The Melbourne Binocular and Telescope Shop – SIDEREAL
Product has been added to your cart.

Collimating a 400mm f/20 Classic Cassegrain

400mm f/20 Classic Cassegrain on collimation bench
Bill Stent

We all know that collimating a telescope that has multiple curved mirrors is Black Magic. I’ve tried it and I can happily confirm I am… not a witch.

Here’s a 400mm f/20 Classic Cassegrain. The primary mirror is a parabola and the secondary is a hyperbola. And if you do the maths, the focal length really is 8000mm!

The last stage of manufacturing for this monster was collimation. To do this, Catalin (definitely a wizard) set it up on a bench so it was facing a flat mirror on the wall opposite. Using an artificial star and a beam splitter, he sent a beam of light from the eyepiece, bouncing off the secondary, back to the primary, out to the flat mirror, back to the primary, then the secondary and through the beam splitter to the eyepiece. This allowed him to align the telescope on the bench first, and then to adjust the mirrors.

The scope has now been crated up and shipped off to its new owner in Europe.

Browse all Sidereal Articles

Subscribe to our Monthly Newsletters and Tutorials

Recent Articles

HAC125 and computer

Sky-Watcher HAC150 – a budget deep-sky fast astrograph

A quick look at the new Sky-Watcher Honders Advanced Catadioptric Astrograph, a 125mm f/2 catadioptric “camera-only” telescope. Is this baby RASA the new grab-n-go sensation?

Read more
Sun in Ha

Using Lunt Telescopes to look at the Sun

With the Australian Solar eclipse coming in 2028, it’s time to get familiar with solar telescopes. With Lunt telescopes you can see the surface of the Sun using white light, or Hydrogen alpha and Calcium-K filters. These all show different aspects of what’s happening there. If you want to photograph it as well, there are cameras for that too.

Read more
Mars opposition is in mid-January next year. But before that happens, Mars is going to enter retrograde motion, where it stops being overtaken by the background stars and starts overtaking them back. Retrograde motion stumped the ancient astronomers and had a hand in stimulating the rise of modern scientific thought.

Mars Opposition 2025: what is Retrograde Motion – and what does it mean?

Mars opposition is in mid-January next year. But before that happens, Mars is going to enter retrograde motion, where it stops being overtaken by the background stars and starts overtaking them back. Retrograde motion stumped the ancient astronomers and had a hand in stimulating the rise of modern scientific thought.

Read more