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Anonymous
Unregistered
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I understand how the Focal Ratio is calculated (Length over Aperture) but I am not sure what the significance is.
I've also heard that lower focal ratios are "Faster". Not sure what is meant by that either.
In general what are the differences between lower F-ratios and higher ones? What does it translate into Telescope performance?
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BluewaterObserva
Post Laureate
Reged: 05/18/04
Posts: 4763
Loc: Zuni Mtns, NM
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The slower / faster comes from photography. Faster means less exposure time needed, slower means more exposure time needed.
On performance, given the same optical qulaity, and precise optical alignment, it would only typically mean less mag / more field of view per given eye piece focal length the faster the scope / lower the f/ratio.
Slower scopes / high f/ratio's, will give more magnification / less field of view per given eye piece.
Now, the other hitch on all this, is the fact that faster optics are more difficult to make accurately.
I hope that helped some.
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dgs©
Postmaster
Reged: 03/29/04
Posts: 13903
Loc: West Monroe, Louisiana
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Overly-simplifying:
Higher f/ratio means longer focal length for a given aperture... so using the same eyepiece, the higher f/ratio scope will yield higher magnification (and correspondingly narrower field of view). For wider FOV - needed for some objects like large nebula (North American Nebula for instance) or large open clusters - a lower f/ratio would be preferred.
Faster refers to exposure times for photography where for a given magnification, faster (lower) f/ratio yields shorter exposure times. The full explanation is a little more involve than that, and maybe someone else can haul me up short and give the full-on explanation.
And welcome to Cloudy Nights! One thing for sure, this is the place to come to get your questions answered correctly (eventually). 
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- david
8"Ø Newtonian on SVP, Moonlite CR2, Telrad
PST Oberwerk Ultra 15x70 Orion Ultraview 10×50
Hand-me-down Sears Refractor (Discoverer) 60mm×900mm
"What we have done for ourselves alone dies with us; what we have done for others and the world, remains and is immortal." --Albert Pike
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12581
Loc: Cumberland, R I , USA42N71.4W
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One other item worth mentioning is this.
In a slow scope, one with a long focal length for its given aperture diameter, it is much easier to bring more of the various wavelenghts of light to focus at closer to the same focal distance. That means you see less color out of focus.
In a fast scope, one with a short focal length for its given aperture, since the focal length is shorter to start with and there is less room for play, it is much more difficult to bring all the wavelengths of light to focus near the same focal length. The end result is fast scopes will almost always show a much larger degree of "false color". That is the ring of color around a bright object created by that wavelenght of light being focused at a different focal length than the other wavelengths.
In a longer focal length it is easier to subdue this undesirable aberration, referred to as Chromatic Aberration. In a short fast scope it is nearly impossible to subdue it.
Apochromatic lenses can also be used to help allieviate this problem, but at increased cost.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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lighttrap
Reged: 02/06/04
Posts: 3833
Loc: cloudy, foggy, humid NC, US
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What Ed is talking about in terms of CA is a problem specific to refractors. Reflectors, generally, do not have that problem. Reflectors encompass all the scopes that use a mirror as their primary, including Newtonians, SCTs, MCTs and variations.
Of interest to reflector owners, slower focal ratios have a larger "sweet spot", or area of acceptable collimation, that makes collimation much easier. Scopes with longer focal ratios also generally are less apt to show aberations in less expensive eyepieces. Very fast scopes (below f/5) often need much more expensive eyepieces and coma correctors. Whereas scopes with focal ratios of f/6 and above often can do just fine with less expensive eyepieces. Beware of really long focal ratios, (over f/10), as the field of view (FOV) will be quite restricted and narrow.
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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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jack45
Carpal Tunnel
Reged: 07/07/03
Posts: 2060
Loc: Lacey WA
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My turn!
I under stand the F/R:
The ratio of a telescope’s focal length to its aperture. Short focal ratios (f/5, f/4.5) produce wide fields of view and small image scales, while long focal lengths produce narrower fields of views and larger image scales.
