erratic FWHM within livestack

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numchuck
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Joined: Sat Mar 16, 2019 2:59 pm

erratic FWHM within livestack

#1

Post by numchuck »

Hey All , I never had this issue before ; but last Thursday night while using livestack to image , my FWHM kept reading high , throughout the night . I always monitor the FWHM tab ; and I usually get great results with a limit set around 4 . When the score would exceed my limit , I would stop and attempt to improve the focus ; but I could never seem to make any improvements . Finally , after wasting a couple of hours , I just upped my limit to around 6 , just barely high enough to allow the stacking program to accept a more reasonable number of frames . When I processed my results , everything looked pretty good , which made me think that my focus scores were exaggerating the amount of error I may have had . Please see the attached links to my photos from that night ; and tell me what you think about the focus . Both were captured with stacks of 8 second frames and dithered every 4 minutes .

Image

Image
Last edited by numchuck on Fri Jul 24, 2020 5:09 pm, edited 5 times in total.
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oopfan
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Re: erratic FWHM within livestack

#2

Post by oopfan »

I don't LS but I do use SC for PA. There are some nights where alignment bounces from Excellent to Good every other frame or so. Other nights it is quite stable. I chalk this up to a turbulent atmosphere. There is nothing much you can do about it except not image.

I can't tell much from looking at your two images since I have nothing to compare them to. Are they both out of focus or is the first in focus and the second is not?

Brian
numchuck
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Re: erratic FWHM within livestack

#3

Post by numchuck »

I thought , for both images , that "everything looked pretty good".
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oopfan
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Re: erratic FWHM within livestack

#4

Post by oopfan »

I agree, I think they look great too.
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admin
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Re: erratic FWHM within livestack

#5

Post by admin »

Hi,

The images look pretty good to me :-) A trick to try next time is to go to the alignment tab and tick the button that highlights the stars that are being detected. While you are live stacking star detection serves two purposes – firstly the stars are used to align each new frame onto the stack and secondly they are used for the FWHM measurements. Nice bright stars are great for the first use, but get excluded from the FWHM measurement (if a star has saturated pixels, you don't know how bright it really should be, so you don't know what the half maximum should be). I'm just wondering if in this particular field-of-view you were picking up mostly saturated stars which leaves very little for the FWHM measurement to go on.

Cheers, Robin
numchuck
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Re: erratic FWHM within livestack

#6

Post by numchuck »

Thank you very much , for your guidance ( and for making SharpCapPro ) On this occasion , I could not improve my focus ; and so very late into my session , I began to think that something was amiss with the FWHM measurement ; but I was slow to come to this conclusion , because , in the past , I've ignored the importance of the focus score and ruined a whole imaging session 🙂
BlackWikkett
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Re: erratic FWHM within livestack

#7

Post by BlackWikkett »

I've seen this behavior several times. I've found this to be due to clouds, usually high thin stuff. Usually still transparent enough to keep imaging but still enough to disturb the FWHM. This is usually accompanied by shift to red in the histogram. Less dramatic shifts to worse FWHM I've had also coincides with seeing conditions. I'm not measuring seeing but the shift would coincide with forecast.

-Chris
numchuck
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Re: erratic FWHM within livestack

#8

Post by numchuck »

Chris , that could have been . I look up at the sky , once in a while ; but since I'm watching the computer so much , my eyes never adapt to the dark ; so thin clouds are harder to see 🙂
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