45 degree diagonal

So much for the new 45 degree diagonal I ordered from Apogee. It's a
complete disaster. I don't recall who said it was meant for terrestial
viewing, but I'll tell you that for star splitting, it's useless.

Unless the star image is in perfect focus, there are two images
uncombined which resemble a split binary with members of equal magnitude.

Last year, I was able to cleanly split Ras Algheti at 5" with my Tasco
and its own junk EPs in 2 minutes, this year it took me 20 minutes to
finally see the companion and after endless tries focusing and
refocusing the Apogee zoom EP. The zoom EP is good though.

What a complete waste! Warning! NEVER buy a 45 deg diagonal for astro
viewing! Guess I had to learn the hard way...
--
I. N. Galidakis
http://users.forthnet.gr/ath/jgal/
------------------------------------------
Eventually, _everything_ is understandable

Typically a 45 degree diagonal has a roof prism to reverse the image
left to right. This requires the roof angle be 90 +/- seconds of arc
to avoid a double image in the horizontal axis. I'm told the roof
needs a coating of some specific properties to avoid a degradation of
resolution in that axis too. I don't understand this, but as Warren
Smith was the one who said it I'm willing to assume it's true. It's
not impossible to make a good prism of this type, but it's not easy.
When you get down to it there are going to be production variations in
this kind of prism, and I've seen very experienced people approve roof
prisms with this kind of error. (It's really obvious on an
interferometer, or even with a star test, but a resolution test needs
to be done with care.) Don't assume that your one example is
representative of the design.

Brian


Ioannis wrote in message news:1092094759.618762@athnrd02...
So much for the new 45 degree diagonal I ordered from Apogee. It's a
complete disaster. I don't recall who said it was meant for terrestial
viewing, but I'll tell you that for star splitting, it's useless.

Unless the star image is in perfect focus, there are two images
uncombined which resemble a split binary with members of equal magnitude.

Last year, I was able to cleanly split Ras Algheti at 5" with my Tasco
and its own junk EPs in 2 minutes, this year it took me 20 minutes to
finally see the companion and after endless tries focusing and
refocusing the Apogee zoom EP. The zoom EP is good though.

What a complete waste! Warning! NEVER buy a 45 deg diagonal for astro
viewing! Guess I had to learn the hard way...