Not Your Everyday Banana

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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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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.

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Are there any rules of thumb around your how long a 'long exposure" is before the rings become an issue for a given size? –  rfusca Jun 12 '11 at 2:29
    
@rfusca no, the atmospheric seeing is an issue for any length of exposure. The longer the exposure is, the more the effects of the atmosphere will blur into a ring. To minimize the effects, you need a telescope with a diameter larger than say 20 cm. Wikipedia on this subject. –  Carson Myers Jun 12 '11 at 4:46
    
Thanks! –  rfusca Jun 12 '11 at 4:46
    
@rfusca no problem! –  Carson Myers Jun 12 '11 at 4:48
    
For unresolved point sources the focal length and f# don't have quite the same effect as for a 'regular' exposure. The thing that matters for astrophotography is aperture. –  Martin Beckett May 4 '12 at 22:00
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