Having difficulty understanding f ratio

[Razorg]

In theory, I understand how f ratio is calculated, but I cannot understand how it affects magnification and the final image. I read that fast telescopes having f ratio f/5 or higher are good for DSOs and slow telescopes are good for planets. How is that deduced? 

[Purplehayze104]

well my thinking behind it is if your think of having 2 tubes one long than the other, but both ends of the tube letting light in are level, the light traveling throu them would reach the end of the shorter tube before the end of the longer one, hence taking less time if you are imaging with a shorter focal length, also shorter focal lengths give a wider field of view.

hope this is of any help

clear skies

john

[Scott]

the f/ratio has no baring on the magnification. magnification is derived by dividing the focal length of your scope by the focal length of your ep.

eg. a 1000mm scope with a 10mm ep will give you 100X magnification (1000/10=100). Sorry if this is obvious.

The aperture of your scope will be an issue in determining what the useful maximum mag will be for your scope. it is recomended that approx 2x the aperure in mm isa useful guide for maximum magnification. eg 60mm aperture= approx 120X mag. there are many other factors such as brightness of the object, sky conditions etcso this is only a guide.

Never owned a long f/ratio scope so I'll leave it to others to explain why its better for planetary

[ronin]

It does not directly effect magnification or final image in terms of size.

The f number is more a carry over from photography, although it does generally imply possible characteristics of a lens or mirror.

Magnification is scope focal length divided by eye piece focal length.

Image size (no eyepiece) is a function of focal length and object size.

The real primary feature of the f number is that as it governs the brightness of the prime image created which is then either brighter or dimmer.

An f/5 lens creates a brighter prime image then an f/8 lens, and for this reason the value is relevant to imagers.

As a somewhat general guide the faster the lens or mirror, say f/5, it gets more difficult to produce, they tend to demonstrate more CA (lens) or Coma for a mirror. Not saying they cannot be made but a good one is more difficult and difficult means costly. In terms of a lens an 80mm f/5 Startravel will show more CA then the 90mm f/10 Evostar. This is what I meant by "characteristics".

You do not need a fast scope for anything other then imaging in specific terms, but selecting the f number to suit your observing or astronomy may lead to a simpler and happier life.

Or you buy 3 or 4 scopes. :eek:

[gnomus]

Magnification is dependent upon focal length. An object will look the same size in two scopes of equal focal length even if they have different focal ratios. Focal ratio is a measure of aperture. So in the case of the two scopes of equal focal length but different focal ratios, one must have a wider aperture. Since it is an inverse relationship, the larger aperture scope has the lower f/ number. Since the scope with a larger aperture captures more light, the image in the larger aperture scope will be brighter, or will require less exposure time for the same result. That is why imagers prefer faster (lower f/ number) scopes. The larger the aperture, however, the narrower is the depth of acceptable focus. Planetary imagers do not need to worry so much about amount of light since their targets are usually pretty bright to begin with.

[acey]

The subject causes confusion because the terminology of photography (e.g. "fast" and "slow") gets carried over into telescopes, and if telescopes are being used for visual observation then photographic considerations are irrelevant. For DSOs the most significant thing is aperture. For planets you want minimal aberration and good contrast. In both cases you want something that is convenient to use and affordable to buy.

A large f-number means a longer tube (for a given aperture), so if you're interested in aperture (for DSOs) you will tend to favour lower f-numbers for convenience.

Low f-number also tends to mean more aberration, for inescapable optical reasons, so if you're interested in planets you may be more likely to go for higher f-number.

Most people are looking for an all-rounder and are happy to compromise.

f-number, in itself, has no bearing on magnification or image brightness.

[rockystar]

Isn't the focal ratio used to determine exit pupil? So 25 mm eyepiece in an f/5 gives 5mm exit pupil, in an f/10 it would give 2.5mm.

[ollypenrice]

The subject causes confusion because the terminology of photography (e.g. "fast" and "slow") gets carried over into telescopes, and if telescopes are being used for visual observation then photographic considerations are irrelevant. For DSOs the most significant thing is aperture. For planets you want minimal aberration and good contrast. In both cases you want something that is convenient to use and affordable to buy.

A large f-number means a longer tube (for a given aperture), so if you're interested in aperture (for DSOs) you will tend to favour lower f-numbers for convenience.

