Gaia DR3 selection for photometry

[Tom Alderweireldt]

Maybe this question has already been raised in some form before (or I overlooked it), but anyway:
Does ASTAP have the possibility for more accurate photometry to make selections or limit the large number of Gaia DR3 used in most solutions:
1) Set max magnitude or min SNR
For a target around V-mag 11-12, it would be nice to be able to set an upper limit like 14-15. (there is no dynamical range in the image to have sufficient accuracy from the fainter stars). In fact maybe this could be done through a minimum SNR limit.
2) Limit range of star color index
Depending on what filter you use, for the best photometric results, it is useful to avoid extremely red or extremely blue stars. So allowing to set a range of color index would be nice.

Best Regards,
Tom

[han.k]

Hi Tom,

1) Set max magnitude or min SNR

It is always good to think about improvements. The development ofthe ASTAP photometry features is going step by step and after each change new improvements are considered and evaluated. But I also have to keep is reasonably easy to use.
Have you tried the new way to"measure all" visible stars in one step and select them in the report window? See attached

2) Limit range of star color index

Do have an sample image set where I could experiment with it and what you would expect?

Cheers, Han

[Tom Alderweireldt]

Hi Han,

Thanks for the quick reply!
I'll try out the manual selection as you indicated.

Attached is an image of V2008 Aql (HADS variable star). I hope it's not too large for this forum.
They are taken with a C14 telescope and R-filter.

I also add a screenshot to indicate location of the variable star, ref star and star 3.
In this case the ref star is extremely red (B-V = 1.6), which triggered my feeling that it's maybe better to leave it out of the average.

Regards,
Tom

[Tom Alderweireldt]

And the screenshot

Tom

[han.k]

Hi Tom,

Now I understand, you want to avoid too red or blue Gaia stars for the comparison. So the ensemble of Gaia stars for the KMAG magnitude if I remember well.

The function is that for all stars typically the V magnitude is calculated from the three Gaia magnitudes G, BP, RP. That could be 100 or 400 comparison stars. Statistically outliers of the measured flux against the expected flux are removed.

Using the online Gaia version any star outside the documented conversion limitations (too red or too blue see Gaia documentation) is not used for comparison

For the local database version I'm ignoring the limitation because all stars are required for solving images. In practise I do not see a difference in the accuracy. This because the statistics are helping me excluding outliers.

Does this match with your proposal?

Han

[han.k]

For local database it is also possible and possibly already implemented. The V50 and V05 contain sthe calculated Johnson-V and the Gaia Bp-RP magnitude. It is already a few years ago it was implemented, so I have to look it up in the code.

Currently clear sky here so more tomorrow.