Ultimate Planetary Telescopes

The debate is as old as the hills. What is the ultimate planetary instrument? Is there such a thing at all and what, exactly, do we mean by "ultimate"? I have touched on this subject already when discussing the relative merits of my own Newtonians and the long-focus Newtonian. One way of answering this question is simply to look at what telescopes the world's leading planetary imagers use. However, when we do this we just see a reflection of the market share of different telescopes. Damian Peach, arguably the world's finest planetary imager, and certainly the keenest, has achieved great success with Celestron's 9.25 (235 mm) and 11 inch (280 mm) Schmidt-Cassegrain telescopes, operated at f. ratios up to f/40 (Figures 3.13 and 3.14). In his view, the optics of Celestron's SCTs are superb, the only

Figure 3.13. Arguably the best value-for-money planetary telescope, the Celestron 9.25's are renowned for their planetary performance. A slightly longer f-ratio primary (f/2.5 not f/2.0) and lower "magnifying" secondary (4x not 5x) is thought to be responsible. Image: Damian Peach.

Figure 3.14. A well-thought-out and well-used planetary telescope. Dave Tyler's 280-mm Celestron 11 and 150-mm Intes Maksutov-Cassegrain mounted on his home-made German Equatorial mounting. The smaller telescope is invaluable for aligning the main telescope on the tiny planetary field. Image: Dave Tyler.

Figure 3.14. A well-thought-out and well-used planetary telescope. Dave Tyler's 280-mm Celestron 11 and 150-mm Intes Maksutov-Cassegrain mounted on his home-made German Equatorial mounting. The smaller telescope is invaluable for aligning the main telescope on the tiny planetary field. Image: Dave Tyler.

disadvantage with the design being that, like all SCTs, collimation does not stay fixed. A nightly check and tweak is always required. Damian has used both Celestron and Losmandy drives to support his Celestron Optical Tube Assemblies. The SCT is the most popular serious amateur telescope design there is, but it is not obvious why this should be. It is, optically, a halfway house in telescope design, not especially suited to either high-power or wide-field observing. However, it does have two huge advantages: SCTs are extremely compact and mass production has made them affordable. A compact telescope is user-friendly, portable, and lightweight, so only a modest drive is needed to mount it. Also, with tube assemblies weighing 15 kg or less (except for the largest models) the instrument can be stored indoors, so no observatory is required. Despite Damian's status in planetary imaging he has never owned an observatory. His telescope is set up outside every single night, even if there are only tiny cloud gaps. This nightly hassle would be impossible with a less compact instrument.

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