Meaning of Numbers in Star Detection

Didge

Hello ...
When using FITSView, clicking on the 'Enable Star Detection' button displays circles around some of the stars, with a number beside each circle, and the summary label : Star Size: x.xx (y.yy").

As you can see in this thread, the number x.xx appears to be in pixels, and the number y.yy is its corresponding value in arc-seconds.

Judging from the size of the circles, they are too big to represent FWHM, or even the diameters of the Airy Disk.
In images where I know FWHM <3", FITSviewer displays y.yy = 8.5".

This leads to a few questions ...

What do the circles represent ?
Why are so few stars enumerated - probably < 25% of stars are detected in the images I've tested
Why are bright stars not enumerated at all ?
What sets the limit on how faint a star needs to be before its not detected ?
Why do stars with the same numeric values have circles with vastly different diameters ?
Why do some stars have a larger number, but a smaller circle, than other stars ? See the link above for examples.
Where is this feature documented ?

I'm sure many users of ASIStudio would find it helpful to have answers to these questions.

Gary

ASIStudio@ZWO

Didge This leads to a few questions ...

What do the circles represent ?
Why are so few stars enumerated - probably < 25% of stars are detected in the images I've tested
Why are bright stars not enumerated at all ?
What sets the limit on how faint a star needs to be before its not detected ?
Why do stars with the same numeric values have circles with vastly different diameters ?
Why do some stars have a larger number, but a smaller circle, than other stars ? See the link above for examples.
Where is this feature documented ?

Conveyed your question to the team.

ASIStudio@ZWO

Didge

Didge What do the circles represent ?
Why are so few stars enumerated - probably < 25% of stars are detected in the images I've tested
Why are bright stars not enumerated at all ?
What sets the limit on how faint a star needs to be before its not detected ?
Why do stars with the same numeric values have circles with vastly different diameters ?
Why do some stars have a larger number, but a smaller circle, than other stars ? See the link above for examples.
Where is this feature documented ?

The circle represents star radius*3.
We will remove some circles, and keep the circle with the value close to the median value, so as not to make the whole picture appear too messy.
We will also remove the over-exposed stars
It is determined by average brightness and variance
The value on the circle is HFD, and the circle represents star radius*3, so they are not equal.
The circle size reflects the visual size of the star, and the visual size is not equal to HFD.
Will feedback to the team to add this feature to the document.

Didge

Thank you ...
A few responses ...

What is the logic behind multiplying radius * 3 ? I could understand * 2 ...
Im not sure that's helpful - FWHM/HFD is much more important for smaller stars.
Could you allow the users to select HFD or FWHM and pixels or arc-seconds
Good - using median or mean is not helpful if saturated stars are included.
What sets the limit on how faint a star needs to be before its not detected ?
'Average brightness & variant' doesnt explain what values are used to set the limits.
The circle should represent HFD / FWHM - otherwise its not meaningful
How do you set the 'visual size' of the star ?
Visual size is the diameter of the Airy Disk.
Good 🙂

Even so, the 'Enable Star Detection' feature is not helpful unless the metrics are explained.

Gary

ASIStudio@ZWO

Didge What is the logic behind multiplying radius * 3 ? I could understand * 2 ...
Im not sure that's helpful - FWHM/HFD is much more important for smaller stars.
Could you allow the users to select HFD or FWHM and pixels or arc-seconds
Good - using median or mean is not helpful if saturated stars are included.
What sets the limit on how faint a star needs to be before its not detected ?
'Average brightness & variant' doesnt explain what values are used to set the limits.
The circle should represent HFD / FWHM - otherwise its not meaningful
How do you set the 'visual size' of the star ?
Visual size is the diameter of the Airy Disk.

Conveyed to the team.

andyMeng

Didge 1.Just to show the requirement, the value could be 2 or 3
2.Only HFD is available at this time
3.The main filtering part of the star point
4.What do you think is a better number
5.The circle is to indicate that there is a star, and the number is the HFD
6.7.The function of star detection is only to measure the average HFD of the current image, and the average value is used to evaluate the quality of an image

Didge

Thanks Andy,
Im not sure I understand you.
Let me be more specific ...

By displaying a circle, the assumption is that the size of the circle means something.
Why are circles displayed at sizes that do not relate to the number displayed ?
It is confusing.
It would be much better to display a circle equivalent to the magnitude of the HFD.
Displaying a circle equivalent to the size of the median is not helpful - See #1.
It would be much better to allow the user to select whether to display star metrics for HFD or FWHM,
and for the numbers to be displayed in " or pixels for each circle displayed.
I don't understand "The main filtering part of the star point".
What sets the limit on how faint a star needs to be before its not detected ?
'Average brightness & variant' doesnt explain what values are used to set the limits.
Many faint stars arrive no circle or number - what is the test for which stars display a circle and a number ?
Displaying circles of undefined diameter is confusing. See #1.
Above was my question : Why do some stars have a larger number, but a smaller circle, than other stars ?
The answer was : The circle size reflects the visual size of the star, and the visual size is not equal to HFD

I do not understand this.
The 'visual size' of a star is the diameter of the star's Airy Disk, which is mathematically related to the HFD.
There is no correlation in ASIFitsViewer between the numbers displayed and the size of the circle.
And the size of the circle is not correlated with the star's HFD.

For 'Star Detection' to be useful ...

the numbers and circles displayed need to mean something quantitative
the metrics need to be documented
the user needs to be able to select their preferred display FWHD vs HFD, and " vs pixels.
There needs to be some explanation why some stars are circled and others are not.

Gary