05-04-2013, 03:33 PM
Astronomical optics tend to be optimised to produce good results at infinity only. They don't have to worry about close focus. Doublet designs give some degree of CA and other aberration correction, and the more exotic the glass used, the better that can get. They can also use more elements too, with the price rapidly rising as the complexity does.
Out of interest, which Sigma 300/2.8 was it? The 120-300 OS isn't critically sharp at f/2.8 but does improve stopped down a bit. I do find AF/OS of a lens to be very useful though, so astronomical scopes wouldn't be my choice unless I had to camp on a single spot.
I've got a 600mm f/5 I can fish out with examples if you want an example of a budget scope (current typical price £250 with basic manual mount/tripod).
Out of interest, which Sigma 300/2.8 was it? The 120-300 OS isn't critically sharp at f/2.8 but does improve stopped down a bit. I do find AF/OS of a lens to be very useful though, so astronomical scopes wouldn't be my choice unless I had to camp on a single spot.
I've got a 600mm f/5 I can fish out with examples if you want an example of a budget scope (current typical price £250 with basic manual mount/tripod).
<a class="bbc_url" href="http://snowporing.deviantart.com/">dA</a> Canon 7D2, 7D, 5D2, 600D, 450D, 300D IR modified, 1D, EF-S 10-18, 15-85, EF 35/2, 85/1.8, 135/2, 70-300L, 100-400L, MP-E65, Zeiss 2/50, Sigma 150 macro, 120-300/2.8, Samyang 8mm fisheye, Olympus E-P1, Panasonic 20/1.7, Sony HX9V, Fuji X100.