6 ms·
With that knowledge, we can do a back of the envelope calculation for what kind of lens it would take to make your camera take a video at 10 frames per second t
by doubleunplussed 10y ago
With that knowledge, we can do a back of the envelope calculation for what kind of lens it would take to make your camera take a video at 10 frames per second that looked the same as its 60 second exposures.
Basically we need 600 times more light. Square root of 600 is about 25. So you'd need a lens system with an initial aperture 25 times larger in diameter than the one you used to do the long exposures.
Big lens, but if you want to brute force this kind of problem, that's the way to do it!
- greglindahl 10y agoCheaper to use a telescope. It's easily possible to grind your own big telescope mirror. A big lens is like trying to build a big refracting telescope -- extremely expensive. The largest refracting telescope ever made is 100cm (40 inches). Mirrors are up to 10 meters.
- tigershark 10y agoMost of the telescopes are extremely dark compared to "normal" camera lenses. The brightest that I remember was the so called "camera Schmidt" with f/2.8. Normal Newtonians usually are in the f/4-f/5 range. The cassegrain and derived are around f/10. The big aperture in the telescopes is needed for the angular resolution, certainly not for the brightness.
- greglindahl 10y agoThe general physics is that the more photons there are (big aperture and/or bright source), the more opportunity you have to have high resolution.
- tigershark 10y agoYes, the resolution is given by the diffraction limit of the optics that is dependent by the diameter. But resolution is different from brightness. If you tried to take a video of the same scene with a telescope chances are that the final result would have been darker, even using the same camera. Granted that it would have had a much much better resolution (but a much much smaller FOV)
- greglindahl 10y agoSo you think astronomers are too stupid to trade resolution for brightness? Hint: downsample your image: you won't lose too much as long as the dark current isn't too large.
- tigershark 10y agoThey have completely different use cases. In one case you are making a video of a scene with a FOV of several tens of degrees. In the other you are observing objects with a maximum size of a couple of primes and with features measuring infinitely small fractions of arc seconds.
- tigershark 10y agoAs I said before the aperture of a lens has nothing to do with the final brightness. You can have a lens 100 times bigger, but if the original lens is f/2 and the humongous one is f/2.8 the first one will be more bright.