What do they mean by larger or small image scales? I just want to make sure I under stand!
Thanks Eurvin!
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16"f/4.5 Discovery Split Tube/TV Paracorr
12.5"f/5 Discovery PDHQ/TV Barlow
Orion SkyQuest f/4.9 XT12"Intelliscope
Orion 120mm F/8.3 Refractor
Burgess BV 24mm aperture/Siebert 4 pc OCA
BV Pairs:26mm,20mm,17mm,14mm,12.5mm
Tele Vue Smooth Side Plossl,10.5mm,13mm,21mm,26mm,TV 11mm
UO Abbe Set,40mm 5000s Plossl,31mm Axiom LX,26mm T/5,LX,23mm Axiom LX,20mm T/2,16mm T/2,15mm Panoptic,14mm Meade UWA,10mm Axiom LX!
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lighttrap
Reged: 02/06/04
Posts: 3833
Loc: cloudy, foggy, humid NC, US
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Quote:
What do they mean by larger or small image scales? I just want to make sure I understand!
Image scale is just the term for how large the image looks in the eyepiece. In your example, above, it's only true that longer focal length scopes produce larger image scales if the eyepiece is kept the same focal length. The reason for that is that in a longer focal length scope, an eyepiece will give higher magnification than if *the same* eyepiece was used in a shorter focal length scope. Remember, magnification is just focal length of scope divided by focal length of eyepiece. Let's look at some examples. An f/5 8" scope has a focal length of 1000mm (8" is 200mm, 5x200=1000). In that f/5 8" an eyepiece with a focal length of 10mm would yield 100x. However, in an f/6 8", the 1200mm focal length of the scope would yield 120x when used with the same 10mm eyepiece. (1200/10=120). If you put that same 10mm eyepiece in an f/10 8" SCT or MCT, you'd get 200x, because the f/10 scope has a focal length of 2000mm.
So when we talk about image scale, we're really just talking about magnification. It means how big the image appears in the eyepiece. But it's not fixed to the f ratio of the scope. For instance, if you wanted 200x in the f/5 8", you'd just use either a 5mm eyepiece, or a the previous 10mm eyepiece with a 2x barlow.
Does that help? Don't let these terms throw you. Ask away.
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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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jack45
Carpal Tunnel
Reged: 07/07/03
Posts: 2060
Loc: Lacey WA
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Lighttarp
Thank you! We need you in the military! I under stood that. Uncle Sam needs you!
Thanks Eurvin!
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16"f/4.5 Discovery Split Tube/TV Paracorr
12.5"f/5 Discovery PDHQ/TV Barlow
Orion SkyQuest f/4.9 XT12"Intelliscope
Orion 120mm F/8.3 Refractor
Burgess BV 24mm aperture/Siebert 4 pc OCA
BV Pairs:26mm,20mm,17mm,14mm,12.5mm
Tele Vue Smooth Side Plossl,10.5mm,13mm,21mm,26mm,TV 11mm
UO Abbe Set,40mm 5000s Plossl,31mm Axiom LX,26mm T/5,LX,23mm Axiom LX,20mm T/2,16mm T/2,15mm Panoptic,14mm Meade UWA,10mm Axiom LX!
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darylf96
Pooh-Bah
Reged: 08/28/04
Posts: 1246
Loc: Danville, California
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Eurvin:
Everyone did a great job explaining the technical stuff
about focal ratios. If you are just getting into this
hobby and are thinking about buying a telescope, one
consideration will be a choice regarding the focal ratio
of the scope. When you first start observing, you may
not be certain whether you like the planets best or deep
sky object that can be very large. If you buy a longer
focal ratio scope, it will do best on planets and smaller
objects, like globular star clusters, but it may prove disappointing if you want to see an object requiring a wider field of view, like a large nebula. There are several scopes that will perform well on both the planets
and larger objects. If you are considering buying a telescope, you should check with the folks in this forum
to help you find a good starter scope.
