Why is atmospheric extinction affecting my photometry on a new 8-inch reflector?

[Tyler.M]

I was trying to calibrate my new 8-inch reflector last night using a well-known star, but my calculated stellar magnitude was consistently off by a few tenths. I'm wondering if atmospheric extinction near the horizon could be having that much of an effect, or if my photometry method is just flawed.

[Edward.H]

Yeah, that can definitely happen. Extinction near the horizon eats into the measured brightness by a few tenths, sometimes more if the transparency is unstable. In my own observing, when the target was far from zenith I saw offsets of about 0.2–0.3 mag even with a decent calibration. The trick is to compare to a standard star at about the same airmass and, if possible, use differential photometry with a couple nearby comparison stars.

[Hannah_R]

Another culprit is the photometry method itself. Fixed aperture can bias results if seeing changes or the PSF isn't stable; sky background subtraction matters a lot too. I once forgot to subtract the sky properly and ended up with a 0.1–0.2 mag error. Flat-field issues and detector nonlinearity can sneak in, especially with a new scope.

[VioletYB]

I did a quick run with a couple of stars at different elevations and graphed magnitude versus airmass. The slope looked off by a small amount and the zero-point drifted over time. I chalked that up to a missing color term in my response function and didn't chase it further.

[LilyW]

Sometimes I wonder if the issue isn’t the catalog magnitudes for the star in your bandpass, or if the star is slightly variable. It feels messy and I keep swapping bits of the setup hoping for a clean answer. Did you try calibrating with stars at similar airmass?