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Seriously, what's up with telescopes and f-ratios?

dementedgr

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dementedgr

dementedgr

Posted
Posted

I've been trying to bend my mind around this one, searching through the internet for a solution but I still don't get it...

How come an x" f/10 scope be 2 times "slower" than an f/7 ??? I get the wider FOV thing but the "fastness", nope!

I mean, in camera lenses, a change in the f-stop means a change in aperture thus fewer/more light entering the sensor!

But in a telescope the aperture is fixed no matter what the f-ratio is, so how can this affect the exposure time needed for

the same brightness of an object? I get that for bigger objects its better because more light is squeezed in one pixel

due to a wider FOV but doesn't that come as a compromise in angular resolution? I mean if i take a 10min exposure

on an f/10 scope and downscale it, shouldn't it be the same as a 10min exp. on an f/7 scope? (assuming exactly the same

optics, photon transmission loss, sensor type etc etc)  Even the Hubble telescope is an f/24 scope, not particularly "fast",

so what's up with all the rush for "faster" scopes??? Please, feel free to go as technical as you like...

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jambouk

jambouk

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It just is :)

For two scopes of the same aperture, if one is f/5 and one is f/10, then the f/10 must have a longer focal length, so the field of view will be smaller. Given the same object being viewed, in the f/10 scope, that object will, as you say, be spread out over more pixels, so will have better [potentially] resolution, but each pixel will have a worse signal to noise ratio. If a given pixel takes 5 minutes to be half full of photons in the f/5 scope, if the same number of photons are spread out over two or more pixels in the f/10 scope, it will take longer for the pixels to become 50% saturated. I've no idea of the maths to validate if an f/10 is teice as slow as an f/7, but it is slower.

That's my limited understanding.

James

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damnut

damnut

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Hi There,

Below copied from earlier thread!

Cheers

Damian

The f number is a photographic term and if you did visual only it may be considered to be of little relevance, not quite irrelevant but less of a concern.

A 150mm scope of f/10 will collect the same amount of light as a 150mm scope of f/5. The aperture of both is the same 150mm.

A 150mm scope of f/10 has to have a longer focal length then a 150mm scope of f/5.

The focal length determines the size of the image formed at the focal plane, basically (but not quite) double the focal length and the image size doubles etc. So the 150mm f/10 scope produces an image that is twice as big as the one formed by a f/5 scope.

However the scopes have the same diameter so that image of the f/10 scope is dimmer then that of the f/5.

In photographic terms that means the exposure time has to alter.

If you stick an eyepiece in the way then it goes out the window.

A 150mm f/10 scope set up to give 100x will give an image the same size and brightness as a 150mm f/5 scope set up to give 100x.

There are characteristics associated with a fast scope and a slow scope, but for visual no real hard and fast rules. For imaging then it is more relevant.

f/6 is a little on the fast side, but there are faster. Fast scopes tend to need a bit more time spent on keeping them set up, but they are shorter (aperture being the same) so a little easier to move round.

My personel preference is for something around f/7 in either refractor or reflector. Although practicality has to come into it - an f/7 200P reflector puts the focal plane at a reasonable position, however if 300mm it is too far if you had to observe near the zenith. So for a 300mm reflector you very likely have to conside a scope no slower then f/5 - otherwise you cannot look through it at times.

I will say be a little wary of f number, it is bounced around as if it is the most imortant part of a scope, it's not. Have been to exhibitions where I was told several times the f number, when I asked a bit more the guy knew the diameter and the f number and just about nothing else. It does however sound good.

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Andy Mitchell

Andy Mitchell

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There are two distinct cases you need to think about.



1) Galaxies, nebula etc.


The larger the aperture of your telescope the more photons you collect in a given time. But, the longer the focal length the bigger you make it look so you spread those photons out further and it looks dimmer. So in this case the f number (defined by the ratio of appeture and focal length) matters. If you have a fast scope nebulas and galaxies will look brighter in the eyepiece and exposure times will be less.



2) Single stars


For any practical amount of magnification it doesn't matter, it still looks like a tiny point. The effective resolution of the optical system (telescope, camera sensors, human eye etc) can't resolve the difference in size. Thus your f number doesn't matter one bit for brightness, just the apeture.


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dementedgr

dementedgr

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So it IS a FOV thing! So let's assume I have 2 images of say a galaxy and one has twice the FOV,

couldn't I just use software and "merge" the values of 4 pixels into one and get the same brightness? (kind of like post-process binning them)

(Not that I know of any software that does that or whatever, just asking...)

It somehow seems a bit of a marketing thing to me!

