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skrap1r0n
2004-Mar-25, 06:10 PM
I am comparing a 10" dob to an 8" dob.

heres what has me confused

8" =1200mm Focal Length, f/5.9
10" = 1200mm Focal Length, f/4.7

How does the f-ratio affect the scope? All I seem to find when googling is referring to f stop on cameras.

Any one have an explanation or advice??

2004-Mar-25, 06:15 PM
I am surely wrong on this; The focal ratio is the distance between the mirror and the eyepiece. The larger the focal ratio, the higher magnification you can use... #-o

slbuczkowski
2004-Mar-25, 06:21 PM
focal ratio functions the same in cameras and telescopes and is defined as the focal length / aperture diameter. Smaller f/stop numbers mean the lens/optical system lets more light reach the detector (i.e. the image is brighter)

In your example, the 10" dob would have somewhat brighter images at the viewfinder and, if you plan to do any astrophotography, would lead to shorter integration times.

tracer
2004-Mar-25, 06:28 PM
Astrophotography with a Dobsonian?

skrap1r0n
2004-Mar-25, 06:28 PM
ok so to understand, the LOWER, the f/ratio the more you will resolve?

Glom
2004-Mar-25, 06:42 PM
f-stop is used to measure non-time dependent exposure. Exposure is all about how much light you let into the thing being exposed. Obviously how long you make the exposure is one factor, which of course affects photography, but that isn't so important just for viewing.

Static exposure depends on two things. First, the size of the aperture. The bigger the primary mirror or lens, the more light you collect each unit time. But aperture alone isn't sufficient to judge exposure, because of the second factor. Second, focal length. Focal length determines your view cone. The bigger the focal length, the skinnier the cone. This is pretty intuitive since longer focal lengths mean more zoom and hence a narrower field of view. But this means less light as well.

So the rule is that bigger aperture means more exposure, smaller focal length means more exposure. Therefore, the best way to summarise the static exposure is to use a ratio of the two, called the f-stop or f-ratio. f-stop is focal length over aperture. This is the sole measure of static exposure on cameras. It is done this way because focal lengths tend to be longer than apertures so doing it this way means the numbers are greater than one, which is more pleasing, and it means that you can use integers more. Of course, it means that higher f-stops mean lower exposures.

I believe as a rule, higher quality optics are found in higher focal ratios. Telescopes that are too fast (low focal ratios) tend to not be as neat in their images.

Glom
2004-Mar-25, 06:58 PM
I am surely wrong on this; The focal ratio is the distance between the mirror and the eyepiece. The larger the focal ratio, the higher magnification you can use... #-o

I think you're referring to the formula for magnification. That is magnification is the focal length of the objective over the focal length of the eyepiece.

The rule about what magnifications you can use I believe is aperture rule. That is the maximum practical magnification you can use on a telescope is 50 times its aperture in inches and twice its aperture in millimetres. This rule is due to limitations in light collected. I believe it works up until you get into the 600X range when things get weird due to atmospheric affects and such.

This rule is golden in watching out for crap-o-scopes. It is common for 4˝'' Newtonians to be presented in colourful boxes with specs like 375X splayed all over it as a way of saying how good it is. By aperture rule, the maximum practical magnification for a 4˝'' is 225X. Beyond that, images will be fuzzy and faint to the point of not being nice. I know this because I own one :-# It was advertised like this because it has a 500mm focal length and comes supplied with a 4mmSR eyepiece and a 3X Barlow lens. With the SR, magnification factor is 125X, which, when the Barlow lens is added, becomes 375X. But, I never use the telescope with the SR and the Barlow in combination because the images are unuseable. I prefer the slightly smaller images from the SR alone (although since joining AstroSoc and using proper telescopes, using my crap-o-scope seems such a come down. I'd like to think that buying a crap-o-scope as a newbie toy is a rite of passage, but it seems a lot of people are smarter than me and bought decent ones first off.).

Of course, advertising in terms of magnification is the major hallmark of a crap-o-scope, whether or not the magnification value is practical. Telescopes should be advertised in terms of aperture and focal ratio.

Glom
2004-Mar-25, 07:00 PM
ok so to understand, the LOWER, the f/ratio the more you will resolve?

