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Need advice on f/number


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The 'F' stands for the focal-ratio of the telescope. This is important if you plan to use the telescope for astro-photography. The lower the 'F' number, the lesser amount of time it will take you to capture a picture with a camera. For visual use, it will tell you that you'll see a small amount of sky through the scope. An 'F4' scope will give you a wider swath of sky through it than an 'F8' telescope. Also the higher the 'F' - the greater mount of magnification an eyepiece will yield. This as the focal-length of the telescope equals the the 'F' number like this:

At F8 your focal-length would be 8 X the size of the lens or primary-mirror. Take a 100mm telescope at F8 is 8 X 100mm = 800mm. Focal-Length. Now take a 10mm eyepiece and divide it by 800. So 800mm / 10mm = 80X. Now in an F4 scope with a 100mm primary-mirror: 100mm X 4 = 400mm Focal-Length so a 10mm eyepiece will do this: 400mm / 10mm = 40X.

Hope this helps!

Dave

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1. - bigger aperture, more light gathering.

As for the 900/960 focal lengths, you'd get similar magnifications for a given set of eyepieces.  And the 'scopes would be of similar size.

"Faster" 'scopes (lower f-numbers) are generally required for photography, and they also "test" eyepieces more, that is need better quality EPs.

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8 minutes ago, Dave In Vermont said:

The 'F' stands for the focal-ratio of the telescope. This is important if you plan to use the telescope for astro-photography. The lower the 'F' number, the lesser amount of time it will take you to capture a picture with a camera. For visual use, it will tell you that you'll see a small amount of sky through the scope. An 'F4' scope will give you a wider swath of sky through it than an 'F8' telescope. Also the higher the 'F' - the greater mount of magnification an eyepiece will yield. This as the focal-length of the telescope equals the the 'F' number like this:

At F8 your focal-length would be 8 X the size of the lens or primary-mirror. Take a 100mm telescope at F8 is 8 X 100mm = 800mm. Focal-Length. Now take a 10mm eyepiece and divide it by 800. So 800mm / 10mm = 80X. Now in an F4 scope with a 100mm primary-mirror: 100mm X 4 = 400mm Focal-Length so a 10mm eyepiece will do this: 400mm / 10mm = 40X.

Hope this helps!

Dave

Thanks dave

What would you suggest then 

6 or 8?

 

Edited by Mayank
minor Mistake
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4 minutes ago, cloudsweeper said:

1. - bigger aperture, more light gathering.

As for the 900/960 focal lengths, you'd get similar magnifications for a given set of eyepieces.  And the 'scopes would be of similar size.

"Faster" 'scopes (lower f-numbers) are generally required for photography, and they also "test" eyepieces more, that is need better quality EPs.

Thanks cloudsweeper..

What would u suggest then 6 or 8?

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For astro-photography - I'd want the F6. For visual observing - I'd take the F8 if I planned to use it on small objects such as planets and comets. If I wanted to view the majestic, wide starfields in the Milky Way, I'd opt for the F6. So it depends on what you want the scope for.

Dave

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I suspect, and no disrespect meant here, for thequestions being asked I doubt complex imaging is going to take place, so I would go with the scooe with greatest aperture as I suspect visual is more likely to be undertaken, and even if imaging is attempted, the 150mm scope will still allow this as long as focus can be achieved depending on the camera to be used.

so i'd say the 150mm scope.

james

 

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F-5 and lower is a regarded as a "fast" focal ratio suitable for imaging the deep sky. F-10 and higher is regarded as a "slow" focal ratio and these sort of scopes tend to be used for viewing and imaging solar system objects. Planets and moon give off a lot of light so short exposure photography is possible - but also you get a longer focal length which gives sharper focusing of brighter, closer objects. Fast focal ratios tend to be a bit more demanding on eyepiece quality (and expense). Hth :)

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The bigger aperture of the 150 should also have a better resolution - a Dawes limit of 0.77 arcsec for the 150mm, rather than 0.97 arcsec for the 120mm. 

I've attached example fields of view, 25mm eyepiece, looking at M45 (From astronomy.tools ). There's not a huge difference.

I'd choose the 150 for visual. f/6 isn't THAT fast. Better resolution, better light gathering.

.astronomy_tools_fov.thumb.png.b6b60d079e

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What are the two telescopes you are comparing?

If they are the same basic design, reflectors for example, I would go for more aperture. 150 versus 120 is a significant step up. However if you are comparing a 150 reflector with a 120 refractor then the difference would be minimal.

There are other factors which may also determine your choice: quality of construction, smoothness of focusing, parabolic versus spherical mirror in a reflector etc. Guarantee and after-sales service is another consideration.

Also, don't overlook the mounting. Dobsonian, alt-azimuth, equatorial etc. The more solid the mount, the better the viewing!

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I'm afraid the people saying the f-ratio is important for the photographic speed are not correct.

This mattered in the old days, when we were using film, but now pretty much everyone uses CCD. Which means we have control over the ratio of focal length to pixel size we never had with film, and we can adjust this so an F8 scope gathers as many photons per pixel as an F4 one does.

