Photometry of dim variables

[Andrew C Murphy]

Hi Han and others, I'm having a problem in doing photometry on a low brightness variable star. I'm trying to measure AR Octans (mag 12 -> 13.4) with a Seestar S50 using 10 sec exposures. Workflow is

1. Download images from telescope.
2. Extract green channel.
3. Stack in groups of 32.
4. Select target star and comparisons and then let ASTAP do the measurements.

This process works just fine for all my other variable star observations, but breaks down for those at dim magnitudes. Let me explain.

For AR Oct I obtained 2025 images over the course of an evening. If I analyse the individual (green) images in ASTAP, the software obtains a valid magnitude in the majority of them.

I then exclude the images that were unable to be measured and use only those remaining, and stack them in groups of 32.

The problem is that in the majority of those 32x stacked images, ASTAP can not determine the magnitude of the target variable.

The aim of stacking is, of course, to increase SNR, so why would it be that the individual images with lower SNRs are able to be measured, but not the stacked ones with a (presumably) higher SNR?

For reference : individual images have SNR of 9>21 for target star. The stacked images (the ones that are measurable) range from 25 to 51.

Any idea what's going on here? Or how to better process variable stars with dim magnitude?

Things I have tried
i) varying aperture.
ii) stacking x8 and x16
iii) excluding individual images with SNR < 16

None of these work :-(

[han.k]

Above a SNR value of about 10 should work. Can you attach one of the stacked images and which star are you trying to measure?

Han

[Andrew C Murphy]

Here you go. A single unstacked image with AR Oct at mag 12.294 and SNR 17.

And a 16x stack that ASTAP is unable to determine a magnitude for. Interestingly if you zoom in on individual pixels, the SNR is around 31 or 32.

[han.k]

Rare case but there is a magnitude 15 star very nearby which causes the star centering to fail. See attached screenshot. The star is measured in your single image but not in the 32xstacked because the magnitude 15 star becomes visible. It will probably work if you stack 2x or 4x but not higher.

As said this is rare. I have no other solution then to stack less at this moment.

Han

[Andrew C Murphy]

Thanks for the quick response. Han. That makes perfect sense.
Damn those mag 15 stars lurking in the background....