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Anonymous
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What does it mean when a scope or mirror is fast? Or slow for that matter? I have seen these terms used quite a few times but the meaning eludes me.
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square_peg
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Reged: 03/26/04
Posts: 23950
Loc: Maple Valley, WA
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It refers to the focal ratio of the objective - focal ratio =focal length/diameter. Example a reflector with a 1200mm focal length and a 200mm objective has an F ratio of 1200/200 or F6. A fast scope is one with a small F ratio. Usually F5 and lower are considered fast scopes while over F7 - F8 or higher are considered slow. The advantage of a fast scope is a shorter lighter tube (or truss poles) and lower eyepiece height for big dobs. The disadvantage is that fast scopes require better corrected eyepieces (more expensive) to combat off axis astigmatism - flaring of point sources out of the center of the field of view and they are more sensitive to slight collimation errors. Slower scopes like a typical F10 SCT can get by with less expensive eyepieces, but sacrifice total Field of View.
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Tom (Pegster)
DSH-8 (GSO Dob)
15x70 Oberwerks
SVP 100 f/6 achro
WO 66 Petzval
Sears Discoverer EQ 60/900
8x42 Regals
History is Philosophy teaching by examples.
Thucydides
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Anonymous
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Cool, that helps a lot still a bit fuzzy but I have an idea now.
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Anonymous
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But why the terms "fast" or "slow"?? Appreciate the insight. Gil
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Anonymous
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I think that has to do with astrophotography. Slow scopes (longer focal lengths) take longer to capture an image on film. Not sure if this applies to CCD imaging though.
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rustynpp
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Reged: 08/27/04
Posts: 1011
Loc: Rye, NY
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Think about it this way, (assuming equal aperture) in a fast scope (shorter focal length) the light takes a shorter amount of time to go through the entire optical train, therefore it's faster (although obviously the light isnt moving any faster). And if the focal length is longer, the light takes a longer time to go through the entire optical train so it's slower.
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Nick P
N8i
Nikon 10x50
PST-SM40
Rye, NY
Edited by rustynpp (03/07/05 03:07 PM)
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Anonymous
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Ok now I have a very good idea. Thanks!
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sixela
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Reged: 12/23/04
Posts: 9499
Loc: Boechout, Belgium
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Quote:
But why the terms "fast" or "slow"??
Straight from photography. At the prime focus of a photo camera lens, the surface brightness of an object¹ depends only on the f/ratio, not the maximum clear aperture (at equal f/ratio, the larger clear aperture will yield a larger image as the focal length will be larger, but in the end, with the same surface bightness for the objects).
As a result, exposure time for a given film sensitivity and a given subject depends on that f/ratio - unsurprisingly, small f/ratios require less exposure time so they're "fast".
That also holds for prime focus astrophotography. Whether you have an 80mm or 406mm scope, the exposure time for extended objects depends only on the f/ratio - though for a given f/ratio, the image scale depends on the aperture.
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¹Assuming they're not point sources diffracted into Airy discs, but outside of astrophotography, the subjects are rarely point sources.
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400mm f/4.46 David Lukehurst truss Dobsonian on Tom Osypowski equatorial platform
Orion Starblast (114mm f/4 reflector, Alt/Az)
Edited by sixela (03/07/05 04:22 AM)
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Patrick
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Reged: 05/16/03
Posts: 6788
Loc: Franklin, Ohio
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Mistmouse,
As someone else mentioned, f/ratio is aperture divided by focal length. Aperture is the diameter of the objective, focal length is the length from the primary mirror to the eyepiece in a reflector and from the objective lense to the eyepiece in a refractor. The term "fast" or "slow" comes from the photography world and has to do with how quickly film is exposed (how quickly light is stored up) on the film thru the lense of the camera.
In astrophotography this is still a valid concept, but in visual astronomy, fast or slow doesn't have the same relevance, since our eyes do not collect and store light.
However, as others have pointed out, the f/ratio effects certain things, in particular, how our eyepieces perform. In slow f/ratio telescopes the cone of light coming to the eyepiece has a shallower angle than the steeper angle found in faster telescopes. Some eyepieces do not respond well to a steeper light cone and may exhibit different types of aberrations. Generally, the eyepieces that have been designed to compensate for the steeper light cone seem to cost more than those that are not designed for it, mainly due to the use of more exotic glass. The advantage to a lower f/ratio is that bigger aperture scopes can be made that aren't so long as to be cumbersome.