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Intes Micro MN66 - Meade 10" SCT
Skywatcher 150mm f8 Achro
CGE Mount - DiscMount DM-6 on Meade Field Tripod
with SkyCommander DSC
Lumicon 80mm Super Finder
ED80 w MoonLite focuser on CG5A-GT mount
WO ZenithStar ST 80mm/WO B-viewer
Astro-Tech AT80 Refractor f6.9 APO
On SV M1 mount & Grab-Go Tripod
Stellarvue 20x85 binos - Unimount
Kunming United Optics 7x50 binocs
Canon Rebel XT, Canon 20D, SB ST8XE
Orion DSCI, Meade DSI
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Anonymous
Unregistered
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See Sky and Telescope, Nov 04 page 150. "let's kill the "f/number"". All you ever wanted to know about f/number.
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ProfLemoi
sage
Reged: 09/26/04
Posts: 351
Loc: Ashburnham, MA
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I have just bought a new LCD75 SN-10 with which has an F/4 thus making it a "Fast" scope. I have seen come excellent pics of Saturn and Jupiter on this website and I am hoping to see these planets up close with my new scope. I just saw a pic on this site of Saturn which used a 5x powermate. What are your thoughts on this item? The image was very large, and very clear. Obviously he/she knew what to do. I am hoping to do both deep space and planets. I have a 4mm,6mm,9mm,15mm,24mm and 32mm lens kit from Celestron and I have a 2x Barlow and a 3X Barlow lens. Is there anything else I should be considering to get sharp images of Saturn/Jupiter?
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Prof Lemoi
10" LX200GPS UHTC
8x60 Pentax Binoculars
ToUCam ProII 840K
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dgs©
Postmaster
Reged: 03/29/04
Posts: 13903
Loc: West Monroe, Louisiana
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I think you probably have it covered. Saturn often looks pretty good in my 8"Ø scope with a 3mm Radian (333×) Jupiter usually wants a little lower power, like the 4mm Radian (250×). Your 6mm with a 2× Barlow will be the same as the 9mm with a 3× Barlow (both effectively = 3mm), but the 9mm will likely have more comfortable eye relief.
Jupiter will be VERY bright in a 10"Ø... you might want to pick up a Variable Polarizing Filter. This will also be really nice for looking at the moon, which is painfully bright in a scope this size... like a strong flashlight directly in the eye. If you get up early in the morning (or stay up late at night), Satrun is rising in the East around 1:30am and is high enough for a decent look by around 3:30 or 4:00am on up thru sunrise.
Be sure to let us know how it works out for you.
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- david
8"Ø Newtonian on SVP, Moonlite CR2, Telrad
PST Oberwerk Ultra 15x70 Orion Ultraview 10×50
Hand-me-down Sears Refractor (Discoverer) 60mm×900mm
"What we have done for ourselves alone dies with us; what we have done for others and the world, remains and is immortal." --Albert Pike
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darylf96
Pooh-Bah
Reged: 08/28/04
Posts: 1246
Loc: Danville, California
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ProfLemoi:
Are you planning to do film, digital, or CCD imaging on the planets?
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Intes Micro MN66 - Meade 10" SCT
Skywatcher 150mm f8 Achro
CGE Mount - DiscMount DM-6 on Meade Field Tripod
with SkyCommander DSC
Lumicon 80mm Super Finder
ED80 w MoonLite focuser on CG5A-GT mount
WO ZenithStar ST 80mm/WO B-viewer
Astro-Tech AT80 Refractor f6.9 APO
On SV M1 mount & Grab-Go Tripod
Stellarvue 20x85 binos - Unimount
Kunming United Optics 7x50 binocs
Canon Rebel XT, Canon 20D, SB ST8XE
Orion DSCI, Meade DSI
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ProfLemoi
sage
Reged: 09/26/04
Posts: 351
Loc: Ashburnham, MA
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Darylf96... I am looking to do CCD photography. Actually after having my LXD75 for awhile I decided this was NOT the scope for me and I sent it back. I was not happy with it at all. I will be upgrading to the lx200 UHTC in a few weeks when I can save off the extra $$$ not sure if I am going to get the 10" or the 8" right now I am thinking 10", but for all of them I am looking at using an LPI for the planets and Meades new CCD imager when it becomes available for deepsky stuff.
