Ann Zabludoff gave a talk in the July 2020 HEN webinar discussing the Hotshots project. ( https://azabludoff.wixsite.com/hotshots )
I'm trying to learn the ways of Transient Astronomy so I figured to signup. One of the questions they ask in the form is what is your exposure time for a mag 19 star.
Hmm. Good question.
Any suggestions on how to determine this? Is there a standard field with calibrated stars down to that level? What SNR would be it be necessary to achieve detection? 50? What level of stacking is appropriate?
I have a 30.5cm scope with a cmos camera.
Thanks for any guidance.
Since the question on that form is not defined very precisely (indeed you would need to have a minimum SNR threshold.. I'd think tho that SNR ~15 is more than enough for mere detection), there is no need to try to come up with a super precise answer. Instead of looking for standard fields or calibrated stars, just point anywhere really and use e.g. Aladin Lite
to look up some of the the faintest stars that you can still safely identify in your image, there will be a lot. Click the checkmark "Gaia EDR3" in the "Catalog" window to select photometry from the Gaia mission. Then click on the squares hovering over the stars you are interested in to get the Gaia photometry. The magnitude you'll want to look at is "phot_g_mean_mag_corrected" near the end of the scroll down list. Indeed the "g" in g_mean_mag stands for Gaia, (not green!) and represents unfiltered photometry. It is close enough for our purpose here.
From experience with my own 20 cm scope and 43cm scopes at iTelescope, I'd think that your total exposure time to get a limiting mag of 19 (unfiltered) is somewhere between 300 and 700 seconds but it really depends a lot on how dark your sky is. But if you take a few frames at your maximum single-frame exposure time and stack them, you should be able to bracket the spot where 19mag stars can be identified confidently.
P.S.: if you are interested in Hotshots, you will probably also be interested in KilonovaCatcher (see the sticky thread in this forum), it has the same science goal and you'll find a link to webinar on it in that thread as well.
I have a 28cm SCT. With an SX CCD camera I can detect mag 19 stars in a 30 sec exposure, if sky is transparent and no Moon. Here I emphasise “detect”: the SNR is too low to measure. But if I can detect , say, an outburst of a CV, I can then follow up with longer (and stacked) exposures to get measurements.