Focal ratio

[the_laughing_crow]

I dont suppose anyone out there can point me to a good explanation of why different focal ratios are considered better or worse for different purposes (ie DSO vs Planetary)?

I've read multiple times about the best uses of different focal ratios but Ive no idea as to the physics behind it.

Thanks!

[Philosophaie]

Some different equations evaluating Focal Ratios:

magnification = (Focal length of the Telescope)/(Focal length of the Eyepiece)

True Field of View = (52 Degrees)/Magnification

Exit Pupil = (Diameter of the Primary Mirror)/Magnification

[Peter Drew]

Short focal ratios produce small high intensity images so are good for wide angle DSO imaging whereas long focal ratios produce much larger photographic images, ideal for lunar and planetary imaging. For visual use, the same magnification on either will give pretty much the same view for a given aperture.

[the_laughing_crow]

Yes, but why?

Surely for a scope of a set aperture the same number of photons are being reflected at the same rate from the primary onto the secondary regardless of how far away the secondary is.

[brantuk]

A faster scope (lower f-number) allows longer exposures. The higher the f-number the shorter you can do the exposures. Fast scopes have f7 or less, slower scopes are above f7.

[Philosophaie]

Try some of the online calculators.

http://www.spacegazer.com/index.asp?pageid=74737

http://www.atmsite.org/contrib/Prewitt/fieldOfView/

[Thomas]

Firstly, I must point out that the second equation given by Philosophaie is incorrect. FoV is obtained by dividing Apparent FoV by the Magnification. None of the equations actually evaluates Focal Ratio, which is Focal length / aperture. Focal Ratio really only matters in prime focus photography. If an eyepiece is used then aperture rules for faint, deep sky objects, while, for planetary work, a long focal length will give high magnification without the use of very short eyepieces, or barlows, etc. I hope that's helpful.

[the_laughing_crow]

I still dont understand where the difference lies

For photography work and for visual as well.... regardless of the target (DSO or planetary) ....what matters is the rate of photons incident on your retina or the CCD, right?

If you have two scopes at 250mm aperture and one has a focal length of 1200mm and the other has a focal length of 3000mm then what is the "x factor" that makes the difference? Theres exactly the same number of photons bouncing off the primary and onto the secondary at exactly the same rate in each scope. Just in one scope those photons have a longer travel path.

[ollypenrice]

I think there are some errors here.

Let's not confuse focal ratio with focal length. The focal length determines the image scale. Longer means the object will be larger on the film or chip.

Focal ratio determines the intensity of the light falling on the chip or entering the eye. It's not true that a faster focal ratio allows longer exposures. It is actually the reverse since a fast focal ratio will over expose an image faster than a slow one. The charm of a fast f ratio is that you need shorter exposures.

Okay, now why? Think of looking at the moon in an eyepiece that gives a half degree field so you get the full moon in the view. Now put in an eyepiece which increases the effective focal length of your system (by changing the fratio) so that you are able to see only half of the moon. It is going to look dimmer because you are no longer getting a full moon's worth of light. The telescope hasn't got any smaller though.

In visual use it doesn't greatly matter what your scope's f ratio is because you can change the effective focal ratio by changing the eyepiece. Yes, it might be marginally better to use a slow f ratio scope for high magnification work and a fast one for deep sky but it won't make all that much difference once you have put in an appropriate eyepiece to give the same image scale in each system.

Olly

[the_laughing_crow]

Longer means the object will be larger on the film or chip

Wait, why does a longer focal length mean the object will be larger? Focal length is the distance of the secondary from the primary, right?

With my limited knowledge about telescopes I was assuming that the secondary mirror reflected light towards the eyepiece as though the light was from infiinity (ie parallel lines). Then the eyepiece converged these parallel lines with the effect of making the object appear larger.

[ollypenrice]

Wait, why does a longer focal length mean the object will be larger? Focal length is the distance of the secondary from the primary, right?

