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What does the f. number of a 'scope actually mean


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I am a semi-pro photographer so I know what the f. No on a camera lens means and its application to photography but what are the implications of a 'fast' and 'slow' scope.

Is it pretty much the same as in photography in that you get more light into a fast scope for the same focal length but that image sharpness can suffer if the optics are not up to it and a shallower depth of field (probably most relevant to lunar obs?) whereas a slow scope lets in less light for a given focal length but generally gives a sharper image?

Thanks for your help

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It actually means exactly the same as it does in photography.  It's the ratio of the aperture to the focal length.  But I guess astronomers tend to use it in a slightly different way since aperture is (mostly) fixed and depth of field is rarely an issue.

For me I don't really tend to worry about it much for visual other than for judging what sort of field of view I might get from a given scope.  For imaging then I guess I tend to work backwards from the image scale I want to achieve.  For example I might want to know what focal ratio is required to achieve 1 pixel per arcsecond, or to match camera resolution to telescope resolution with a given camera.  In those cases you might end up with a target focal ratio that you try to match as best you can given the other limitations of your kit (eg. it's fine to know you need to aim for f/8, say, but will your kit guide sufficiently accurately to allow long enough exposures at f/8 to get a decent image?)

James

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Yes, you have interpreted it correctly. More light for a given focal length at faster f ratio :) It is calculated the same way as well, focal length / aperture just with a telescope system the aperture is fixed.

For deep sky AP the idea is to go for the fastest f/ratio you can, taking into account the mechanical quality of the equipment. E.g. for my medium 750-800mm focal length I use a 6" f/5 Newtonian because I can't afford an 8" f/4 with a good enough build quality to keep everything in alignment with sufficient accuracy. I just take longer exposures to compensate.

I don't know if image sharpness is affected, but I don't think so. Shallow depth of field probably isn't as relevant as shallow depth of critical focus. It is much harder to get (and keep) sharp focus at fast focal ratios, hence my comment about build quality above.

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Completely agree with the above.  It is the same in Astronomy as Photography, at least as far as exposure times are concerned.  You can of course vary the F ratio of a scope by adding a barlow or focal reducer and an increase/decrease in exposure times.

I would have thought that stopping down a refractor would have exactly the same benefits as stopping down a camera lens in terms of depth of field and reducing distortion, but remember a good refractor has been designed to operate at full aperture, whereas a camera lens may be more of a compromise.  I am not sure that the same will hold true for a reflector.

Robin

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I don't know if image sharpness is affected, but I don't think so. 

This is of course complete cobblers! Larger aperture will net you higher resolution in planetary images upto the limit imposed by atmospheric distortion, and by clever use of software (very fast frame rates and drizzle stacking many hundreds to thousands of frames) you can compensate somewhat even for atmospherics.

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yes both the same meaning. 

i think of it as this. 

in astronomy your aim is to capture as much light as possible, so a low F number like f4 -f5 is ideal.

higher the f longer the time to collect the same amount of light. 

in photography its reversed, we dont want to much light so we keep the say f5 but we ramp up the shutter speed.

so as not to saturate the sensor, we also use the faster F number of bokeh and DOF 

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For me I don't really tend to worry about it much for visual other than for judging what sort of field of view I might get from a given scope.  

James

I gather James means, what field of view with any given eyepiece he will get as f/ratio visually has absolutely nothing to do with FOV of any given scope.

Visually an 8" f/4 has exactly the same max wide ffield as a 8" f/6 it's just they will both will require different eyepieces to reach max FOV.

Each given aperture has a minimum magnification. All 8" scopes have the same minimum magnification regardless of f/ratio. How they reach that min magnification is down to eyepiece used. The AFOV (apparent field of view) now comes into play. If two 8" scopes are picked at random, say an f/4 and a f/12 both are using an eyepiece of a focal length suitable to get them down to min magnification, then the AFOV of the eyepiece will determine which has the widest FOV. If the f/4 scope is using a 55deg plossl and the f/12 scope using a 100deg hyper wide type, then the f/12 scope will yield a wider FOV.

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I gather James means, what field of view with any given eyepiece he will get as f/ratio visually has absolutely nothing to do with FOV of any given scope.

Yes, I worded that very poorly.  Trying to concentrate on too many things at once :(  Thanks for clarifying.

James

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Saying it is the same as photography can be a bit misleading. If you change the f-number on a camera then you  change the aperture of the lens i.e. a low f-number physically lets more light into the camera. However, an f4 and f8 10" Newtonian will collect the same amount of light - they will just have different image scales.

NigelM

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Saying it is the same as photography can be a bit misleading. If you change the f-number on a camera then you  change the aperture of the lens i.e. a low f-number physically lets more light into the camera. 

Or you change the focal length of a zoom lens without changing the aperture (similar to adding a barlow or focal reducer to a scope)?

That's what prompted me to say in my initial post that whilst it means the same thing, interpretation is probably different.  I'd guess that when photographers talk about changing the f-ratio they generally mean by altering the aperture, letting in more or less light, whereas when astronomers talk about changing the f-ratio they're perhaps most often thinking about doing so by altering the focal length, keeping the amount of light entering the scope the same, but "spreading it out" more or less.

James

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The OP still has the right understanding though.

For a given focal length, different f-ratio means different aperture and hence different image brightness on extended objects.

For a given aperture, different f-ratio means different focal length and hence different image scale.

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Is it pretty much the same as in photography in that you get more light into a fast scope for the same focal length but that image sharpness can suffer if the optics are not up to it and a shallower depth of field (probably most relevant to lunar obs?)

