F.stop/ratio confusion!

[pipnina]

Hello, I joined this forum nearly two months ago and since being here, I've noticed that F.stop for telescopes seems to be different than the kind I'm used to (For cameras).

I understand that higher F.stop in cameras leads to darker images and less off-focus blurring and that higher F.stops in cameras opens the aperture, lets more light in and creates a nice blur outside of the focal point. This F ratio for telescopes, however, confuse me.

I've heard that telescopes with low Fs tend to be less sensetive to collimation not being perfect, but telescopes with high Fs tend to require collimation be much more accurate.

Could someone clear this up for me? As a convert from camera optics to telescope optics this is very confusing!

        ~pipnina

[ronin]

They are the same but I think people use the terms fast/slow rather then low/high.

Where Fast = Low and Slow = High.

Fast scopes are just more critical of being collimated right, and small deviations will cause larger then expected problems.

Also the faster (lower) the scope f number the more curvature is involved which means the edges and it is the mirror and lens edges that are the problem.

[wxsatuser]

It's just the simple ratio between the focal length and aperture for both camera lenses and telescopes.

My little Borg 67mm is 255mm focal length so is f/3.8

My 105mm Sigma macro lense has an aperture of 26mm at f/4.

[russ.will]

I understand that higher F.stop in cameras leads to darker images and less off-focus blurring and that higher F.stops in cameras opens the aperture, lets more light in and creates a nice blur outside of the focal point. This F ratio for telescopes, however, confuse me.

I've heard that telescopes with low Fs tend to be less sensetive to collimation not being perfect, but telescopes with high Fs tend to require collimation be much more accurate.

None of this changes, because it's a telescope. As stated above, the Focal Ratio is the ratio of the objective/primary mirror diametre, to the focal length of the scope/lens. It's just that scopes are generally used at full aperture, whereas a camera has the opportunity to stop down the effective aperture with an iris.

If you took two scopes of equal focal length, but one had half the diametre of objective/mirror, the image will be four times duller - Just as it would if you stopped a cemera lens down from F4 to F8. Now there is an attending increase in depth of field, but as everything we view in a scope is basically at infinity, this manifests itself as an increased depth of focus, or to put it another way, there's a bit more latitude in achieving a sharp focus at infinity.

The ease of, or lack of sensitivity to, collimation comes down to the a longer focal ratio scope having a less steep light cone and therefore a flatter focal plane. As you move the steeper curved focal plane of a faster scope away from optimal collimation, the very narrow depth of focus is rapidly shifted away from the focal point of the EP (or CCD) and it all goes south very quickly. In the slower scope, the flatter field with the attendant extra depth of focus can be out of whack a bit more before it becomes as noticeable.

In another camera related analogy, you can consider changing the eyepiece to vary magnification, as much the same effect as changing the size of the camera film/chip on a given lens. For instance a medium format camera treats 80mm as a 'standard' lens, but the same lens on a 35mm camera is a mild telephoto, on APS-C chip it's a telephoto and by the time you get to the tiny things you find in a compact, an 80mm lens is ridiculously telephoto. In this analogy, the Medium format camera is your 30mm finder/wide-field EP, the 35mm camera is your general nebula/open cluster EP, the APS-C your small fuzzies/globs EP and the compact camera your planetary high magnification EP.

I'll leave someone else to fill in the differences in the way an imaging chip (with exposure times) varies from the continuous sampling of MkI Eyeball as I'm no imager and don't really care about it!

Russell

[Moonshane]

I believe that the depth of focus holds true too with telescopes. At f4 the point of best focus is sometimes harder to find than at f11 in two of my newts and I put this down to a wider field of focus in the latter compared with the former. For visual there's no difference in brightness of the image at the same magnification and aperture but this does not apply I understand to imaging.

[ollypenrice]

There is confusion, though, and pipnina asks a sensible question. The problem arises from daytime photographers taking a shortcut with the terminolgy since they speak of the F Stop (and so, implicitly, the F ratio) as the aperture, which it simply isn't. The F Stop (say F5.3) is the focal length of the lens divided by its aperture. That applies to any and every system, be it a telescope or a lens. It does not specify the aperture, which is the diameter of  the open light beam allowed in by the system.

Why do they do this? They do it because their focal length is fixed (for a given shot this applies to zooms as well) and their aperture is variable since it is governed by an adjustable iris. This is not the case with telescopes which have a fixed aperture and a fixed focal length. Lens makers don't want their users to have to calculate the focal ratio for each movement of the iris so they do the sum in advance for that lens and give the F ratio arising from each position of the iris. In doing so they introduce the tendency to call the aperture the F ratio in camera circles. This is a useful shortcut because it is the F ratio which controls the exposure speed and the depth of field. A high F number is faster and has a shallower depth of field. (It's easy to visualize why. A larger aperture (open iris) lets in more light but makes a sharply angled cone as it comes to the focal plane. Move the focal plane even slightly and you move it to a wide part of the cone and are out of focus. If the cone is shallow from a stopped down iris then the same movement doesn't move you to a wide part of the cone and you are still more or less in focus.

For the reasons you say, all this is useful to daytime photographers who may well not want everything in focus.

Astrophotographers are always working at infinity so they have no depth of field to capture. They want everything in focus and they always want as much light as possible so they never stop down. (Well, big Newt users may stop down on the planets...)

