Whether dealing with telescopes, eyepieces, filters, or focusers, nearly every amateur astronomer is bound to come across a large and sometimes confusing array of machined threads. Sorting out which type of thread is which, and which component fits into another, can make it difficult to find and order the right parts and make sure they all fit together.

To avoid such frustrations, it helps to understand the most common thread types found in astronomy equipment. Such information exists in bits and pieces in catalogs and on websites, but it's hard to find a single good 'primer' that explains astronomy threads. This guide solves that problem. It explains everything you need to know about the configuration of astronomy threads to help you make informed buying decisions as you acquire new equipment or make the most of what you already have. If there's one article or guide to read on the Agena AstroProducts website, this is it. It may save you a lot of time and headaches over the long run.

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For many years, Baader Planetarium offered just a small handful of premium quality star diagonals. Over the past few years, however, they have introduced the innovative T-2 system of diagonals and accessories along with a half-dozen otherdiagonals to the product mix. There are now some 13different Baader diagonals in production, which offers amateur astronomers a wide range of choice and configurations.

Because all the light from your telescope objective is reflected by your star diagonal, the choice of a star diagonal is at least as important as the choice of eyepiece. Baader makes excellent diagonals, but with so many choices, it might be hard to figure out which diagonal is right for your situation. Should you choose adiagonal with a right-angle prism or Amici prism? Or 45-degree or 90-degree viewing angle? How about Maxbright or BBHS coatings? Or T-2, 1.25", or 2" diagonal? Baader or Zeiss prism? If you’re confused, you’re not alone. We were too. That’s why we sat down to sort out all