[Totoro]

Originally Posted by AstroApprentice

Thanks guys,
The i-star lenses look very promising
http://www.istar-optical.com/r30%2c-...-doublets.html
I particularly like the look of the 180 mm F/8 - most of the others would require incredibly long tubes - such as the 250 mm F/20 5000 mm FL!!!

If I'm correct 180mm F8 achromatic objective has CA ratio(F / Dia in inch) about 1.13, according to this widely referenced chart, is classed as unacceptable. I'm not sure if the claimed Strehl ratio 0.91 is for single wave length or weighted full spectrum.

https://www.cloudynights.com/uploads...3834301145.jpg

The first one in your link 5" F12 wich CA ratio 2.4 should work decently though.

My 2 cents