma?"

That looks like coma (only or mostly) and normal without a coma corrector.

#8 carlosm

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Posted 07 August 2020 - 09:59 AM

"That looks like coma at the field center due to miscollimation."

 

I think you are right, yesterday I was more careful with collimation and the stars in the center do not look like before.

 

"That looks like coma (only or mostly) and normal without a coma corrector."

 

I have a doubt here, I know some coma correctors add inward or outward focus, in my case i'm using a low profile T2 and low profile T-Ring to be able to achieve focus in my DSLR (I have only like 1 mm more of inward distance for the focuser), I was looking at the "Baader Mark III MPCC".

 

If I'm not wrong the CC would be threaded directly to my DSLR T-RING (Instead of the Low profile T2), is there a possibility that I will not be able to achieve focus with the CC? Would a Coma Corrector that adds outward focus be my best choice?


Edited by carlosm, 07 August 2020 - 09:59 AM.


#9 Starman1

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Posted 07 August 2020 - 12:14 PM

Some coma correctors reviewed:

https://www.astrofot...?p=856#more-856

https://www.cloudyni...iosn/?p=7506297

https://www.cloudyni...ctor/?p=6945811

You might not want the Baader.


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#10 SteveG

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Posted 07 August 2020 - 09:52 PM

That fourth picture tells us nothing. It's far too far out of focus to tell us anything, including secondary centering. What I see in the second picture is an oval star, possibly astigmatism.

 

The third picture shows what looks like off-axis coma to me, but not really sure.



#11 carlosm

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Posted 08 August 2020 - 01:18 AM

That fourth picture tells us nothing. It's far too far out of focus to tell us anything, including secondary centering. What I see in the second picture is an oval star, possibly astigmatism.

The third picture shows what looks like off-axis coma to me, but not really sure.


I tried to apply all the advices to my collimation, I also had the secondary mirror way to tightened and the collimation screws way too untightened I think now my only problem is just the coma.
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#12 Asbytec

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Posted 08 August 2020 - 01:39 AM

I tried to apply all the advices to my collimation, I also had the secondary mirror way to tightened and the collimation screws way too untightened I think now my only problem is just the coma.


Remember, the purpose of collimation is to minimize or eliminate coma on axis. You can do that with good axial alignment and good tools. Or use a star test.

The defocused image above easily shows miscollimation. But as you get much closer to good axial alignment, that large amount of defocus becomes inconclusive. Miscollimation becomes difficult to read. You may evaluate and residual coma on axis much closer to and in focus to the extent seeing allows.
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#13 carlosm

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Posted 08 August 2020 - 12:14 PM

Thanks to all of you for your answers, as I was using my scope mainly for visual, collimation was not that critical to me as now for imaging.


Edited by carlosm, 08 August 2020 - 12:14 PM.


#14 Asbytec

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Posted 08 August 2020 - 04:18 PM

Thanks to all of you for your answers, as I was using my scope mainly for visual, collimation was not that critical to me as now for imaging.

You're welcome. I'd argue high magnification visual "imaging" does have pretty high tolerances for collimation so the image (ideally centered) in the FOV remains diffraction limited for coma. 


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#15 carlosm

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Posted 22 August 2020 - 01:01 PM

Update:

After research about cheshires, barlowed lasers, start tests and so on, I got new collimation tools and a ComaCorrector as well, now my stars do not look like that anymore.

 

I think my problem was the difference between plugin in the camera with the screws of the 2inch barrel of the ota and the way I was plugin the laser collimator.

 

Also the ComaCorrector solved the coma in the edges.

 

Thank you all again for helping me out.




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