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'Fast' scopes and exposure time.


CanesVenatici
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Hi Folks,

Could anyone explain to me - in simple terms - why a small 'fast' scope gives shorter exposure times than a bigger but slower scope? On the face of it, it seems that a bigger scope would collect more photons than a smaller scope over an equal period of time and so build up an image more quickly, so I am obviously missing something!

Cheers! ;)

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I think you're just a bit mixed up with the terminology (bigger v smaller). A fast scope is one that has a lot of light-gathering capability. The direct measurement of this is the f number. A fast f5 scope will gather light quicker than a slow f10 scope.

The f-number is the ratio between focal length of the scope and diameter of primary mirror or lens. So, a scope with a longer focal length (smaller field of view) needs a correspondingly bigger primary to maintain the same f-number.

For a given focal length (and telescope design), a faster scope will typically physically bigger than a slower scope.

James

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Fast telescopes have focal ratios below F/6 and slow telescopes tend to come in at F/10 and greater. While fast telescopes are more compact and have wider fields of view, they also demand much of the eyepieces and require exact collimation to get good views. It's also more difficult to make truly good optics for fast telescopes than slower telescopes in general, at least when refractors and Newtonians are concerned. Catadioptrics have very fast primary mirrors and complex corrector plates that require very accurate surfaces to perform properly. Slower telescopes work well with less complex eyepieces, are more tolerant of miscollimation and are in general easier to make to a high standard of quality. Against the advantages, a slow telescope can be very long, and bulky and they are not practical for telescopes over 8 or 10-inches in aperture unless you place them in an observatory or have a large vehicle. You wouldn't want to haul a 24-inch F/8 Dob around. So for really large apertures, F/5 and below are the only realistic options because otherwise you would need a massive ladder or a scissor lift to get to the eyepiece when aimed high in the sky. In general, slower telescopes are usually best for planetary, lunar and double stars, whereas the faster telescopes are more suited for deep sky objects such as nebulae and galaxies. But this is not set in stone, if the optics are of high quality they are up to any observing challenge you take on.

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A 3" diameter lens will collect the same amount of light whether it's a f5 or f10 design.

With the lower f ratio it concentrates the light from extended objects and gives a brighter image for diffuse objects (ie nebulae etc), but the amount of starlight (from a pinpount source) recorded is exactly the same as an f10.

So "fast" lenses/scopes are popular for Deep Sky Imaging where the focus (pun intended!) is on extended nebulae and wider fields of view.

HTH

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I think you're just a bit mixed up with the terminology (bigger v smaller). A fast scope is one that has a lot of light-gathering capability. The direct measurement of this is the f number. A fast f5 scope will gather light quicker than a slow f10 scope.

James

I am still a bit confused. I have a 'large' (relatively speaking) 8" F 9.8 VMC 200L. From what you seem to be saying that has less light gathering capacity than a small 70mm F5 APO. ;)

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A 3" diameter lens will collect the same amount of light whether it's a f5 or f10 design.

So far so good. In turn, an 8" F10 design will collect more light than a 70mm F5 design. Yes?

With the lower f ratio it concentrates the light from extended objects and gives a brighter image for diffuse objects (ie nebulae etc), but the amount of starlight (from a pinpount source) recorded is exactly the same as an f10.

So, if I understand correctly, what is important is that a 'fast' scope effectively magnifies less than a slower scope? (A 1000mm focal length scope with a 20mm eyepiece gives a magnification of 50x, whilst a faster, 500 mm focal length scope with the same eyepice gives a magnification of 25x).

I am not sure, but isn't it the case that when one uses a high magnification eyepiece, one is effectively looking at just a small part of the image that is being reflected by the primary mirror, 'spreading the light out' to make it look bigger, but in turn also making it dimmer?

To add to my speculation, given that imagers often don't use eyepieces, what would seem to be important is having a light cone that fills the chip of the camera being used, which I am also guessing is a larger area than the human retina, and so demands less magnification?

Or something like that! ;)

Edited by CanesVenatici
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First question - Yes - Aperture in King when it comes to collecting light.

Second question.....

You're mixing visual with imaging.....

The "fast" scopes are considered when imaging - as I said they give better "concentration" for deep sky objects.

