Does anyone use Sky Quality Measuring devices - what's your experience with them?
Pondering what folks experience with SQM devices are? How do you best leverage their use during a imaging session - like as the sky gets darker do you automatically adjust image durations / does your software store the SQM per shot in the file names etc?
I use ASTAP to review , calibrate, align and stack my images. It has a feature to calculate your Sky Brightness in SQM but I don’t use it. In Sydney under B8 skies the Sky Brightness levels throughput the evening and early hours are vastly different.I don’t bother varying sub exposure lengths as the Sky Brightness it’s a constant moving target , setting different exposure lengths periodically all night long IMO would not yield better results so generally a waist of time. Also for multiple nights capture the Sky Brightness varies greatly as well
I leave the variables in Sky Brightness throughput a single session or multiple night session to my Stacking program ASTAP. You can stack using Average for a single night only which averages out your Sky Background or Stack using Average for multiple nights then Stack the averages using Sigma Clip. Never had an issue with Sky Brightness
ASTAP is an excellent Stacker and Plate Solver and has excellent support from its developer Han.
I have a SQM, had it for ten years. It has been very useful for documenting the sad degrading of the night sky in my backyard observatory. It is also useful for correlation with the Global Light Pollution map, which is available online,
I found that the years I spent camping with fellow astronomers at Bretti Reserve near Gloucester NSW, provided a great field data gathering exercise for the SQM. Each trip I would take readings with the SQM and it gave me a real appreciation for its accuracy and just how dark, dark really is.
While my observatory has lost some darkness rating over the last decade, it still delivers occasionally a night which comes close to the old days. And the SQM is still in use when I roll the roof back.
PS. If you have a SQM, always make sure it has fresh batteries at the start of each season, they are sensitive to poor voltage.
Thanks for sharing that - SQM is my next project. My understanding is SQM may vary across the sky - but I guess I am most interested in SQM in the region where my OTAs are pointing.
So in shared shots - I can see from start of night (ASTAP solved 60 second image versus deep at night 300 second solved Luminance filter image) the SQM varies from a Bortle 9 SQM 16.32 to Bortle 8 SQM 18.08 Sydney surburan night sky.
My second thought was SQM for a stable sky should be a slowly changing piece of data - so I am hoping APT can leverage ASTAP to calculate the SQM periodically.
Re the batteries comment - keep them fresh - are batteries still needed on the SQM-LE model which nowadays has a 5V DC input source?
I am playing with ASTAP in batch mode - to add SQM to my fit files. Han's program seems to work well in some cases - but in other the data isn't updated / displayed. Yet if I click on any unsolved image and asks it to solve SQM it does - strange!
So sorting by SQM - an example of the highest value, the avg value, the lowest SQM value and no SQM values...
On the SQ failures it looks like ASTAP somehow missed the calibration files. The images that were successful have "DF" (Dark/Flat) in the Calibration column. Without the calibration the process will always fail.
As I pointed out elsewhere, only exposures with your LUM filter can give accurate SQ readings. BUT you may still be able to use the results for the others to determine good or poor SQ. It's only an untested idea, but it may work & be useful for you.
Now, your RGB filters will give you incorrect SQ readings overall, but the error should be consistent across all exposures using each filter. So while they can't give you an accurate SQ reading, it may work using them for a comparison across exposures for that particular filter. So by comparing only the exposures taken with one filter you should be able to determine good to bad SQ for that filter.
So try comparing just the reds against the other reds, blues against blues & greens against greens. The actual SQ figure will be wrong, but I think it should be consistently "wrong" across all exposures with the same filter, giving you a way to compare them.
It may not work, but just throwing it out there for you to try if you wish.
As an aside, it is possible to use a conventional digital camera and some trickery/algorithm/correlation to obtain a reasonable estimate of the SQM reading as if it were a Unihedron SQM Meter. I tried it a few times and obtained SQM readings similar to those charted for my area in lightpollutionmap.info.
I guess if I cycle through a few hours of LRGB - and see the deltas between the accurate L and the incorrect R, G and B I can get a sense of if there is a linear correction or if its just a random wander.
So looking at several concurrent shots taken around midnight a week ago - the readings sure don't look random...
So data from 9pm - 11pm on the 20 Jan for NGC 1566 - I would guess the variation is high cloud cover.
