I have heard and read lots of favourable reports regarding the Orion/Synta ED100. Has anyone here used them for deep sky astrophotography at f9?? Is this too "slow" a f ratio for DSLR photography? I suppose the thing that appeals to me is the 900mm focal length would give a good image scale on a lot of deep sky objects.
No it isn't
All the f ratio determines is the image scale. If you put a focal reducer on you just get a smaller image of what you photographing. The light is more concentrated resulting in less exposure time. This is OK for wide field images of nebs etc but not for anything that is a bit smaller. If you only ever want to image the orion neb or similar size items then the faster scope would be beter. At 800mm it is still hardly a long focal length and tracking and guideing should be easy on any decent mount.
From my understanding, the focal length of the scope would determine how "accurate" the guiding would need to be, whereas the focal ratio would determine exposure time. For instance f9 versus f7.5 would mean 1.44 times the exposure to obtain the 'same' exposure at a given ISO. I think this makes sense.
You're right Daniel .... the focal ratio describes the speed of the lens i.e. the smaller the focal ratio, the brighter the image produced by the lens, the shorter the exposure required.
As Mick said you can use a reducer to widen the fov and thus increase the speed of the lens.
