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How should the HFR of your Frame & Focus compare to the HFR of your actual images? (SGP)

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#1 dan_hm

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Posted 12 July 2019 - 09:35 PM

I am imaging right now and I noticed something odd.  Before starting my sequence, I did an autofocus and got an HFR of 0.89, which I figured was quite good so I went with it.  I'm imaging in H-alpha and I use 25s exposures for autofocus, but 300 second exposures for my actual images.  When my first, 300 second image came up the HFR was 2.01.  Seems really high.  Could this just be rapid focus change due to temperature change, since it's still early in the night?  The temperature gauge on my focuser says I focused at 20.50 C and it's now 20.00 C.  For what it's worth, seeing is above average tonight. 


Edited by dan_hm, 12 July 2019 - 09:35 PM.


#2 2ghouls

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Posted 12 July 2019 - 10:41 PM

Sounds to me like something went wrong with the autofocus routine to give you a value that low. For Ha, I typically have HFR of between 1.6 and 1.8 on good seeing nights and anywhere from 1.8 to 2.3 for mediocre to poor seeing. 2.01 sounds like good focus to me. What does the sub look like visually?

Edit: to answer your original question. The HFR value of my frame and focus exposures and my actual subs are usually quite close. Usually within 0.2 of one another.

Edited by 2ghouls, 12 July 2019 - 10:44 PM.


#3 Alex McConahay

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Posted 12 July 2019 - 10:44 PM

I would expect that a focus shot of four seconds would have better HFR than a long exposure of a few minutes just because there is less time for things to get messed up by seeing, tracking (guide failure) and whatever. But that is a pretty dramatic difference. 

 

Let me ask......is your autofocus run at the same binning as your light exposure?

 

Alex



#4 Alex McConahay

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Posted 12 July 2019 - 10:45 PM

And, no, I cannot imagine temperature affecting a tube that much. 

 

Alex



#5 2ghouls

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Posted 12 July 2019 - 10:51 PM

Whoops forgot that HFR is in pixels and not arcseconds. So forget about me trying to compare my values. In any case something seems weird if the values are that different. In that case, I would probably examine the frame and if it looked out-of-focus, re-run the autofocus routine to make sure.

#6 jerahian

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Posted 12 July 2019 - 10:52 PM

I have the same experience though my AF number is never that low.  I’ve chalked it up to the impact of seeing over a longer period of time.  Mine can have roughly a 1 HFR difference.

 

-Ara



#7 NorthField

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Posted 12 July 2019 - 11:15 PM

Yup.. I see the same thing every night..

My AF’s are bin 2

What I don’t get is that 2.03 hfr equates to ?.?? In subframeselector

If I were using the mn190: 2.03 hfr in sgp would be about 1.50 in sfs

Svq100 : 2.03 would be about 1.95

#8 dan_hm

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Posted 12 July 2019 - 11:26 PM

I would expect that a focus shot of four seconds would have better HFR than a long exposure of a few minutes just because there is less time for things to get messed up by seeing, tracking (guide failure) and whatever. But that is a pretty dramatic difference. 

 

Let me ask......is your autofocus run at the same binning as your light exposure?

 

Alex

I have my autofocus binning set to 2x2 and I take my lights at 1x1.  

 

Incidentally, just a couple frames in tonight my HFR went down to good levels and has been stable since.



#9 Alex McConahay

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Posted 13 July 2019 - 12:40 AM

>>>>>> I have my autofocus binning set to 2x2 and I take my lights at 1x1.

 

Well, then you are not as far off as you think. Remember, your pixels binned at 1x1 are smaller than those binned at 2x2. So, if a star is .89 pixels across at 2x2 (during focus), it will be 1.78 at 1x1 (during imaging). Your 1x1 pixels are half the size (in both directions) as your 2x2.

 

If you were to focus and image at the same binning, you would find you were much closer in HFR. Although, as several of us have pointed out, the longer exposure would probably be a bit larger because of binning, tracking, and so forth. 

 

Alex




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