Easy way to (automatically) display faintest and brightest star magnitude usable for photometry in an image ?

[Thomas Rudolph]

Hello,

Photometry only works if a star is not saturated, but also not too faint. Is there a simple way to determine or display these two values in ASTAP for a specific image that you have loaded?
This would be very helpful to optimize parameters like gain/exposure time/defocus to reach a certain range.

I think ASTAP can already determine these values, but I don't know how to display them. Or, if this is not yet possible, I think this would be a very helpful thing for a new feature ?

CS
Thomas

[han.k]

You can display the limiting magnitude if you do in the viewer

1)solve,
2) magnitude (measured) annotation CTRL+Alt+M

But that is once you have the images. I would look to the pixel values of the variable star when your imaging. Which imaging acquisition program are you using?

Han

[Thomas Rudolph]

I use a Seestar S50. I'm especially interested in the high end, maximum limiting magnitude. I sometimes see nice diagrams, one axis magnitude (from Gaia) and other axis measured flux. Over all stars in an image. This is a straight line in a specific area. I am looking for a way to display something like this. But actually only the two ends (low and high) of the linear range are of interest. ASTAP measured annotation shows a 'faintest magnitude', it would be nice also see a 'biggest magnitude' or so. ?

CS
Thomas

[han.k]

I have added the estimated saturation magnitude to the text log only:

20:47:07 Limiting magnitude is 17.58 ( σ=0.0028, SNR=7, aperture ⌀1.7) Saturation at ≈ 10.9

Windows development version:
http://www.hnsky.org/astap_setup.exe

Does this work for you?

Cs, Han

[Thomas Rudolph]

Yes, thank you very much, that's a great help. One last question, there is a "high saturation level" checkbox in Photometry tab, is the new sat. value with or without this selection ?

CS
Thomas

[han.k]

That should not influence the saturation measurement.

Many sensors are 12 bit and then the maximum value is then 4096. 14 bit has range 0..16384. But many camera suppliers multiply this range with 16 or 4 to get the 0 ..65535 range.

What ASTAP does is to measure the maximum value in an image and assumes this is the maximum possible. This doesn't always works. A clean 14 bit image could stay far below 16384 or below 65535. The check mark high saturation just lifts the saturation value to 64000. Then it is up the user to decide if the star is saturated.

The maximum value is stored in DATAMAX keyword.

[Thomas Rudolph]

Ah ok, thank you very much :-)