Focal ratios

[timsky]

Looking to purchase a scope and I'm trying to get my head around focal ratios. I'm a keen photographer so I understand the technical side but I don't know how that translates into reality with using telescopes.  I've read that scopes with focal ratios of 10 or more aren't good for imaging or deep sky, faint objects, etc yet the marketplace seems to be full of high end scopes of f10 or more. Is the f10 of one scope different from the f10 of another?

[Moonshane]

the principles are the same as photography in that an f10 scope/lens gathers data for an image at a much slower rate than an f5 scope/lens. longer exposures = harder to track etc for the same data.

there's no real difference visually other than a slower f10 scope has a much longer focal length than a faster scope of the same aperture.

[JamesF]

Focal ratios of 10+ are commonly used for planetary imaging -- narrow field of view, large image scale on bright targets.  They're not good for imaging faint targets though because in photography terms the optics are very slow.  For that you ideally want something nearer, say, f/5.

The principle is the same as photography really, but you're much more limited in terms of how you can change the focal length and generally you don't want to "stop down" because doing so loses resolution as well as the amount of light that can be collected in a given time.

James

[jambouk]

It is confusing.

The way i think of it is in astronomy equipment, the higher the focal ratio, the narrower the field of view.

So f/4 has a wide field of view good for many of the deep space objects as many are quite big; f/15 has a much narrower field of view so lends it self to smaller [and brighter] objects like the things found in our solar system (planets, moon, sun if used with appropriate filters and precautions).

There is also the issue about the "speed of the optics" which i'm still unclear about no matter how many times it's been discussed on here!

SCT and maks are generally slow scopes (f/10 or slower), where as newtonians are faster (f/5 and faster); there are things in between.

So it depends what sort of things you want to look at. If you want to look at everything, you probably need more than one telescope.

That's my very amateurish take on it.

James

[timsky]

So would f10 be ok for observing galaxies and nebulae?

I was wondering if high focal ratios on scopes reduced the light as much as stopped down DOF preview mode does on an SLR camera- I had an old manual focus lens mounted on a DSLR by way of an adaptor with no mechanical or electronic connection between the camera and the lens. After about f8 it became too dark to focus...

[Moonshane]

the focal ratio has no effect upon the visual image other than the focal length affects magnification at the same aperture.

a 6" f10 at 100x has the same brightness of image and resolution as a 6" f5 scope at 100x (visually)

my experience also suggests that slower scopes provide slightly more contrast than faster scopes but I think this is likely to be an effect of a smaller exit pupil creatibng a darker sky in the slower scope

[jambouk]

Imd certainly look at stuff through various telescopes before spending any money.

My opinion is that observing most deep space objects (nebulae and galaxies anyway) is pretty dull in anything under 10 inches; and the the narrower the field of view, the more spread out an object will be and it will have less contrast than the surrounding sky....

[CodnorPaul]

I use an 8" sct and get some fabulous views of nebula and galaxies - excellent optics are up there with importance on aperture IMHO. Do I want a C11 - yes, am I disappointed with my C8 visually - not at all.

dark skies are equally important

[Moonshane]

as long as the object you want to view fits in the available field, the focal ratio (and to a large extent the design of the scope) is irrelevant for visual observing.  darker skies and larger aperture in that order matter more.

[jambouk]

I always worry people think that when they look through a telescope they will see the stuff they've seen in the members photographic gallery in the sky at night magazine. I like to lower their expectations so much that when they look they are overwhelmed...

Is it proven that visually for a given aperture scope, the same object looks the same brightness at the same magnification in scopes of very different focal ratios?

(I told you this bit confused me!)

Jd

[Moonshane]

it's proven in as much as I have tried it and yes, they were the same. comparing a 6"f11 newt with a 6" f5 scope at the same magnification. as I said above thought the f11 has to my eyes better contrast.

with 12" or more aperture there are several objects that look a bit like images / close anyway - larger planets, moon, globular clusters, the sun, open clusters

[CodnorPaul]

Yes that is generally accepted. Even the effect of larger CO is overstated apparently, and I am certain reflector users forget they have a substantial CO also.

slower scopes need less expensive EPs also which helps

[jambouk]

Yes i looked at M42 through a C11 and even though there was light pollution and a full moon, i could clearly see the nebula, more clearly than i'd ever seen before visually.

I'd like to do the focal ratio comparison for myself, side by side, as i've always found dso's utterly disappointing my 180mm f/15 mak, but more rewarding in a 150mm f/5 newt; but not done side by side comparison.

Jd

[Moonshane]

the CO affects contrast generally with smaller meaning a little less loss of contrast. my 6" f11 has a 25mm secondary so 17% by diameter but <3% by area.a shorter, faster 6" would have a bigger secondary by diameter.

[timsky]

What does CO mean?

Just been reading the Wikipedia page on focal ratios and it says that f ratios for scopes and cameras differ. Brightness of the image through a telescope is a property of the physical size of aperture only and independent focal length.

[Moonshane]

Yes i looked at M42 through a C11 and even though there was light pollution and a full moon, i could clearly see the nebula, more clearly than i'd ever seen before visually.

I'd like to do the focal ratio comparison for myself, side by side, as i've always found dso's utterly disappointing my 180mm f/15 mak, but more rewarding in a 150mm f/5 newt; but not done side by side comparison.

Jd

you need to ensure similar magnification. e.g. 100x in the mak needs a 27mm eyepiece and in the 6" f5, a 7.5mm eyepiece.

[jambouk]

Imm sure i'll never get to do a side by side; there are many interesting, academic things like that i'd like to try but time and available kit limits this.

James

[Moonshane]

sure, me too. on the whole I'd sooner be observing!