Focal length, ratio, etc.

[Tim Armes]

Thank you all for trying to sort out my head.

The book in question which got me into this mess is in French, called "Guide de l'astronome débutant". I don't recommend it!

Reading it again I realise that I misunderstood part of it, since at one point the author does state that the resolution is dependant on the aperture, not the ratio. On the other hand, he also states the following:

"Even if the ratio isn't directly responsible, we often see that a large ratio corresponds to an instrument of high resolution, and a lower ratio to an instrument of lower resolution

The ratio also dictates how easy or hard it will be to obtain a lot of luminosity. A large ratio = difficulty to obtain luminosity, a small ratio = easy to obtain luminisity.

The larger the ratio, the higher the contrast.

We quickly see the dialemma - impossible to have high resolution, constrast and luminosity at the same time."

From the above, he then concludes that for planetary viewing:

"To view the planets we need high magnification, but to do the effectively we need to start with an image of high quality - in other words, a high resolution". Resolution is therefore important, whereas luminosity isn't since the planets are already very bright.

To obtain these characteristics we favorise:

1) A large diameter - doubling the diameter will double the resolution

2) a high ratio"

So, here there are my questions/sticking points

a) is he right when he states: "we often see that a large ratio corresponds to an instrument of high resolution, and a lower ratio to an instrument of lower resolution"

why? We've established that resolution is dependent on aperture, not ratio. Even given that this is a generalisation, I don't see how he's arrived at it.

c) his point 2, above, therefore seems wrong to me. Surely diameter is the only important factor (optical qualities aside).

d) it seems to me that focal length determines only how wide an angle the telescope can view with a given eyepiece, and has no other importance - is that reasonable?

e) Am I correct in understanding that the only affect that ratio has on the image is the sweet spot size. This is only of interest to photographers who which to match the size of the pixel sites on the sensor with that of the focal point size of the telescope in other to get the greatest possible image quality.

[Moonshane]

a) is he right when he states: "we often see that a large ratio corresponds to an instrument of high resolution, and a lower ratio to an instrument of lower resolution" I suspect he is confusing contrast and sharpness with resolution. my 6" f11 has more contrast and sharpness on planets and the moon (and double stars) generally than my 12" f5.3 dob and is often better when the seeing is poor on bright objects. that said, when the seeing is good, the 12" is awesome and much more detailed than the 6" - this is resolution!

why? We've established that resolution is dependent on aperture, not ratio. Even give that this is a generalisation, I don't see how he's arrived at it. see above which I believe is correct.

c) his point 2, above, therefore seems wrong to me. Surely diameter is the only important factor (optical qualities aside). I agree.

d) it seems to me that focal length determines only how wide an angle the telescope can view with a given eyepiece, and has no other important - is that reasonable? for visual observing yes. eyepieces also affect the field of view of course.

e) Am I correctly in understanding that the only affect that ratio has on the image is the sweet spot size. This is only of interest to photographers who which to match the size of the pixel sites on the sensor with that of the focal point size of the telescope in other to get the greatest possible image quality.

I would say yes, but in my experience, and as Olly says, it's easier to find focus with a slow scope than with a fast scope at the same magnification. this feels like more depth of field but is 'sweet spot related'.

[Tim Armes]

suspect he is confusing contrast and sharpness with resolution. my 6" f11 has more contrast and sharpness on planets and the moon (and double stars) generally than my 12" f5.3 dob and is often better when the seeing is poor on bright objects.

Ah, ok, that makes sense. It's sharper because the mirror/lens is easier to make for slower scopes. Why do they have better constrast though?

Given that, that's probably why he prefers the higher ratio too. Luminosity isn't as important has sharpness. Now it all makes sense.

Thanks,

Tim

[Moonshane]

as I understand it, basically a longer focal length allows a smaller secondary obstruction so there is less contrast lost.

I think this illustrates that you can have specialist scopes and generalist scopes and everything is a compromise one way or another. e.g. a very small secondary would have excellent contrast but you would lose a lot of illumination at the edge of the field. for planetary use this would not matter, might for lunar but certainly would for DSOs, especially extended ones. a very large secondary would (I think anyway) be better for photography but you'd lose a lot of contrast for visual where you really need as much as you can get. most off the shelf scopes have a mid range secondary which covers most of the situations we need the scope for pretty well but don't excel in any area (other than excellent value for money).

[ollypenrice]

Thank you all for trying to sort out my head.

The book in question which got me into this mess is in French, called "Guide de l'astronome débutant". I don't recommend it!

