In VPhot using the Time Series photometry there is the option to fix the aperture.
What settings do the sky annuli use as these have no option to change in TS?
I’m getting some differences between Single image photometry and TS in the measured mags for the same aperture.
The sky annulus settings in the Tools settings are used in TS. It is only the measurement aperture that will change IF you are using aperture = 1.5x FWHM? Yes, you may get some mag differences between single image and time series.
I usually inspect the Single image photometry(SIP) FWHMs and pick a fixed aperture to suit.
Right or wrong I have done similar in the Time Series and always used a fixed (number of pixels) aperture, not 1.5x FWHM. (I assume the latter adjusts each Co, Ck and target to 1.5x its FWHM.
This latter approach seemed rather different from SIP so I avoided it.
Should I use 1.5x FWHM in Time Series?
I use 1.5x in time series. The time series may be a series of 1B, 1V, 1I. Or it can be 100 of each. They are always stacked 2 to 30 images unless it is a particularly interesting and “good enough” datapoint taken during poor conditions.
My optics are poor and focus is variable with altitude, temperature. I find that setting VPHOT for 1.5x FWHM handles that better than me going through 300 images to determine an average FWHM and applying a fixed aperture radius.
Sometimes I get some images from CMOS cameras having little tiny pixels on fast optics. Very rarely I have just one image. In that case I set the aperture radius to 1.5x the FWHM that VPHOT measures for stars selected in the field. But most images are focused nicely so FWHM is something like 3.6 pixels and I leave the Tools aperture radius at 5 pixels.
If I were to defocus for some reason, I would adjust aperture for a messy single image FWHM carefully.
There are rare instances where I adjust apertures for crowding and to keep the sky annulus clear of more than one star.
Just my experience. Milage may vary for other observers or equipment.
Ray