Quote:
Originally Posted by Wavytone
Fast Newtonians... As low-to-medium power wide-field light buckets they're fine but don't expect them to perform well at high magnifications. They are popular only because:
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I think this is a very old fashioned approach and shows some narrow-mindedness and naivety in regard to modern Newtonian telescope design, construction and performance.
I don't particularly like Newtonians faster than F4, for a whole range of reasons, the main one being the fact that I have seen more poor quality sub F4 mirrors, than I have seen decent ones and these have all been from reputable US opticians. Myself and a large number of other observers share the view that some of their best lunar and planetary views have been in medium to large aperture Newtonians ranging in size from 12" to 20" and in F ratio from F4 to F6.
I have used all types of telescopes including Maks (Questar, Quantum, Intes & Intes Micro) ; countless SCT's from both Meade and Celestron, some of the best refractors money can buy up to 15" aperture (AP, Takahashi, TMB, Televue, TEC and D & G) and countless Newtonians ranging in quality from abysmal to exceptional in in aperture from 3" to 36" and in F-ratio from F3 to F12.
My best lunar and planetary views have come in my own 18"/F4.5 Obsession (OMI mirror) , my own 14"/F4.5 SDM (Zambuto Mirror) and James Pierce's 16"/F4 SDM (Lockwood Mirror). The problem is for a Newtonian to deliver its best images everything has to come together all at the same time. The scope has to be well built, it has to be properly cooled and collimated and the seeing has to be very good as large aperture scopes are more severely affected by poor seeing than small aperture scopes. To get a medium to large aperture fast Newtonian to properly strut its stuff takes some know how and patience, which many people that own them can't accommodate because they don't have the know how, patience, or both. Unfortunately 90% of Newtonians on the field at public star parties are poorly set up and will not be capable of delivering top quality high power views because of a combination of poor set up, poor collimation and inadequate cooling and air circulation.
When you get a newtonian in the 12" to 20" class that has a high quality thin mirror (faster cooling), in a good telescope structure with a good mirror cell that is properly collimated under excellent seeing, it will deliver better high power views by virtue of its greater aperture, hence greater resolution, than a smaller telescope of a different design.
An 18" AP refractor would undoubtedly give slightly superior views to my 18" Obsession under the same conditions, but unfortunately Roland isn't making them at the moment and I couldn't afford one if he was.
Cheers
John B