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8" Acromatics?

Started by tomdisco, 05/24/2003 04:48PM
Posted 05/24/2003 04:48PM Opening Post
There are several 6" f/8 acromatics made today. Why are there no 8" models? An 8" f/6 would require the same length tube. Would the color problems be all that much more severe than in a 6"? Are there other reasons for no mass produced 8" refractors? Really curious about this.
Posted 05/24/2003 05:19PM #1
The cost of materials and labor for producing a refractor with optics better than 1/4 wavelength grows dramatically with the diameter of the aperture. Coating big lenses is much more difficult, also. The matter of spurious color would be greater in a f/6 refractor than in a f/8, although many achromat enthusiasts would say a Chromacorr would fix most of the problem. It would be an interesting exercise for a producer such as Burgess to attempt, but I would guess it would be a pricey item. Furthermore, the weight of an 8" OTA would require serious mounting, increasing an already steep cost curve. Nontheless, I would be willing to bet there is a market for such a creature out there.


Walter Locke
"The Fearless Spectator"

There are 10^11 stars in the galaxy. That used to be a huge number. But it's only a hundred billion. It's less than the national deficit! We used to call them astronomical numbers. Now we should call them economical numbers.
Richard Feynman
US educator & physicist (1918 - 1988)

Posted 05/26/2003 06:22PM #2
I have the 8.5" f 12.5 Chinese made refractor lens in cell in a tube I made. It tests well, except for a slight turned edge which led me to stop it down to 7 3/4". It gives very nice low power views when seeing allows, and can be good on planets to over 300 X. It shows some slight color on planets, and on stars at higher powers, though not much at under 100 X.

There Ain't No Such Thing As A Free Lunch [TANSTAAFL], a principle of the universe [see Einstein, Murphy, Feynman, et al.] applies. You can't have larger aperture and shorter focal length without increased color, all things being equal.

This, in itself, is not necessarily a problem, if colored halos don't distract; but the color is also only a symptom of what's worse, loss of detail and contrast resulting from colors coming to focus at points perpendicular to the image plane. Since the colors most affected are at the ends of the visible spectrum, the effects may not be immediately obvious, until you compare the view to what you see in a similar aperture/focal length apochromat.