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Focusing problem, DSLR through C8

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#1 Porkkala

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Posted 17 January 2021 - 07:26 AM

I’m trying to get a bit into astro and my first target would be the moon. I have a Celestron adapter and a T-ring for the DSLR (Nikon D7500). 

 

To avoid the frustration of trying out things in the dark, I decided to practice in daylight. So I picked a target half a mile away, made sure that I can focus on it visually through an eyepiece. Then I removed the visual back, attached the adapter and the camera body to the adapter. The c8 is 2032 mm f/10. I shot manually and by trial and error I got the exposure right. However, it was very hard, and in practice impossible, to get a good focus. I used Live View and zoomed in. I’m not talking about reaching focus, I can defocus in either direction, but I don’t seem to be able to get a pin sharp image. I took a comparable image with a Nikon p900 and the latter was much sharper, even though I took it hand held. I can get decent images of the moon with the p900, but I was expecting to get much better results with a DSLR and the C8. Now it seems that I can’t even get a sharp image of a terrestrial subject.

 

I don’t think thermal equilibrium is the problem, the C8 had stayed at the same temperature for more than six hours. I also don’t think this is an issue with atmospheric seeing, as in this case the distance was rather short and in any case the P900 should encounter the same conditions. And I don’t think it’s vibration, the Evolution mount is quite good and I used exposure delay of 10 sec. The collimation is OK. I get sharp images when using the scope visually. 

 

Am I expecting too much? Is the C8 not really sharp? Will I always get better images with a P900 (with a really small sensor) than with a DSLR through the telescope?



#2 Dynan

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Posted 17 January 2021 - 08:24 AM

You might try BackYard Nikon and a Bhatinov mask combinaiton. BYN is free to try. The mask will cost a little bit, but well worth it in man-hours. Focus with Bhat mask on a bright star.



#3 Boeglewatcher

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Posted 17 January 2021 - 09:16 AM

Don’t forget that you don’t get a sharp picture with a C8 across an APS size sensor due to the curvature of the focal plan of the C8. only a small inner part of the pic will be sharp. A focal reducer / flattener will help to get a larger area focused.


Edited by Boeglewatcher, 17 January 2021 - 10:01 AM.

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#4 kevinbreen

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Posted 17 January 2021 - 11:23 AM

Your C8 collimation may be off. If your collimation is very bad you will not be able to focus, even on the moon. Here's an excellent guide to collimating 

 

https://www.cloudyni...-a-photo-guide/

 

If your collimation is spot-on and you're still not happy with focus, it may be that the seeing may not be as good enough as you would like it to be. Personally I never could get razor sharp focus on the moon in live view, due to poor seeing, with my 8SE and DSLR. You just have to decide what you think is best.

 

So I'd go through a collimation test on a bright star like Vega for example. It's not the most enjoyable process to have to go through but it is satisfying to suddenly realize your collimation is out and all the more so when you think you have it nailed.


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#5 Andy Lucy

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Posted 17 January 2021 - 12:31 PM

Vibration originating from the mirror on the D7500 may be the problem.  Try using it in Mirror-up mode.

Andy


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#6 kevinbreen

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Posted 17 January 2021 - 12:43 PM

I thought Porkkala was experiencing focus problems in live view.
But you're right, Andy, mirror lock-up is a good idea of course.

#7 acat

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Posted 17 January 2021 - 03:02 PM

I have nearly the identical setup as you, a de-forked C8 and Nikon D5100. I highly recommend Backyard Nikon as it has a focus assist feature built-in using either star diameter FWHM, Bahkitov as well as several other methods, also address long exposures, mirror lockup etc., You certainly have taken the right approach trying things in daylight. Just remember that even modest temperature changes will change the focus of the scope. I would not rely on the viewfinder at all, not only is it hard to do, the results are unreliable. Daylight is one thing night time quite another. I have a Lumix camera with live view which I thought great until I hooked up with BYN. Before you go much further you'll also want an on-axis guider. The one for the Celestron C8 by the firm is excellent IMHO. Also small, sometime unnoticeable things such as a very small layer of frost or dew will play havoc with an image. Even with the dry cold weather in Colorado The cooling atmosphere will place an almost invisible layer of moisture on the front lens that messes things up badly.


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#8 kevinbreen

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Posted 17 January 2021 - 03:14 PM

I have nearly the identical setup as you, a de-forked C8 and Nikon D5100. I highly recommend Backyard Nikon as it has a focus assist feature built-in using either star diameter FWHM, Bahkitov as well as several other methods, also address long exposures, mirror lockup etc., You certainly have taken the right approach trying things in daylight. Just remember that even modest temperature changes will change the focus of the scope. I would not rely on the viewfinder at all, not only is it hard to do, the results are unreliable. Daylight is one thing night time quite another. I have a Lumix camera with live view which I thought great until I hooked up with BYN. Before you go much further you'll also want an on-axis guider. The one for the Celestron C8 by the firm is excellent IMHO. Also small, sometime unnoticeable things such as a very small layer of frost or dew will play havoc with an image. Even with the dry cold weather in Colorado The cooling atmosphere will place an almost invisible layer of moisture on the front lens that messes things up badly.


A small amount of moisture on your
Front collector will cause blurring on the sensor and hence your image, granted. Use a hairdryer obliquely on it when needed.
I reiterate, check your collimation. Best of luck.

#9 Porkkala

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Posted 17 January 2021 - 04:24 PM

OK, thanks everybody. I'm still not sure. As I stated, collimation seems to be OK. And I'm not really convinced the mirror could be the culprit, because doesn't look like motion blur. But I will certainly try that. I have a bahtinov, but it's not useful in daylight. But the Backyard Nikon sounds interesting. I'll download the trial version. If it helps me finding focus it's definitively worth the price. 


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#10 bignerdguy

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Posted 18 January 2021 - 07:18 AM

You still do need a Focal Reducer / Field Flattener to get sharper images across the entire focal plane.  This is because the image you get without one is curved because of the circular mirrors.  So the center will be sharp but it gets progressively worse the farther from center you get.  With the flattener/reducer, the image will focus better and also be slightly lower powered and will look better overall.  Also when focusing on a daytime land based object with that powerful of a scope, be sure it is sufficiently far enough away as anything too close may not focus well anyway.


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