Affiliation
British Astronomical Association, Variable Star Section (BAA-VSS)
Mon, 07/15/2024 - 14:39
I seem to be having to re-focus using my ASIair autofocus function.
They are pretty small adjustments and I wondered if this necessitated new flats.
I wouldn't think so. There might be a small difference in the flats and corresponding calibration, but I don't think it will make a difference in the photometry. Some focus change over the course of the night from temperature change is normal. You might shoot both twilight and dawn flats and compare to see what that difference in temperature does.
-Walt
Thanks Walt
My usual answer is that it depends.
If you took your flats when the telescope was properly focused, then your autofocusing routine will provide another good focus, and the dust and other defects should be calibrated properly. If you took your flats, and then ran autofocus something not too long afterwards, the flats could have been out of focus and the dust, etc. might not perfectly divide out. As Walt suggests, take some dawn flats as well, after you have been focusing all night, and see how well the dusk flats and the dawn flats "ratio" (divide one by another to look for changes).
As a subtle change, if the temperature changes during a night and you follow focus, then the focal length of your telescope also changes slightly. This shows up as a slightly different pixel scale from dusk to dawn, especially for aluminum tubed refractors. A focal length change can result in slightly different sized dust donuts, so they may not divide perfectly. For most photometry, this effect can be neglected.
Arne
Thanks Walt,
I have carbon fibre tube so focal length changes should be minimal.
I must admit, I thought flats were out of focus given that the screen sits at the end of the tube.
I also only take flats every few weeks using flat white paper over the tube end. I have never done dusk or dawn flats.
Kevin
The ultimate test of the effects, if any, of changes in a setup or changes in callibration frames is to run photometry on the same field under the two conditions, then subtract the original measured magnitudes from those measured after the change. I have done this occasionally when a question arises. If the delta mag values are smaller than errors, they can be ignored. Of course, in the case of different sets of flats, any changes in dust donuts may not occur at the positions of measured stars, but that doesn't matter, because you may be testing precision for a particular situation. Likewise effects of changes in vignetting may depend on the distribution of stars across the field. These statements are hypothetical, but the result of the test is the important thing.
Roy