With my limited knowledge about telescopes I was assuming that the secondary mirror reflected light towards the eyepiece as though the light was from infiinity (ie parallel lines). Then the eyepiece converged these parallel lines with the effect of making the object appear larger.

No. Focal length is the distance needed for the optics (mirrors or lenses) to bring incident rays to focus. In a Newt the secondary doesn't do anything optically other than send the rays to a place which will not cause the observer's head to obscure the view!

Olly

[xboxdevil]

Think of it as a projector on a table pointing at a wall, the further you move the projector from the wall then the larger the image appears.

[GazOC]

Think of it as a projector on a table pointing at a wall, the further you move the projector from the wall then the larger the image appears.

And the "fainter" it gets.

[brantuk]

"It is actually the reverse since a fast focal ratio will over expose an image faster than a slow one"

Hi Olly - thanks for getting me the right way round. Can you explain a little further please why we use fast scopes imaging dso's and slow ones on planets? Re: length of exposure v's exposure. Or I'm gonna make a pratt of myself at Kelling lol

Is it a case of we want to overexpose the dso's for longer periods?

Cheers

[ncjunk]

My bet would be fast for dsos to pick out the faint stuff and minimise exposure times (same amount of detail in less time...a plus with all these clouds)

Slow for planets as they are bright enough already so you can push the focal length and get more magnification out of them thereby taking hundreds of frames and picking out the ones with the best seeing. I assume that having a faster scope would over saturate the image requiring shorter exposure times in order to see any detail in said planet but also leave you with a smaller image.

If i am wrong burn me at the stake...or steak...with pepper sauce....and let me know where i got it wrong!

[FraserClarke]

For planets you want long focal *length*, so that you can get sufficient resolution on your camera. Focal ratio doesn't really matter here (because planets have high surface brightness); but you tend to end up with large focal ratio because you can't afford a large primary mirror. Remember that Focal Ratio = Focal Length / Mirror Diameter ; so if FL is large, FR will be large, unless MD is large (== expensive!).

Why does focal length = more resolution? Consider beams of light from two stars, 1 arcminute apart. The beams come in to the primary mirror with a relative angle of 1 arcminute, and go out with a relative angle of 1 arcminute. The longer the focal length, the further the beams will travel before they come to a focus, so the larger the physical separation between the two stars will be in the image. So, the longer the focal length, the physically larger the image is, and the higher the resolution of your camera.

You might want to try and draw this; it's easy to understand when you do the drawing...

For DSOs, you're worried about surface brightness. This means you want to get as much light into each pixel as possible. So first consideration is that you need a large mirror, to gather lots of light. OK, but if that large mirror also has a long focal length (hence large focal ratio), the image is very large at the camera, so the light is more spread out. So, you want a large mirror, with a short focal length, so that you have lots of light concentrated into a small area. So, for DSOs you want small focal ratio telescopes.

In fact, the maths shows you something interesting a lot of people don't appreciate;

Focal Ratio = Focal Length / Mirror Diameter

Amount of Light = (Mirror Diameter)^2

Size of Image = (Focal Length)^2

Brightness of Image = Amount of Light / Size of Image

Brightness of Image = (Mirror Diameter)^2 / (Focal Length)^2

Brightness of Image = 1 / (Focal Ratio)^2

Brightness of Image, which is what drives imaging speed, depends ONLY on focal ratio; it does not depend on the size of the primary mirror!

[brantuk]

Blimey! - thought I understood it till this thread lol. I'll have to re-read that a few times but it's beginning to make sense now though. So thanks guys (NCJunk and TeaDwarf)

[ncjunk]

It still takes a bit of effort to get your head around tgis sometimes.

My ed80 is 80mm with a focal length of 600 so its an f7.5.

