A beginners follow up question – CCD suitability

[Darren Jehan]

Hello all

Firstly, best wishes to all for 2021, hope everyone had a safe festive period.

Following on from my initial post I am looking to make some changes to my equipment to give me a more permanent set up for my observations, and would value some opinions on my thinking…

My AtikOne CCD and Esprit 100ED (f/5) combination appears to give a good FWHM combination of 1.7″/pixel (focal length of 550mm with pixels of 4.54 microns). However, in my f/12 Cassegrain, this comes in under-sampled at 0.51″. If I BIN 2×2, I get back to 1.02″/pixel – which is on the border of under-sampling.

Therefore I am considering a 2nd CCD to allow me to take both pretty pictures and do photometry at the same time, so my question is:

Should I look for a camera with slightly bigger pixels (e.g. 6.45) to avoid the need for binning, or does binning not really matter – provided my calibrations and calculations take this into account?

Are there other factors I should take into account?

Many thanks!

Darren

[Dr Paul Leyland]

You can always do post hoc binning.  What you can not do is undo any binning already applied.

In particular, performing NxN binning in software afterwards gives you the opportunity to increase your dynamic range N^2 -fold by either summation if you can avoid integer overflow or averaging to a floating point format if you can’t — the two are equivalent from a signal processing point of view.

[Robin Leadbeater]

I think you mean oversampled ie there are more than 2 pixels per star FWHM. (Why do you consider 1.7 arcsec pixel optimum? What is your actual best seeing?)

Undersampling is to be avoided for science applications but the effect of the additional read noise on signal/noise ratio due to oversampling  (this is the only disadvantage I know of) depends on the observation. Playing with Michael Richmond’s calculator for example entering actual numbers for different cameras and observing conditions will give you an idea of the significance for photometry.

http://spiff.rit.edu/richmond/signal.shtml

Note with CCD cameras (not CMOS), in camera binning (as opposed to post binning) reduces the read noise  as there is only one dose per binned super- pixel, though CMOS cameras generally have lower read noise 

Cheers

Robin

[Dr Paul Leyland]

Note with CCD cameras (not CMOS), in camera binning (as opposed to post binning) reduces the read noise  as there is only one dose per binned super-pixel

That is true, but the read noise adds in quadrature whereas the signal adds. The post-binning signal to noise ratio per NxN binned pixel is N times that of the unbinned pixels.

That is why I was careful to state that the dynamic range can be improved by a factor of NxN for N-fold post-binning, and not the signal to noise ratio.  Sometimes the dynamic range is particularly important, such as when trying to detect extremely low contrast objects for instance.

FWIW, I generally use in-camera binning for photometry and post-binning for simple detection. The reason for the former is primarily for faster download times and smaller image sizes, rather than any read noise consideration. Read and dark noise is so small on my cooled CCD camera that it is completely overwhelmed by photon noise from the object and the sky. It is way down in the noise, to coin a phrase 😉

Entirely agree about the under/over confusion but failed to mention it earlier. I am pleased that you have done so.

[Dr Paul Leyland]

Robin, thank you very much for posting the link to http://spiff.rit.edu/richmond/signal.shtml which I had not seen before and have now bookmarked.

Typing in the values for my set-up confirmed my prejudice that the read noise is almost entire unimportant for the photometric work I do, where I generally take 30s to 60s subs and co-add.

[Robin Leadbeater]

>That is why I was careful to state that the dynamic range can be improved by a factor of NxN for N-fold post-binning, and not the signal to noise ratio.  Sometimes the dynamic range is particularly important, such as when trying to detect extremely low contrast objects for instance.

Indeed and this is a limitation of in camera binning where unlike post binning, you cannot see the individual pixel counts, so to be sure of not having any individual saturated pixels you have to limit the total counts in a super-pixel to the full well depth of an individual pixel.(Well I suppose one could perhaps assume a gaussian distribution of brightnesses in the star image and allow a higher maximum count per super-pixel which would bring some dynamic range benefit.) 

The in camera binned exposure could be split  into NxN shorter sub-exposures to match the post binned dynamic range without risking saturation but that then returns you to the same read noise contribution as for post binning. In camera binning can be advantageous  though for weak signals in a dark background where the read noise contribution can be significant (in spectroscopy at low SNR for example.)

 

[Dr Paul Leyland]

I think what we are really saying is that you have to pay attention!

Astrometry could well be different from photometry which could well be different from spectroscopy which could well be different from bare detection which could well be …

To summarize: think about what you wish to achieve and make your decisions in that light.

[Jeremy Shears]

I wouldn’t worry too much Darren. For photometry try what you have at hand first before shelling out on more stuff. If you want to go to the nth degree in photometry you can try something else later. If it helps, I generally run my CCD camera 2 x 2 binned for photometry. There is nothing inherently wrong with binning.

[Dr Paul Leyland]

I whole-heartedly agree!

As already stated, I also generally run in 2×2 binning mode for photometry.

[Author]

Posts