Starting CMOS photometry

[Kevin West]

I finally have my system ready for CMOS photometry.

StellaMira refractor, 90mm ED triplet, FL 600mm

ZWO – AISI 1600MM Pro (cooled) camera

Baader Bessel Photometric B&V filters

Controlled by ASIAIR Plus

Now I am trying to find my way through the labyrinth of settings available for CMOS imaging/photometry.

I’m really just looking for some simple starting points to my settings.

I’m thinking of optimum or at least somewhere to start.

The following are of relevance to darks, flats, bias, and dark flats.

Gain, Temperature(cooling), Offset, Binning,

I was trying prepare some of these on the cloudy nights we are stuck in.

I have to select values that will match my lights (science images)

I’m guessing there isn’t a simple answer but any starting points would be helpful.

I took some test images of M11 and with 30s unguided exposures and could “see” stars down to 13th mag.

I am in the middle of the brilliant AAVSO VPhot course which really starts with the assumption of calibrated images.

That’s the step I need to go back to.

Thanks

Kevin

[Mr Ian David Sharp]

Hi Kevin,

Section 3.3.10 in the AAVSO CCD/CMOS Photometry guide is very informative with regards to the Gain and Offset settings in CMOS cameras:

https://www.aavso.org/sites/default/files/publications_files/ccd_photometry_guide/CCDPhotometryGuide.pdf

Once you choose a gain and offset, just stick with it for all lights and calibration files.

With regards to temperature, just go with a value that your camera can cool to reliably. I use both -10C and -20C with my CCD. I use -10 in the Summer months when I find the camera can’t cope with -20. I have a QHY268C CMOS camera which I image with and I find that it can’t get down reliably cooler than -15C, so I tend to stick with -10.

Binning – I use both 1×1 and 2×2 for fainter targets.

Cheers
Ian.

[Robin Leadbeater]

Kevin West wrote:

StellaMira refractor, 90mm ED triplet, FL 600mm

ZWO – AISI 1600MM Pro (cooled) camera

This gives a plate scale of 1.3 arcsec per pixel unbinned so unless your seeing is particularly poor you would be undersampled with 2x binning

[Kevin West]

Thanks Robin,
Could I press you to expand on the binning a little please?
Does it mean I should stick with 1X1 or is the camera a poor match for this short FL.
Regards
Kevin

• This reply was modified 1 year, 1 month ago by Kevin West.

[Robin Leadbeater]

Kevin West wrote:

Does it mean I should stick with 1X1 or is the camera a poor match for this short FL

Hi Kevin

Keeping in mind that I am only a casual photometrist and perhaps more experienced observers can comment, but my understanding is undersampling (ie less than 2-3 pixels relative to the star image FWHM) can be problematic for photometry. So on that basis I would say you should not bin unless your seeing is worse than 5 arcsec and if your in focus star image under good seeing is smaller than 2.6 arcsec FWHM you might have to consider mitigating measures even when unbinned. The AAVSO manual also discusses this in section 3.2.2 – 3.2.3

Cheers
Robin

[Dr Paul Leyland]

By far the easiest mitigating is just to defocus! This is very often done by photometrists.

[Kevin West]

Thanks Paul,
How do you calibrate defocussed images? Presumably you have to take all the flat frames at that the same defocus?

[Kevin West]

Thanks Ian,
Very useful starting point for me.
Some imagers have advised to cool to only zero, but I believe that there is some advantage in going cooler with CMOS cameras, but I’m not sure about this.
Regards
Kevin

[Dr Paul Leyland]

Kevin West wrote:

Thanks Paul,
How do you calibrate defocussed images? Presumably you have to take all the flat frames at that the same defocus?

Not really. All flats are completely out of focus anyway!

That is the whole point of flats: they should have no extrinsic structure whatsoever and all non-uniformity over the flat image arises from sensor sensitivity variation, dust rings, vignetting, etc, and not from anything outside the optical chain.

[Dr Paul Leyland]

Kevin West wrote:

Thanks Ian,
Very useful starting point for me.
Some imagers have advised to cool to only zero, but I believe that there is some advantage in going cooler with CMOS cameras, but I’m not sure about this.
Regards
Kevin

You should cool to the point where the thermal noise is negligible compared with other sources of noise, such as sky background, Poisson noise, and so on.

Where that point lies depends strongly on the camera and, to some extent, on the sky background. For my present camera (a SX-814 CCD) -10C is easily cold enough and well within the reach of its Peltier cooler except in the very hottest of summer nights. Air temperatures of 25C are not unknown in La Palma and were reached this last summer on a few nights, but they are not common.

It’s easy enough to measure the thermal noise of your camera as a function of temperature. Take some (say) 60 second darks at a variety of temperatures and look at the variance of the pixel values.

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