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FITS Image Grader


Kaliska
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On 18/08/2018 at 00:08, Kaliska said:

Do any of you guys use the FITS Image Grader to analyze you data.

Do you trust it 100% and if so, at what %age point do you start deleting your images?

Or...do you use another program ?

After analysing some images series you will know what HFR (or HFD:=HFR*2) you can achieve. In the zenith they will be better then at low altitudes. In general I like to keep all images and only throw away the real outliers caused by loss of tracking or clouds. So in some image series, all images good or maybe 5% is bad. If it is all running smooth the quality will be consistent. Sometimes the mount HEQ5 has a bad position or day and 50% is bad. Or I try to track on on a hot pixel.

My telescope focal length is 580 mm. So seeing has not so much influence. For longer length the problems will increase. If you have a very good mount almost all images will be acceptable. The HFD achieved in my setup is  1.8 to 2.0, so lets say 1.9.  This is reported by FITS grader as HFR at around  0.9  Outliers are images with a HFD of 2.3 or HFR of 1.1

So what I'm trying to say is that for deep sky imaging keep all images which have historical reasonable HFR or HFD value. Only skip the outliers in HFR/HFD or star count. You can sort on HFR/HFD by clicking on the top of the column. It is not like planetary imaging where you have abundant amount of images and are just looking for that short moment(s) of stable air and perfect seeing.

 

The FITS grader is new for me, so I did some testing to compare it with my own program ASTAP with similar analyse/rename capabilities.

This program work well and fast. The only major drawback is there is no viewer included and can’t be linked either.

+ Can include subfolders

+ Report and average, SD values

+ Fast

-   No viewer
 
The HFR (Half flux radius) should 50% of the HFD (Half flux diameter) value. Compared with ASTAP, FITS Grader reports typical maybe 10% better HFR or HFD/2 values so better values then ASTAP.

It can only read 16 bit files which is not a problem since these will produced by almost all image acquisition programs like SGP.

The star detection routine behaves a little different for problematic images.

Los of tracking, so streaks in the image are reported as an image with a high HFD but normal amount of stars. In ASTAP streak images can be detected by the low number of detected stars.

Minor clouds in the image result in a low star count. In ASTAP the number of stars detected stay roughly the same but the background value as a third quality parameter is reported as abnormal high.

The latest version of FITS grader is from 2012.

Edited by han59
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