Sat, 01/13/2024 - 21:45
During the AAVSO CCD Photometry Parts 1 & 2 course, I measured the linearity of my QHY600M CMOS camera
and it was linear through 64,000 ADU. I would like to utilize ADU counts up to 64,000.
What threshold would you recommend I use for Photometry and Exoplanet work?
Note this agrees with the manufacturers supplied linearity specs.
Most modern CMOS…
Hello,
Most modern CMOS cameras show similar characteristics.
Even though the camera has shown linearity up to 64000, to be on the safe side, it is a good idea to keep the ADU value below 55000.
Kind regards,
Nikola
Yeah - other consideration is that, with how long exoplanet observations are, you would almost never want to PLAN to be anywhere that close to saturation, nonlinear or not. With the duration of most observations, swings in airmass and seeing conditions can easily cause you to hit saturation if you intentionally don't give yourself some breathing room (like the 55k suggestion implies). You might be OK with not tossing out frame where the values happen to be > 60k (before dark subtraction), for example, but you'd probably want to not plan to be anywhere near that high.
There are several reasons why you would want to stay below the saturation limit of your QHY600. Some examples:
- are you binning? Then a subpixel in your binned pixel could saturate, while the binned pixel seems like it is below saturation.
- are you doing a time series? Usually this means following a star for multiple hours over a wide range of airmsss. If you push your exposure towards 64K when the star is low in the sky, it might exceed that limit as it rises.
- are you doing a time series? Quite often the seeing is variable, and so you can get moments of good seeing, which will peak the star above the saturation limit
- are you doing a time series? The sky transparency can change. You might have set the exposure when there was a bit of cirrus or other absorbing material present, and later in the night, the sky gets more transparent and the count rate goes up.
My usual rule of thumb is to keep exposures so that the peak count rate is ~1/2 linearity, or about 32K in this case. Hopefully, you are well sampled, so that the star flux is spread over many pixels, and so you have plenty of signal/noise for bright objects.
When you push close to saturation, I typically find ways to bring the count rate down, whether shorter exposures or defocus or what.
Arne
I will keep the ADU's below 55,000.
Steve - HSTG
I have a cooled 178 zwo camera. My plan was to use a .63 focal reducer and bin 2x2 with my C6 for a .94 pixel scale. I had not considered the issue with a saturated pixel ruining the super pixel. The 178 doesn't have a deep well depth and is only linear to 45,000 adu.
I have a .33 reducer that gets me to f3.6 and nearly 1 pixel scale unbinned, but I understand that under f4 should be the lowend for variable star photometry.
Is this the same for exoplanet work? What would you recommend with my equipment? I'm not opposed to a new camera but would prefer to get some experience to understand the process better before investing in new gear.
See what the experts…
Hi TDL
See what the experts respond with but, for now, I recall that we generally want the FWHM of the Target Star to cover in the range of 3-5 pixels. When you zoom in on an exposure test image of the target in some acquisition applications (I use TSX), you can see how your pixel coverage looks as well as measure the adu value of each pixel. This helps with binning decisions.
Gary
Having a 1" pixel scale should be OK. With the C6, you should be able to do some exoplanet detections of "hot Jupiters" depending upon your seeing conditions. So, it really depends on whether you want to do exoplanet detections for the thrill of it or do it in order to contribute to exoplanet science. In any case, I would suggest you try such deep exoplanet transit observations before upgrading equipment.
I will be conducting a six-week CHOICE course beginning Feb. 5 on exoplanet observing that I recommend you consider. Although it's labelled as "advanced," I have had in the past newcomers who have taken it and have gotten a lot out of it.
Dennis
Thank you Dennis, I am already signed up!
It is a CDK17 FL-2934 with FR-6.8
The QHY600M is binned 2X2
This yields a resolution of 0.53"x0.53" per pixel and a FOV of 0.7° x 0.47°
Steve - HSTG
Hi,
I use a QHY600M on a 16 inch f/6.8 remotely. I bin 4x4 to reduce file size (still about 7.5MB per file). I do about 700 to 1000 images a night of 30-40 targets.
I never go to 55000 ADU in a single exposure. If SNR is in the order of 500 thats more than enough for most of the photometric targets.
I have a remote installation with more than 300 clear nights a year. SO file size is an issue n my case. Compared to previous CCD cameras the QHY600M is about twice as efficient although with smaller pixels (about 3.8 microns for CMOS compared to 9 microns for the CCD). I binned the CCD by 3x3 so a 4x4 binned CMOS is still about only half the size per pixel for the CMOS and more than twice the file size compared to the CCD camera.
Josch