Low f-number also tends to mean more aberration, for inescapable optical reasons, so if you're interested in planets you may be more likely to go for higher f-number.

Most people are looking for an all-rounder and are happy to compromise.

f-number, in itself, has no bearing on magnification or image brightness.

Exactly. I think F ratio causes confusion because it can only usefully be considered in the context of the values which make it up, namely focal length and aperture, and not outside them.  The classic case is the F ratio myth, born of focal reducers which reduce F number without inceasing aperture.

Olly

[rockystar]

Also, as a Barlow lens technically increases your focal length (which has the side effect of increasing magnification), does that then affect the f/ratio?

[gnomus]

Also, as a Barlow lens technically increases your focal length (which has the side effect of increasing magnification), does that then affect the f/ratio?

Yes. When you see the planetary chaps quotong f/20 or much higher, they are usually using Barlows.

[steppenwolf]

Exactly. I think F ratio causes confusion because it can only usefully be considered in the context of the values which make it up, namely focal length and aperture, and not outside them.

[minor hijack]

It is always a source of frustration for me when I see planetary and Lunar imagers describing the parameters used for their wonderful images when they talk about the focal ratio used - of much more interest and value is the effective focal length, surely? OK, if they also tell you what aperture their telescope is, a simple calculation will produce the desired figure but why not miss out the middle man and tell us something meaningful?

[/minor hijack]

[Razorg]

Also, as a Barlow lens technically increases your focal length (which has the side effect of increasing magnification), does that then affect the f/ratio?

This is exactly what I'm concerned with. If I have a 130mm scope with 650 mm focal length(f/5), attaching a Barlow both increases the magnification and decreases the image brightness, along with the change in focal length. So doesn't that indirectly relate f ratio with brightness and magnification? sorry if I'm being stupid, but this whole f ratio thing is pretty tough for a noob like me  

[gnomus]

This is exactly what I'm concerned with. If I have a 130mm scope with 650 mm focal length(f/5), attaching a Barlow both increases the magnification and decreases the image brightness, along with the change in focal length. So doesn't that indirectly relate f ratio with brightness and magnification? sorry if I'm being stupid, but this whole f ratio thing is pretty tough for a noob like me

No, the Barlow inreases the focal length, thus increasing magnification. The aperture doesn't change, therefore the f/ ratio (which is a simple piece of arithmetic) must go up. The f-ratio is changing because the arithmetic says so. The f-ratio is not the thing affecting magnification, that is the focal length. Perhaps it would be better to forget about f-ratio and only think about aperture.

[John]

Perhaps it's more straightforward to think in terms of focal length then, with the barlow lens added, effective focal length as Steve / Steppenwolf suggests ?

[ollypenrice]

If we are talking about imaging then magnification is meaningless anyway. In visual observing something is magnified. The size of the image formed on the retina. This is the baseline image we magnify wth our optics. In imaging it has no equivalent so we should talk about image scale.

Yet another source of confusion comes from the camera world in which 'aperture' is used synonymously with F stop. This is forgivable as a shorthand in a system in which the focal length is fixed and the aperture is varied by opening or closing the diaphragm. (We know that the focal length is fixed so the only way to change the F ratio is to change the aperture. In this sense they are effectively synonymous though F ratio is unitless, being a ratio.) But since astrophotographers rarely stop down their aperture (and then only to fight optical defects) 'aperture' and 'F ratio' should never be used synonymously because they simply aren't if the focal length isn't pinned.

If it's of any help I can refer you to some graphic efforts I use here.

1) Why is 'fast' fast?  https://ollypenrice.smugmug.com/Other/Takahashi-EM200TEC140/i-RnGN8nD/0/O/F%20RATIO%20AT%20FIXED%20FOCAL%20LENGTH%3B.jpg

2) The F ratio myth. (Why focal reducers may not do what you want them to do.) https://ollypenrice.smugmug.com/Other/Takahashi-EM200TEC140/i-vsgSRm6/0/O/THE%20F%20RATIO%20MYTH.jpg

3) What focal reducers can usefully do: https://ollypenrice.smugmug.com/Other/Takahashi-EM200TEC140/i-MjtB7zd/0/O/reducers%20used%20properly.jpg

Olly