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f stops and scope speeds.

Myopicus

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Myopicus

Myopicus

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Hi all, 

So, my SW 200p is f5.9, apparently.  Is that fast or slow or something else? And, compared to what?

I'd love to make sense of this at last, especially as I'm starting to seek out better EP's.

thanks, mace

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Moonshane

Moonshane

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it's actually medium! I'd say scopes start to get fast at f5 and slow at f8.

the 'speed' of a telescope has no bearing on visual observing as the view at 100x in an f4 scope and at 100x in an f10 scope will be pretty much the same at the same aperture.

as you hint though scopes faster than f5 tend to show anomalies in the construction of eyepieces more readily than slower scopes. f6 is a nice position as most eyepieces of decent quality will work well, although all eyepieces will work in all scopes in truth, just show more aberrations at faster 'speeds'.

focal ratio (speed) is focal length of scope (1200mm) divided by aperture (200mm) = f6.

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todd8137

todd8137

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Mines f5 and old plossl have a bit of field curvature at the edges ,but the celestron x cel lx show only a tad color is great I thought they would be horrible but they turned out to be brill wide eps are not great of they cheap one s I found

Pat

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JessicaFaye

JessicaFaye

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Brilliant. I get it. Been trying to work that out for weeks.

I hope that I'm not doomed as an amateur astronomer because of my fear of maths?

Amen  to that... Maths is seriously not my strong point!!! with the astronomy and the photography I'm doing my brain is fried from maths, especially ratios! Bear with it... it gets easier (I hope...) ;)

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bogdog

bogdog

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Amen  to that... Maths is seriously mot my strong point!!! with the astronomy and the photography I'm doing my brain is fried from maths, especially ratios! Bear with it... it gets easier (I hope...) ;)

mot?  looks like spelling is up there with math :grin: ..now back to school with you

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Perseus_m45

Perseus_m45

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Hi all, 

So, my SW 200p is f5.9, apparently.  Is that fast or slow or something else? And, compared to what?

I'd love to make sense of this at last, especially as I'm starting to seek out better EP's.

thanks, mace

Mace, your scope would give very nice wide field views with the 5.9 f/l . Maybe you could try one of the wide field eyepieces out on the market now? There are some relatively inexpensive brands that would give you some very nice views indeed.

                                                                     mike

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Charic

Charic

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Mace.......Hi, f/ratio is the result of dividing the telescopes focal length by its aperture. The end result is the f/ratio. You could also use the f/ratio times the aperture to find the focal length, working backwards.

F/Ratio`s  are  more important for photographic use. In Camera's alone. the f/ratio describes the aperture stop and how wide it is and effectively how much light can enter ( shorter exposure times ) or how closed it is for depth-of-field.
Our Dobs are f/5.91 but advertised as f6. This should put us at the fast end of the medium range?
What the aperture can show,  is how the cone of light is shaped? How steep the curve is, and is often mentioned in relation to the focal point and what eyepieces are used. 
This  basic picture shows  f/6 and f/4. light cones (shaded green).
A short f/ratio telescope is supposed to be better for wide field viewing, and a  long f/ratio telescope is more suited for higher-powered observations (such as planets and double stars).
Newtonian's have Coma due to their design, this can be tweaked out by the use of certain lenses, but only if the telescope is properly collimated. If there is any coma in my scope, I'm not looking for it. I focus centrally, and track with ease.
f/ratios under f/6 are considered fairly short, f/6 -  f/10 medium, and over f/10 long.
Our f/6 (f/5.91) Skyliner is good for general purpose use without the uses of special corrective lenses. The only special lenses I have are my BST's.
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Blazar

Blazar

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By the looks of it you have a copy of Turn Left at Orion (TLaO). In there all the basic telescope maths should be explained with some examples. Moonshane has answered the speed question however there are other 'sums' that might be of use to you in the future.

Good luck!

