First off the assumption that fast is better is often wrong. The HST works at F/45 or so for its deepest images (varies with camera upgrades). The reason is that an asteroid is a point source at that distance so as the focal length increases the size of the asteroid on the sensor doesn't increase but the background does thus it gets darker when the asteroid stays the same. I learned this back in film days when I used a 10" f/8 and 10" f/5 scopes. The f/8 went deeper in the same time than did the f/5 even though the f/5 had higher efficiency coatings more than making up for the larger secondary shadow. In fact I often gained a full magnitude! Of course with digital pixel size enters into this calculation. A f/10 scope with a 9 micron pixel is the same as the same aperture at f/5 with a 4.5 micron pixel. I now work only at f/10 yet can reach on a very good night 23.5 magnitude in only 40 minutes working with a 18 micron pixel at 3650mm (about 1") Moving to a 0.5" pixel at 9 microns I've reached 24.3. Extending time to 2 hours only got me to 24.5 and that took very good seeing on a super transparency night. I've not seen such a night now in over 2 years.
My deepest image I have magnitudes for is of gravitational arcs in Abell 2218. I'm suspicious of magnitudes listed by NED but they are all I have to go on. I've attached the HST image and mine taken at 0.25" per pixel, well beyond my resolution but needed to go this deep.
These are NED's magnitudes for the arcs I've identified in the images.
A Not listed
H Not listed
L Not listed
Exposure time was 2 hours in 20 minute subs using my 14" LX200R. As I saw little difference at 1 hour I didn't go further. NED had nothing on the stars which may have gone fainter. I have other images picking up galaxies about 24th magnitude but they are few and far between. None in the last few years as my transparency has taken as hit a solar activity increased increasing my sky background and humidity has risen significantly for unknown reasons.
While I pick up a lot of asteroid in my images they are usually in the main belt and moving rapidly which greatly reduces the time one spends on any one pixel limiting my magnitude to about 22 on a good night. I have gone fainter by tracking the asteroid and streaking the stars. I do this mostly for NEOs moving too rapidly to catch otherwise. Not needed for Kuiper Belt or beyond like Sedna, Eris etc. Big and bright they are rather easy since their motion in a 10 minute sub is about at my resolution limit so they stick mostly to the same pixels for 10 minutes when tracking sidereal rate. If the sky ever cooperates I should see if I can find my limit for a point source. Quasars are point sources but the really faint ones are high z and thus severely reddened and my sensor is very poor in red light. Some listed as even 22 in K band (near IR) are invisible to me I suppose because they are too red and likely even fainter in visible red light.
It's remotely possible that on a perfect night of 1" seeing (never has happened) and super transparency (happened a few times) I might hit 25 given enough time. I don't expect that to happen however.
Edit: Somehow the attachment didn't make it or vanished. I've tried again.
Edited by Rick J, 01 February 2016 - 05:58 PM.