Diagonals

Can someone explain to me the practical difference(s) between a prism and a mirror diagonal?

I just got a NexStar C6 SE which comes with a prism diagonal and based on favorable comments was thinking on upgrading to a 99% dielectric. Are the 2 interchangable or are they mutually exclusive?

Dennis

You'd use one OR the other so I guess you can say they are interchangable.

I used to have a 1.25 prism diagonal for my 6 in APO. That Prism came with my 20 year old $100 Celestron 80mmF5 short tube refractor. Upgraded to the 97% WO 2 in diagonal. Can't say I notice any dramatic improvement. I don't think a 10% gain at the EP makes THAT much of a difference.

I would recommend using the prism diagonal with your new SCT for a while to see if you like to scope first. There is no need to rush and spend $$$ on a mirror diagnal that quickly. You may decide you like other type of scope better.....

Erik D

A prisim diagonal has 3 mirrors inside it to flip the image to correct (naked eye) view. A mirror diagonal has a single mirror that will flip the image so what you see on the star chart is not how it looks in the eyepiece.
When using a prisim diagonal you will experience some light lose due to the bending by the additional mirrors. Here is a site that explains it better than I can.
http://sctscopes.net/SCT_Tips/Eyepieces/Diagonals/diagonals.html

Fred

This is a complex question. In principle, a well-made
prism with flat surfaces, good coatings, etc. will be
hard to beat for one of these SCT scopes. The
light throughput with even modest quality coatings
will be something like 98%. The difference from 98%
(prism) to 99% (dielectric mirror) will not be noticable
at all.

How flat are the prism surfaces? This is hard to know,
but the same could be said for the mirror diagonal
also.

I think, unless you suspect a problem with the prism,
or you want to up-size from 1.25" to 2", I would just
stick with what you've got now. Get some nice eyepieces
instead.

Thanks everyone for the info. I have a much better idea now on the different types and how they work.

Dennis