Siegfried, in my opinion, Erik's only misstep was equating an obstructed 70" scope to an unobstructed 41 inch. Large unobstructed telescopes are not practical. It would be better to compare apples to apples and just leave the secondary obstruction out of any arguments. Keep the playing field level. There's nothing to be gained from downgrading a particular configuration because it isn't a theoretical, unobtainable best case. The hypothetical 63.7" scope if obstructed similarly would have even less light-gathering power. The same argument can be made for resolution. Don't compare large obstructed reflectors to large unobstructed reflectors unless it's just a mental exercise. Otherwise the only practical information will come from talking about small shiefspieglers and refractors, entirely different animals from huge, fast reflectors. On Tue, Oct 29, 2013 at 6:22 PM, Siegfried Jachmann <siegfried@jachmann.org>wrote:
Eric, that is dead wrong. You are getting the resolution of a 70" and the light gathering power of a 63.7" scope.
On Tue, Oct 29, 2013 at 1:29 PM, Erik Hansen <erikhansen@thebluezone.net
wrote:
I was not suggesting a smaller secondary, I was suggesting a smaller primary with a more efficient design.> The true aperture is 41 inches, so you are getting the resolution of a 41 inch scope, my point is a 45 inch primary with a more conventional secondary would give same light gathering with probably same ladder height as a 70 inch with a 29 inch secondary. I doubt it puts the Grim to shame, it would be interesting to compare the 2 at SPOC, and hear what the comments are.
The other issue, is what is the largest field of view with the scope? With some eyepieces any object centered will not be there long. The time it takes someone to climb down the ladder and another one up, the object will be out of the field of view....will they have more than a minute or less than a minute.
True, but a smaller secondary would also mean a taller ladder. As built,
the observing position when pointed at the zenith is only about as high as you'd need to go with the 22" that Brent built.
Reducing the secondary diameter also only gains a very small percentage of the total area of the 70" mirror.
And the diffraction effects from a 40% obstruction won't be an issue with deep-sky objects, and should be offset by the increased resolution of a massive 70-inch aperture.
There are always trade-offs.
On Tue, Oct 29, 2013 at 12:45 PM, Erik Hansen <erikhansen@thebluezone.net>wrote:
of course the draw back is you get a shorter tube but a 40% obstruction of the primary, probably could have a used a smaller mirror, to reduce tube length, and get similar light gathering.
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