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There's a whole host of inexpensive mirror lenses available in the 500 mm - 800 mm range (this for example). They have slow max apertures (f/5.6-8) and a donut shaped bokeh (not a problem for astronomy work), but it seems that with suitable tracking they could be excellent for DSLR astrophotography.

However, I can't find anything one way or the other in regards to this.

Are these lenses appropriate for DSLR astrophotography work?

Apart from longer (and more accurate because of the focal length) tracking times because of the slow aperture, is there some glaring issue with them?

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rfusca
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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.

Carson Myers
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