SQM measurements

[Lammertus de Vries]

Hi all,

Since I just finished a little Weather Observing Conditions device that also measures
the SkyQuality or SQM value I would like to ask how precise ASTAP´s SQM value correlates
with the well known Unihedron SQM-L device.
I wrote an ASCOM driver for it and it works with NINA to automate many things.
Is there some tip as to do exposures and afterwards use ASTAP's SQM value to calibrate
my device?
Thanks for any feedback, much apreciated!!

--Mert

[han.k]

Hi

I did some test in the past. E.g here:

https://stargazerslounge.com/topic/370621-sqm-sky-background-measurement-using-an-image-and-the-astap-program/#comment-4024962

Note that there could be 0.2 magnitude differences between the SQM-L devices. Some older types of the SQM-L seem to age.

ASTAP uses the stars as reference. It assumes an air transparency based on the elevation. So a poor (air) transparency (foggy) could result in an measurement offset. But the measured values will never drift by aging or dirty optics.

And finally the pass-band will be different. But I noted that for example the measuring with an H-alpha filter gives a very similar value compared with clear view.

Note that for an accurate measurement the sky glow should have increased the background value of the image. So expose a few minutes to achieve that. Secondly the background of a dark should be entered correctly to be able to measure the difference correctly. (or add a dark in the dark tab.)

Han

[Lammertus de Vries]

Hi Han,

Thanks a lot for your feedback, much apreciated!!
I will do as you suggest and lateron give feedback on
how it goes.

Yesterday from 21:something during 2 hours I pointed my
device to the sky from inside through a window and got
this graph using the logging function.
It was totally clouded so the lights of the city did their job.

[Lammertus de Vries]

Ok, I have tested the device under relatively dark skies and the values are within 0.2 Magnitudes so that looks promising.
I still have some stability problems where the value mesured is not very stable eventhough the skybrightness doesn't vary.
Will have to check the electronics maybe.
CS,
--Mert