Are 500–800mm mirror lenses a good choice for DSLR astrophotography?
Asked 6/11/2011
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I'm considering one of the inexpensive 500–800mm catadioptric (mirror) lenses for DSLR astrophotography. These lenses are typically around f/5.6 to f/8 and produce donut-shaped out-of-focus highlights, which doesn't seem important for astronomy.
With a suitable equatorial mount and accurate tracking, can a mirror lens work well for astrophotography? Aside from needing longer exposures because of the slower aperture, are there any major drawbacks compared with more conventional telephoto lenses or small telescopes?
Originally by rfusca. Source · Licensed CC BY-SA 4.0
rfusca
15y ago
2 Answers
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A slower aperture means you need to take a longer exposure to get the same brightness as a faster aperture with a shorter exposure. As long as you have an equatorial mount with smooth, accurate tracking, this shouldn't cause problems.
The atmospheric seeing is also an issue for ground based telescopes, and with long exposures it causes rings to appear around point sources. However the resolution of long exposure images is inversely proportional to the diameter of the scope, up to a certain point. That means that as your telescope gets smaller, the atmospheric seeing has a larger impact on the resolution of your image. Since reflector telescopes have larger diameters for less money than refractors, they are better equipped to overcome the seeing.
It's worth noting that all of the major telescopes are very large reflectors, including Hubble and JWST.
Originally by Carson Myers. Source · Licensed CC BY-SA 4.0
Carson Myers
15y ago
0
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Yes—they can be used for astrophotography, but they’re not automatically an ideal bargain.
The main practical issue is speed: at f/5.6–f/8 you’ll need longer exposures than with a faster lens to reach the same image brightness. If your equatorial mount tracks smoothly and accurately, that by itself is not a deal-breaker.
At these focal lengths, tracking accuracy becomes more critical, so the mount matters a lot. Atmospheric seeing also limits sharpness in long-exposure images of point sources, and that can become noticeable at longer focal lengths.
A broader point is that aperture diameter matters for resolution. Larger-diameter optics can better overcome seeing limits up to a point, which is one reason reflector telescopes are popular for astronomy. So while a mirror lens can work, a purpose-built small reflector telescope may offer better performance for the money.
In short: a mirror lens is appropriate if you already have good tracking and accept longer exposures, but it may not be the best astrophotography option compared with dedicated telescope setups.
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