[Shiraz (Ray)]

Its not just initial collimation with the f4 - that is just routine after a while.

Some other problems are:
1. the focal depth at f4 is about 40 microns. that means that the mechanical system (focuser etc) has to keep to about that tolerance all over the focal plane. you will need to replace the standard focuser since it will almost certainly sag way out of tolerance with a normal camera and give you odd shaped stars across the field. You will also need a camera with the focal plane exactly aligned with the camera body - my QHY8 was not well enough aligned at f4. Even with an aligned camera, slight misalignment in the locking collar will throw it off enough to slightly tilt the focal plane and distort the stars.
2. you will need a coma corrector and the tolerances on placement and alignment of that element are very tight. the standard focuser is not good enough to keep an MPCC in the right place and you also might be unlucky and get an MPCC with slight misalignment built in - I think mine is off a little bit, but I haven't yet devised a test for this. The MPCC also requires that the collimation be spot on - if the MPCC is not on the centre of the optics, it will distort the stars in very odd ways.
3. the standard GSO mirror mounting systems do not keep good collimation at f4 when you move to a different part of the sky. It is not just collimating once a night, you really need to recollimate if you change to a different target or even if you image for a long period: camera off and warm up, take it out, recollimate, replace camera and hope collar has locked properly, cool camera again and hope no fogging, refocus, possibly redo PHD calibration.
4. the short focal length means that you have significant field curvature - the MPCC does not do so well with coma correction under these conditions and you end up with some coma creeping back in as well as slightly unfocused stars at the extremities of an APS sized chip.
5. low cost eyepieces tend to run out of steam before they get to f4 - you will probably need to buy some that are designed for fast optics (read are expensive) if you want to do visual observing - or buy a good quality Barlow.

Having said all this, f4 is really fast and I still like my scope for imaging wide field DSOs.....mostly. But I would not recommend it as an all-rounder.