More Coma at lower targets lower in the sky

zishor
17 Jan, 2018 19:32
Hi, maybe someone of you can help me.

I seem to have a problem with coma at constellations that are lower in the sky with my GSO 6" f/4. When imaging the Orion I seem to have a lot more coma then then later the evening a M101 which was quite a bit higher (and pointing north) at that time.

You can quite see the difference in these two images:
https://www.astrobin.com/329514/?nc=user
and
https://www.astrobin.com/329492/C/?nc=user

Does anyone of you have an idea how to fix this? Collimation looked fine, checked it before imaging Alnitak. Only refocused at M101.

GPU Coma Corrector distance is within 0.3mm of the suggested working distance. Checked the primary seems not to be pinched (I can rotate it in the mirror holder).

Looking forward to your answers. Thanks in advance
Frank
Starminer68
18 Jan, 2018 04:46
Are you sure that the secondary mirror is not frozen? This is one of possible explanation. Another - cone error or poor allignment. I had terrible coma problems this night as well, but I think my poor secondary mirror collimation is the reason, the primary is ok.
zishor
19 Jan, 2018 08:59
Thanks Adel, your answer is very much appreciated!

Yesterday I checked everything again and made two changes: I had to change the focuser and noticed the height of the secondary was not correct again (I just assumed the height has not changed and just collimated with a laser). Fixed that. Also I added another 0.5mm fine tuning spacer to increase the working distance a bit. TS Optics recommended doing this in my case.

Hopefully that was it, just have to wait for my replacemend ASI1600 now from ZWO (the old one had a massive sensor glow, hardware error) and then I am able to test again smile Man this hobby is driving me nuts, so hard to get everything right smile

Best Frank
Starminer68
19 Jan, 2018 22:33
You are welcome, Frank, hope you fixed the problem. Newtonian scopes are great instruments, but constant need to collimate and re-collimate again makes me nuts. I have  a small Maksutov scope - almost carefree (of course, termostabilisation takes longer) and sometime I use achromat refractor (also easy to use). I am still thinking to buy a decent APO to save time wasted on collimation and fight with coma corrector ( so far for me unsuccesfull).  smile
Edited 19 Jan, 2018 23:32
Starminer68
19 Jan, 2018 22:58
Here is a good collimation guide for Newtonians: http://astro-baby.com/collimation/astro%20babys%20collimation%20guide.htmsmile
Starminer68
19 Jan, 2018 23:06
When I usually finish reading such guide - the only idea left: screw you, Sir Newton  smile
zishor
20 Jan, 2018 09:39
So true, especially the f4 ones are hard to get right every time. Generally I follow a different approach collimating the OTA, based on Tommy Nawratils guide:

1. Get the secondary centered and round with a concenter
2. Use a (proper collimated) collimation laser that can be screwed onto the coma corrector and then collimate via laser with the coma corrector attached. I was amazed what a difference this makes

However, I generelly screwed up with the secondary mirror, when you don't use enough lighting, it is hard to see the circles from the concenter and the secondary is easily wrongly centered / tilted. It really takes some practice.

Best Frank
Starminer68
20 Jan, 2018 11:47
smile Thanks, Frank, for really good idea to collimate with coma corrector on, I have to try on.
 
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