If you are the owner of a tripod mounted refractor telescope then chances are you’ll need to own an erecting prism, either that or you’d better make friends with a very good chiropractor.
Most refractors have their eyepieces positioned at the opposite end of the optical tube to the objective lens and as such the eyepiece will almost always be the lowest point on the scope in terms of height. Trying to observe with an eyepiece slotted directly into the telescope is incredibly uncomfortable, when viewing objects near the zenith (highest point in the sky) you are literally almost sitting on the floor, this is clearly undesirable and that is where the erecting prism comes in.
The erecting prism slots into the barrel of your refractor via a threaded metal cylinder, the inside of which is covered in a matte black coating to avoid internal light reflection, this helps keep maximum contrast through the telescope and keeps images at their sharpest.
The metal cylinder sits on the main body of the erecting prism, this is a small triangle made from a thick course plastic that houses two small mirrors in a V shape. Jutting out from the other side is a black plastic barrel with two circular metal pegs in it, this is where you insert your eyepiece. The pegs screw in and anchor the eyepiece into the prism. With the erecting prism installed your eyepiece is now positioned at a 90 degree angle to the line of the telescope. This means you don’t have to become a professional limbo dancer to get comfortable views through your refractor.
As well as the erecting prism you may hear about a device called a star diagonal, this is very similar to the erecting prism but differs in two key areas. Firstly the star diagonal produces an image that is reversed from right to left (in other words flipped around) while the erecting prism yields views that are correctly oriented. Secondly and most importantly the star diagonal produces slightly better views, largely due to the way the mirrors are positioned.
For most people the star diagonal is preferable to the erecting prism despite its flipped images, since purchasing my erecting prism when I was still relatively new to the hobby I have bought a star diagonal that’s comparable in price to the prism (around £30), the diagonal does produce noticeably better images and the reversed images are a small price to pay.
To conclude, this erecting prism from Celestron is a nice cheap way to observe correctly oriented images comfortably, but if you don’t have a problem with which way round your images are then I would recommend a similarly priced star diagonal. If you’re willing to spend hundreds of pounds it is possible to get a prism that works as well as a diagonal but in the budget category the diagonal is the better choice.