Quote:
Originally Posted by Wavytone
+1 agree with Steffen.
Comparing my 7" f/15 Mak with 8-12" newtonians:
A long Newtonian, say f7 or f8 with a single curved vane spider, or alternatively a maksutov Newtonian (no vanes at all) should be considerably better. By way of example the 16" at Mt Bowen (f7) provides superlative views of the planets and easily better than a C14.
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If you think you can only get exceptional planetary images from a long focus Newtonian you are still in the stone age. With modern mirror making techniques and scope construction techniques it is easy to have Newtonians as fast as F4.5 delivering exceptional planetary views, the equal of long focus Newtonians. The critical things are that the ability and skills of opticians has improved enormously in their ability to make mirrors in the F4 to F5 range very well and the scope designs and low profile focusers have enabled the secondary obstruction to be kept under 20%. Both my F4.5 scopes have an 18% Central Obstruction.
Assuming you have found a good optician and have a central obstruction under 20% the only optical disadvantages of a fast Newtonian these days are:-
1) Collimation is more critical to produce good performance. It's no harder to collimate a fast scope but the faster the scope the better the collimation needs to be to produce high grade images.
2) Depth of focus is reduced making it more difficult to hold a good focus at higher powers under variable seeing conditions.
There are of course a large number of mechanical factors which make a fast scope more desirable than a slow scope
Cheers,
John B