There has been a lot of comment recently in the Forum on photometry with smart telescopes.
These instruments use OSC (one shot colour) sensors, and therefore the principles of DSLR photometry would generally apply.
One of those principles is that images should be slightly defocussed, to spread the star image over more pixels because of the issue created by the Bayer matrix where only one in four pixels is blue (or red) and only one half of them is green.
However, smart telescopes track the sky using astrometry, and thus I presume would work best with in-focus images.
What are smart telescope users actually doing? Has the smart telescope working group discussed this?
I don’t have a smart telescope, but out of curiosity ran tests on in-focus images with my DSLR imaging through a 200mm f/3.5 lens. I was surprised to find that transformed magnitudes measured using photometric standard stars were pretty good when averaged over a number of images (15 in the tests).
I’ve been experimenting with a Seestar S50 to measure T CrB over the past week. At least from my site, the FWHM of stacked images is around 3-4 pixels, so my guess is that the light is spread out over plenty of pixels. If the system were producing images of FWHM ~ 1-2 pixels, then I might worry.
On the other hand, there might be another reason to defocus: a 10-second sub-exposure causes T CrB (at V ~ 10) to have peak pixel values not too far from saturation. If one is observing a star of similar brightness, one might defocus to provide a larger window of safety to prevent saturation.
The S50 does not allow shorter exposures?
Wow! I almost ordered one. Thanks for saving me.
I suppose you could expose and calibrate with neutral density filters.
No, one CAN take shorter exposures. The 10-second value is simply the default. At some point, if one observes very bright stars, decreasing the exposure time can cause problems, and so choosing to defocus becomes a good idea.
Yes, you should defocus and not just for very bright stars. Since you are separating your color channels, defocusing assures that your stars are adequately sampled. I used my Seestar S30 on an eclipsing binary last night that was 10.1-10.6 mag. I defocused by adding 50 to the autofocus position. That gave me a FWHM of 9 pixels for my target and comp stars. I used an exposure of 30 secs and it gave me a SNR of ~55.