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jambouk

jambouk

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That is called binning; you will lose resolution, but gain in "brightness". No idea what happens technically to the signal to noise ratio as you still have 4 pixels (2x2) each of which will be susceptible to noise...

What issue are you trying to solve / address?

James

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Andy Mitchell

Andy Mitchell

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Each pixel will produce as much noise, but as noise is an uncorelated signal totalling it will on average be much less than 4 times as bad. Think it would work out at double as the formula is something like you square the amplitude of each noise sources, add them all together, and take the square root.

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ronin

ronin

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The F number is simply a reference to camera's and the exposure times that occurred when taking a photograph.

If you got a picture of the correct result on a 50mm lens of F/6 and exposure of 1/250 sec then you should get the same result on a lens of 100mm f/6 and 1/250 second. Although the focal length has altered the same exposure time and the same f number arethe items that are "important". The focal length just gives a bigger or smaller image.

There are a few other "features" that generally go with the f number but the f number is a photographic term.

Someone at some time has applied it to scopes (sounds good = makes the reader think we know somehing) and it has stuck.

The "problem" of a fast scope tends to be coma on a reflector, CA on a refractor.

But these are as much mechanical as anything.

Neither a parabola surface of a mirror or the spherical surface of a lens at the ideal so the errors caused by this "machanical" aspect start to show up more and show up most on fast scopes then on slow scopes.

If you want to get some idea of f number then search out photography and the effect of the f number on depth of field, depth of focus, exposure times. The book will also have to refer to manual lens. Not to the use of present day cameras and lens that attempt tp do it all for you without your intervention or knowledge.

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Charic

Charic

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dementedgr........Hi, Focal ratios are not printed on a telescopes label plate as it's generally not of importance, especially for visual work.  The focal ratio is the result or ratio of dividing the telescopes focal length by the same telescopes aperture.


In photography it is a measure of the 'speed' of the lens, which is important to know for photographers. A lens with a greater focal ratio will project a darker image! Any aperture difference or change you make,  introduces the inverse square law. The brightness of your image decreases with the square of the f-number. ie when using  a flash gun, if you double the distance to your photographic model, you need 4x the power to maintain the same measured illumination , otherwise the luminance of the target reduces by four times, requiring four times the exposure! Thats the Inverse square Law. In cameras you adjust the focal ratio (unless its in 'P' mode?)  to control the aperture (the amount of light entering the lens) and to control the depth of field.


Now the telescope. The f/number or focal ratio of the telescope is not listed, as stated above, you have to calculate this number. The principles are the same but work differently. Where the camera's focal ratio varies the amount of light and the depth of field [the area of 'In-focus] between foreground and horizon, these issue are not adjustable in your telescope. Therefore the  focal ratio in your telescope only tells you the speed of the telescopes optics, without actually allowing you to change anything. The smaller the focal ratio, the lower the telescopes magnification that can be achieved, and the wider the field of view and brightness of the image. All this depending on the eyepiece one uses.


As the focal ratio number reduces, so the speed goes up. So my f6 telescope is slower than the telescope with f/4.8 ratio. Faster f/ratio telescopes are generally  better for low power user's, wide field observing and deep space photography. whilst the slower focal ratios are better suited to higher magnification, ie.  work on the Lunar surface, any planetary work and Stars. To me, my f/6 telescope sits between fast and slow, and is easier to use and easy on my eyepiece choices.


This is more or less how I see f/ratios in telescopes, but as I'm not adept (yet) at astrophotography, I tend not to bother too much with it. The ratio just  tells me the speed of the telescope, and,  fast or slow, determines its end use?

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bingevader

bingevader

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I understand that f ratio is meaningless visually,

Not so.

Faster 'scopes mean better corrected EPs = more expensive!

Also, there's the size issue.

As people have already stated, for the same f ratio, a larger mirror will require a longer focal length.

To keep 'scopes at a practical height, without the requirement for a large step ladder, the f ratio may decrease as the mirror size increases.

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dph1nm

dph1nm

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2) Single stars

For any practical amount of magnification it doesn't matter, it still looks like a tiny point. The effective resolution of the optical system (telescope, camera sensors, human eye etc) can't resolve the difference in size. Thus your f number doesn't matter one bit for brightness, just the apeture.

Not if the star is resolved. i.e. your pixel size is smaller than the instrumental/atmospheric seeing. Then stars behave just like galaxies. Double the focal length and they cover twice the area.

NIgelM

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Moonshane

Moonshane

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I think cameras and scopes are quite different as with a camera, you have a variable aperture (the iris) which is why the focal ratio varies.

as above, these elements are fixed in a telescope but you can buy different scopes with different parameters for different applications.

as a visual astronomer it matters little to me so in honesty I have not got my head around how it works.

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