Aperture Sizes

[Gary Hawkins]

Hi Hans,

I recently had the photometry capabilities of ASTAP demonstrated to me by a fellow member of the Red Dwarf Group flare star project. During the demonstration, a few questions arose regarding aperture sizing, and I wanted to seek clarification and share my observations.

Does the visualization of the aperture in ASTAP accurately represent the size of the aperture used during photometric analysis? If so, it seems that even when the maximum aperture size is selected, the aperture might still be too small, potentially cutting off the lower parts of the bell-shaped curve of the star's light distribution.

In my experience with AIJ (AstroImageJ), the recommended aperture sizes fully encompass the entire bell-shaped curve to ensure accurate photometric measurements. I’m curious if ASTAP could include additional aperture size options such as 2.5, 3, and 3.5 times the FWHM for testing purposes. I’d like to experiment with these larger aperture sizes to evaluate their impact on photometric results.

I look forward to hearing your thoughts and suggestions.

Best regards,
Gary
Gary Hawkins
Coordinator, Red Dwarf Group
Blossom Valley Small Telescope Observatory

[han.k]

Hi Gary,

In ASTAP the aperture size can not be larger then the diameter of the detected flux so it is capped even when set larger. The maximum effective aperture is determined by the flux measurment. For this the program builds a flux histogram from the star "center of gravity" and stops when the flux signal is less then 3 time the noise. Any aperture beyond this value is ignored/capped.

The star algorithm was originally developped for astrometric (plate) solving so detecting the amount of flux was not essential but more the location of the stars. But the best photometric SNR is achieved when the aperture is set around one HFD or FWHM.

Do you think the photometric accuracy will increase if the diameter is set larger for stars with an high SNR? If so that would be interesting to test. How does the noise of photometric measurements in ASTAP compare to that of Astroimage? That would be an interesting experiment to test on a stable star.

Cheers, Han

[han.k]

For ASTAP the optimum aperture is around 1.6 to 2 times HFD and can best found for your setup by the test button next to it.

[Gary Hawkins]

Hi Hans,

Could you also clarify whether the aperture visualization in ASTAP accurately represents the actual aperture size used during photometric analysis?

Regarding accuracy improvements, I'm not entirely sure at this stage. We'll need to conduct detailed testing. We're currently planning these tests as we investigate some minor, unexpected fluctuations in our ASTAP results. While these variations could stem from a range of factors (software, equipment, environment, etc.), we're being thorough and systematically reviewing all elements to ensure reliable analysis.

Best regards,

Gary

[han.k]

The inner space of the aperture circle should match:

[Gary Hawkins]

Hi Hans,

Thanks for that additional information. Let me do some testing and get back to you with comments.

Best regards,

Gary

[han.k]

For an other development I noticed an discrepancy in the star detection routine. For a diameter of 1xHFD the flux should half but it isn't. I will look into this the next days.

[han.k]

False alarm I confused radius and diameter during testing. . It works as desired.