The long moment arm of this tube means that a very robust mount is needed to avoid having the scope twist in every breeze. The 6” f/15 is not an easily portable telescope. Even with a big mount on a tall tripod, when looking at objects near the zenith you may be lying on your belly to look through the scope. Handling heavy components above your head in the dark is no fun. The tube assembly requires a robust mount on a tall tripod putting an equatorial head high above the ground. The most problematic characteristic of a 6 inch f/15 is the extremely long tube about 90 inches (nearly 8 feet). I have a six inch f/10 Jaegers which is a superb optic but the CA really bothers me. The CA is far less than what is seen in a 6 inch f/10 or a 6 inch f/8. But, most people will never notice it, unless they are straining to see the very slight purple fringe around Jupiter or some such. You don’t need hyperfine focus and a multispeed focuser.Ī perfectionist will point out that the Chromatic Aberration (CA) is less than perfect in a 6 inch f/15. When using such a scope you don’t have to constantly readjust the focuser. Defocus due to atmospheric disturbance is less pronounced than in faster scopes. You are not tempted to tweak the focus all the time as you are with a shorter focal ratio scope.The forgiving optical behavior of such a scope reduces focusing problems when the seeing is less than ideal. Of course, it cannot defeat all the effects of seeing but it seems to achieve that because the defocus range is more generous. Some people say an f/15 refractor “beats the seeing”.And, the eyepieces can be relative long focal length for the same power making the eye relief much more comfortable. This design delivers superb views with even the simplest ones. No need for expensive eyepieces either.The focal length is generous, making the observation of planets and the moon at high power easy with comfortable, long focal length eyepieces.The color (Chromatic Aberration) is nearly zero far better than an f/10 achromat.There is no secondary obstruction or spider to reduce contrast in a refractor.Such a scope has many desirable features: Though the aperture is a bit small by modern standards, such an instrument still makes a great choice for a backyard astronomer. For many years a traditional 6 inch f/15 achromatic doublet refractor like the one on the left has been a leading choice for observing planets, the moon and double stars.
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