Hmm. Well theoretical resolution is determined purely by aperture, but I doubt we're on that level, when it comes to amateur telescopes. The rule is that faster telescopes will produce brighter images under a given magnification. I believe resolution is purely a product of aperture and quality.

Wally
2004-Mar-25, 07:03 PM
ok so to understand, the LOWER, the f/ratio the more you will resolve?

Actually, a telescope with a higher f-ratio means it has a longer focal length. this in turn means you'll obtain a higher magnification for a given eyepiece. all this because f-ratio is simply focal length divided by aperture. And magnification is focal length of the scope divided by focal length of the ep. Bottom line, f-ratio really doesn't mean much in visual astronomy. Just concentrate on aperture and type of scope you want to buy.

frogesque
2004-Mar-25, 07:24 PM
Just had a quick scan of this site but it seems to be OK for basic interpretation of the terms used without being too technical.

http://www.absolutebeginnersastronomy.com/math.html

skrap1r0n
2004-Mar-25, 07:26 PM
I have the answer (I called the astonomy shop :D ).

Both scopes have a 1200mm focal length. In order to keep the scopes the same length, the f/ratio on the 10" has to be lowered. Its a portability issue.

Glom
2004-Mar-25, 07:28 PM
Portability is the reason (besides the fact it would also be expensive) we don't all go out and buy 30'' Dobs. What's going on is that the telescopes have certain fixed focal lengths, but one has a larger aperture and hence by the formula, that gives a lower f-ratio.

slbuczkowski
2004-Mar-25, 07:57 PM
Astrophotography with a Dobsonian?

Yes, perhaps not the best example. :D

(although I have seen it done.)

Wally
2004-Mar-25, 08:22 PM
I have the answer (I called the astonomy shop :D ).

Both scopes have a 1200mm focal length. In order to keep the scopes the same length, the f/ratio on the 10" has to be lowered. Its a portability issue.

You don't "lower" an f-ratio persay. Again, it's simply the focal length of the scope divided by the aperture. You're likely correct in that the manufacturer decided to make both scope the same focal length so the 10" could remain portable. Doing so results in a lower f-ratio. Splitting hairs here probably, but just want to make sure you understand it's simply a ratio of the focal length and the scopes aperture.

skrap1r0n
2004-Mar-25, 08:55 PM
Quite right Wally, Lowered- should have been Lower for it to be technically correct.

As for astrophotography with a Dob? Well Thats part of my long range plan.

I figure I'll go the Webcam/Registax route. If that doesn't work...Well Necessity is the mother of invention. (or I'll put the damn thing on an equitorial tripod)

Sam5
2004-Mar-26, 03:18 AM
How does the f-ratio affect the scope? All I seem to find when googling is referring to f stop on cameras.

The lower the number, the brighter the image.

I’m trying to remember this from my old manual photo days... f 4 on a camera lens will let in twice as much light as f 5.6

If you change your camera lens setting from f 4 to f 5.6, then you need to change the shutter speed to double the shutter opening time, like from 1/4 second to 1/2 second, to let the same amount of light reach your film.

If the telescopes follow the same general rule, it seems to me that f 4.7 will give you almost twice as bright an image as f 5.9 Actually, not quite twice as much.

This should allow you to cut your astro photography exposure time almost in half with the f 4.7, or use almost double the magnification with the 4.7 for the same brightness of the image you would get with less magnification on the 5.9 scope.

bmillsap
2004-Mar-26, 02:25 PM
If you change your camera lens setting from f 4 to f 5.6, then you need to change the shutter speed to double the shutter opening time, like from 1/4 second to 1/2 second, to let the same amount of light reach your film.

Remember, though, on a camera changing the f-stop really means changing the aperture since the focal length is fixed (for a given lens, forgetting zoom or anything like that). So while it's true that f/4 lets in more light than f/5.6, it's not because of the f-ratio per se but because the lower f/ratio results from increasing the aperture for a fixed focal length.

Sam5
2004-Mar-26, 03:37 PM
If you change your camera lens setting from f 4 to f 5.6, then you need to change the shutter speed to double the shutter opening time, like from 1/4 second to 1/2 second, to let the same amount of light reach your film.

