Maybe i should get a new diagonal?

[Monki]

I don't know why, but after playing with a webcam with my scope it got me thinking...i'm still using my stock prism that came with my celestron refracter. It's hard plastic in a spherical shape with 90° angle; has anyone come across one of these? I do like the fact that everything i view is the "right way up", but i think i've been neglecting quite an important piece of my scope....after all no matter what eyepiece i use the light still has to travel through my diagonal first=/

I was looking at this prism...just for the sake of it being correct rotation

http://www.firstlightoptics.com/diagonals/william-optics-125-45-degree-erecting-prism.html

Then i saw a 90° (would suite me better comfort wise) dielectric for a bit more

http://www.firstlightoptics.com/diagonals/william-optics-125-carbon-fibre-dielectric-diagonal.html

Then i found a gso dielectric for £14 less...not carbin fibre so more money in the optics perhaps?

http://www.scopesnskies.com/prod/Revelation/astronomy%20-accessories/mirror-star-diagonal/125dielectric.html

It's going to be a week before i can get it so i'm going to do some research, but i still always like ask on here

Sent from a Galaxy S 2 far away.

[Capricorn]

Just for information: I am assuming that the unit you have has a prism in it's construction. The unit will have an optical path of some length, and the various tubes that make it up have a length. This will alter with a new diagonal. So you may need an extender to get the set up so that the eyepieces can achieve focus again.

I say extender as the optical path that was in the prism is absent in a mirror.

Most scope use a simple mirror as this adds nothing to the optical path and there is no refractive index of the glass to be concerned about.

Just saying that it is not usually a straight swap.

[Monki]

Thanks for that! Yes it's a prism. I never even thought about it affecting the focus.

Am i right in thinking.. if i focus with the diagonal, then remove it and physically measure the amount of back travel i need to refocus the image again, this will give me an idea of the length of extender tube i'd need?

[newman]

A diagonal will give you a horizontally reversed image. It doesn't really hurt anything when observing the sky, but it's something to keep in mind. Another thing to keep in mind is that a 1.25" diagonal is going to keep you limited to 1.25" eyepieces, while a 2" one would allow you to use both 1.25" and 2" ones if you ever need to expand your ep collection that way. But I guess that depends on which refractor you have. I own a GSO 2" quartz dielectric 99% reflectivity diagonal (this one: http://agenaastro.com/gso-2-90-99-quartz-dielectric-refractor-mirror-diagonal.html) and it works great.

[Monki]

Well, I took the plunge and ordered a revelation 1.25" quartz from telescopehouse.com. After reading various opinions around the web it looked like my best option

I also already have spare revelation barlow I can unscrew the lens from and use as an extender

Thanks again for your helps guys.

[barkis]

There is some light loss too in using a prism, A reflective diagonal is to preferable I would think. The dielectric ones are fairly expensive though.

Ron.