focal lenths

[red dwalf]

hi all,

can i ask, is it better to have a short focal length scope or a long focal scope for imaging planets.

i have a skywatcher 5.1" 650 focal length f/5 newtonian and a 6" 1500 focal length f/10 celestron 6se.

i asking which would be best to use with a webcam ?

[GazOC]

Long focal lengths are better for planets, the SCT is the one to use...)

[brianb]

Long focal lengths are better for planets, the SCT is the one to use...)

And even then you'll probably need a barlow, at least once you get experience. You need to work at f/20+ to wring out all the detail that the objective can resolve though that does take good seeing, especially on lower surface brightness targets (Jupiter, Saturn). Many of the really great images taken in excellent seeing are at focal ratios around f/40.

[Doc]

You could do with a X4 powermate Rob.

[red dwalf]

Thanks chaps,

I do have a 3x TAL barlow so will try this tonight if the clouds don`t roll in, would i also be better when imaging to not use the diagonal when attaching the barlow and webcam ?

[brianb]

would i also be better when imaging to not use the diagonal when attaching the barlow and webcam ?

Yes, the less in the way the better.

[GazOC]

Thanks chaps,

I do have a 3x TAL barlow so will try this tonight if the clouds don`t roll in, would i also be better when imaging to not use the diagonal when attaching the barlow and webcam ?

Yes, but you may find you don't have enough focus travel without the diagonal as the barlow will push the point of focus "outwards".

Its worth a try though, SCTs have a very good range of focus so you might well be OK.

[brianb]

Its worth a try though, SCTs have a very good range of focus so you might well be OK.

I'd be amazed if you're not.

For those scopes that do have "too much backfocus" issues, get an extender tube to use up the extra length.