<p class=”wp-dark-mode-bg-image”>Right so I was woken at 3am by the youngest sprog and couldn’t get back to sleep, so I did a load of internet searching and reading around SNR, AstroImageJ and aperture photometry (totally normal behaviour, right?).
<p class=”wp-dark-mode-bg-image”>Stumbled across this useful looking calculator…<br class=”wp-dark-mode-bg-image”>
https://mirametrics.com/sn_calculator_mvn.php<p class=”wp-dark-mode-bg-image”>Plugging in the pertinent data, or estimates of, and it thinks using a 102mm refractor operating with 2 minute exposures, and assuming my minimum acceptable SNR to be 500, then it looks like I should be aiming for stars of at least mag 12.5 with a 2% transit depth.
<p class=”wp-dark-mode-bg-image”>Edit: Plugging in numbers for my 300mm/1500mm Newtonian says it should be good down to mag 14 ish with a 2% depth.
Perfectly normal behaviour, IME.
Your numbers sound about right to me and consistent, though not identical, with what I posted earlier.
Your noise will depend greatly on your sky brightness, of course. Don’t expect as good results from a brilliantly moonlit Bortle-7 location as a new moon B4.
I would recommend installing a proper photometry package (I use and recommend APT, Aperture Photometry Tool) but there are several others available. Any of them will give a good measurement of the SNR of any given star in any given image.
Incidentally, and this doesn’t apply to exoplanetary transit work, stacking many unsaturated images in addition mode will increase the SNR by a factor of the square root of the number of images. By adding the images you preserve the photometric accuracy and let the counts per pixel exceed the detector’s saturation limit.
