Quote:
Originally Posted by OneCosmos
Thanks Glen,
Whilst having my coffee, dog patiently at my side, I’m browsing about this topic and whilst long ‘fracs have their advocates, there are more than a few folk who say there is virtually nothing to compare between the views through a classic f/15 and a Skywatcher ED equivalent size. The latter is easier to mount and probably way cheaper.
That isn’t the whole story of course but it would be foolish to not recognise just what good value SW refractors are these days. Pride of ownership is something else again. Being proud of a ubiquitous SW is less likely than a rare Unitron et al.
PS I suppose what I’m saying is classic long focal length refractors are great value compared to exorbitant APO triplets from the likes of APM, Astrophysics, Tak etc but not when compared to an SW ED. If long fl is better however, then that’s another story. For example achieving higher magnification with a larger EP is one reason to prefer long FL.
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Chris, you make some good points. In the SW ED range, the largest available is the 150mm f8 ED, currently selling for $2999 @ Andrews. Still is it really a long refractors in the classic sense, being an f8? It has come into some criticism because of SW's refusal to discuss the glass used in the Doublet, it's reputation for blue tinge. Early SW ED models, up to the 120mm, did use an FPL element I believe, but SW does not advertise them like that anymore. It could be argued, and has on the CN forum, that SW wanted to move to a lower cost objective, lessening the imaging community interest (after all they can push the Esprit as the scope for imagers.)
Looking at the economics, you can go to larger diametre Doublets, but many of the big objectives are expensive. Building your own long refractors is possible, as I did with my iStar. And iStar still builds good Stehl Doublets, but there is a cost.
I noticed that iStar are now offering Flourite APO objectives, but the cost for an 8" f7 is $11k USD. You can get an 8.7" f15 R35 for $4800 USD. The bargain of the bunch is imho, the 150mm f15 Achro Doublet at $700 USD, with a Strehl ratio of 0.95, and focal length of 2250mm - now that is a long refractors.
IStar website here:
http://www.istar-optical.com/achromatic.html
Use the top tabs to select which type of objective you want to see, full spec are there for them all.
D&G still sell long objectives as well.
http://www.dgoptical.com/refractor.htm
Their 8" f15 complete scope, seems pretty reasonable at $3700 USD, but the shipping would be a killer, then ad GST, etc, which is why it is so cost effective to build your own, imho.