Skywatcher Star Diagonal with Adjustable Angle Setting

[igorigs]

So I've got one of these (Skywatcher Star Diagonal with Adjustable Angle Setting) with purchase of my telescope (used). Got A regular diagonal and this one. My question is, does it degrade image and is it worse or better than this one Amici prism (31.7mm, 1.25") with 90-degree viewing..

[Louis D]

I think it's based on an oversized mirror, so it's not going to flip the image left for right like the Amici prism diagonal.  As such, it won't introduce a spike on bright stars.  Also, it wouldn't be a replacement for an Amici prism for terrestrial use since it doesn't flip left for right.

[igorigs]

Thanks for your reply mate. How about the light reflection?

[Louis D]

Not sure what you mean by "light reflection".  Based on the price, I would think it has a budget first surface mirror.  I have no idea how well collimated it would be nor how optically flat it is.  I would think it performs best at 90 degrees, getting slightly worse toward the 45 degree end of things.  I would be concerned about vignetting at 45 degrees if the mirror wasn't made large enough.

[igorigs]

How much of the light gets reflected.. Never mind on my question, your answer is what I was looking for. 

[Ruud]

I'll assume that the second diagonal you show is the Amici. (I'm not sure)

I think that the adjustable diagonal contains a gear system that turns the mirror over half the angle that you turn the top part. That way the light can always reach the eyepiece. However, the precise alignment of the mirror is important and a fixed mirror would be better. 

An Amici diagonal has an erecting roof prism. Those are much more difficult to make than an ordinary non-erecting prism. And again, ordinary prisms are more difficult to make than mirrors. The most economical way to a high quality view is a simple 90° star diagonal: a single first surface mirror.

The bad news is that average Amici and adjustable angle diagonals are likely to cause some image degradation at least. You can do a test though: Try with the eyepiece directly in the focuser (without using either diagonal). If you can get a focused image that way, and it looks brighter and more contrasty than with whatever diagonal you test it against, then there definitely is some degradation going on.