The advantage of a 'faster' scope, visually, are its lighter, shorter (hence easier to mount), and it's a bit easier to use for low power/wide field views (very long focal length eyepieces have their own problems, and there will be limits on how wide a field of view you can get). If you want to view planets, you will probably be using a barlow anyway.

The advantage of a 'slower' scope is usually cost - its easier to make a longer focal ratio lens than a shorter one. With a reflector, some of the aberations like coma and astigmatism are reduced the longer the focal ratio. They also have a smaller central obstruction, so give better planetary views.

You don't say if the two scopes are refractors or reflectors (or a mix). That's probably a lot more important than F-ratio, depending on what you want to observe and how experienced you are.

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7 minutes ago, theastrodragon said:

I'm afraid the people saying the f-ratio is important for the photographic speed are not correct.

This mattered in the old days, when we were using film, but now pretty much everyone uses CCD. Which means we have control over the ratio of focal length to pixel size we never had with film, and we can adjust this so an F8 scope gathers as many photons per pixel as an F4 one does.

The advantage of a 'faster' scope, visually, are its lighter, shorter (hence easier to mount), and it's a bit easier to use for low power/wide field views (very long focal length eyepieces have their own problems, and there will be limits on how wide a field of view you can get). If you want to view planets, you will probably be using a barlow anyway.

The advantage of a 'slower' scope is usually cost - its easier to make a longer focal ratio lens than a shorter one. With a reflector, some of the aberations like coma and astigmatism are reduced the longer the focal ratio. They also have a smaller central obstruction, so give better planetary views.

You don't say if the two scopes are refractors or reflectors (or a mix). That's probably a lot more important than F-ratio, depending on what you want to observe and how experienced you are.

Tysm

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6 hours ago, Ricochet said:

If you're still building your own with a spherical mirror f8 as I think f6 needs a parabolic mirror. 

Yes I am on a DIY..but why so...preference of parabolic mirror is always higher..i know that...but as you state..IT NEEDS A PARABOLIC MIRROR.. 

Will spherical mirror cause trouble? 

Thanks 

Mayank 

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A parabola is the shape that will cause all the rays of light to converge at a single focus point.  I think that with a spherical mirror there will be a range of focal points that lead to a blurry image, but with a long enough focal ratio (f8) a spherical mirror approximates a parabola closely enough that you can get a sharp image.

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A spherical mirror cannot bring parallel rays to a single point. However, you can "get away with" a spherical mirror if the difference between the sphere and parabola is less than 1/4 wave. I think the limit is about 4" f/10.

I'm afraid f-ratio *is* important for photography, since for two 'scopes of the same focal length a f/4 scope will collect 4 times as many photons as a f/8 'scope, hence needing only 1/4 the exposure for the same S/N.

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23 hours ago, DaveS said:

I'm afraid f-ratio *is* important for photography, since for two 'scopes of the same focal length a f/4 scope will collect 4 times as many photons as a f/8 'scope, hence needing only 1/4 the exposure for the same S/N.

This isn't the thread to get bogged down into this long-standing "discussion", and I am certainly not the one to have the debate as I am far too thick when it comes to optics, but... if two scopes have the same focal length, and one is f/4 and one is f/8, then of course the f/4 will collect more photons as its aperture will be twice that of the f/8 scope, and its light collecting area will be four times that of the f/8 scope.

James

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31 minutes ago, DaveS said:

Yes. I was actually correcting an earlier post that said f-ratio wasn't important in digital AP.

It isn't. Your 'correction' is in fact wrong.

The number of photons collected/second has nothing to do with the focal ration per se, its defined by the focal length of the telescope and the size of the pixel on the CCD.

If we have a 150mm F4 scope, and a CCD with 5micron pixels, and a 150mm F8 scope on a CCD with 10micron pixels, they get the same number of photons/second into the CCD pixel bucket. For imaging, they are equally fast.

The 'longer focal ratio scope is slower' argument ONLY applies if the CCD is already fixed, AND it cant be binned in software (very unlikely). Depending on the CCD, a longer focal ration scope can be FASTER.

The 'F4 is 4 times as fast as F8' argument is a holdover from film, where the effective resolution is fixed and unchangeable, and you didn't have fancy software to fiddle with things.

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47 minutes ago, DaveS said:

Yes. I was actually correcting an earlier post that said f-ratio wasn't important in digital AP.

DaveS, sorry, I hate to be argumentative, but the information you posted is giving the wrong message to anyone new to astronomy. If focal length is fixed, the only way to adjust the focal ratio is to alter the aperture, and therefore it is not fair to compare the focal ratios in terms of "speed of acquisition of data" as there are now two variables, aperture and focal ratio.

I suspect you meant to say "in two telescopes of equal aperture, one f/4 and one f/8..." in this regard it is fair to compare focal ratios, but as the astrodragon above has indicated, if one is able to alter the pixel area / use binning, then one can make adjustments to combat the rumored benefits of faster scopes [at the expense of "resolution"].

James

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Gents,

Let's not get derailed by a discussion which I doubt is relevant to the OP. I may be wrong but I believe the OP requirement is for visual, so not related to imaging 'speed'

Feel free to start another thread to continue the discussion which of course is relevant to imagers.

Thanks!

Stu

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