Beat the Heat: Conquering Newtonian Reflector Thermals — Part 1

Newtonian thermal gradient

What you need to know when it comes to optimizing your scope’s thermal behavior.

Generations of backyard astronomers have debated why, inch-for-inch, the performance of a high quality refractor usually edges out an equal-quality Newtonian reflector. This disparity is most apparent when viewing low-contrast planetary detail — the images in a good refractors often have a touch more snap to them. Is there some intrinsic shortcoming in the design of the Newtonian reflector that makes this inevitable?
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How To Build A Curved-Vane Secondary-Mirror Holder

Curved spider

The secondary mirror holder and spider on my 12¾-inch truss-tube Dobsonian is made with scrap wood, a few nuts and bolts, and a stainless-steel ruler.

The curved-vane secondary mirror holders I use on almost all my telescopes never fails to excite curiosity. Most people know that the principal benefit of the curved spider is spike-free stars, but they often wonder if it really works. The “points” adorning bright stars in telescopes with straight-vaned spiders are diffraction artifacts that don’t seriously affect the image but do impose an aesthetic quality that may not appeal to you. Luckily, the remedy is easy to make, works like a charm, and can be retrofitted to virtually any reflector — commercial or homemade.
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My 8-inch Travelscope


Have scope, will travel! This Dobsonian not only gives great views, it also fits into an airplane’s overhead storage compartment.

One of the best reasons for learning to build telescopes is that you can make instruments that perfectly match a particular observing need or circumstance. As an editor at Sky & Telescope, my “circumstance” happily involved a lot of travel, and as a result I found myself dreaming of a telescope that I could take with me as I zig-zagged across North America from one star party to the next. It seemed a shame to arrive under the dark skies of the Texas Star Party or Mount Kobau without a telescope of my own.
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Save Your Scope!

Scope cover

With star-party season upon us once again, you’re probably confronting the age-old problem of how to protect your prized telescope from the triple threat of dust, rain, and heat. The traditional big, ugly trash bag or blue tarp works for two of those three (dust and rain), but looks bad and lets your scope to get way too toasty on a hot summer’s day. Alternatively, you can plunk down $50 or so on a proper dust cover, but if you’re like me, that cash almost always seems to go towards another eyepiece instead. Stumped? Don’t worry — a cheap, effective telescope cover is only as far away as your local hardware/camping-supply store.
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A Simple Double-Plate Mirror Cell for Your Reflector

Mirror cell

Remounting your Newtonian reflector’s primary mirror could lead to better performance and easier collimation.

Many telescopes have mirror mounts that are less than ideal. Some hold the mirror in place with clips that project over its surface and introduce image-harming diffraction effects. Others have adjustments that require tools or are so frustrating that observers rarely attempt to properly collimate the optics. Some cells effectively seal the mirror from the outside air. With no provision for ventilation, it is virtually impossible for the primary to cool down to the ambient air temperature — crucial for sharp images. Some mirror cells have all these problems!
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Gary’s Easy-Go-Round Binocular Mount

Easy-Go-Round Mount

Build this simple device for steady binocular views of the night sky.

I love binocular astronomy. At least, that’s my excuse for cluttering the house with a dozen (at last count) of these double-barreled optical wonders. Recently, my collection expanded to include particularly heavy 10×50s and inexpensive 15×70s. For the first time I really felt that I needed some kind of binocular support.
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Sizing Up the Newtonian Secondary

Secondary mirror

Too big, too small, or just right? Making sure your reflector’s secondary mirror is the correct size is a straightforward task.

The Newtonian reflector has many strengths, not the least of which is that it consists of just two elements: a precisely shaped paraboloidal primary mirror and a flat diagonal secondary mirror. Yet for all its intrinsic simplicity, confusion abounds when it comes to the optimum size of the diagonal. Many amateurs, and apparently even some telescope manufacturers, seem unsure as to how to choose the correct size for the diagonal. So how big should it be? That depends on several design parameters and some personal preferences.
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