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How to focus on what you can't see visually

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#1 Kcormier19

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Posted 20 October 2020 - 11:47 AM

I am very new to astrophotography, and I am starting on planetary photos while I learn more about both pre and post processing. The other night I had a good seeing night and for fun pointed my C8 telescope with a ZWO asi385 color camera at the ring nebula. (No filters or reducer was used)
When looking through the eyepiece I could see the faint grayish blob that was the nebula and the stars in the eyepiece seemed to be in focus. I then took several 2 minute exposures using Sharpcap but when I looked at them, they were all out of focus. Looking again in the eyepiece, the surrounding stars around the nebula were in focus. So the question is what is the best method to know something is in focus when you cannot really view it visually?

#2 Deesk06

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Posted 20 October 2020 - 12:02 PM

The focal plane of a camera is going to be different than that of your eye. Not only that but when taking an image it needs to be stable. Any amount of movement will causing it to "blur" and look out of focus, among other things. You would also need to have excellent tracking with added guiding. A 2 minute sub without guiding is going to show trailing etc, especially at the native focal length of 2032mm which is mighty difficult to image with for a beginner(which I am as well). Focus through the camera, not through your eye and that should get you going in the right direction. 

 

As bob said, I am finding it hard to believe the stars were in focus. They may have seemed in focus but if you compared it to another image that has them in focus then I am sure you would have seen the difference. 


Edited by Deesk06, 20 October 2020 - 12:14 PM.

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#3 bobzeq25

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Posted 20 October 2020 - 12:02 PM

I am very new to astrophotography, and I am starting on planetary photos while I learn more about both pre and post processing. The other night I had a good seeing night and for fun pointed my C8 telescope with a ZWO asi385 color camera at the ring nebula. (No filters or reducer was used)
When looking through the eyepiece I could see the faint grayish blob that was the nebula and the stars in the eyepiece seemed to be in focus. I then took several 2 minute exposures using Sharpcap but when I looked at them, they were all out of focus. Looking again in the eyepiece, the surrounding stars around the nebula were in focus. So the question is what is the best method to know something is in focus when you cannot really view it visually?

If the surrounding stars were in focus, and the nebula not, something is wrong other than focus.  Two things you can take to the bank.

 

It's physically impossible for the star focus to be different from nebula focus.

 

All serious DSO imagers focus on stars.  They use a number of methods, but they use the stars.  Two good ones for you.

 

A Bahtinov mask.

 

Looking at the FWHM (full width at half maximum) measurement in Sharpcap, and minimizing it.  The more advanced imagers use autofocus methods that work that way, but you can do it manually.

 

Something else to know.  A C8 is (however unintuitive this may be to you) a really hard telescope to start out with in DSO imaging.  That's not something I thought up myself.  I collect quotes to better advise beginners, and I have a lot of ones just like this.

 

"I regret spending the first 6 months trying to learn <DSO> imaging with an 8" Edge.  With that scope it was a losing effort. Fortunately got a nice little refractor, and not only have the quality of my images improved but I'm actually enjoying the process of learning how to do it!"

 

Among many other things, focus is easier with the small refractor.  The big scope makes every last thing harder.  It's not helping you learn, it's getting in your way.

 

Planetary is very different.  The planets are _very_ small and _very_ bright, the C8 works fine for learning (and doing) planetary.


Edited by bobzeq25, 20 October 2020 - 12:14 PM.

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#4 kathyastro

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Posted 20 October 2020 - 12:13 PM

You aren't focusing the telescope.  You are using the telescope's focuser to focus the camera.  Therefore the only way you can tell if the camera is in focus is to look at the image coming from it.

 

Focus on something bright and sharp, like stars.  If you have a Bahtinov mask (you should have one!), use it on a first-magnitude star.  Once the star is in focus according to the camera's image, lock the focuser, slew to the faint fuzzy object, and begin imaging.


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#5 BinoGuy

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Posted 20 October 2020 - 07:16 PM

+1 KathyAstro  

 

If you can set up the telescope and camera in the daytime you can focus on a terrestrial object about a mile away.  This should get you pretty close, just don't change anything before you get out under the stars.  Then take some sample images with the camera and adjust from there.



#6 kel123

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Posted 20 October 2020 - 07:32 PM

Just a previous posters pointed out, you have a number of things that are out of place. First and foremost, the eyepiece and the camera are at different focus points. Hence, you seeing stars focused with your eyes does not mean they are focused with an eyepiece.
Even after getting that out of the way,you need to the deal with the problem of shooting at 2000+ focal length for 2 minutes without guiding. There will be star triails, making your image seem out of focus.
At that focal length, polar alignment I'd very critical, meaning that you need to get it very right.
So, shooting with that scope is not is not for the faint hearted as you need to get plenty things right.

#7 Kcormier19

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Posted 15 November 2020 - 02:10 PM

Thank you for the advice everyone. I am trying to do some live stacking to view that way as well as waiting on a Bahtinov mask for better focus.


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