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wolfpaw

wolfpaw

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it's actually medium! I'd say scopes start to get fast at f5 and slow at f8.

the 'speed' of a telescope has no bearing on visual observing as the view at 100x in an f4 scope and at 100x in an f10 scope will be pretty much the same at the same aperture.

as you hint though scopes faster than f5 tend to show anomalies in the construction of eyepieces more readily than slower scopes. f6 is a nice position as most eyepieces of decent quality will work well, although all eyepieces will work in all scopes in truth, just show more aberrations at faster 'speeds'.

focal ratio (speed) is focal length of scope (1200mm) divided by aperture (200mm) = f6.

This is only half the story though. Better quality eyepieces might well show fewer anomalies than cheaper ones but what they WILL show is the natural coma built into the mechanics of a faster telescope. Expensive eyepieces remove eyepiece astigmatism only to reveal OTA coma instead.

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Moonshane

Moonshane

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This is only half the story though. Better quality eyepieces might well show fewer anomalies than cheaper ones but what they WILL show is the natural coma built into the mechanics of a faster telescope. Expensive eyepieces remove eyepiece astigmatism only to reveal OTA coma instead.

true but if the OP had a scope faster than f5, I'd be recommending he get a coma corrector for wide field/low power. he hasn't so there was no need to complicate matters by bringing that into the equation. also, I see hardly any coma in my f4 scopes with no coma corrector when using say a £50 15mmTV plossl (at 80x). you don't need expensive eyepieces to get good results with a fast scope, just better quality and accept narrower fields.

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ollypenrice

ollypenrice

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A short f/ratio telescope is supposed to be better for wide field viewing, and a  long f/ratio telescope is more suited for higher-powered observations (such as planets and double stars).

Well, not necessarily. It's focal length which determines field of view. I have an F7 apo with a focal length of 714mm and an F4.1 Newtonian with a focal length of just over 2 metres. Clearly the apo gives a much wider field. It is best, I think, to be formally accurate and not confuse F ratio with focal length. 

However, it probably is best, for high powered viewing, to construct your effective focal length (EP and scope together) from a long FL scope rather than a short. At least it is likely to be cheaper for a given quality because fast optics are hard and expensive to make.

Olly

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Moonshane

Moonshane

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it's certainly easier to attain a wide field at low power with a smaller short focal length scope as there are more eyepiece choices available to do so. the larger the scope, the longer the focal length for the same focal ratio and therefore the harder it is to get a wide field. e.g. to get a field of more than 1.5 degrees with my 114mm f4.3 I can use anything wider than a 15mm plossl but with my 16" f4 dob I'd have to use a 31mm Nagler or wider.

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Charic

Charic

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Well, not necessarily. It's focal length which determines field of view. I have an F7 apo with a focal length of 714mm and an F4.1 Newtonian with a focal length of just over 2 metres. Clearly the apo gives a much wider field. It is best, I think, to be formally accurate and not confuse F ratio with focal length. 

However, it probably is best, for high powered viewing, to construct your effective focal length (EP and scope together) from a long FL scope rather than a short. At least it is likely to be cheaper for a given quality because fast optics are hard and expensive to make.

Olly

ollypenrice........I never mentioned that focal length affects the field of view? I said focal length divided by aperture equals focal ratio, and I don't believe I'm confused between the two!
The smaller the f-ratio, the faster the telescope ,f/4 to f/6 providing wider fields of view (FOV) making them more suitable for deep space observations.
The larger the f-ratio, the slower the telescope. The FOV becomes smaller as the f-ratio gets larger. Telescopes with large f-ratio's (e.g. f/8 or f/10) are most suitable for high power planetary work.
My use  of the word 'short' for fast and 'Long' for slow focal ratios  may have mislead you. 
Its my understanding  the focal length controls the field of view of the instrument and the scale of the image that is presented at the focal plane. The fitting of an eyepiece and the formula AFOV/Mag=FOV does the rest.
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Charic

Charic

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........missed a bit, The advantage of a "fast" / 'short'  focal ratio with a visual  telescope like my Skyliner, is that it will deliver a wider field of view than telescopes with long/ "slower"  f/ratio's. hence my OP

I hope this clears any confusion

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