Warped stars - only on long exposures - Forums

Topic starter

Mar 3, 2021

Hello everyone,

yesterday it was the second consecutive night I tried to photograph M97. During the first night, I took some 2 minute and 3 minute exposures and everything worked fine. Last night I went for 10 minute subs and the results were surprisingly poor (all 25 images I took look the same). I took the same settings and Equipment during both nights, apart from the exposure times.

The image below taken right after the one above, but instead of 10 minutes I took a 3-minute exposure just to see if it makes a difference...and the results where fine to me.

Right after taking these images I also checked the polar alignment. It was just perfect.

My Equipment:
Guiding: Lacerta MGEN 2
Camera: Nikon D750 (unmodded) - I always choose a 3 mirror lock and a 10 second break between images
Telescope: TS-PHOTON 6" F5 Advanced Newtonian
Mount: Skywatcher EQ6R-Pro

Maybe someone had the same trouble in the past and has a suggestion how to fix the problem. Thanks a lot!

Regards,
Steffen

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andrea tasselli

Frequent contributor

Mar 3, 2021

How was your guiding graph/curve? Were you guiding off-axis or with a guide scope?

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Steffen

Topic starter

Mar 3, 2021 · Edited: Mar 3, 2021

Unfortunately, I didn't track my guiding. I am not sure if the lacerta is internally storing the quality paramaters so that I can have a look at it afterwards. The ortho after calibration was 99%.

I am using a 8X50 guide scope (no off-axis guiding).

regards
Steffen

dkamen

Mar 3, 2021

Hi Steffen,

The fact of the matter is it is not an easy thing to keep a 700mm telescope with a very heavy camera pointing at the exact same spot for 10 whole minutes. Have you compared an 3 minute image with another 3 minute image taken 6 minutes afterwards? I bet if you supersede them you will see something very similar to the 10 minute image. But no matter the reason, many things can go wrong in 10 minutes. Even the telescope might flex because its angle changes significantlys, by the way flexing issue with metal tube Photons because the material is very thin.

The most probable suspect is your polar alignment though: unless it is perfect (which it can never be) there will be field rotation which guiding simply cannot compensate for after X minutes. Maybe X=3 is a bit on the small side of things meaning your PA is way off and that would be the first thing to check. But if minimizing polar alignment error and eliminating flexing still does not get you further than say 5 minutes, I wouldn't really sweat over it. I would just take 2x5 minute subs instead of 1x10 minutes. Or even stick to 3 minute subs. Your photos indicate Bortle 7 or above, very little point going above 3 minutes IMO.

Cheers,
Dimitris

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Steffen

Topic starter

Mar 3, 2021 · Edited: Mar 3, 2021

Hi Dimitris,

thanks a lot for your detailed answer. I think it is very helpful learn from others in the astrobin-community!

To be honest, I am not sure if a 10-minute exposure is too much to carry for the mount (I am below 9kg of equipment, cables…

and for my PA-abilities. In the past I had a few nights where I successfully guided 10-minute subs (ie my image of the Butterfly Nebula and Sadr). But there are so many factors that vary from night to night when I set up the equipment that I really cannot tell why it sometimes works out just fine and sometimes it does not.

The reason why I sometimes try to stick to longer exposures is the fact, that my LPS-D2 filter does work very well in Bortle 7 skies in combination with my D750. I am aware that this is not a dedicated astrocam (and it is quite heavy). I try to compensate the disadvantages of an unmodded DSLR with a bit longer exposure times.

Best regards
Steffen

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andrea tasselli

Frequent contributor

Mar 3, 2021

The most likely cause is differential flexure or mirror-cell slop. I'd keep shorter exposures at low ISO if I were you. In my test with similar filter there isn't much light loss even with broadband objects. There is no net gain in going above 5 min in your imaging conditions.

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AwesomeAstro

Mar 4, 2021

Andrea beat me to it. It's almost certainly either mirror movement on your primary scope (which the guide scope cannot see), or differential flexure between the two. This is apparent because it's worse with longer images (all else being roughly the same), and because I can actually see this on your shorter image too. I know it looks pretty good, but those stars are actually not perfectly round either. They have "small" versions of the streaking you see in the longer image, even in the same direction. You're just not noticing it much. This streaking corresponds to the direction in which either 1) the mirror is moving or 2) the setup is flexing.

This problem is almost unsolvable in the current setup. I had the same issue before switching to off-axis guiding (the true solution to all such problems) and it wasn't solvable otherwise. If you luck out and it's 100% due to differential flexure, and not mirror movement, and you manage to suppress all flexure, that's your ideal situation. Practically, however, you only have 2 solutions: stick with the shorter exposures (you don't need 10 minute subframes with a DSLR and no filters anyways), or switch to off-axis guiding. The latter is the perfect true solution, although you should weigh the benefits given that you don't need, or want, 10 minute subs in this setup.

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Steffen

Topic starter

Mar 4, 2021

Thanks a lot everyone for your replies!

Based on your information I tend to say that the reason for the warped stars is a a flexure problem between the main scope and the guide scope. There are two attachment points between those two, as I attached a Y-shaped adapter to use the guide scope and a small viewfinder at the same time. In the past I also had some trouble with the primary mirror - thats why I perform a collimation every evening.

What I am going to try ou:

1. Shorter Exposures
2. Leave the Y-Adaper away
3. Tighten the lock screws a bit more after collimation

Thanks again. I will post M97 in the gallery once I was succesful.

CS,
Steffen

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