Hi All,
Quote:
Originally Posted by Wavytone
Chris,
For the 5"-7" aperture range mak-newtonians are underrated, IMHO. The basic geometry is the same as any newtonian, ie can have a smallish secondary, however the use of spherical surfaces means no coma, and a flat focal plane is possible ...
... The alternative is a newtonian with coma corrector of one kind or another near the focal plane. This solution is cheaper but inferior, and there's no practical limit on the aperture (big cheap mirrors are available).
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While I agree that the Maksitov design eliminates coma, coma only becomes truly obtrusive in a Newtonian when the f/ is very short (ie f/5 and faster) and the star is far from being on axis -- ie when using a wide visual field at low power.
A Newtonian with a coma corrector the only other solution ???
What about a "slowish" Newtonian -- like one I used to own a 10" f/6.2 (1570mm f.l) with a 46mm 18% central obstruction? At f/6.2, I concede the tube wasn't "short" and needed a good mount to bear its mass and length, but the planetary images were absolutely stunning.
A 27% obstructed 9.1" Intes Makistov isn't a "smallish" central obstruction -- it is moderate verging on large and will affect contrast particularly on planetary images. This is settled physics -- though people still tend to try and argue it away with their own perceptions at the eyepiece or some form of argument from authority. You can arger' with me son but you can't arger' with figures -- as Foghorn Leghorn was wont to say.
In a perfect (unobstructed ) telescope, 84% of the incoming light will make it into the centre of the Airy disc and 16% into the surrounding diffraction rings. This effect is caused by the wave nature of light, the circular shape of the entrance pupil (aperture) and nothing else. If you introduce a central obstruction first and then progressively increase it in diameter more and more light is redistributed from the central disc and into the rings. At a 20% obstruction a further 8% is re-distributed -- ie 76 -v- 24%. by the time you get to 30% an additional 16% is re-distributed from the disc to the surrounding rings -- ie 68 -v- 32%. The difference between say my old f/6.2 and your Intes is about 10% more energy illuminating the rings rather than the central disc.
So how does this affect a visual planetary image? Well the image of a planet you see in the eyepiece is a mosaic of adjoining/overlapping airy discs and the more energy/illumination there is in the rings, then the less contrast you will perceive because the rings overlay the adjacent discs.
http://www.brayebrookobservatory.org...m/c-o%27s.html
If you are doing planetary observing coma is no issue at all -- the planet is in the centre of the field and you are using a narrow field with a small exit pupil. Nobody observes planets out near the edge of the field. Coma only become obtrusive at low f/ratios at very low power.
One of the nice things about building a Newtonian from the ground up rather than buying one off the rack is you can deliberately optimise various attributes by changing focal lengths and secondary obstructions.
In the U.S (in particular) there is a strong movement (fashion?) toward large ultra-fast Newtonians. If you want a 24" telescope **and** you want to observe with both feet on the ground **and** you want increased portability that's fine -- but there is a price to be paid for that -- a much increased secondary mirror **and** coma **and** decreasing tolerance of slight mis-collimation (and much increased fabrication costs for the mirror). If people want to pay that price, that's their business and its a matter of priorities I guess. But here's the thing: It is a price that must be paid no matter how you argue it -- that's simple physics
My latest 'scope is the first one I never designed and had no part in building, a 25" f/5 "classic" Obsession. The price I have to pay for exquisite image quality (with a 14% central obstruction -- almost negligible) is a fairly narrow native field (smallest low power is x104 and 48 arc-minute field), poor portability and being several steps up the ladder nearly all the time.
Many people are not willing to pay that price but I do not need portability. I observe in my front paddock 80m from the house and the 'scope moves only 5m fully assembled from storage to observing. My own personal view is that the exquisite image quality exceeds the nuisance value of trips up and down the ladder. That was my choice to pay that price rather than the other one: 31% obstruction at say f/3.2 (7" secondary) producing decreased image quality + vastly increased mirror fabrication cost + zero tolerance to mis-collimation.
But to re-iterate -- there are a lot of very good and attractive reasons to own a Maksutov -- or for that matter a Newtonian, a Refractor or indeed a Schmidt-Cassegrainian. Maksitovs are excellent performers for several applications and they are very good general use telescopes, but given a 27% central obstruction, they are not super-outstanding planetary performers compared to similar-sized 'scopes with a much smaller or better no obstruction.
Best,
L.