Paul – I suppose limiting mag depends a bit on your skies and a lot on other things. Using a RedCat 51, so the same aperture, from here on a good night a star at 16.0 G has an SNR of 6 in 10 mins using a cooled sensor (ASI2600MC). That is with a sky background of 18.1 mag/arcsec^2. In the same time, in similar conditions, on my 0.28-m SCT I can get similar SNR for a star of mag 20. That’s about right given the ratio of apertures.
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