Astrophotographers use the terms, aperture, F ratio and focal length correctly but get confused when they pick up focal reducers. These reduce the focal length and speed up the F ratio, but do they reduce exposure times? Aha, not necessarily. It depends on the size of your target, but let's leave that for now!

Olly

[pipnina]

None of this changes, because it's a telescope. As stated above, the Focal Ratio is the ratio of the objective/primary mirror diametre, to the focal length of the scope/lens. It's just that scopes are generally used at full aperture, whereas a camera has the opportunity to stop down the effective aperture with an iris.

If you took two scopes of equal focal length, but one had half the diametre of objective/mirror, the image will be four times duller - Just as it would if you stopped a cemera lens down from F4 to F8. Now there is an attending increase in depth of field, but as everything we view in a scope is basically at infinity, this manifests itself as an increased depth of focus, or to put it another way, there's a bit more latitude in achieving a sharp focus at infinity.

The ease of, or lack of sensitivity to, collimation comes down to the a longer focal ratio scope having a less steep light cone and therefore a flatter focal plane. As you move the steeper curved focal plane of a faster scope away from optimal collimation, the very narrow depth of focus is rapidly shifted away from the focal point of the EP (or CCD) and it all goes south very quickly. In the slower scope, the flatter field with the attendant extra depth of focus can be out of whack a bit more before it becomes as noticeable.

In another camera related analogy, you can consider changing the eyepiece to vary magnification, as much the same effect as changing the size of the camera film/chip on a given lens. For instance a medium format camera treats 80mm as a 'standard' lens, but the same lens on a 35mm camera is a mild telephoto, on APS-C chip it's a telephoto and by the time you get to the tiny things you find in a compact, an 80mm lens is ridiculously telephoto. In this analogy, the Medium format camera is your 30mm finder/wide-field EP, the 35mm camera is your general nebula/open cluster EP, the APS-C your small fuzzies/globs EP and the compact camera your planetary high magnification EP.

I'll leave someone else to fill in the differences in the way an imaging chip (with exposure times) varies from the continuous sampling of MkI Eyeball as I'm no imager and don't really care about it!

Russell

So, does this mean in my scope (f-6.9) I should be more worried about my collimation than I thought before? (As stated, I got confused with people here calling scopes fast/slow and thought I had a slow scope :x)

I don't have any collimation tools, but I can see that my primary mirror is out a bit. Maybe I should make a collimating cap from the dust cap for my scope's EP socket (I normally keep the 25mm in ther all the time and put the ep's dust cap on, I'm lazy!)

[ollypenrice]

F6.9 is not 'fast' in most peoples' books. It's extremely (  ) moderate...

Olly

[pipnina]

F6.9 is not 'fast' in most peoples' books. It's extremely (  ) moderate...

Olly

Ok then, I guess I have some bigger issues right now than collimation anyway. (My forgetfull brain keeps leaving the red dot finder on, it's now out of battery from at most 7 observing sessions over 2 months )

Thanks for the help, guys! I'll be sure to keep this stuff in mind and not get confused

[gnomus]

.... I understand that higher F.stop in cameras leads to darker images and less off-focus blurring and that higher F.stops in cameras opens the aperture, lets more light in and creates a nice blur outside of the focal point. This F ratio for telescopes, however, confuse me.

.....

Regarding the bit I have underlined, I believe that the opposite is true.  In a camera, f/16 represents a smaller aperture than f/5.6.  I don't know if that is a typo.  If not, then it could be the source of your confusion.

[Peter Drew]

Whilst not wishing to add to the "confusion" there is often another F stop mentioned occasionally. This refers to field stop which is the baffle ring at the focus of the eyepiece which determines the apparent field of view. Not to be confused with the other F's. 

[russ.will]

So, does this mean in my scope (f-6.9) I should be more worried about my collimation than I thought before? (As stated, I got confused with people here calling scopes fast/slow and thought I had a slow scope :x)

In terms of modern Newtonians, yours is definitely the slow scope that is less fussy about collimation, but that does not mean you should ignore doing it. It just means you'll probably get away with it less often and close enough will be good enough. And don't think a refractor sidesteps collimation either, as so many owners do. If the lens cell was aligned at the factory, I'll bet you the proverbial pound that the focuser won't have stayed put and most diagonals can be well out of whack too. The difference is that a Newt will tend to benefit from an occasional collimation tickle , whilst once set, the refractor will stay that way for longer unless who give it a hard knock.

Either way, £20-30 spent on a Cheshire Collimator (and a few hours on Youtube trying to work out how to use it!) will make a lot more difference than spending an extra £30 on any EP, especially when you're looking at planets. And before you ask - Don't get sidetracked by lasers....

I'm not sure why the even slower F8-F10 Newts have virtually disappeared - presumably it's because there's some sort of fashionable bragging right about having a fast scope? I mean, a 6" F10 Newt isn't really any longer or heavier than a 5" F10 Achro refractor. But it is potentially cheaper, will have zero chromatic aberation, be essentially coma free and won't have you grovelling around on the floor, with cranked neck, just because you chose to point it near zenith.

Likewise, the race to ever shorter Newtonian focal ratios for use in a Dobsonian leaves me completely non-plussed. It makes sense with really big mirrors, as it avoids the use of step ladders, but for purely visual purposes an F7 10" Newt makes a lot more ergonomic sense than the generally available F4.7. I guess it's an economy of scale thing.

Russell