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EAF NINA Auto Focus Routine: Are 6 Second Images Optimal?

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#1 ChiTownXring

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Posted 24 July 2021 - 09:28 PM

Is there a formula or science being how long one should take auto focus images to get the best possible focus?



#2 Alex McConahay

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Posted 24 July 2021 - 10:43 PM

I do not think it is all that tricky.

 

YOu want them long enough to gather stable data. But that is not much more than a split second for seeing conditions that would allow you to image. You want them long enough to get a good image of the stars that rise above background noise.

 

But you do not want them so long that the stars are saturated. And you certainly do not want them so long that you waste time focusing. 

 

And you may have to adjust for narrowbands, or for extreme focal lengths. 

 

Six seconds sounds long to me. Three or four sounds better. 

 

I should say that I do not know NINA, and it may have special requirements. But I cannot imagine what would be so special. 

 

Alex


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#3 ChiTownXring

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Posted 25 July 2021 - 12:20 AM

I do not think it is all that tricky.

 

YOu want them long enough to gather stable data. But that is not much more than a split second for seeing conditions that would allow you to image. You want them long enough to get a good image of the stars that rise above background noise.

 

But you do not want them so long that the stars are saturated. And you certainly do not want them so long that you waste time focusing. 

 

And you may have to adjust for narrowbands, or for extreme focal lengths. 

 

Six seconds sounds long to me. Three or four sounds better. 

 

I should say that I do not know NINA, and it may have special requirements. But I cannot imagine what would be so special. 

 

Alex

Thank Alex for the reply.. Since it's nebula season I am shooing in Narrowband now using Astronomik 6nm filters and I think I got the 6 second focus exposure from Cuiv but do see that a lot of youtubers are set at 5 secsonds..



#4 ks__observer

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Posted 25 July 2021 - 03:53 AM

For LRGB is use around 5sec.

For narrowband i use around 10sec.



#5 WadeH237

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Posted 25 July 2021 - 07:58 AM

I don't know that there is any one answer.

 

I've been using NINA for most of the year now, and am really enjoying it.  Up until recently, I've been using a 130mm, F/6.3 refractor and focusing with 3 second exposures through the luminance filter.  This is with a QSI 690 mono CCD camera.  That works great.

 

I'm in the process of setting up a new camera, an ASI2600MC Pro.  Since this is a one shot color camera, there is no luminance filter.  I have the driver set for mono output when binning, and I take my focus exposures binned 2x2.  This camera is on an 80mm refractor at F/4.8.  I was not surprised that when using just an IR/UV cut filter, the same 3 second exposures work great for focus.  Last night was first light with a full, automated session.  Since we're in full moonlight, I put in the L-Extreme filter.  I did some test plate solves and focus runs.  I expected to have to adjust the exposure time for both with the filter, but was pleasantly surprised that it both solved and focused fine with the same exposures that I use without the filter.  That's 3 seconds for focus exposures and 5 seconds for plate solve exposures, both binned 2x2.

 

Whether 3 seconds works for you or not is going to depend on the camera you are using, the aperture of your scope (star intensity is more determined by aperture than focal ratio), the filters you are using and the sky conditions.  My suggestion is to start with fairly short exposures, to avoid incurring a more of a time penalty than necessary.  Pay attention to the shape of the focus curve.  If it looks good, then you are probably good to go.  Also, try to do your testing in the worst conditions that you expect to see.  In my case, I didn't plan on doing first light with a full moon, but I'm glad that it worked out that way.  Things will only get more reliable when the sky gets darker.


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