Quote:
Originally Posted by RogerLightyear
Hi all,
I'm interested in a good value/performance refractor of approx 4"' aperture and wondered if there's any recommendations available in Australia other than f/9 doublets that are ~90cm long and triplets.
Do we have similar here? Thanks for any suggestions
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Apochromatic essentially means to be free of any false colour and spherical aberration. Some sources expand and define this to be free of spherical aberration at 2 wavelengths and free of false colour at 3 wavelengths.
The laws of physics (optics) determine that a 2 element objective needs a slower f-ratio (to be longer for a given aperture) to be apochromatic, than a 3 element objective. This is why the 3 element APO's (triplets) can be shorter than the 2 element APO's (doublets).
Despite the fact that newer technologies and glass types have narrowed the gap a lot and allowed for manufacture of some "decent" 2 element refractors at around F7 to F8, IMO if you want high quality optical performance in a shorter tube refractor, it needs to be a triplet.
As you mention you are just starting out and don't want to spend a fortune, which is understandable, which means for "decent" optical performance it needs to be F7 or F8, which is where I would be heading. I think you're better with something decent optically in priority to something short. If you want both it will be a triplet and a lot more money.
As Greg Bradley mentioned there are some pretty decent scopes available 2nd hand. I would prefer a high quality 2nd hand scope (unless its a gift to someone) as opposed to a new scope made in China or Taiwan. While optically some of the new stuff is reasonable, they don't have the same build quality and quality control as the premium Japanese manufacturers. I would be backing a 10 year old Takahashi FS102 to outlast and outperform a new Skywatcher doublet scope, every day of the week. It will also hold resale value better. The Takahashi FS102 is F8.1 but very good optically. Another scope to keep your eyes out for is the Vixen FL102S. Again this is a Japanese made Flourite doublet of the very highest quality and the equal of the Tak FS102.
Cheers
John B