For visual is doesn't matter as much. The magnification and field of view, as well as the background light pollutuion all come into play...A 10" f5 will give you the same views as a 10"f8 - IF you use the same magnification and type of eyepiece.

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Hi Merlin,

Thanks for your patience!

Ok, so we are agreed that aperture is king when it comes to collecting light, and collected light is what we need to make a good image, even though imagers are able to gather light both by virtue of aperture AND over time.

If you could explain further, what do you mean by 'concentration'. Is this 'concentration' a function of the magnification given by the scope?

If the actual number of photons being collected by the objective is the same however slow or fast the scope is, why exactly does the exposure time needed vary between the two?

;):icon_scratch:;)

Apologies for my idiocy! :)

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I think of it like this

Image a window 2 foot by 2 foot.

The amount of photons hitting the glass will always be the same.

Imagine the concentration or F stop as how opaque the glass is. The more opaque the longer it will take to get the same image forming.

Not an exact anology, but works for me.

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yep - the concentration (photons per area of image receptor) is higher in the fast scope - wiith a larger image scale (arc seconds per pixel) even if the same total number of photons are captured.

so - from the image receptors point of view - the image is brighter and quicker to capture faint details with respect to background noise.

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I am not an imager but think my logic and understaning of this is correct:

if you image with a larger aperture, this will gather light more quickly.

if you image with two scopes of the same aperture but different focal lengths, the faster scope will have a larger 'exit pupil' using the same camera than the slower scope in a similar way to using the same eyepiece. a smaller 'exit pupil' means less light per square mm and therefore longer exposures.

the problems with larger apertures are longer focal length for a given focal ratio and weight which are harder to mount and control hence the preference for smaller faster scopes. also, smaller faster scopes can provide wider field images which helps with many subjects.

hope this is correct and makes sense but someone correct me if not! I'd be interested to understand why.

Edited by Moonshane
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I alway like to add the important consideration that point sources such as stars have a brightness that depends only on aperture and is not affected by F ratio or eyepiece choice (unless you use an EP with an excessive exit pupil).

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There are many calculators for this, I use this one:

The New CCD Astronomy Home Page

There you can play around with different apertures and focal ratios to see how it affects the field of view. Soon the relationship between these variables will become at least a little bit less confusing.

Start for example with a small scope in the calculator. Say 100mm aperture and F8. This telescope is 800mm long. Ok double the aperture to 200 mm, keep F8; now you have a scope 1600mm long. You will notice that the field of view is smaller with the big one, so although it's big it's effectively looking at a smaller part of the skies and that limits the light that can possibly reach it. So these two scopes will produce a similarly lit image using the same exposure time, but the big one covers less sky, so only thanks to its bigger lens does it keep up with the little one in producing an image as bright - and that's the F ratio for you!

/Jesper

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if you image with two scopes of the same aperture but different focal lengths, the faster scope will have a larger 'exit pupil' using the same camera than the slower scope in a similar way to using the same eyepiece. a smaller 'exit pupil' means less light per square mm and therefore longer exposures.

Given that both scopes collect the same amount of light, wouldn't it be the scope with the 'larger exit pupil' that had less light per square mm, as it was spreading the incoming photons over a larger area? ;)

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no, because the area from which the light is gathered is magnified more in the case of a longer focal length. imagine looking at the moon with two 100mm scopes (one f10 and one f5) and the same 10mm eyepiece. the area of the moon 'seen' by the scope will be twice as large in the f5 as it is in the f10 and therefore is by definition twice as bright in the eyepiece. if you replace the 10mm eyepiece with your DSLR chip then the effect is the same.

I'll gladly be proved wrong on this as whilst I am sure that I am correct I am no expert by any stretch.

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Start for example with a small scope in the calculator. Say 100mm aperture and F8. This telescope is 800mm long. Ok double the aperture to 200 mm, keep F8; now you have a scope 1600mm long. You will notice that the field of view is smaller with the big one, so although it's big it's effectively looking at a smaller part of the skies and that limits the light that can possibly reach it. So these two scopes will produce a similarly lit image using the same exposure time, but the big one covers less sky, so only thanks to its bigger lens does it keep up with the little one in producing an image as bright - and that's the F ratio for you!

