A finder for visual astronomy, the Celestron Right Angle Correct Image (RACI)

By Martin Whillock, York Astronomical Society, with Robin Scagell

Anyone who uses a telescope knows that a good finder scope is essential. Some like to use zero-power finders such as the Telrad, which projects a target onto the sky, while others prefer to use a small low-power telescope, which gives a magnified view of the sky.

I have used a trusty Rigel Quik Finder for years and can thoroughly recommend it. The Quik Finder is similar to the famous Telrad.

I decided to add an optical finder to my Newtonian, to complement the Quik Finder. So I got a Celestron illuminated Right Angle Correct Image (RACI), which has an advantage over most optical finders because it gives a correct view of the sky, just as you see it with the naked eye. The RACI has a 50 mm aperture and comes with a fixed X9 illuminated eyepiece, with focusing adjustment. As it has a right angle prism, and can rotate in its frame it’s easy to use even when looking up to the zenith. The illuminated eyepiece has double cross hairs, so the star is not hidden by crossing single hairs. It has good quality optics. The 50mm objective lens screws in and out on a fine thread with a locking ring, giving a second focussing facility.

Celestron Right Angle Correct Image finder scope

Celestron Right Angle Correct Image finder scope

The RACI has a dovetail base so this had to be modified to fix onto my 200mm Newtonian’s tube. It filed off easily as it is made of soft aluminium.
The collimating mechanism is effective and simple – only two screws to adjust. But at first light I found the X9 magnification too high for me – I could not recognise the star patterns I could see.

This is where the fun started. The downside of the RACI’s design is that it does not have a standard eyepiece holder, because of the illumination. So to get a lower magnification eyepiece in I had to alter the holder, not easy, and the barrel of a 32mm eyepiece. By removing the original eyepiece I have neither illumination nor cross hairs, but the viewing with the 32mm eyepiece is OK.

If a X9 magnification eyepiece fixed in a finder is too narrow a field for you, I do not recommend the Celestron RACI.

It’s a good idea, well designed and made, if it had a normal eyepiece holder for normal eyepieces it would be very good.