A Question About Diagonals

Hi all

With my current set up, Celestron 9.25 SCT, I am using the mirror diagonal that came with the eyepiece kit that I purchased along with the scope, in my mind I feel that I am not getting the best out of the scope with this diagonal, to my eyes the sky does not look dark enough, and I am giving serious consideration to changing it for one of better quality. I have looked online for information about this particular diagonal, reflectivity, etc., but have come across nothing.

My question is this; is it worth the cost of buying another diagonal of better quality, or does it really not make so much of a difference?

Paul

Hi Paul. I can only tell you I have Dielectric mirror diagonals on my scopes made by Orion. At the time they were of the 2nd highest standard of their range. I find them very effective.

Aubrey.

What make is the diagonal?
It would want to be a particularity bad diagonal to give you a visible degradation of the image.
One thing that can sometimes help darken the fov is to use a good sized dew shield.

The diagonal is a Celestron make, perhaps it's just me, maybe I am thinking of other scopes I've owned that were of a faster focal length and believed that the view was somehow better.

This is the diagonal in question.

www.telescopes.com/telescope-accessories...gonalmirror2inch.cfm

Paul

I can't imagine that been a bad diagonal.
A bad diagonal would dim the star field, so would actually darken the field. A bright field is been caused by something else like light scatter or an actual bright sky.
What you can do is get a visual back where you can put the eyepiece directly into the back of the scope and not use a diagonal at all, that would be the ultimate test.

Diagonals, and their ability to improve views, is one of those subjects which crop up constantly on other forums. Some believe that dielectric diagonals are a great improvement, and others reckon they are a waste of money. It seems to me that most Celestron diagonals, rather like most Skywatcher finderscopes and the eyepieces supplied with cheaper scopes generally, are poor quality, and rather let down the rest of the equipment. However, the diagonal you have seems pretty reasonable.

I use a William Optics diagonal in my Nexstar, and I think it does show a marked improvement over the original. A mate also bought a 2" Williams for his refractor, and he also thinks it is an improvement.

One possible solution would be to collimate the diagonal you already have, but you may need a refractor to do it. I did this with the diagonal on my TAL refractor., and the view improved considerably.

Make a paper circle for the front lens end of the refractor, and make a tiny hole in the centre. Take out the four screws at the back of the diagonal, there should be another four screws underneath. Put the diagonal into the eyepiece end, and put a red collimating laser into it. See where the laser hits the paper at the front. Mine was way off. Adjust the screws until the laser passes through the hole in the paper. I set up the scope so that it pointed towards a mirror, which meant I didn't have to keep going from one end to the other to see where the laser was hitting the paper.

I don't know if it would be possible to bounce the laser off the secondary mirror of the SCT and collimate it by centring the reflected light in the black dot at the centre of the collimator, rather like collimating a dob, but it might be worth trying.