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Coma or collimation?

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#1 Ryan1776

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Posted 18 September 2019 - 09:15 AM

In these images you can see the brighter stars have some light traveling into the center of the image. Not sure the technical name. Aberrations? 

I'm not smart enough yet to determine if that's coma, that can be fixed with coma corrector, or if this is a bad collimation? 

Meade LXD55 8" F4. 

Part of me hopes this is coma....as that means I did a good job on my collimation. 
Part of me hopes this is collimation as it means I don't need to buy something else. hahahaha 

In the end, with that fast of a scope, I know a coma correcter/field reducer will be something I'll need anyway. 

https://www.amazon.c...DKIKX0DER&psc=1

 

60 seconds

ISO 400

with light pollution filter.

IMG_1799.jpg

 

8second

ISO 1600

with LP filter. 

IMG_1816.jpg

 

I was trying to find the Ring Nebula and play around with settings at the same time. 

Never found it. Plate solved with these today and I pretty much just swung around it. The GOTO on my mount is atrocious. BUT it seems to track decently enough.

 

Thank you,

Ryan

 

EDIT: I don't know if it matters, but I believe my sensor was pretty hot during these as it was +36°C after I got done shooting some planets. And this was 5min after...... 


Edited by Ryan1776, 18 September 2019 - 09:22 AM.


#2 Hesiod

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Posted 18 September 2019 - 10:23 AM

Could provide a stretched crop of the central area?

Your stars look elongated (maybe as a mix of asthigmatism and drift: it seems that there is a patter pointing the top-right angle, but the actual shapes are not constant across the frame) and there is coma even very close to the optical axis.

Since both aberrations increases with miscollimation, I would first check the collimation with the imaging train in place; a coma corrector will be useful (I would get a specific one for Newtonian reflectors however) but take note that these are designed to clear up the edges of the frame which, at least for a small* circle around its center, should show point-like stars always.

 

 

*how small depends on the telescope's own design



#3 Ryan1776

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Posted 18 September 2019 - 10:50 AM

Could provide a stretched crop of the central area?

Your stars look elongated (maybe as a mix of asthigmatism and drift: it seems that there is a patter pointing the top-right angle, but the actual shapes are not constant across the frame) and there is coma even very close to the optical axis.

Since both aberrations increases with miscollimation, I would first check the collimation with the imaging train in place; a coma corrector will be useful (I would get a specific one for Newtonian reflectors however) but take note that these are designed to clear up the edges of the frame which, at least for a small* circle around its center, should show point-like stars always.

 

 

*how small depends on the telescope's own design

1st image zoomed and cropped

IMG_1799.jpg

 

2nd image same.

IMG_1816.jpg

 

Thank you for looking into this. 

I did use a bahtinov mask on Vega prior to these. 

I'll have to learn how to collimate WITH my camera in place....I don't know how to do that. I have a laser collimator and every time I check, it's pretty good. It WAS bad when I first checked it. 

My focuser is so weak that I'm sure  it does move when I mount up the camera. 



#4 Noobulosity

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Posted 18 September 2019 - 11:03 AM

I'm still learning to read these various signs, but maybe it's coma + astigmatism?

 

Here's an interesting page as a reference for various aberrations:

https://www.handprin.../ASTRO/ae4.html



#5 Hesiod

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Posted 18 September 2019 - 11:32 AM

The brightest stars show obvious coma (I suppose that if stretch the picture the fan-shaped halo will pop up also around fainter ones), and stars are clearly elongated but more in the longer exposure, so I guess that there is both a bit of asthigmatism and trailing.

Because the aberrations can be detected also in the middle of the field, I would check  collimation again as first step



#6 Ryan1776

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Posted 18 September 2019 - 11:42 AM

The brightest stars show obvious coma (I suppose that if stretch the picture the fan-shaped halo will pop up also around fainter ones), and stars are clearly elongated but more in the longer exposure, so I guess that there is both a bit of asthigmatism and trailing.