Remember, though, on a camera changing the f-stop really means changing the aperture since the focal length is fixed (for a given lens, forgetting zoom or anything like that). So while it's true that f/4 lets in more light than f/5.6, it's not because of the f-ratio per se but because the lower f/ratio results from increasing the aperture for a fixed focal length.

With a telescope there is no iris to close down, and the “aperture” is always wide open. Like on a camera lens, a “wide open” aperture f 1.4 will let in about twice as much light than a “wide open” f 2.8 lens of the same focal length.

Since the focal lengths of the telescopes are the same, but the mirror diameters are different, the “wide open” aperture of the 10 inch will produce a brighter image than the “wide open” aperture of the 8 inch.

aurora
2004-Mar-26, 10:45 PM
In practical terms, for visual observing, there's only a few differences between otherwise identical scopes that have mirrors with different focal lengths.

A reflector with a higher f ratio is slightly less sensitive to miscollimation, and slightly easier to collimate, than a scope with a lower f ratio.

A reflector with a higher f ratio will have a longer tube, and therefor be less portable.

Using an identical eyepiece, the scope with the higher f ratio will provide a higher magnification view. This is why scopes with small f ratios are often called "rich field" scopes. Of course, identical magnifications can be obtained in the two scopes by using eyepieces with different focal lengths.

For those that grind their own mirrors, it takes a lot of work to get a mirror to a small f ratio because you have to remove a lot of glass.

As with everything else in selecting a scope, it is all about tradeoffs.

Tensor
2004-Mar-26, 11:22 PM
In practical terms, for visual observing, there's only a few differences between otherwise identical scopes that have mirrors with different focal lengths.

A reflector with a higher f ratio is slightly less sensitive to miscollimation, and slightly easier to collimate, than a scope with a lower f ratio.

A reflector with a higher f ratio will have a longer tube, and therefor be less portable.

Good points Aurora. In Refractors, a longer f ratio scope will have less Chromatic Aberation than a smaller f ratio.

As with everything else in selecting a scope, it is all about tradeoffs.

Truer words have never been spoken.

JohnW
2004-Mar-27, 12:23 AM
Using an identical eyepiece, the scope with the higher f ratio will provide a higher magnification view. This is why scopes with small f ratios are often called "rich field" scopes. Of course, identical magnifications can be obtained in the two scopes by using eyepieces with different focal lengths.
The corollary to this is that the scope with the lower f ratio will provide a wider field of view. With an identical eyepiece, you'll see more of the sky. Not a bad idea if you're learning to find things.

ChesleyFan
2004-Mar-27, 04:34 PM
I should point out that the smaller the f/ratio, the more difficult it is correct aberrations created when the light passes through the lenses/bounces of the mirrors of the scope. If it's poorly corrected, you'll get stars that look like ovals or tiny comets, etc.

In refracting telescopes this is usually corrected using triplet lenses with exotic types of glasses or coatings (flourite triplets come to mind) and will cost you primo money. In a Dob a smaller f/ratio means you have to be very vigilant in collimating the scope.

skrap1r0n
2004-Mar-27, 11:56 PM
I'll have to learn how to collimate the scope sooner or later. I think I am going to go with the 10" I have never had anything but refractor scopes before and usually got frusrated with them because they were pretty low quality.

I have put myself on an 8 month savings plan for the Orion 10" dob and will make due with Binoculars until then

milli360
2004-Mar-28, 11:29 AM
With a telescope there is no iris to close down, and the “aperture” is always wide open. Like on a camera lens, a “wide open” aperture f 1.4 will let in about twice as much light than a “wide open” f 2.8 lens of the same focal length.
Not twice. Four times. If you were to go from f/1.4 (notice the slash--it's a reciprocal), to f/2.8, you'd have to quadruple the exposure time.

Since the focal lengths of the telescopes are the same, but the mirror diameters are different, the “wide open” aperture of the 10 inch will produce a brighter image than the “wide open” aperture of the 8 inch.
That's true no matter what the f/ ratios, as far as stars are concerned, because their images are essentially points. For instance (reversing the values from the OP), a 10" f/5.9 would still let in more light and give a brighter star image than an 8" f/4.7. That doesn't follow for extended objects because the size of the image also changes, as others have noted.

