Why can a telescope outperform a long DSLR lens for astrophotography?
Asked 7/31/2020
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If a telescope is used as the camera’s lens for prime-focus astrophotography, why is it often better than using a long photographic lens such as a 150-600mm zoom with a 2x teleconverter? A lens+teleconverter can reach very long focal lengths too, especially on a crop sensor. Technically, what makes a telescope preferable for planets or deep-sky imaging?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
5y ago
2 Answers
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In principle, there's no difference between a camera lens and a refractor telescope.
While focal distance is related with the magnification of the image, in astronomy, resolution is related to aperture (not the F-ratio, but the area where light enters). So, if you increase your focal distance, without increasing your aperture, then you are not increasing resolution at all!
Every glass element also have some transmission loss. That is: reflected or absorbed light that does not reach the sensor. And for faint distant galaxies and deep sky objects, is very important to minimize transmission loss.
At last, remember that teleconverters not only increase your focal distance, they also increase your f-ratio. Since the lens/teleconverter device has more focal distance with the same aperture area.
In the end, a telescope has some serious advantages over a zoom lens with teleconverter:
- No teleconverter means fewer elements and less transmission loss. Also, teleconverters may, and usually will, degrade image resolution and quality
- Zoom lenses have way more elements than prime lenses and telescopes. More transmission loss and more image degradation.
- Same quality and aperture tends to cost less, since you don't have unneeded features like autofocus and variable aperture.
- If you use reflector telescopes, same apertures cost even less. And you may reach 10 or 12 inches of aperture.
- Telescopes are compatible with astrophoto-dedicated cameras, and astrophoto-dedicated filter rings.
Of course, for wide field astrophotography, a camera lens is the best option. But I'd use a prime lens.
Originally by user83011. Source · Licensed CC BY-SA 4.0
user83011
5y ago
0
Generated from our catalog & community — verify before relying on it.
You don’t strictly need a telescope for astrophotography. For wide-field night-sky shots, camera lenses are often the better choice.
Where telescopes usually win is at high magnification and low light:
- Aperture matters most. In astronomy, resolution and light-gathering depend primarily on the entrance aperture, not just focal length. Making focal length longer without increasing aperture does not add detail.
- Teleconverters cost light. A 2x teleconverter increases focal length but also raises the f-number, so the image gets dimmer.
- Fewer optical elements can help. Telescopes, especially reflectors and simple refractors, often have fewer elements than a zoom lens plus teleconverter. Fewer elements generally means less transmission loss, scatter, reflections, and aberration.
- Planetary vs. wide-field use differs. For planets and other small targets, telescopes are usually better because they provide large aperture and long focal length efficiently. For wide-field astrophotography, a good camera lens is often excellent.
So the key point is: a telescope is not magically different from a lens, but for many astro subjects it gives you more useful aperture and cleaner high-magnification optics than a long zoom with a teleconverter.
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