Erect Image Diagonal

[Mark.Tanner]

Hey everyone,

I have a Celestron Nexstar 8SE. I want to buy a 45 degree (that's the degree Celestron seems to recommend) erect image diagonal for terrestrial viewing. I've looked around and there seems to be a few types that are essentially the exact same but are priced anywhere from $39 - $90. What are the differences? Should I just get the cheaper one, or is more expensive branded ones typically better?

Also, what effect do these have if used while looking at planets?

[astro744]

I’m not sure which ones you are looking at but most in that price range will be be of similar design using an Amici prism to provide the correction. The angle may be 90 deg but is also available as 45 deg. The problem with most is that the clear aperture is limited and far less that what a standard non-correcting diagonal would give. This limits your wide field eyepiece use. The clear aperture is often around 20mm whereas a standard diagonal can provide around 27mm. You can get a 2” amici diagonal but these are expensive and then require a 2” visual back. You would use the 2”-1.25” adaptor for your 1.25” eyepieces and effectively have no obstruction.

The other issue is that on bright planets you will see a bright spike through the planet especially at low power. This is due to the design of the Amici prism whether 1.25” or 2”. A standard prism or mirror star diagonal is best for planets.

Note if using an SCT for terrestrial viewing with the tube facing forward and you standing behind looking into the eyepiece in a standard diagonal you will only get left/right reversal with the up/down being correct. This is only a problem if you are reading signs. The image is not upside down, only left right reversed. An Amici diagonal corrects the left/right too.

[Renato1]

The 45 and 90 degrees ones I've got are pretty hopeless on planets.

At low power I'm pretty happy with them - makes it much easier to find objects manually, as the view matches that in the star atlas.
Regards,
Renato