Affiliation
British Astronomical Association, Variable Star Section (BAA-VSS)
Fri, 03/01/2024 - 21:53
I read somewhere that there is a problem with CMOS camera photometry at exposures less than about a second.
I wanted to check if this is true and why please.
Kevin
But gathering light for less than a second on any sensor may subject you to random twinkle, which is different than Twinkies.
I used to do read at ~200Hz with a SIM SBC hooked to a 1P21, but averaged 1000 of them. Wasn't too terrible.
Ray TRE
Hi Kevin,
If you can find where you read that there was a short-exposure problem with CMOS, that would be helpful.
As Ray said, scintillation poses a limit for ALL cameras, not just CMOS. The amount of scintillation noise depends on the exposure time, the aperture of your telescope, and the atmospheric conditions. I usually don't recommend anything shorter than 5-10 seconds with small telescopes for the highest precision (say, an exoplanet transit).
Another issue with CMOS is if you continually take short exposures, like doing planetary imaging where you might take several thousand very short exposures in a row. That will heat up the sensor and so limit the ability to cool it.
With very short exposures, how the sensor creates the short exposure can some into play, whether it is a rolling shutter or a global shutter. At one second, this is not normally an issue
Other than that, I don't see short-exposure problems with the CMOS sensors that I've used. For example, I've taken thousands of 0.2second exposures with a QHY600 CMOS camera, and the photometric results after stacking look just as good as single long exposures.
Arne
Ah great. Thanks Arne
I think I may have heard it in a video on Youtube. (Not a great source maybe) I'll dig further.
It was in relation initially to making flat darks which I understand need to be very short exposures.
The advice was that CMOS should not be used for this for less than about 1-2 sec. Sorry it's a bit vague.
There are just a few quite bright stars from my PEP days that I would like to follow with CMOS set-up.
They need a few exposures of 0.5 to 1s
Coincidentally the calibration software (ASTAP) did not accept exposures less than a second, recording them in the fits file as zero.
Kevin
Kevin and all,
Given that Kevin thought the issue may have been raised in the context of taking flat darks, I speculate that the dark current behaviour of (at least some) CMOS cameras may be behind it.
The following link is to a screenshot I took a while back for this Forum (can't remember if it was Photometry or another).
https://www.flickr.com/photos/199603914@N08/53336755119/in/dateposted-public/
It shows the results of an experiment on dark current with my ZWO ASI294MM (CMOS) camera. The experiment was done as part of a successful trial of scaled darks, the Forum topic that prompted the experiment.
In the screenshot, both left and right panels show the same data, but the right panel is zoomed in to the shortest exposures.
Roy
I meant to include the following in the previous post:
The plot shows that dark current for the ASI294MM does not become linear until the exposures are as long as 5 - 10 seconds.
Roy