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I have two Titans, One for the Alpy600 and one for the LISA in IR-mode. From Mt. Pinos, CA (Bortle 3-4 sky)
I can see targets down to 15.5 magV with my 33cm aperture classical cassegrain with a 6 sec exposure.
Its amazing that Robin can get down to 20th magnitude with a C-11 at F/5.5, even if there stacked.
I'll try for an ultra dim target (mag<17) with my LISA on a high red shift BH!

James

James Foster wrote: Its amazing that Robin can get down to 20th magnitude with a C-11 at F/5.5, even if there stacked.

I can't guide on that though. The target is invisible while guiding. I take a deep stacked image using the guiding camera and measure the offset to a brighter star that I then guide on.

This is the 30x30sec stack I used to calculate the guiding offset for PS1_J134743+495621 (The total exposure for the spectrum was 140 minutes)
Good luck with your faint targets !

Cheers
Robin

Robin Leadbeater wrote:I take a deep stacked image using the guiding camera and measure the offset to a brighter star that I then guide on.

I think I used the "bright" ~mag14 star at the top of the image to guide on

Robin