Question about focal length

[Kirkus]

If I'm imaging a planet at prime focus with, let's say a scope with a FL of 750mm, would the image be twice as large using a scope with a focal length of twice that -- 1500mm?

[mental4astro]

Sorry to do this to you, but it will be smaller. The way it works is the longer the focal length of the scope (not the eyepiece), the greater the magnification.

It is like the giant telephoto lenses used on cameras, the longer the lens focal length the bigger the image.

If you are after info on imaging, ask the gurus in the 'cameras, photography & imaging' forum. I do no astrophotography, & the few Q's I've had I've asked there. Incredible resource.

Mental

[Kirkus]

That's what I thought. I just phrased my question all goofy. Thanks, Mental.

[ausastronomer]

Yes that is correct. A scope at prime focus with a focal length of 1500mm will give double the image scale of a scope having a focal length of 750mm. A 2X barlow in a scope of 750mm focal length will achieve exactly the same result of doubling the image size.

Cheers,
John B

[hulloleeds]

There's no point buying an expensive telescope, just add more successive barlows, foolproof :D

[mjeremy]

The Apparent magnification is the focal length of the Tube/focal length of the eyepeice - so if you double the tube focal length then you double the magnification. Equally you could halve the focal length of the eyepeice to acheive the same result.

Magnification is everything though - If you don't have the aperture to capure enough light then constantly stepping up the magnification (using the smaller focal eyepeice logic) tends to result in fainter and fainter images

A 1500mm Tube Focal length with a 25mm Eyepeice should result in an apparent magnification of 1500/25 = 60x If you double the OTA focal length to 3000mm or halve the EP to 12.5 you get

3000/25 or 1500/12.5 both of which are 120x

Although I have discussed EPs the same rule applies to imaging when using Barlows

Mike

[Kirkus]

Thank you all for your answers.

So I assume the same principle would apply with aperture (f ratio?). Using the same focal lenth of 750mm, let's say it was an f/5. If I were to use a 2X barlow that would double the FL to 1500, but would that also reduce the aperture by half?

In other words would a 750mm-f/5 scope using a 2X barlow be the same as using a 1500mm-f/10 scope without a barlow?

[Kirkus]

LOL.

In my amateurness I tried that once. By the time I got my camera attached to the top of it, it looked like the Leaning Tower of Pisa.

[astro744]

No. aperture remains unchanged but yes, your last statement is essentially correct. Note though the distance from the Barlow or Powermate to the focal plane can change the amplification factor. See http://www.televue.com/engine/page.asp?ID=42

In a given camera lens the iris can control the aperture and therefore f ratio but a given telescope has no iris and only a change in focal length can control f ratio unless you manually mask the front of your telescope with a smaller aperture mask (basically cardboard with a hole in it).

Note a smaller aperture will lower the resolving power of the telescope.

[Kirkus]

Ohhhh. OK. So I shouldn't be interchanging the terms "aperture" and "F Ratio", because they are actually different things.


Very interesting stuff (my family and friends might argue that point).

[Mike21]

Different but related. The f-ratio is the aperture per focal length. A fast f-ratio has big light collection capacity and by comparison does not try to magnify too much. A slow focal ratio, the opposite.

This explanation may sound whacky but it initally helped me. Photography is analagous to making cordial. If cordial was aperture and focal length water. A fast ratio is like concentrated cordial in a small amount of water, you don't need to taste much before you've had enough; fast exposure. Whereas a slow f-ratio is like a drop of cordial diluted to buggery, you'll need to drink heaps before you can even work out what flavour it is.

[astro744]

That's correct.

f ratio = focal length / aperture.

eg. C8 = 2000mm focal length, 200mm aperture, f ratio = 2000/200 = f10

This is considered 'slow' photographically.

Put a f6.3 focal reducer on the C8 and you get 200mm aperture and f = 1260mm, f ratio = f6.3.

(Note I've rounded the numbers for simplicity; C8 aperture is 8 inches which equates to 203.2mm and f.l. = 80 inches which = 2032mm)

A Newtonian of the same aperture, 200mm has typically a focal length of 1200mm, ie. f ratio = 1200/200 = f6.

A Barlow, Powermate or tele-extender increase focal length and therefore f ratio for a given aperture.

A focal reducer decreases focal length and therefore f ratio for a given aperture.

See also http://www.televue.com/engine/page.asp?ID=237 for other methods of changing the effective focal length of a system particularly for imaging.