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2" Correct diagonal
Okay, the price is likely to be high, and check out the 'clear aperture' of the light passing through the unit before you jump. (I am unsure of the characteristics of the diagonal.)
This left to right (mirror reversal), is a real pain in the aXXX for someone who grew up with Newtonians. Upside down, I can handle, but mirror reversed totally loses it! (Leonardo Da Vinci wrote mirror reversed so his noted were coded. That give you an idea how hard it is to relate stuff when mirror reversed.) Okay I could take out the diagonal, look straight through, and crick my neck!
Now there is a cheap, but inferior alternative that does not involve sacrificing first- borns. This is the 45 degree correct image versions. I did an assessment some time ago on my unit:
Celestron 45-Degree Erect Diagonal (1.25") Review
By PeterA VERIFIED BUYER Would be great for terrestrial viewing and the low price attracted me, but...
The main item of note is that the undersized prism has a clear aperture 18mm in diameter compared with 28mm for the standard 1.25" 90 degree model supplied with Celestron SCT's. Also,contrary to the illustration it has only one screw to affix the eyepiece and no grubb screw in the side to adjust the prism. The point of focus is quite different than when using the standard 90 degree prism.
This 18mm diameter prism means that when using low magnification eyepieces you suffer vignetting around the edges. For example the field lens of the 40mm Plossl that came with my C11 uses the full inside diameter of the barrel of 28.5mm and vignetting is bad, and the edge of the field is not visible. A standard 25mm plossl has a 23mm wide lens and vignetting is very apparent. It is not until you get to the 8-24mm Celestron zoom with a small field lens of 16mm (narrow low power field) or higher powers like a 12.5mm plossl (11mm wide lens) that vignetting is not apparent.
So this effectively rules out low power wider angle views. Examining the image critically I formed the view that the vignetting encroached a tad more at the lower edge of the field. The images also appear to vary very slightly in focus and quality over the field, but not on one axis as you might expect if it was an alignment issue.
I felt that the image was not quite as bright nor quite as sharp as with the (standard) 90 degree model, but it was a very close call. I was trying to be as critical as I could, switching from one diagonal to the other using the same eyepieces. I believe, that the differences were subtle, but apparent. I used Jupiter and the galilean satellites on my C11 for my comparison and they are always a good test.
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