I have just bought a c11 at focal length (with 0.63 focal reducer) 1700 mirror size 280 therefore f 6.3

So when i eventually get to use the c11 it should be faster than the ed80 and the dso will appear a lot bigger on the ccd. I can't wait to do some photos of the same sub length and see what happens. Typically i have run out of money and cant afford the losmondy adaptor for the eq6 so i can only polish the c11 at the mo!

[brantuk]

Lol Neil - yeah the only problem with this hobby is money. Look forward to seeing your pics with the C11 - thanks again

[ollypenrice]

Sorry, I lost track of this thread and didn't reply to Brantuk, but others have said it all better anyway. (I've been at war with an M33 image!)

Of course another reason for wanting a fast scope for DSO is that it reduces the demands on your guiding. In the extreme case of the Hyperstar, a system working just below f2, exposures on DSOs can be so short that you can get away with an Alt Az mount, or so say Starizona.

Another thing; if you want to set a max focal length in order to frame your object on the chip, having a bigger aperture is desirable 1) because it reduces f ratio and 2) because it enhances resolution.

So when you win the lottery do you buy a small Tak FSQ f5 refractor or a larger Tak Epsilon corrected Newt at f3.3? They both have the same focal length, more or less. More speed and resolution in the Epsilon but not plug and play because the fast f ratio needs precision collimation... We have to remember mechanical as well as optical concerns...

I'll have both, I think, Santa.

Olly

[brantuk]

Thanks Olly - and yes - if it was a lottery win - I'd get both and more too lol

Laughing_cow - sorry I hijacked the thread - but it's in context and I hope the answers to my question helped you

[ollypenrice]

I think f ratio is so important that we need to get a good handle on it. My fastest is option is f3.3 in the reduced Baby Q and that is bliss, fast and furious. Even in the one shot you have a picture in 2 hours.

Olly

[Capricorn]

How can size of image = (focal length) ^2

Does a cow at 10 feet away make an image the same size as a ladybird at 10 feet?

Because that is what the equation says!

Thought that for small angles the image size was proportional to the focal length, not the square of it. Isn't it:

Image size = 2*F*(Tan alpha/2) where Alpha is the angle subtended by the object. So is directly proportional to F not F^2 ?

Brightness of Image, which is what drives imaging speed, depends ONLY on focal ratio; it does not depend on the size of the primary mirror

Been known in the photographic area for a quite some time, 100+years, and does work in astrophotography unless you stick an eyepiece in the way. In which case it goes out the window. That "eyepiece" can be a real eyepiece, a barlow of the lens of a compact camera.

I really wish that using photographic terms in astronomy was outlawed. As most seem to have the idea that getting a fast scope then blowing the magnification to something huge means they still get a bright image.

[ollypenrice]

Been known in the photographic area for a quite some time, 100+years, and does work in astrophotography unless you stick an eyepiece in the way. In which case it goes out the window. That "eyepiece" can be a real eyepiece, a barlow of the lens of a compact camera.

I really wish that using photographic terms in astronomy was outlawed. As most seem to have the idea that getting a fast scope then blowing the magnification to something huge means they still get a bright image.

Yes, obviously nothing changes when your lens began life as a telescope. But does anyone really think that a fast scope remains fast when its focal length/ratio is extended? I haven't come across this error myself, though all DS imagers are well aware that a focal reducer speeds up the system. That's why they buy them, though having a wider FOV is sometimes the intention, sure. However, persuading people that the measure of 'light bucketness' (to coin a forgettable phrase) is f ratio and nothing else is often nigh on impossible. They look at our 85mm Takahashi and say, 'You took Andromeda in that???' I've had this conversation twice already this week, funnily enough. It carried on, 'Well why do the professionals use huge telescopes?' - to which the answer is Rresolution and focal length,' I suppose. 'Or resolution at a long focal length.'

I think we'd be a bit stuck without photographic terms in astrophotography though!

Olly

[ncjunk]

The Keck telescope is a 10M morror with a focal length of 17.5 M giving it an f1.75...now whats the arcseconds per pixel on that?