Advice on Focal Length

[Gweilo]

Hi All,

I read in Astronomy magazine that for imaging Jupiter the focal length of the scope needs to be f30. Is this right?

I have a Celestron c9.25 which I believe is f10 so I guess a 3x Barlow is in order and the camera is an Orion Solar System Imager II . I'm new to planetary imaging, well all of it really, so any advice would be greatly appreciated.

[SteveKinder]

Yes but check out the Televue Powermates. The 5x would do great things with Jupiter on your scope (ok with excelllent seeing and tracking and stability too).

Cheers,

Steve.

[Cornelius Varley]

It's actually the focal ratio that needs to be in the f20 - f30 range to get the best resolution. A C9.25 + a 5x barlow might be pushing things a bit.

Peter

[JohnnyD333]

Cornelius is right,

Focal length will determine your magnification. The Focal Ratio will determine how fast your scope collects light. A fast scope at F/5 will show a very bright Jupiter with no detail through a camera. By using a slower scope, say F/10, and using barlows to slow it further, Jupiter will be less bright and show detail. But dont let the magazine confuse you, it is very possible to have a fast scope and get images of bright objects, you just need to make adjustments like using barlows and changing the settings on your camera.

[steppenwolf]

I've never really understood why planetary images talk about the focal ratios they use - it's a meaningless figure unless they also let you know either the focal length or the aperture that it pertains to!

It is the 'effective' focal length that they are really interested in so why not just talk 'effective focal length' as no other parameter is then required to get across the requirement?

Hope that didn't sound too much like a rant

[ollypenrice]

^^ Sounds bang on the money to me.

Olly