Hope this helps,
Patrick
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Discovery 10" f/6 Split Tube Dob
Celestron C6 SCT
Denk Binoviewers
AT66ED Refractor
Oberwerk Ultra 15x70 Binocular
475B Geared Tripod & 501HDV Head
Oberwerk 9x60 Binocular
Celestron Regal 8x42 Binocular
Canon 30D DSLR
Mini EQ1
My Astronomy Pages
Edited by Patrick (03/07/05 06:37 AM)
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matt
Vendor (Scopemania)
Reged: 07/28/03
Posts: 10022
Loc: Chaville, France
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Quote:
Think about it this way, in a fast scope (shorter focal length) the light takes a shorter amount of time to go through the entire optical train, therefore it's faster (although obviously the light isnt moving any faster). And if the focal length is longer, the light takes a longer time to go through the entire optical train so it's slower.
I ask the jury to disregard this statement and consider it a joke! good one!
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rustynpp
Pooh-Bah
Reged: 08/27/04
Posts: 1011
Loc: Rye, NY
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Quote:
Quote:
Think about it this way, in a fast scope (shorter focal length) the light takes a shorter amount of time to go through the entire optical train, therefore it's faster (although obviously the light isnt moving any faster). And if the focal length is longer, the light takes a longer time to go through the entire optical train so it's slower.
I ask the jury to disregard this statement and consider it a joke! good one!
Why? Did I get something wrong?
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Nick P
N8i
Nikon 10x50
PST-SM40
Rye, NY
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Patrick
Postmaster
Reged: 05/16/03
Posts: 6788
Loc: Franklin, Ohio
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Quote:
Why? Did I get something wrong?
Nick,
yes...think of a 300mm f/5 scope. It has a focal length of 1500mm. Then compare that to a 150mm f/8 scope, which has a focal length of only 1200mm. Yet the 150mm scope is considered to be "slower" then the 300mm scope.
I can't say that I completely understand all the physics involved but it has to do with the rate at which light accumulates on the photographic plate, whether CCD or film.
Maybe someone else can chime in here to explain the physics of f/ratio and light accumulation?
Regards,
Patrick
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Discovery 10" f/6 Split Tube Dob
Celestron C6 SCT
Denk Binoviewers
AT66ED Refractor
Oberwerk Ultra 15x70 Binocular
475B Geared Tripod & 501HDV Head
Oberwerk 9x60 Binocular
Celestron Regal 8x42 Binocular
Canon 30D DSLR
Mini EQ1
My Astronomy Pages
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Patrick
Postmaster
Reged: 05/16/03
Posts: 6788
Loc: Franklin, Ohio
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Quote:
Straight from photography. At the prime focus of a photo camera lens, the surface brightness of an object¹ depends only on the f/ratio, not the maximum clear aperture (at equal f/ratio, the larger clear aperture will yield a larger image as the focal length will be larger, but in the end, with the same surface bightness for the objects).
I should have read through all the posts more carefully. Sixela actually did a good job explaining it.
Thanks,
Patrick
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Discovery 10" f/6 Split Tube Dob
Celestron C6 SCT
Denk Binoviewers
AT66ED Refractor
Oberwerk Ultra 15x70 Binocular
475B Geared Tripod & 501HDV Head
Oberwerk 9x60 Binocular
Celestron Regal 8x42 Binocular
Canon 30D DSLR
Mini EQ1
My Astronomy Pages
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rustynpp
Pooh-Bah
Reged: 08/27/04
Posts: 1011
Loc: Rye, NY
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Quote:
Quote:
Why? Did I get something wrong?
Nick,
yes...think of a 300mm f/5 scope. It has a focal length of 1500mm. Then compare that to a 150mm f/8 scope, which has a focal length of only 1200mm. Yet the 150mm scope is considered to be "slower" then the 300mm scope.
I can't say that I completely understand all the physics involved but it has to do with the rate at which light accumulates on the photographic plate, whether CCD or film.
Maybe someone else can chime in here to explain the physics of f/ratio and light accumulation?
Regards,
Patrick
Ok I changed my post. I assumed equal aperture was a given but I can see how it could be interpreted differently. My bad.
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Nick P
N8i
Nikon 10x50
PST-SM40
Rye, NY
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Jarad
Post Laureate
Reged: 04/28/03
Posts: 3858
Loc: Atlanta, GA
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Quote:
Ok I changed my post. I assumed equal aperture was a given but I can see how it could be interpreted differently. My bad.
Uh, no, that wasn't the problem.... The "fast" or "slow" has nothing to do with how long it takes light to go through the system (the difference would be measured in picoseconds for any reasonable sized scope, anyway).
The "fast" or "slow" refers to how long film has to be exposed in a photographic system. A "fast" system (low f-ratio) puts the same amount of light into a smaller area than a "slow" system (long f-ratio). Thus, film reaches the needed exposure faster. In cameras, a fast lens can use a shorter exposure time, and a slow lens needs a longer exposure time. This is calculated purely based on the f-ratio, regardless of aperture (i.e. an 80mm aperture f5 needs the same exposure time as a 160mm aperture f5). Of course, the larger aperture f5 will have a larger image scale, but the same exposure time.
