Fixing focal length

Paul Money discusses the latest astro equipment news

Pacman Nebula by Graham Green, Waterlooville, Hampshire, UK.
Published: November 11, 2012 at 12:00 pm

How fast can it go?


You might think I’m referring to the latest sports car or motorcycle but hey, this is an astronomy blog.

In terms of fast or slow, we’re dealing with the different focal lengths of telescopes and in particular, how these variations affect astrophotography.

Long focal length systems tend to be regarded as slow, as it takes longer to record faint galaxies or nebulae.

That’s why these systems are quite often used for bright subjects such as the planets and lunar and solar imaging.

Short focal lengths are classed as fast systems that gather in a lot of light and are especially good for capturing nebulous or faint deep sky objects.

Fast mirrors are OK, but can suffer from distortions towards the edges.

There seems to be a trend now for manufacturers to design and build telescopes with mirrors that are very fast, highly corrected and ideal for imaging with.

One such system is the Officina Stellare RH200 that we reviewed in our December issue.

It is a 200mm (8 inch) f3 system.

In other words it has a focal length of just 600mm and all in a tube just 222mm long.

It’s not just compact but makes for a short focal length fast astrograph, or imaging system.

If the above tempts you, you’ll probably want a mount and CCD camera too, so our other reviews fit the bill nicely; the Vixen SXP mount with Starbook TEN and the Orion Telescope and Binoculars Starshoot G3 CCD camera.


Make sure you enter any images you take with the above equipment into our Hotshots competition.

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