[pluto (Hugh)]

Hi Chris,
The catch is that longer focal length means tracking needs to be more accurate and longer focal length and more weight means guiding becomes more difficult. Good guiding is very dependant on good balance, and balancing a large/long and heavy scope on a SA would be difficult.

With my A7s on a WO71 (350mm FL) the pixel scale is about 5 seconds​ of arc per pixel - so if the error of the mounts tracking/guiding is more than that then you won't get round stars. In my experience that was about the amount of error I had when guiding and so my exposures were a bit hit and miss - and that's if everything else was perfect.
If I had my camera on an ED80 then the pixel scale would be smaller and I think most, if not all, of my subs would not have been usable. Of course most DSLRs have much smaller pixels (photosites) than the A7s so this would make the problem worse.

The rule of thumb is that, for astrophotography, you halve whatever the manufacturer rates their mount at.

I started out with a 5DmkII on an ED80 on an EQ5 (not HEQ5, bigger than a SA) and getting round stars was a challenge. Even getting nice round stars with a DSLR on an ED80 on an HEQ5 is not trivial let alone on a tiny mount like a SA.

Hope this makes sense! (Typing this on a phone is tricky!)

P.S. the Star Discovery mount, being an AZ-ALT mount and not an equatorial mount, is not really designed for astrophotography - so for visual use it's probably fine to push the mount to its limit weight-wise.