Erecting eyepiece diagonal: opinions?

[sophiecentaur]

I have this nice little ED80 OTA and it gives smashing images. I want to use it for terrestrial viewing. Are there any opinions about the Amici (roof) prism type erecting diagonals? I can't find much in the way of diagrams of the optics; the only one I found shows a 90° version, which is not as convenient as a 45° version
I only have nice 2" eyepieces, which limits what's available. Seems like there is no cheap solution. There is apparently a limit to magnification with these prisms. Are the cheaper ones worse in this respect?

 

[davenn]

I have this nice little ED80 OTA and it gives smashing images. I want to use it for terrestrial viewing. Are there any opinions about the Amici (roof) prism type erecting diagonals?

hey sophi

not sure if this is the sort of thing you are after ?
Am not sure if it will erect the image ?

http://starizona.com/acb/Enhanced-Aluminum-Mirror-Diagonal---2---Satin-Finish-P2506C105.aspx


Dave

 

[sophiecentaur]

Thanks for getting back but that's a normal star diagonal, as far as I can make out. It is a posh (high reflectivity ) mirror and I already have one (reasonable quality, I think). It produces a mirror image and, for terrestrial viewing. the 90 degree angle is a pain - you have to look directly down into it or the image slopes drastically up/down hill. The erecting versions often has a 45 degree angle, for convenience and, like with binoculars, the roof prism gives a normal view.
This link has a picture of an amici prism system. There is a lot wrong with the system - hopeless of astronomy I think because of the streaks it gives stars and not good for high magnification because of the stripe down the middle but roof prism binos are absolutely fine with normal magnification.

 

[Drakkith]

This link has a picture of an amici prism system. There is a lot wrong with the system - hopeless of astronomy I think because of the streaks it gives stars and not good for high magnification because of the stripe down the middle but roof prism binos are absolutely fine with normal magnification.

Are you sure it's not suited for astronomical viewing? I have a hard time believing they sell them for that exact purpose if there are large problems with the design.

Seems like there is no cheap solution. There is apparently a limit to magnification with these prisms. Are the cheaper ones worse in this respect?

They could be, but I have no personal experience with them. I can understand why 2-inch eyepieces have expensive diagonals. That's a lot of added glass compared to the 1.25-inch eyepieces. Probably almost double the amount of glass and possibly more.

 

[sophiecentaur]

I have a hard time believing they sell them for that exact purpose

That's just it. Afaik, the erecting variety is more used for spotting scopes or for low magnification 'finder' purposes. They don't seem to be as available as the non-erecting, 90° versions on the market and I am just wondering if that's because the quality is not so good.
What I was hoping for was that someone would actually tell me they have one and it's brilliant or terrible.
It was interesting to find out about the Roof Prism principle but there are very few diagrams that make much sense. It's a very clever idea though.

 

[Drakkith]

They don't seem to be as available as the non-erecting, 90° versions on the market and I am just wondering if that's because the quality is not so good.

What do you mean a "non-erecting" version? All versions should be erecting.

As for quality, the only thing I know about them seems to be that they are more technically demanding than other types of prisms, especially the roof edge. Which makes sense, as any light that hits the peak of that roof and isn't reflected will result in a loss of light and alteration to the final image (possibly a bright diffraction spike according to some sources). The difficulty of making high-quality Amici prisms appears to be the reason that they are often marketed for terrestrial viewing or for low magnifications.

What I was hoping for was that someone would actually tell me they have one and it's brilliant or terrible.

Sorry, I wish I could.

 

[sophiecentaur]

A 'normal' star diagonal gives a mirror image. That is not an erecting diagonal (afaik). To get a terrestrial view, you have to be looking straight down which gives a crick neck. The 45 degree versions give a normal ('erected') image and save your neck. Good for spotting and finding. I have read that they give streaks on stars (roof effect) and also a band across the middle of a terrestrial view. I just wonder how much I will need to spend.