Help to troubleshoot stars

[Rhinottw]

Hi all,

I am having some troubles with my new(used) camera, a Nikon D5300. I have tried getting it to work for 2 nights now, but i am getting the same result each time with donut shaped stars. Example image: https://i.imgur.com/v1mpnXY.jpg - 40s exposure - After having captured this image i put on my tried Canon 450D and after re-focusing i got this: https://i.imgur.com/ITxUY7N.jpg - 120s exposure.

I know i have a bit of tilt going on causing elongated stars in the lower left of the image, but it is really the donuts that bother me, the tilt can be corrected. My first intuition is that it is a focus issue and that i am seeing the secondary mirror in the middle of the stars. My issue with this is that the camera was focusd using Backyard Nikon and FWHM focusing assitant. When doing this there was no sign of the donuts and i could focus it just fine - like i have done several times before with my Canon. The only thing that was changed between the two images was the camera (and camera adpter). My optical train is TS Photon 6" (GSO) --> TSMaxfield Coma corrector --> M48 to camera adapter --> Camera.

This is very puzzeling to me, since there should not be any difference between focusing and imaging from a focus perspective. Does anyone have any idea what could be goind on, could the camera be defective, i have not really used it much before this, only a few shots of daylight images that showed no issues. Any help would be much appreciated.

 

[vlaiv]

Hi and welcome to SGL.

I'm not sure what the issue is. I mean, I can see that stars are out of focus slightly and yes, there is some tilt that I suspect is not the issue you are referring to.

You say that you are using focusing assistant and when you do focus exposure you have "pinpoint" stars. What happens next? Do you do exposure right away and get doughnuts or does it happen on some of the frames later on (after some time)? Do you slew your telescope between focusing and taking light exposure?

There are two things that can cause focus going off between focusing exposure and later light exposures:

1. Focuser slip. Depending where in the sky you point your scope and how the camera is attached (or rather where gravity pulls it) - you can have focuser slip (if for example camera is pointing straight up or down).

2. Temperature issue. With changing temperature OTA changes its length - materials expand and shrink with different temperature. This is enough to throw off focus, particularly on fast telescopes. If temperature drops rapidly over the course of the night, you might need to refocus on every hour or even half hour on fast scopes. Did you let your OTA cool prior to doing focus exposure? It might be that you are focusing while scope is still "hot" and not at ambient temperature. If so - it will cool rapidly and throw off focus.

Another thing that might be happening that you need to check for - is focuser lock mechanism. Do you lock your focuser after doing focusing exposures? (this sort of rules out case number one above). If so, tightening focuser lock screw can mess up with your focus sometimes, so you need to investigate if that happens, and when does it happen (tightening fast / slow, very tight or whatever). If it happens regularly, you might be able to figure out "offset" that you need to create - thus not focusing perfectly and letting lock screw "mess it up" just right (yeah, I know, not a perfect solution, but can help until you sort out your focuser to eliminate this if it is indeed issue).

[Rhinottw]

Thank you for the welcome and input.

Yes, the tilt is not the issue, i will have to deal with that sepperately, probably with a tilt-correcting adaptor, it is also in the Canon exposure.

I use the FWHM assistant to get the value a small as possible and then i lock down the focuser. I then re-check the value to make sure it is has not changed, if it has i re-focus and lock again, repeat until i have the smallest value possible. This is working just fine for my Canon and has always been my focusing method since i started using backyard canon/nikon. This rules out focuser slip and focus shift from locking down the focuser. I believe the focus mode uses the livewiev from the camera and sends it to the computer, but i am not completely sure how it works technically.

After this i do some test exposures - this is where the Nikon 40s exposure is from with the donut stars. The telescope sat outside for 30-40 minutes before i started polar alignemet and focusing, so i would guess at least 50min. to an hour of  cooling down before the image was captured. This is extremely strange and i have not seen this before - this is my second season doing AP, so i am not completely new anymorem but not excactly an expert either this is just a mystery to me.

[Rhinottw]

Another image showing the same issue, i also get double diffraction spikes... :

[Rhinottw]

So, mount and scope is in the yard and i had a chance to do some further experimentation in my pursuit to solve this. I think i am dealing with a spacing issue. I thought to myself that my method is good and i know it is working, what i did not think about is just because the FWHM value is as low as i can get it does not mean the camera is in focus. Turns out my camera adapter rings are not equally thick and i think this is the issue. I happened to have a 1mm and a 0.5 mm spacer rings that i fitted between the CC and the camera, this kinda helped and the stars improved a bit and the spikes got close together. So i tried unscrewing the camera a bit to further increase the distance to the CC and i got rid of the ring stars and the diffraction spikes came together. It is of course very tilted because of the camera is not screwed all the way in, but the star look much better in some parts of the image. I will order some additional spacers and try again when they get here.