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Found an affordable 2” erecting prism

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#1 mkothe

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Posted 13 September 2019 - 09:30 PM

Hi all,

 

I have been looking for a while for a 2” erecting prism that does not break the bank for occasional terrestrial use of my 4” refractor. Mostly what I found was in the $300 ballpark, and that was more than I was willing to spend for this application. Recently I came across a promising-looking unit on Ebay for $63 including shipping from China. Search for “2 Inch Erecting Image Prism 45 Degree Metal Diagonal Corner Zenith Mirror A Z4T3” from edieret-us”. lol.gif

 

It took about 2 weeks to get here and I have to say it fits my needs very well. Pretty solid, with compression rings, not too bulky (as 2” diagonals go), and it even comes with a little pouch. The only thing I did is exchange the set screw for a shorter version. The original one was long enough to fit through the 1.25” adapter, so in 2” mode it was sticking out.

 

I tried the diagonal briefly in the daytime and the view was nice and clear. I did not notice any vignetting with my 40mm Paragon. The aperture near the prism is only about 1.25”, but I think the light cone is narrow enough there to not cause issues?

 

Overall a nice piece at a good price, so I wanted to share.

 

Michael

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#2 petert913

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Posted 13 September 2019 - 11:22 PM

Not bad !  Thanks for the tip. 



#3 Jon Isaacs

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Posted 14 September 2019 - 05:02 AM

I tried the diagonal briefly in the daytime and the view was nice and clear. I did not notice any vignetting with my 40mm Paragon. The aperture near the prism is only about 1.25”, but I think the light cone is narrow enough there to not cause issues?

 

The light cone is converging until it reaches the focal plane so it's actually larger passing through the diagonal than it is at focal plane.  There will be vignetting with a 40mm SWA.  It may be difficult to see during the day because the image is so bright and your eye is stopped down to a 2mm pupil or so.  

 

At night, the only thing going for it is that the aperture restriction is out of focus but I suspect you will see significant vignetting.  With a 1.25 inch RACI prism with 20mm clear aperture, it looks good with an eyepiece with a 25 mm field stop but the vignetting is very noticeable with a 27mm field stop.

 

Jon



#4 mkothe

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Posted 14 September 2019 - 06:31 AM

Thanks for clarifying, Jon, makes sense. I’m not planning to use it at night, but I will try it out just to see what it looks like.

Michael

Edited by mkothe, 14 September 2019 - 08:00 AM.

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