Thanks for your help

I don't think depth of field is going to be too much of an issue when you're focussed on something 1/4 million miles away...  :tongue:

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.....Because we have a focal lengths and  apertures on  telescopes, it stands to reason that something being more specific to photography will cause some confusions and more conversations.
My take on the matter is that if your Newtonian telescope is 4" or 8" aperture, the same quality of light from your target is going to enter the OTA and hit the mirror! Its the size of the mirrors surface area  that matters to me , and the bigger the better, as the larger aperture will allow a larger image scale and better contrast for DSO and extended objects.
We know the telescopes f/number is the ratio of its aperture to its focal length, and is most probaly fixed, whereas on a DSLR the aperture adjustment will affect the volume of light,  which in turn controls the depth of field, with the shutter controling the time of exposure.
I too, only use the ratio as a guide, and its non importance (to me) to my visual observations, but it may have a bearing (for some folk) on what type of eyepiece they should choose, due to the width of the field of view at the focal plane, and the inherent coma present from a Newtonian, so  having a very fast ratio, may require some form of coma correction and possibly the purchase of more expensive eyepieces to help alleviate the effects of the coma, if its off-axis aberation is a concern to the user.
My picture shows field of view. Hopefully this image shows that the faster ratio scope, f/4 gives a slightly wider field of view at the focal plane over the smaller FOV from the f/6! This should achieve a slightly larger image scale, and better contrast to create the perfect image at the focal point. Then just the simple task of selecting an eyepiece to frame my subject.  This wider field of view at the focal plane can  make eyepiece choice more critical for some folk and collimation more critical ( although no-one intently collimates to less than 100%  but they may not achieve 100% )
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I gather James means, what field of view with any given eyepiece he will get as f/ratio visually has absolutely nothing to do with FOV of any given scope.

Visually an 8" f/4 has exactly the same max wide ffield as a 8" f/6 it's just they will both will require different eyepieces to reach max FOV.

Each given aperture has a minimum magnification. All 8" scopes have the same minimum magnification regardless of f/ratio. How they reach that min magnification is down to eyepiece used. The AFOV (apparent field of view) now comes into play. If two 8" scopes are picked at random, say an f/4 and a f/12 both are using an eyepiece of a focal length suitable to get them down to min magnification, then the AFOV of the eyepiece will determine which has the widest FOV. If the f/4 scope is using a 55deg plossl and the f/12 scope using a 100deg hyper wide type, then the f/12 scope will yield a wider FOV.

Gosh, I'm going to sound horribly pedantic but, in a totally gentlemanly kind of way, ;-), I don't agree with what you are saying. Notwithstanding the fact that it is focal length rather than ratio which determines mag and therefore fov (from afov), I tend to agree with Janes that the focal ratio does give an indication of the likely fov with a given eyepiece.

Actually, this will end up in semantics if I'm not careful, specifically I think that an 8" f4 scope and an 8" f12 scope have very different properties. The f4 with a hypothetical 21mm Plossl, 55 degree afov will still have a wider fov than an 8" f12 with a 21mm ethos. My 8"f20 mak would only achieve around 0.6 or 0.7 degrees whereas an f4 would manage far wider. Similarly, even with a 41mm Panoptic or 55mm Plossl the minimum magnification was still around x97 or x72.

I know in extreme, with a 3" or 4" focuser etc etc you could achieve the same so I agree with your comments in principle, but in practise they are very different.

To repeat, I totally understand what you were trying to say but just think the examples were not necessarily valid.

Back to the main point of the thread, I agree that the it's understanding is correct.

Cheers,

Stu

Sent from my iPhone using Tapatalk

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Increasing the focal length of a telescope (and hence the F ratio) will reduce the field of view. The amount of light from any given object will also be reduced, but to the eye, distant galaxies may be seen more easily with the longer focal length due to the larger imaging scale.

Primary mirror size or larger objective lens will gather more light and if you have a 12" and a 6" at the same F number the 12" will have a focal length double that of the 6" and hence the FoV will be narrower and the object will be fainter, i know strange but true. If they are at the same focal length then the 12" will have a much lower F number and have a wider FoV and the target will seem brighter.

F number is king among imagers who have to set the exposure on their cameras, the human eye is a little more adaptable and there it is more about getting the scale of the target right, hence non imagers tend to look at bigger scopes which have longer focal lengths.

Robin

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Gosh, I'm going to sound horribly pedantic but, in a totally gentlemanly kind of way, ;-), I don't agree with what you are saying. Notwithstanding the fact that it is focal length rather than ratio which determines mag and therefore fov (from afov), I tend to agree with Janes that the focal ratio does give an indication of the likely fov with a given eyepiece.Actually, this will end up in semantics if I'm not careful, specifically I think that an 8" f4 scope and an 8" f12 scope have very different properties. The f4 with a hypothetical 21mm Plossl, 55 degree afov will still have a wider fov than an 8" f12 with a 21mm ethos. My 8"f20 mak would only achieve around 0.6 or 0.7 degrees whereas an f4 would manage far wider. Similarly, even with a 41mm Panoptic or 55mm Plossl the minimum magnification was still around x97 or x72.I know in extreme, with a 3" or 4" focuser etc etc you could achieve the same so I agree with your comments in principle, but in practise they are very different.To repeat, I totally understand what you were trying to say but just think the examples were not necessarily valid.Back to the main point of the thread, I agree that the it's understanding is correct.Cheers,StuSent from my iPhone using Tapatalk

Stu my example stated an eyepiece to take that both scopes to the minimum magnification for that aperture. This is set by exit pupil so both scopes will be using different flength eyepieces. The f/4 will be using a much shorter flength eyepiece to reach this than the f/12. Perhaps the f/12 may be an extreme case as a long enough flength eyepiece may be tough to acquire. So perhaps I didn't give enough thought to my example, sorry. :)
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