/Jesper

And yet:

There is a prevalent misconception that short f/ratio 'scopes because they are photographically faster, yield brighter visual images, and are therefore preferable in the field of deep sky observation. Visual image brightness is governed solely by the diameter of the exit pupil, and has nothing whatsoever to do with the objective's f/ratio. The problem with using a 6-inch f/4 Newtonian say, for wide field RFT work on deep sky objects is its lousy off-axis performance. Unless the outfield is adequately corrected for the gross coma and astigmatism exhibited by an f/4 paraboloidal mirror, that outfield is unusable. Yet it is feasible to obtain a 2 degree field of view with an 8-inch f/8, and enjoy a fov to all intents and purposes free of coma and astigmatism without employing a coma corrector. The design is less prone to veiling glare, the focal plane is flatter, and because there is little off axis coma, even the outfield is passable. An f/4 telescope requires a 28mm focal length eyepiece to yield a 7mm exit pupil, and an f/8 requires a 56mm focal length eyepiece. All else being equal, image brightness will be the same in either telescope.
long vs short f/ratios

Confused? You bet!

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After all, both a fast and a slow scope of the same objective diameter collect exactly the same number of photons.

Precisely wrong ;-). The faster one is looking at more sky, at a wider field of view so draws more photons from that bigger region. And this wider field picture of the fast scope is still crammed into the same image chip, so each pixel is bombarded with photons from more sky and thus the resulting picture gets bright quicker.

Sure, on the chip a galaxy for instance appears smaller, but this means it's also brighter.

/Jesper

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Precisely wrong ;-). The faster one is looking at more sky, at a wider field of view so draws more photons from that bigger region. And this wider field picture of the fast scope is still crammed into the same image chip, so each pixel is bombarded with photons from more sky and thus the resulting picture gets bright quicker.

Sure, on the chip a galaxy for instance appears smaller, but this means it's also brighter.

/Jesper

Ah! I perhaps misinterpreted what Merlin meant when they wrote:

A 3" diameter lens will collect the same amount of light whether it's a f5 or f10 design.
Whatever, it does seem that this is an issue that causes a lot of confusion. As is shown by this other thread that I found on Cloudy Nights:

Telescope Reviews: Why are fast f/ratios brighter?

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Precisely wrong ;-). The faster one is looking at more sky, at a wider field of view so draws more photons from that bigger region. And this wider field picture of the fast scope is still crammed into the same image chip, so each pixel is bombarded with photons from more sky and thus the resulting picture gets bright quicker.

Sure, on the chip a galaxy for instance appears smaller, but this means it's also brighter.

/Jesper

The faster scope is not looking at more sky. It will show a wider field for any given chip or EP than another slower scope of identical aperture, but the same amount of light still comes in the front of the scope. By selecting different EPs to compensate for the focal length difference between the two scope, I can get identical views regardless of speed.

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I don't think that's right Ags. If you project the image of these two scopes with no eyepiece onto paper, the slower scope will project onto a piece of paper a smaller area of e.g. the moon than the faster scope. all an eyepiece does is magnify the image that's created by the focal ratio/aperture and this is why the image covers more area for a fast system and a smaller area for a slow system with the same eyepiece. plus in your analogy, the exit pupil is still smaller when the objects are the same size in the slower system and it would take longer to image the object.

that said, I wish someone would explain what happens when the photons travelling straight hit the same amount of glass area but with a different ratio. where do the 'other' photons 'go' in the slower system?

Edited by Moonshane
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Guys,

We could keep this discussion going forever!!

When we talk focal ratio and different focal ratio - when comparing scopes - bear in mind the two variables - aperture and focal length...which one do you keep constant for the discussion?? Camera lens have a variable focal ratio with a fixed focal length, telecopes usually have a fixed aperture and variable focal length....big difference.

I have a great book which certainly does a better job explaining the issue than me (and some of the stuff on the web!!) :

"Tools of the Astronomer", G.R. Miczaika & W.M. Sinton, Harvard, 1961

Give me a couple of hours, I'll scan the relevant pages and upload them.

I'm 100% sure when you read the explanation, you'll nod your head and agree.

Watch this space......

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I have a great book which certainly does a better job explaining the issue than me (and some of the stuff on the web!!) :

"Tools of the Astronomer", G.R. Miczaika & W.M. Sinton, Harvard, 1961

Give me a couple of hours, I'll scan the relevant pages and upload them.

I have a feeling that the 'simple explanation' I was hoping for will be a long time coming! ;)

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