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Question regarding Star Quality Analysis in upcoming v1.076 release  

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(@mestutters)
Red Giant Customer
Joined: 3 years ago
Posts: 89
January 13, 2020 13:27  

Hi Mabula,

I have been reading your draft 1.076 release notes: in particular the last section dealing with Star Analysis Quality. I have never previously fully understood how quality was calculated so I was hoping for enlightenment but have to say I am still somewhat unclear about the calculation?

You state that 'The star size and roundness are based on the median star profile of all stars analysed'. Does this mean the median of all the stars in all frames currently selected? Or is the median calculated for all the currently selected frames for a specific filter and/or session?

I am probably missing something but it also seems to me that for a given set of frames the median star size and median star roundness will be constants. So to me this is saying that the quality value is thus proportional only to the number of stars? In determining 'quality' I am thinking that there should be something to compare the median star size and roundness of a particular frame with the corresponding median values of all the stars in the overall population of frames, i.e. a frame with smaller and rounder stars than average should score higher than a frame of average or below average values. If you agree this is relevant I am not seeing how it is being taken into account?

Despite these questions I have been using APP for some 2 years now and still think it is a great product and I am very much looking forward to the next release.

Best Regards

Mike

This topic was modified 3 months ago by mestutters

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(@mabula-admin)
Quasar Admin
Joined: 3 years ago
Posts: 2364
January 15, 2020 20:53  

Hi Mike @mestutters,

You state that 'The star size and roundness are based on the median star profile of all stars analysed'. Does this mean the median of all the stars in all frames currently selected? Or is the median calculated for all the currently selected frames for a specific filter and/or session?

This is quality calculation for each frame individually, so of a single frame, the median star profile is found. The star size and shape of that star profile determines the score. The frame quality after star analysis is a combination of both the size and roundness of the star multiplied by the number of stars.

I am probably missing something but it also seems to me that for a given set of frames the median star size and median star roundness will be constants. So to me this is saying that the quality value is thus proportional only to the number of stars?

Indeed, that interpretation is not correct. In a single frame we can find let's say 250 stars. Of those 250 stars, we find the median star profile, because that is not sensitive to bad detections (like a small galaxy) and outliers in the starlist. The median profile therefor is a very good indication of the star quality in the entiry frame.

In determining 'quality' I am thinking that there should be something to compare the median star size and roundness of a particular frame with the corresponding median values of all the stars in the overall population of frames, i.e. a frame with smaller and rounder stars than average should score higher than a frame of average or below average values. If you agree this is relevant I am not seeing how it is being taken into account?

So for each frame we calculate this star size and roundness of the median profile and multiply it by the amount of detected stars. Those frame scores are calculated per frame. So after star analysis, these scores still don't say a lot because no check has been made relative to the other frames, right?

After registration, the relative FWHM min, max, shape are calculated based on the registration parameters between the frames. If there are image scale differences (due to different camera/OTA, focal length etc) in the dataset, the scores are now corrected properly.

Your suggestion

something to compare the median star size and roundness of a particular frame with the corresponding median values of all the stars in the overall population of frames, i.e. a frame with smaller and rounder stars than average should score higher than a frame of average or below average values.

is not enough when you process data of different optical setups at the same time so the registration engine corrects for the quality scores found after star analysis ;-).

The smaller and rounder the stars, the better off course. The higher the star density, the better off course. The star density is also the correction for the absolute star count after registration.

I hope this clarifies this more ?

In the soon to be released documentation all will be explained in detail 😉

Kind regards,

Mabula

Main developer of Astro Pixel Processor and owner of Aries Productions


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(@mestutters)
Red Giant Customer
Joined: 3 years ago
Posts: 89
January 17, 2020 18:58  

Hi Mabula,

Thanks for the detailed reply.  That is in line with what I thought should be happening.  For clarity might I suggest you append the words underlined to this sentence in your release notes. 

The star size and roundness are based on the median star profile of all stars analysed within each frame

I am looking forward to the release of v1.076.

Regards

Mike


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