Reading it again I realise that I misunderstood part of it, since at one point the author does state that the resolution is dependant on the aperture, not the ratio. On the other hand, he also states the following:

"Even if the ratio isn't directly responsible, we often see that a large ratio corresponds to an instrument of high resolution, and a lower ratio to an instrument of lower resolution

The ratio also dictates how easy or hard it will be to obtain a lot of luminosity. A large ratio = difficulty to obtain luminosity, a small ratio = easy to obtain luminisity.

The larger the ratio, the higher the contrast.

We quickly see the dialemma - impossible to have high resolution, constrast and luminosity at the same time."

From the above, he then concludes that for planetary viewing:

"To view the planets we need high magnification, but to do the effectively we need to start with an image of high quality - in other words, a high resolution". Resolution is therefore important, whereas luminosity isn't since the planets are already very bright.

To obtain these characteristics we favorise:

1) A large diameter - doubling the diameter will double the resolution

2) a high ratio"

So, here there are my questions/sticking points

a) is he right when he states: "we often see that a large ratio corresponds to an instrument of high resolution, and a lower ratio to an instrument of lower resolution"

No, he is entirely wrong. Or at least, we might just as often see the reverse. The Keck is f1.75 but has (not that I've used it!) rather nice resolution. What he is doing is confusingly using f ratio as a synonymn for focal length. As I said earlier, if you pin focal length then reduce f ratio by increasing aperture you will improve resolution, not the reverse.

why? We've established that resolution is dependent on aperture, not ratio. Even given that this is a generalisation, I don't see how he's arrived at it.

c) his point 2, above, therefore seems wrong to me. Surely diameter is the only important factor (optical qualities aside).

He's at it again. A high ratio will increase focal length and so resolution at a given aperture. But you are right. A better way would be to leave f ratio alone and increase aperture to increase focal length and that would improve resolution for two reasons, not just one.

d) it seems to me that focal length determines only how wide an angle the telescope can view with a given eyepiece, and has no other importance - is that reasonable?

Yes. Maybe he had reflectors in his head because the slower the f ratio the smaller the central obstruction and the higher the contrast. In unobstructed systems this does not apply.

e) Am I correct in understanding that the only affect that ratio has on the image is the sweet spot size. This is only of interest to photographers who which to match the size of the pixel sites on the sensor with that of the focal point size of the telescope in other to get the greatest possible image quality.

I'd get a new book!

Olly

[acey]

"a large ratio corresponds to an instrument of high resolution, and a lower ratio to an instrument of lower resolution

Replace "ratio" by "aperture" and the statement becomes correct.

The ratio also dictates how easy or hard it will be to obtain a lot of luminosity. A large ratio = difficulty to obtain luminosity, a small ratio = easy to obtain luminisity.

If "luminosity" means image surface brightness (hence retinal illuminance) then the statement is completely wrong.

The larger the ratio, the higher the contrast.

Completely wrong. Contrast is (essentially) the ratio of object surface brightness to background surface brightness. Any telescope will reduce both of these by the same amount, so that contrast is unaltered. The reason why DSOs become easier to see as they are magnified is that we are better able to perceive low contrast objects when they are larger. In practice, contrast is also affected by stray light and diffraction (not to mention thumbprints on eyepieces), but that's a separate issue.

To obtain these characteristics we favorise:

1) A large diameter - doubling the diameter will double the resolution

Correct (where "resolution" is taken to mean the inverse of the Dawes or Rayleigh limit) - and doubling aperture will also increase light-grasp by a factor of 4. Only downside is that you'll end up with a bigger, heavier and more expensive scope.

So, here there are my questions/sticking points

a) is he right when he states: "we often see that a large ratio corresponds to an instrument of high resolution, and a lower ratio to an instrument of lower resolution"

why? We've established that resolution is dependent on aperture, not ratio. Even given that this is a generalisation, I don't see how he's arrived at it.

c) his point 2, above, therefore seems wrong to me. Surely diameter is the only important factor (optical qualities aside).

d) it seems to me that focal length determines only how wide an angle the telescope can view with a given eyepiece, and has no other importance - is that reasonable?

e) Am I correct in understanding that the only affect that ratio has on the image is the sweet spot size. This is only of interest to photographers who which to match the size of the pixel sites on the sensor with that of the focal point size of the telescope in other to get the greatest possible image quality.

a) No.

You've correctly answered this.

c) You're right - he's wrong.

d) Yes - and low f-ratio makes for a smaller scope.

e) No, you've already said it has an effect on field of view, which is also a consideration.

Conclusion - ditch the book.

[Bazzaar]

Either a lot of fact didnt translate well from french or it was just plain wrong to start with.

Either way, find another book.

Perhaps the best that could come from this thread is recommendations for a reference source?

Barry