Fast glass

[poider]

G'day, I am still learning photography and have a basic understanding of shutter speed and aperture, I understand that fast glass means smaller f numbers (larger aperture) and means the aperture will open wider to let in more light in less time thus reducing blur.
I hear all the time that for astrophotography you need fast glass....why?... from what I understand the main 'tool' for getting more detail is to leave the shutter open for longer periods of time.....wouldn't this negate the need for fast glass???
Peter

[janoskiss]

f-number determines how much light per unit area falls on your detector/CCD/film. This figure is independent of focal length and aperture and the f-number is all that matters.

Each 1.4-fold* increase in f-number (each f-stop increase) requires doubling your exposure time; i.e., at f/5.6 you get half as much light as at f/4.

*The exact figure is square root of 2 = 1.4142... but in practice 1.4 is plenty good enough.

PS. If you really want to learn this stuff properly, get an elementary textbook on optics to begin with. The technical aspects of photography are all about basic geometry and physics/optics.

[thegableguy]

Longer exposures open you up to guiding problems, particularly at long focal lengths. Other things being equal, a larger aperture will allow shorter exposures, reducing the possibility of streaky stars.

The reason we use long exposures is because we have to, as the targets are very very dim. It's not really by choice!

[gts055]

"f-number determines how much light per unit area falls on your detector/CCD/film. This figure is independent of focal length and aperture and the f-number is all that matters."

Hmmm, I would have thought that given two telescopes, one having twice the diameter of the other but both with the same f ratio. Then the larger diameter will give greater resolution given the same imaging time. Mark

[thegableguy]

Yes, but from a section of the sky twice as large. The amount of light will be the same, but one is using a larger area to gather it and you'll get less magnification.

That's why it costs a fortune to have both long focal length AND large aperture, both in photographic lenses and telescopes. You can get one at the expense of the other, or you can pay big dollars to get both.

[Camelopardalis]

It's the other way around...if both scopes have the same f-ratio, but one has larger aperture, the larger aperture will have longer focal length and consequently smaller FOV...effectively zooming in further, thus spreading the light from a smaller area over the sensor.

[Camelopardalis]

In theory...although what you actually get will depend on the angular resolution of your pixels (i.e. your pixel size) and whether or not you are seeing-limited...seeing is the great equaliser to resolution.

[thegableguy]

Sorry, my bad.

So yeah, compare a 102mm f/5 achro to a 200mm f/5 Newt and you see the vast difference between them. One fits in a backpack, the other is a meter long and 11kg. Same focal ratio but very very different telescopes,one costing about triple the other. To keep a fast focal ratio as you increase the focal length gets very expensive very quickly. Same for camera lenses - compare prices for f/2.8 zooms to f/4 zooms.

Point remains. The faster your ratio, the shorter exposures you can take, and that's a good thing - but with long focal lengths the fast tubes cost a lot more.

[janoskiss]

As far as resolution goes, in the image plane (i.e. at the detector/sensor/CCD/film), it is again only the f-number that determines the absolute resolution (in microns; if you want a formula, ask me; I don't know off the top of my head but I can work it out).

For typical modern DSLRs you start seeing the diffraction limit of your lens/scope at approximately f/20 or slower. Sometimes that is desirable (e.g., planetary imaging): it amounts to oversampling the image plane, which can make processing easier.