3 ms·

200mm are 7.9 inches. Are the focal length and the diameter roughly the same?

carl_dr 5y ago
Not at all, my 9.25” telescope has a focal length of about 2.4 metres - a focal ratio of f/10. You’ll recognise f/x from camera lenses and such. The diameter does control how much light the telescope “swallows” though, you’ll need a larger telescope to see less bright objects (or longer exposures, although when you are using your eye, that has a limit of course!) So in this case, the post about is saying you’ll need something about 200mm diameter to see the comet. Anything smaller, and you’re just not getting enough light to see it.
- SAI_Peregrinus 5y ago
  That said, if you don't care about seeing it in real time you can use a MUCH smaller lens and stack exposures. You'll likely want a star tracker (MASSIVE decrease in effort) to go with your camera, lens, tripod, intervalometer, and Bahtinov mask (cheap, makes focusing MUCH easier). https://www.youtube.com/watch?v=iuMZG-SyDCU https://www.youtube.com/watch?v=iuMZG-SyDCU is an excellent tutorial on how to do this without the tracker.
deleted 5y ago
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malfist 5y ago
Nope. Focal length is how far the light has to travel, diameter is the aperture, or how big the opening is to collect light. The XT8 telescope I references has an 8 inch aperture, and a 1200mm focal length
lmm 5y ago
GP is trying to point out that "200mm telescope" here means 200mm in diameter, not whatever arbitrary thing "200mm" apparently means in photography. The unit conversion adds more confusion, I don't know why they did that.
- SAI_Peregrinus 5y ago
  In photography, 200mm is the focal length of the lens. f/X is the focal ratio, with X being some number/ So a 600mm f/4 lens means it's got a 600mm focal length and 600/4 = 150mm aperture diameter. The reason for this is that camera lenses have an "iris" that reduces the aperture; that blocks some light but increases the depth of field. This means that unlike a telescope a camera lens's aperture can vary, so it doesn't make much sense to specify the absolute aperture for most situations. It's also because the focal ratio tends to be a more accurate way to estimate how bright an image will be than the absolute aperture. A 35mm f/4 lens will take an image with just about the same brightness (LV) as a 600mm f/4 lens, even though the 35mm f/4 is only 8.75mm in diameter compared to the 150mm diameter of the 600mm f/4. The shorter focal length means light gets gathered onto the sensor from a wider angle, which exactly compensates for the decreased diameter. Since photographers typically can't take hours-long exposures (or stack hundreds of photos to get the same effect) the way astronomers can, this system works better for photography.

Not at all, my 9.25” telescope has a focal length of about 2.4 metres - a focal ratio of f/10. You’ll recognise f/x from camera lenses and such. The diameter does control how much light the telescope “swallows” though, you’ll need a larger telescope to see less bright objects (or longer exposures, although when you are using your eye, that has a limit of course!) So in this case, the post about is saying you’ll need something about 200mm diameter to see the comet. Anything smaller, and you’re just not getting enough light to see it.

SAI_Peregrinus 5y ago
That said, if you don't care about seeing it in real time you can use a MUCH smaller lens and stack exposures. You'll likely want a star tracker (MASSIVE decrease in effort) to go with your camera, lens, tripod, intervalometer, and Bahtinov mask (cheap, makes focusing MUCH easier). https://www.youtube.com/watch?v=iuMZG-SyDCU https://www.youtube.com/watch?v=iuMZG-SyDCU is an excellent tutorial on how to do this without the tracker.

deleted 5y ago

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malfist 5y ago

Nope. Focal length is how far the light has to travel, diameter is the aperture, or how big the opening is to collect light. The XT8 telescope I references has an 8 inch aperture, and a 1200mm focal length

lmm 5y ago

GP is trying to point out that "200mm telescope" here means 200mm in diameter, not whatever arbitrary thing "200mm" apparently means in photography. The unit conversion adds more confusion, I don't know why they did that.

SAI_Peregrinus 5y ago
In photography, 200mm is the focal length of the lens. f/X is the focal ratio, with X being some number/ So a 600mm f/4 lens means it's got a 600mm focal length and 600/4 = 150mm aperture diameter. The reason for this is that camera lenses have an "iris" that reduces the aperture; that blocks some light but increases the depth of field. This means that unlike a telescope a camera lens's aperture can vary, so it doesn't make much sense to specify the absolute aperture for most situations. It's also because the focal ratio tends to be a more accurate way to estimate how bright an image will be than the absolute aperture. A 35mm f/4 lens will take an image with just about the same brightness (LV) as a 600mm f/4 lens, even though the 35mm f/4 is only 8.75mm in diameter compared to the 150mm diameter of the 600mm f/4. The shorter focal length means light gets gathered onto the sensor from a wider angle, which exactly compensates for the decreased diameter. Since photographers typically can't take hours-long exposures (or stack hundreds of photos to get the same effect) the way astronomers can, this system works better for photography.