Erect Image Diagonals

[Shasta55 (Chris)]

As I've mentioned here before I'm pretty new (18 months) to this hobby. So a while ago I had a mishap and put a crack in the plastic EP housing in my Celestron 102 refractor. I decided to get a new dielectric coated model, even though the old one successfully super-glued. I knew that most flip the image left-to-right, like any mirror. But I'd forgotten that after 18 months of observing. I was then surprised to find that with the new one, objects now traveled west to east in the sky . The diagonal shipped with the scope is bulb shaped and objects seem to travel normally east to west. I prefer the new one - the images are sharper but, can anyone explain this, probably obvious, feature of the original diagonal?



Chris.

[Saturnine (Jeff)]

Hi Chris
The original diagonal is what is referred to as an Right Angle Correct Image ( RACI ) diagonal, it has a series of prisms inside to bounce the light around to give a correct perspective view in the eyepiece. I have one the same. A cheapy but is mainly for terrestial viewing anyway, what I use mine for is in a cheap 70 mm refractor to make it an RACI finder scope.
The new diagonal that you have is a simple mirror diagonal that provides an upright image though reversed left to right, because it only has one reflective surface instead of several in the case of the prism diagonal, there is less light loss, presuming the mirror is reasonable quality.
Hope that helps.

[Shasta55 (Chris)]

Originally Posted by Saturnine

Hi Chris
The original diagonal is what is referred to as an Right Angle Correct Image ( RACI ) diagonal,

I had checked the specs on Celestron's site and it is listed as a star diagonal - thus the confusion. I had suspected what you have just confirmed so thanks for that.


Due to an error by a supplier, I have ended up with 2 x 9mm WA eyepieces. The vendor does not want one returned due to the expense of tracked postage, so one for the grandson's scope.


Last night I put one in the prism diagonal and one in the dielectric mirror diagonal for comparison. The difference in sharpness and clarity is quite obvious. So all good, thanks again.