Long F Numbers

[pbyrne]

Hi there

I have seen images on the web of the moon and planets where the details reveal that the image was taken with a particular scope operating at f/25 or so.

I think I understand how this is, if the scope is f/10 and a 2.5X barlow is used then it makes it f/25. Am I correct in what I'm thinking or is there something else?

Thanks.

Paul

[Psychobilly]

Yep thats about it

Billy...

[steppenwolf]

Yep that's it but personally, I find the 'F number' meaningless i this context even though I probably go against the grain on this!

To me, a more meaningful description is to give the 'effective' focal length with the accessories included as this means something on its own whereas the F number is meaningless unless the aperture is quoted as well!

Now, where did I put that parapet.

[MrEd]

Yep that's it but personally, I find the 'F number' meaningless i this context even though I probably go against the grain on this!

To me, a more meaningful description is to give the 'effective' focal length with the accessories included as this means something on its own whereas the F number is meaningless unless the aperture is quoted as well!

Now, where did I put that parapet.

Heh heh i know what you mean about the focal length Steve.

I just think its easier to type less digits

Ed

[steppenwolf]

That can't be my mate Mr Ed surely? Nah, you never agree with anything I say! You make a good point about typing less characters though ...... (see you on Monday)

[RichieJarvis]

Oh, you 2 will be bashing spots of each other again soon

See you Monday!

Richie

[jgs001]

To me, a more meaningful description is to give the 'effective' focal length

Steve, is that the actual or 35mm equivalent Focal Length ...

I'm with you, and the focal length makes more sense.

See you all on Monday

[Dangerous-Dave]

Yep that's it but personally, I find the 'F number' meaningless i this context even though I probably go against the grain on this!

To me, a more meaningful description is to give the 'effective' focal length with the accessories included as this means something on its own whereas the F number is meaningless unless the aperture is quoted as well!

Now, where did I put that parapet.

I think you're spot on Steve. There is a world of difference between image size in a 3" scope at f25 and a 12" at f25. In my mind's eye I envisage image scale in terms of the focal length and have acquired a range of scopes and lenses to cover these:

16mm Zenitar fisheye

28mm Takumar (c. 45mm Effective Focal Length)

135mm Takumar (c. 216mm EFL)

200mm Takumar (c. 320mm EFL)

(also own a 2x teleconverter to double these if necessary)

Megrez 90 FD with MkIII reducer - 500mm focal length

Megrez 90 FD (no reducer) - 620mm focal length

OMC140 - 2000mm focal length

That should cover me from half the sky up to a galaxy outside the local group.

[acey]

Hi there

I have seen images on the web of the moon and planets where the details reveal that the image was taken with a particular scope operating at f/25 or so.

I think I understand how this is, if the scope is f/10 and a 2.5X barlow is used then it makes it f/25. Am I correct in what I'm thinking or is there something else?

Thanks.

Paul

I thought it meant that the aperture had been reduced with a mask, so that the f-ratio becomes longer for the same focal length. I'm not an imager, but I know that for visual observation of planets it can help to reduce the aperture, since it reduces aberration. Telescope lens caps often have a mini-hole with cap attached so you can do this.

[SteveL]

Not much point in that unless you are solar imaging. There is nothing to gain by blocking light when doing any other imaging. When the planetary guys say they are using C11s or C14s @ f/30 or more, they REALLY are using the full aperture of the scope, plus adding barlows into the imaging train.