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Celestron are known for their high quality refractors at a reasonable price and if observing through such a refractor there is one piece of kit you won't be able to do without, and that is a simple star diagonal.
When observing through a refractor, the bussiness end (where you place your eye) is usually situated on the lowest part of the telescope off the ground (unlike most reflectors where observing is done at the highest point of the telescope).
If trying to observe without a diagonal you will quite often find yourself twisted and contorted into incredibly odd and uncomfortable postures.
The star diagonal's role is to provide a 90 degree angle to the telescopes light path meaning you can observe even the highest objects in the sky far more practically and comfortably.
The diagonal is inserted into the drawtube of your scope via a metal cylinder, a selected eyepiece is then inserted into the solid black plastic tube jutting out from the oppsite side and is easily secured with two silver thumb screws.
In the triangular body of the star diagonal lies two high quality polished mirrors that reflect light up into the observers waiting eye. During the process of this light reflection somehow the image becomes flipped around, North and South remain correctly aligned but East and West become reversed.
For most astronomers this topsy turvy orientation doesn't present a problem, once you're aware of it. There is a similar device available known as an erecting prism, this keeps images correctly positioned but at the cost of reduced optical quality.
Considering that this is one of the cheapest star diagonals available
(a little under £30) it performs fantastically. Foolishly I've allowed mine to get dusty and scratched, there really is no excuse for this though as Celestron include a solid black plastic cap as well as a see through thinner plastic cap to keep this exact thing from happening.
If you take good care of it this is the finest sub £50 star diagonal available. It doesn't noticeably degrade image quality (as introducing any extra glass to an optical system can potentially do) and the design is solid yet light enough not to cause balancing issues (say when removing the diagonal to insert a CCD camera).
Highly recommended, there really is no need to shell out over £100 for a decent star diagonal that gets the job done.