Hi Matt,
Yes, I love fracs, and Chris Lewis' review on this cheaper and versatile frac has a very important role. I am not an expensive taste amateur, believing in 90% can be obtained from modest scopes, leaving the next 10% to expensive scopes. Astronomy can be accessible to most on very modest budgets with decent brands catered for mass produced scopes, and Saxon, Bresser and Skywatcher are good brands with their achro fracs.
I only venturing out recently to reflectors and now trying short(er) tube fracs recently. Reason is I have one of those little 50mm f360mm (f7.1) Chinese refractor scopes with 0.965 focuser and 90 degree diagonals as a finderscope, replacing my original straight thru 50mm finder scope.
It is pain to use a straight thru finder when you are pointing up the zenith. I was quite happy with the quality of f7.2 on these 50mm fracs, ie, f7 is versatile as a grab and go and wide field short(er) tube scope. That's why I picked up this 70mm f500 to try it out.
Agree wholeheartedly, very common or the norm to stop down short tube frac so they deliver better images. In my case, by stopping 70mm to 60mm on my unit by the maker, it is no longer f7.1 but f8.3 and using better part of a lens margin in a 70mm as opposed to the edge of a 60mm to form an image, as such, it should deliver nice images. The Q is what happens if we open up and let it be a full 70mm instead? Time will tell, will keep you and raymo posted if I managed to get to that baffle with my basic tools.
I have a Tasco 60mm f15 scope, but it is a dud at the moment, the doublet lens needs to be rotated to find the sweet spot as it is badly out of alignment when I got it, QA is not a necessity or a must with mass produced units out of China.
Wow, you have 6 inch f12! The crisp and sharpness must be fantastic on a f12. Yes, I love my 70mm x f900, the Airy disks on stars and splitting double stars is a joy on this frac. Long achro fracs has its place with some of us, as much people move onto the modern bg $$$ frac cousins such as Apo, ED, fluorite etc.
The old Crown and Flint doublet design in long f ratios will last a long time if well care for. We have our 40 inch Yerkes and 36 inch Licks doublet fracs as testament to their durability and purpose. Extract below is a reminder of the old doublet frac versus the new fracs:
http://www.jayreynoldsfreeman.com/Au...s/GBU.text.pdf
They were comparing high end 5 inch modern Tak, Meade ED, AP refractors on various objects including Jupiter, then trained the 36 inch Lick Observatory doublet frac on Jupiter as a comparison:
There is one final bottom line: 3.) APERTURE WINS. Remember, this test was conducted at Lick observatory. At the end of the evening, we all went inside and had a look at Jupiter through the 36-inch refractor. This instrument was loafing -- its 55 mm Plossl eyepiece delivered 316x and an exit pupil nearly 3 mm in diameter -- one we associate more commonly with looking at galaxies and emission nebulae than at planets. Even at f/19, a three-foot conventional doublet has a lot of secondary color, and although the seeing was very good, it was not perfect for such a large instrument. Nevertheless, the 36-inch blew us all away. Features only hinted at in our puny five-inch instruments were shown clearly and with lots of detail in the big refractor. How humbling, to be reminded that the best of our modern, high-tech, high-end instruments was barely qualified to replace the finder on this century-old leviathan.
Thanks again, you must be real proud of your f12 yard cannon, must be a real joy to own such a fine precision scientific instrument.
Kind regards,
Bill