Ideal F ratio for planets

[madjohn]

Hi all,

:scratch: Just wondering what the ideal F ratio would be for observing the planets.

Seems a lot of people reckon the higher the F ratio the better, is this puely for greater magnification or is there a more sutble reason? maybe better colour correction.

In an imaginary Apo refractor of say 100mm objective size, what would you think would be the ideal F. ratio?

John

[ringz]

John,

The rules of thumb regarding planetary telescopes were coined before apochromatic refractors were generally available.

Before this achromats were the norm and to limit chromatic abberation, a long focal length was required. It is easier to make long FL lenses with an accurate shape/figure so spherical abberation and astigmatism were well controlled as well.

With modern apochromats and eyepieces you can mostly do what you want, if you have deep pockets

--

Martyn

[Kaptain Klevtsov]

f/2 would do for me, but that would need a lot of aperture. There is no ideal f/ ratio, just the focal length that fits the planet onto your camera chip with room to spare. You get this by using Barlow lenses which changes the native f/ ratio of your 'scope, then you work with that.

Joking aside, and I see that you said observing, you can use whatever you have to hand and use bits and pieces to make it work for you. I've seen Saturn at f/5 and f/10, but in different aperture 'scopes, that means that the f/ ratio comparison was meaningless visually. Anyway, how do you do f/ when you are using eyepieces? Seriously, you can Barlow an f/10 SCT and use a 20mm EP, or use a 10mm EP. Does one work out at f/20 and the other at f/10? Seriously confused now about the focal ratio WRT visual stuff.

Kaptain Klevtsov

[GazOC]

I agree KK, I've looked at planets in f5 Newts and f15 Maks at the same magnification and the only really obvious difference you notice is the different eyepieces you've had to use to achieve that magnification in the different scopes.

[Lady Isabella]

Below f/7 in a APO refractor, you will find that the focussing will become more critical. In a side by side test with a friends Televue NP-101 and a Takahashi TSA-102 it was very clearly noted. The Tak would easily come into focus, while we had to fiddle about a bit with the NP scope. The longer focal ratio of the Tak, allowed the use of cheap Plossl eyepieces, while the NP was a bit more fussy.

[madjohn]

thanks for all the comments everyone, seems f7 plus would be about right but being careful not to end up with a long unwieldy scope.

john

[nitram100]

I've viewed Saturn through an Orion Optics F6.3 10" Newt and it didn't look much different to the view i get in my F4.8. The only difference was the F6.3 tube was very long.

[lunator]

Hi John

For planetary if you want to use a reflector the longer the focal length the better, but the difference will be slight. From my observing sight the sky is a much more important limiting factor than the F ratio. The caveat to that is that if you do get a great night and you have the well collimated long focal length Newt the view would be excellent.

Cheers

Ian

[narrowbandpaul]

with a newt, a longer f ratio means a smaller secondary which leads to higher contrast. Vital for planetary imaging....

a F/10 newt would be good, but also very long....but the views are good

[GazOC]

A smaller secondary is important for viewing planets but not so much when imaging them. Look at the images that people with SCTs with ~35% obstructions put up on here. When you have 1000s of frames and PS curves and levels you can bring out the low contrast detail that the eye may miss.

[narrowbandpaul]

i really meant observing not imaging...im in a world of my own

however, processing shouldnt be a more important factor than having a small secondary to begin with