Star Diagonal Comparisons


Kent Blackwell
 

I’ve always felt we often overspend on diagonal mirrors, or star diagonals for SCT and refractors so I ran a few test. I compared how the 6th star in the Trapezium in the Orion Nebula looked and also compared a very tight 0.5” separation double star. My test diagonals were a run-of-the mill Chinese 2” Celestron (same as Orion) and a Tele Vue Enhansed Aluminum diagonal. Comparing the two side-by-side I saw no differences whatsoever.

Mark O might want to chime in since he has experience comparing mirror diagonals to prism diagonals. 

 

Takahashi TOA 150 telescope

5mm Pentax 5mm XW eyepiece at 220x.


jimcoble2000
 

That's an excellent issue and I am not really sure of the answer. I do use a very good (Baader) on my four inch but use a very standard Williams Optics on the 5 inch. I really can't see much difference. I did once have a Televue 1.25 inch diagonal that somehow had a lot of small visible lines that looked like they were in the coating and I felt it did not perform as I would want. I don't think I scratched it as I rarely clean the mirror surface save for using a soft brush.

I do see differences in Prism diagonals and mirrors. Theoretically there is no more light loss using the prism
instead of the mirror surface. That said, the prism may not be appropriate for the rare fast focal ratio telescope. There the color correction may not be quite as good. I think it is really a matter of prism size and vignetting mostly. Roy's answer "depends" applies here. I do see on planets a difference but only at the most critical observing of planet or lunar observing. For the very toughest details I do use my prism on the five inch reflector. But this is only for very very critical observing using Othroscopic design eyepieces. So if you want to take on something like the central rille  in the Alpine Valley, on the moon, I would use the prism. I have only seen it once in decades of observing. There are so many variables in a visual observation.

The prism is not appropriate for my 4 inch which has a very fast focal ratio. Prism diagonals may be a product of the past when there were no good mirror coatings available. At one time they were the only way to get a good image. Today not so much.

On Wednesday, January 26, 2022, 03:01:34 PM EST, Kent Blackwell <kent@...> wrote:


I’ve always felt we often overspend on diagonal mirrors, or star diagonals for SCT and refractors so I ran a few test. I compared how the 6th star in the Trapezium in the Orion Nebula looked and also compared a very tight 0.5” separation double star. My test diagonals were a run-of-the mill Chinese 2” Celestron (same as Orion) and a Tele Vue Enhansed Aluminum diagonal. Comparing the two side-by-side I saw no differences whatsoever.

Mark O might want to chime in since he has experience comparing mirror diagonals to prism diagonals. 

 

Takahashi TOA 150 telescope

5mm Pentax 5mm XW eyepiece at 220x.