Because the aberrations can be detected also in the middle of the field, I would check  collimation again as first step

So the "fan shape" is coma. Got it. 

I will certainly check colimation again. 

Certainly some trailing. My mount isn't great. A stock-o Meade LXD55.

 

Thank you. 



#7 Hesiod

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Posted 18 September 2019 - 11:49 AM

Yes, it is called so because turns stars into sort of "comets" (coma is the latin word for hair); as for the trailing, it is helpful to know where is the N/E/W/S in the picture because the most obvious cause is not much the mechanical trailing due to gears (periodic or not periodic) but the PA being not accurate enough



#8 Ryan1776

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Posted 18 September 2019 - 01:29 PM

Yes, it is called so because turns stars into sort of "comets" (coma is the latin word for hair); as for the trailing, it is helpful to know where is the N/E/W/S in the picture because the most obvious cause is not much the mechanical trailing due to gears (periodic or not periodic) but the PA being not accurate enough

I know how my camera was oriented in the scope. 
I know which way I was pointing. (Almost straight up) but everything is mirrored correct? 

If so, this is pretty close. 

IMG_1816.jpg

 

According to the plate solver...

2338770.png



#9 John Tucker

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Posted 18 September 2019 - 03:09 PM

If your GOTO is horrible and you have a SynScan mount, try to polar align very carefully. Everyone will tell you that polar alignment and GoTo alignment are completely unrelated, but my GoTo performance on my EQ6-R mount is like night and day since I got Sharpcap and started getting my polar alignment spot on. 


Edited by John Tucker, 18 September 2019 - 03:09 PM.


#10 Ryan1776

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Posted 19 September 2019 - 01:36 PM

If your GOTO is horrible and you have a SynScan mount, try to polar align very carefully. Everyone will tell you that polar alignment and GoTo alignment are completely unrelated, but my GoTo performance on my EQ6-R mount is like night and day since I got Sharpcap and started getting my polar alignment spot on. 

I see SynScan being thrown around a lot, I confess I don't know what that actually means. 

 

Oh I think the GOTO and polar being related makes a lot of sense. And leveling. 

I'd say my polar alignment is actually pretty good. It goes to Arcturus within the FOV and then I usually use Altair as it's in a different part of the sky. Bot easily visible to my location. 

After that, it's a crap shoot. Sometimes it's pretty good. Other times its awful. I've been shooting planets mainly and I have been going to Jupiter after my 2nd alignment star. 

I've had it get pretty close. Tuesday night it was pointing at my garage roof. About 10° off  vertically. 

 

When I do get Jupiter or Saturn in my FOV. Hook up the camera, zoom in the 5x I can image for probably 30min without having to adjust a thing. I've had the polar alignment be off some and the tracking was awful. So that's why I think my polar is pretty good. 


Edited by Ryan1776, 19 September 2019 - 01:38 PM.


#11 Hesiod

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Posted 19 September 2019 - 02:22 PM

The drift seems to appear along the East-West line, so afterall the PA is a less likely culprit than a tracking issue .

Guiding could solve the issue, but if are willing to train the mount may give a try to the periodic error routine



#12 Ryan1776

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Posted 19 September 2019 - 04:15 PM

Checked with my laser just now. 

 

57E1EA6B-C7B7-49C4-93D0-65A421B25AB9.png

 

Looks good to me. 



#13 Noobulosity

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Posted 19 September 2019 - 04:42 PM

I see SynScan being thrown around a lot, I confess I don't know what that actually means.


SynScan is the firmware that drives the mount in the hand controller. It includes certain features that other hand controllers may not. The controller will generally say SynScan if you have that type of mount.

#14 Hesiod

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Posted 21 September 2019 - 05:19 AM

Checked with my laser just now. 

 

attachicon.gif 57E1EA6B-C7B7-49C4-93D0-65A421B25AB9.png

 

Looks good to me. 

I suggest to check rather on a star with the imaging train. Also, it may be just a matter of perspective, but the laser's dot seems to fall a little off (down-left in the pic)




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