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Help troubleshooting image Nikon D5300

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#1 Rhinottw

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Posted 22 September 2019 - 04:33 AM

I got myself a camera upgrade from a Canon 450d to a Nikon D5300 and has been looking forward to trying it out. Last night was clear so i decided on shooting the Andromeda galaxy. PA was good and the mount was tracking as good as it normally does(less than 1" rms). But my stars came out weird. I went through my whole setup to make sure everything was ok: rechecked balance, new PHD2 calibration etc. but no matter what i did the stars came out weird. At first i thought it was down to a tracking issue since the Nikon has smaller pixels than the Canon and requires better tracking, but after some shots all the pictures looked exactly the same. The focus was done like i always do with the FWHM assistant in backyard canon/nikon and telescope was freshly collimated and had cooled down to ambient temperature.

 

Optical train is: Camera --> TS Maxfield Coma Corrector --> TS Photon F5 (GSO) - Both camera's using a standard M48 to bayonet adapter screwed on to the CC.

 

Nikon shots.

120s exposure: https://i.imgur.com/CLtuJe7.jpg

40s exposure: https://i.imgur.com/v1mpnXY.jpg

 

After looking at the 40s exposure today the stars look like little circles.

 

The only thing i had changed on my setup was the camera, so i went inside and dug out trusty Canon 450d i have used many times before and put it on the telescope. After re-focusing the stars came out fine, like they used to do.

 

Canon:

120s exposure: https://i.imgur.com/ITxUY7N.jpg

 

No processing was done to the images linked, just converted to jpg. I am a bit baffled by this, so i am hoping someone can point me in the right direction of what to look at to resolve the issue. Any help is much appreciated.


Edited by Rhinottw, 22 September 2019 - 04:34 AM.


#2 tonyt

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Posted 22 September 2019 - 05:23 AM

The bayonet m48 adapter might be introducing tilt. If it has the small screws around the edge check that the inner part is seated properly and that the screws are firm.


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#3 Rhinottw

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Posted 22 September 2019 - 07:45 AM

The bayonet m48 adapter might be introducing tilt. If it has the small screws around the edge check that the inner part is seated properly and that the screws are firm.

I did think briefly about sensor tilt but dismissed it again since the other equipment was the same. Did not think about the bayonet itself. It does have small set screws, i have loosened them and made sure the inner ring was flush before tightening them again. Do you know if there is a way to check for this without imaging at night?



#4 tonyt

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Posted 22 September 2019 - 08:33 AM

I don't know how to check without imaging but if you end up needing a new T ring the copper T rings sold by william optics are of high quality.


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#5 Rhinottw

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Posted 22 September 2019 - 03:43 PM

I will give it another try when i get a new chance after adjusting the adapter and then have a look at a better one if it does not work out. Thank you for helping.



#6 Rhinottw

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Posted 01 October 2019 - 03:22 AM

So, i had a chance to take the telescope out again last night after having adjusted the adapter and it was just horrible, long stars, double diffraction spikes from bright stars. I can post a picture later when i get home.

 

Having looked a bit closer at the Canon image i can see there are some elongated stars in the lower left corner, they seem elongated in the same direction as the Nikon image. I guess the Nikon is just more sensitiv to tilt?

 

Is this an indication that there is an issue with the optics? Could it just be down to collimation? I never saw any of these issues with my old telescope which i have taken a good number of images with using the Canon. I did however not have a Coma Corrector, so i would always crop out the edges and not look too closely at them.

 

How can i determine if this is an out of alignemt optical issue or something else? I am considering contacting the vendor because this is a new scope purchased specifically for astrophotography and sold as such, so i would expect it to work for this. I would just like to be sure it is not something i have done wrong or can adjust before doing so.



#7 Rhinottw

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Posted 01 October 2019 - 04:44 AM

I just had another though, because i had double diffration spikes(still at work, will post the new picture later). I remember seeing this before i started using software for focusing and got this when the focus was not right. When zooming in on the 40s exposure in the OP, to my eye it kinda just looks like the not so bright stars stars are out of focus and it is the secondary mirror showing in the middle. Lower left is definately elongated and that could be related to tilt, but the rest of the image the stars seem like round doughnuts across the rest of the field.

 

I just dont understand if this is the case. I focus using backyard Nikon in focusing mode and getting the start fwhm as low as possible, same method as i used with the Canon and that always produce good results. What could possibly cause a difference in focus between focusing mode and capture mode, that seems really strange to me. Could the camera be defective? I did buy it used. But still i cannot think of anything causing a difference in focus from one mode to the other.

 

Weather forecast is looking fine tonight, so i might be able to go out for another round of testing if anyone has any suggestions to how i can test so i can find the issue.



#8 Rhinottw

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Posted 01 October 2019 - 10:50 AM

Another image, 10s exposure: https://i.imgur.com/elOhVpi.jpg



#9 Rhinottw

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Posted 01 October 2019 - 02:37 PM

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.




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