What Focal Length

[valleyman]

What focal length is best for luner/ planet photography.

[brantuk]

1500mm will give sharp contrasty views - you can allways barlow it if you want more length. A 5mm-8mm ep's would be good in your Lightswitch (in good seeing)

Edit - for webcamming the chip gives an image size equivalent to that of a 6mm ep.

[nightfisher]

kim would you say 1500 is ideal, better than say 1800 or even 2000, this is of great interest to me

[JimStan]

Perhaps a better question to ask is "refractor vs. reflector?" For planetary viewing, you cannot beat a refractor, since there is no secondary mirror or spiders, ( not the insects, the mirror supports ! ) in the light path, which lead to defraction artifacts.

However, a real long FL refractor can cost a lot of money, so many people will accept a Mak style reflector as a good substitute.

You can get some magnification by going with a short tube and eyepiece, and barlowing it to death, or with a long tube, and moderate to short eyepieces.

You will find the longer the tube, the better it will be for planetary observation.

If you see two different 4 inch scopes, the F:10 would be a better choice for planetary use, and the F:7 would work best for deep sky objects. The one would give greater image scale, and the other, wider fields.

JMHO - Jim S.

[TheThing]

I find a 200mm aperture, 1000mm focal length reflector is perfect. That's what I use with, IMHO, passable results!

TheThing.

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[spaceboy]

I find a 200mm aperture, 1000mm focal length reflector is perfect. That's what I use with, IMHO, passable results!

TheThing.

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Ditto.

[JohnnyD333]

For Planetary and Lunar, go big in focal length. SCTs are great with their huge focal length, anything over 1000mm, and compact size. A tracking mount would save you some headaches as well for imaging.

[Ags]

I think it is more accurate to say that aperture determines resolution, light gathering and relative contrast (when comparing scopes of similar optical design), and so limits the maximum magnification that can be used. The focal length is nearly irrelevant.

There is the argument that a relatively smaller refractor stands up to poor seeing better than a larger instrument (because it is smaller) and has better contrast because it is unobstructed, so depending on your viewing style you may prefer the smaller refractor over the larger reflector.

EDIT: Sorry - my comments were skewed to visual observing which was not the question. For imaging of planets the sweet spot (I'm told) is F20 to F30. So for a 200mm scope, the ideal focal length would be 4m to 6m, typically achieved with barlows.