Erect image diagonals

[yagon]

Why aren't erect image (or correct image diagonals) used more often for astronomy?

It seems to me that an erect image would be more intuitive for navigation and finding obejcts.

Does the image quality reduce if an erect image diagonal is used vs a standard diagonal?

[[1ponders]]

Good question yagon. Haven't heard it put quite that way before. Maybe erecting prisms aren't produced to the quality of star diagonals. Maybe it's because there are more surfaces to reflect of to erect the image???

[janoskiss]

Degrades the image a lot more or costs a lot more for the same performance. Like Paul said, you need more reflections to "correct" the orientation of the image. Don't get one, unless you have to.

[yagon]

I'm very interested to see a diagram of the light path in an erect image diagonal. I just can't figure out how they do it.

Does anyone know of a diagram on the net that I can review?

[avandonk]

You could put two mirror diagonals in series and the only degradation would be loss of intensity if the mirrors are flat. You also need a fair amount of in focus distance.

Bert

[Miaplacidus]

I have the WO 2" 45 degree correct image diagonal, and it is a ripper. Only astro use I put it to sometimes is solar. A significant problem with erect image diagonals, I think (not 100% certain), is that there is a vertical join in the middle of the prisms which means that there is almost always a pronounced line, like a diffraction spike, on star images.

[yagon]

How bad is the diffraction spike? Does it become worse with magnification?

[Miaplacidus]

Geez, I don't know, but have a look at this article that just appeared on Cloudy Nights.

http://www.cloudynights.com/item.php?item_id=1428

[Roger Davis]

The line is called a roof. Hence the term "Roof Prism".
The "smearing" effect is due to the "roof" across which the erecting of the image occurs. See the attachments.

[Miaplacidus]

Hi Roger,

I always thought that the definition of a roof prism was one with two totally internally reflecting surfaces at 90 degrees to each other, and that was where the name "roof" came from. Is this wrong?

Cheers,

Brian.

[Roger Davis]

Sorry grabbed the wrong diagrammes. The 45° prisms are known as SCHMIDT prisms, not AMICI prisms. A common misconception. As I said Schmidt prisms produce a correct image through 45°, but the Amici does it through 90°. The roof is the erecting reflection and any inaccuracies on it are "reflected" (pun intended) on the image quality. I suspect that the roof diffracts the image acting similarly to a slit, thus producing the lines as seen in the ronchigram on Cloudy Nights (copied in the attachment). I would be open to discussion on this. I think I will try a few experiments and ray traces!