Quote:
Originally Posted by ausastronomer
No C14's or big Meade's for me either. They wouldn't even be allowed onto the same observing field as my 14"/F4.5 Zambuto powered SDM.
It doesn't need to be F7. My 14"/F4.5 Zambuto powered SDM gives planetary views the equal of anything I have looked through over the past 50 years, which includes Newtonians to 36", Refractors to 15" and Classical Cassegrains to 20". Very closely followed by James Pierce's 16"/F4 Lockwood powered SDM. Rod Berry's 20"/F5 SDM (The Mary Rose) with OMI Mirror is right up there as well. Unfortunately with a 20" x 2" thick mirror it took quite a while and favourable thermal conditions to properly stabilise and deliver its best views. Also with 20" of aperture seeing needed to be good as well. When thermals and seeing conditions co operated, the planetary views of Jupiter and Saturn through the Mary Rose were exceptional.
You haven't compared it to my 10"/F5.3 Suchting powered SDM in that case.
I have time and money, if you care to try it on ?
The days of extremely long focal length Newtonians are long gone. The main criteria to getting good planetary performance from a newtonian are:-
1) Premium Optics (primary, secondary and eyepiece). The optical system
is only as good as its weakest link.
2) Get the secondary obstruction under 20%
3) A thin mirror, which cools and stabilises quickly.
4) Good Collimation.
5) Good Baffling.
5) A good solid telescope structure (motorised tracking helps enormously)
The two advantages to a long focal length Newtonian over a short one is that the long focal length one will have a greater depth of focus, which makes things a bit easier for the observer. It doesn't improve the image quality. Similarly, the diffraction limited field in a long focal length Newtonian is larger than in a short focal length one, due to coma, but with a tracking scope and the image centred in the FOV, that is irrelevant in any case.
The disadvantages of a slow focal length Newtonian are numerous.
Years ago the number of medium to larger aperture high quality Newtonian mirrors in the F4 to F5 class was pretty small, hence people favoured slower F-ratios, as the mirrors were generally better, than the faster mirrors. That doesn't apply these days. Mark Suchting actually independently tested my 14"/F4.5 Zambuto mirror many years ago, at about 1/40th wave.
A well tuned high quality newtonian in the 12" to 20" class between F4 and F6 will give planetary views the equal of anything money can buy.
Cheers
John B
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Amen & Amen Bammo -- I've been preaching this here for some time. Leaving aside a similar aperture refractor, in terms of image quality on planets, a well made, optimised Newtonian with high-quality optics and a small secondary obstruction <20% will be the winner all other things (like a good mount) being equal.
One thing to be said for longer focal length newtonians is that it is easy to get a small secondary while maintaining a good-sized 100% illuminated field **and** that to get higher magnifications, a longer focal length eyepiece is allowed providing a bit more comfort on the eye and usually longer eye relief.
Best,
L.