This carry over from photography isn't that useful for visual telescopes, since you can adjust the magnification with different eyepieces (effectively varying the focal ratio at your eye). It really only applies for prime focus photography (for eyepiece projection, you have to use the effective focal length and focal ratio produced by the whole system, much more complicated....).
Jarad
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rustynpp
Pooh-Bah
Reged: 08/27/04
Posts: 1011
Loc: Rye, NY
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Quote:
Quote:
Ok I changed my post. I assumed equal aperture was a given but I can see how it could be interpreted differently. My bad.
Uh, no, that wasn't the problem.... The "fast" or "slow" has nothing to do with how long it takes light to go through the system (the difference would be measured in picoseconds for any reasonable sized scope, anyway).
The "fast" or "slow" refers to how long film has to be exposed in a photographic system. A "fast" system (low f-ratio) puts the same amount of light into a smaller area than a "slow" system (long f-ratio). Thus, film reaches the needed exposure faster. In cameras, a fast lens can use a shorter exposure time, and a slow lens needs a longer exposure time. This is calculated purely based on the f-ratio, regardless of aperture (i.e. an 80mm aperture f5 needs the same exposure time as a 160mm aperture f5). Of course, the larger aperture f5 will have a larger image scale, but the same exposure time.
This carry over from photography isn't that useful for visual telescopes, since you can adjust the magnification with different eyepieces (effectively varying the focal ratio at your eye). It really only applies for prime focus photography (for eyepiece projection, you have to use the effective focal length and focal ratio produced by the whole system, much more complicated....).
Jarad
Right, but I wasnt trying to explain it in a scientific or photographic way. I was trying to explain it in a way which could be understood easily, especially for someone without expierence in photography. Despite the extremely small difference in time that it takes light to travel through a long versus short focal length scope, it's still there. Besides, it was the concept that I was trying to make clear. I wasnt trying to base anything I said on scientific data. I was by no means attempting to explain exactly why various focal lengths are considered fast and slow. I was just trying to explain an easy way to remember fast and slow.
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Nick P
N8i
Nikon 10x50
PST-SM40
Rye, NY
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Anonymous
Unregistered
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cool lots more info  so as far as everyday (night) observering with a fast or slow mirror wont make any differance to me. 
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lighttrap
Reged: 02/06/04
Posts: 3833
Loc: cloudy, foggy, humid NC, US
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Well, mist, it will make a difference to you. Forget the orgins of the terms and any references to photography for now. Just know that with the same eyepiece a fast scope will give less magnification and a wider field of view, (FOV), than a slow scope. Put another way, achieving high magnification with fast scopes can sometimes be difficult, while achieving wide FOVs with a slow scope can sometimes be difficult. Something else to consider is that faster scopes, (say those of about f/5 and faster), are often more demanding in terms of eyepiece selection, and often require more expensive eyepieces for best satisfaction. Also, faster scopes are more demanding of precise collimation.
Personally, I'm a fan of medium scopes with focal ratios in the f/6-f/8 category. In scopes under about 250mm diameter for reflectors, and of about 120mm and under for refractors, such mid-focal range scopes are often the best of all the worlds.
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18" Starsplitter II f/4.5
8" Hardin Dob f/6
C5 workhorse mini SCT f/10 or f/6.3
70mm TV Ranger dual purpose birding/astro
77mm Leica Televid APO
16x70 Fujinons on UA Deluxe Mt.
12x50 Nikon SE
8x30 Nikon E2s
and many others
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Anonymous
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Cool, Thanks Lighttrap ! I think you gave me the anwser I was looking for. I will try to be more precise with my questions in the future. 
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Jarad
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Reged: 04/28/03
Posts: 3858
Loc: Atlanta, GA
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Quote:
so as far as everyday (night) observering with a fast or slow mirror wont make any differance to me.
In practical terms, as Lighttrap mentioned, the main impact of f-ratio is on eyepiece choices. You can calculate the exit pupil by dividing the eyepiece focallength by the f-ratio. So for an f4 scope, you probably don't want to use eyepiece much over 28mm (7mm exit pupil, lowest power for most uses), and for an f10 scope you wouldn't want to go much below a 5mm eyepiece (0.5mm exit pupil, highest power for most uses).
Also, slow scope can use cheaper eyepiece designs while faster ones (f6 or less) need more expensive designs to avoid astigmatism around the edges of the field. Finally, fast newtonians can benefit from using a coma corrector (say less than f5 or so).
Jarad
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