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Prof Lemoi
10" LX200GPS UHTC
8x60 Pentax Binoculars
ToUCam ProII 840K
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David Knisely
Postmaster
Reged: 04/19/04
Posts: 6784
Loc: Beatrice, Nebraska
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Quote:
See Sky and Telescope, Nov 04 page 150. "let's kill the "f/number"". All you ever wanted to know about f/number.
I thought that the article was one of the worst cases of "tilting at windmills" I have ever seen. He keeps getting hung-up on the slash of the f/ as meaning some numerical operation when it means nothing of the sort. "f/" has always mean the f ratio (or f/ratio as it is often referred to as) for at least 50 years (or more), and it is the ratio of the focal length (the "f" part) to the aperture of the instrument. Like it or not, the author of that article has to realize that this is the standard convention. Clear skies to you.
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David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info
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Scott Beith
SRF
Reged: 11/26/03
Posts: 33056
Loc: Gulfport, MS
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David,
No offense intended towards the author - but I also thought it was a waste of paper. I read it twice to make sure I didn't miss something, and I ended up disappointed 2x. I am not a literary critic, so my opinion in this matter doesn't hold much weight.
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  Scott
"The only thing necessary for the triumph of evil is for good men to do nothing." -- Edmund Burke.
"People sleep peaceably in their beds at night only because rough men stand ready to do violence on their behalf." -- George Orwell
"The measure of a man’s greatness is not determined by what he accomplishes for himself, but by what he accomplishes for others.” -- Some Bald Guy
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Don W
Postmaster
Reged: 05/19/03
Posts: 13235
Loc: Wisconsin, USA
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I agree, this method of notation has been firmly entrenched in this hobby/science (can I use a slash there?) for a lifetime. I browsed the article and then dismissed it as carping over something that no one else I know of is bothered by. It's sort of like "Is Pluto a planet or something else?". It's been called a planet since its discovery in the 30's. Why change now?
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Don Wyman
Obsession 18" f/4.5 #1166
W/Argo Navis DSC and Torus Primary
William Optics Megrez 90
Coronado PST
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Jon Isaacs
Postmaster
Reged: 06/16/04
Posts: 12070
Loc: San Diego, California
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David et. al.
Didn't see the article yet but..
It has always seemed to me that the proper expression was F/ratio simply because it is short for the
Aperture = Focal length/Focal ratio or as you point out, it is a ratio.
Maybe next month they will have an article discussing the issue of lambda/
jon
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markf
Carpal Tunnel
Reged: 10/13/04
Posts: 1935
Loc: Houston, TX
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Quick question:
Does the f/ration have any effect on how "fast" or easy it is to bring an object into focus?
Just curious, as I'm always fiddling with the focus (guess we all do!), but it seems even trickier when using my 2x barlow
-Mark
Celestron Nexstar 130gt
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Celestron C6N on a CG5-GT
Orion 80mm Refractor (guidescope)
ToUCam Pro II
Canon Digital Rebel
http://www.carsandfish.com/astroweb
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erik
telescope surgeon
Reged: 01/30/04
Posts: 24019
Loc: Hawaii 19 N lat -155 Long.
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yes, longer f/ratio scopes can have a wider "sweet spot" of focus, but it also depends on the quality of the optics. higher powers always have a smaller area of focus, but a scope with good optics will "snap" into focus, whereas a scope with mediocre optics will be more difficult to find focus...
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-Erik Wilcox
Homebuilt 16" Truss Dob
SV 80mm ED Nighthawk NG on M1 ALT/AZ
Nikon Prostaff 65mm spotter on Trekpod
Konusvue 20x80 binos/Peterson pipemount
Orion 10x50 binos
Homebuilt 80mm f/5 refractor
Mirador 60mm f/12 1960's refractor
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