Why not smaller reflectors for AP?

[RikM]

Sorry - must be from a different era as I haven't a clue what you're on about but presume = 85mm OG not much bigger than my secondary - why would I go backwards? I haven't bought any special gear for nearly two decades but I'm happy with my results.

not backwards, but different. The FSQ85ED is believed by some to be the finest widefield imaging scope available.

[ollypenrice]

Conversely exposures can be greatly reduced with a large aperture AND fast f/ratio - I run my 12" SCT @ f/3.6 but it's not portable

How do you get it down to F3.6? There's a 3.3 reducer but it only works on tiny chips and is of little use on even medium sized ones.

Stupid question maybe guys, but from reading 'Making Every Photon Count' I noted that aperture isn't as important for AP as it is for visual use. This makes sense I guess because you could instead take a longer exposure to compensate for the reduced light grasp.

What confuses me a bit though is that the author of that book, and seemingly the majority of the posters on here are using 8 - 10 inch reflectors for AP.

Why not use smaller scopes like a 130 or 150? Wouldn't the lighter weight allow for a more portable mount? Is there a sweet spot in terms of aperture for AP?

Sorry if its a dumb question but I'm curious...

Maybe it's just because reflectors are inexpensive anyway (relatively) so folks go for fairly large ones. I'm not sure how field flatness plays out with small reflectors. In refractors you can expect the flat image circle to be larger in larger apertures.

Olly

[ollypenrice]

Sorry - must be from a different era as I haven't a clue what you're on about but presume = 85mm OG not much bigger than my secondary - why would I go backwards? I haven't bought any special gear for nearly two decades but I'm happy with my results.

To get better images? You'd be hard put to beat the FSQ85 at the focal length it offers (450mm or 328mm with reducer.) It is also very easy to use, needs only a light mount, needs no collimation, doesn't catch the wind, etc. It also has a 44mm flatfield image circle at native FL so it can nearly cover the Kodak 11meg chip which is about 37x26mm in size. That is very different from what you are used to, which is 8.6 x 6.5mm if I'm not mistaken. (This may explain why you can use the 3.3 reducer if that is indeed what you're using.) The FSQ allows you to image from Alnitak, the Flame, the Horse, to Running Man and M42 in a single frame. A different world in terms of sky coverage. I'm happy with the quality, I must say. This image was on a 15mm square chip but you can get exactly this resolution all the way from Alnitak down to here using a full frame chip; http://ollypenrice.s...NfQT5R&lb=1&s=O

Olly

[Peter Drew]

Rockrae.

If you bring your telescope along again sometime I would be pleased to assess modifying it so that you could use it for AP without a Barlow. Extension tubes, I would have thought, would make the focus position worse.