[Aavso-photometry] Large pixel size for photometry: what about defocusing?

[Matilde Fernandez]

Dear colleagues,

   we are working on the design of an infrared camera that will be used
(among other projects) for photometry, hopefully with precision of a
few millimag.

   We are right now discussing about the pixel size.

   From my experience, a pixel size that matches the average seeing does
not give good results (as many or all of you already know) and a pixel
size that is half the average seeing works much better. I have
made some tests in order to show this effect and the results are plotted
in the attached figure: it shows the dispersion of the data when the
ratio seeing/pixel_size increases from 1 to 4. We did not reach very
high precision because the stars were faint, but the point was to show
the improvement of the photometry.

   Now my question:
since some colleagues want a large field of view for their images, they
ask for a large pixel size and suggest us to defocus in order to reach
the photometric precision that we need. I wanted to ask those of you
that have used this strategy. How well does it work? What precision do
you reach? How much do you defocus?

   I am using (on well focused images) apertures that are only a bit
larger than the FWHM (Full Width Half Maximum) in order to get 'the
best' of each star, and this technique has helped me to increase my
photometric precision. I am afraid that defocused images might not allow
to use this small apertures, unless the defocused stars keep the shape
of a Gaussian. Am I wrong?

  Thanks a lot for your help and best wishes!
Matilde