Charlie in Dayton
2004-Mar-29, 08:38 AM
I look at all the explanations given, and everyone's talking camera lenses and exposures and brightnesses, but I don't really see the answer to the question: "How does the f-ratio affect the scope?" until you get down to JohnW's post...

f-ratio is the ratio between the distance it takes to focus the light to a point (with either a lens(es) or a concave mirror ) and the aperture of the telescope/diameter of the primary mirror (for purposes of this argument, they're identical).

Let's take a common little scope like my basketball-and-smokestack Bushnell Voyager. It has a 105mm aperture, and a 445mm focal length. 445/105 = 4.2. That means the focal length of the mirror is 4.2 times as long as the aperture is wide.

Let's take my Orion SpaceProbe 130. The aperture is 130mm, the focal length is 900mm. 900/130=6.9. The focal length is 6.9 times as long as aperture is wide.

Let's take my old Tasco. The aperture is 76mm, and the focal length is 700 mm. 700/76=9.2. The focal length is 9.2 times as long as the aperture is wide.

There is a relationship here. The longer the focal length, the narrower the view. Cut a 1/2" piece off a soda straw, and look through it. Now look through the rest of the straw. Heckuva difference, ain't it? The apertures are the same, the (apparent) focal lengths are different. Here's why scope
manufacturers make differing f-ratio but identical aperture scopes. Want a great wide angle light bucket? That 8" f4 scope will fill the bill quite nicely. Going solely for the planets? That 8" f8 will do you better.

"Fast" (low f-ratio) scopes (reflectors AND refractors) have short f-ratios -- and by the nature of a short ratio, MUST bend light more than a fast ratio. Remember, the f-ratio is an expression of the length necessary to focus the light to a point (that point being just inside the eyepiece of the scope).

Short f-ratios mean the focal length approaches the value of the aperture, which means greater bend angles. (To grossly oversimplify things, imagine an isosceles triangle with a height twice its base.The base represents the aperture, the height represents the focal length. As you stretch the triangle longer, the angles made by the sides with the base get larger -- which means the angles made by those sides with the verticals representing the telescope tube get smaller. Those are the angles that represent how much the light must get bent to reach focus. Draw it out and see for yourself.) The less bend, the less chance for error.
Less of a bend of the light means less distortion introduced, both in focus and in colors -- different colors' wavelengths focus at different distances. It's just enough to make noticeable differences with a faster-than-a-certain-value focal length. You'll see color aberrations that aren't there in longer f-ratio systems, especially in refractors.

So...want a wide-angle viewer? f5 or less.
So...want a narrow-angle viewer? f5 or more.
f5 seems to be the commonly-accepted dividing line between a 'fast' and a 'slow' ocular.

Got it all figgered out? Good. Practical exercise.

492mm aperture scope. 7239mm focal length. What's the focal ratio, is it a wide angle or deep sky instrument, and don't you wish you could get your grubby little paws on it?

http://www.unavowed.net/charlie/VariousesForTheWeb/MeBigScope50pct.jpg
Focal ratio = f14.7
Deep sky narrow angle viewing instrument (we're working on a video system with a 6mm-equivalent electronic eyepiece -- that gives 1206x magnification! -- our big problem now is focus and collimation)
...heh heh heh...

Alan
2004-Mar-29, 03:48 PM
As for astrophotography with a Dob? Well Thats part of my long range plan.

I figure I'll go the Webcam/Registax route. If that doesn't work...Well Necessity is the mother of invention. (or I'll put the damn thing on an equitorial tripod)

Here is a page with photos taken with a webcam through a dob and stacked.

http://www.comedyentertainment.com/toucamimages.htm

mike alexander
2004-Mar-29, 05:45 PM
As ChesleyFan points out, one problem with a scope of low f ratio is that aberrations increase more rapidly the farther you move off the central axis. Especially apparent with low-f paraboloidal mirors.

Interesting historical ploop: The Hale telescope on Palomar has a paraboloidal mirror at f 3.3, very fast. The original design required a special corrector lens to flatten the field and reduce the abberations. The problem was that for it to work the mirror and corrector lens had to be held colinear to 0.040" or less with a tube length of 55 feet when pointed in any attitude. The problem was that a 55 foot long structure weighing in at 100+ tons will always sag somewhat. The solution was the Serurrier truss design, where the ends do sag, but by the same amount.

In addition to the truss, mark Serurrier also invented the Movieola.