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The Advantages of Observing with Correct Image Diagonal
The Advantages of Observing with Correct Image Diagonal

After some 47 years of observing, Roger Ivester finally purchased an Orion 1.25" Correct-Image Star Telescope Diagonal for his Orion 102mm Refractor. The precision 90 degree diagonal is meant to deliver superior image fidelity, or a right-side-up, non-reversed image. Ivester, who observes visually and sketches through the eyepiece on a regular basis, was pleased with results. His full review is as follows:

I love to observe with my 102mm refractor, but I don't like the reversed mirror image view presented, due to the 90 degree diagonal. On many occasions I choose my Newtonian, as it's so easy to make a correctly oriented pencil sketch. For many years, I've wanted to try a correct image 90 degree diagonal, but thought the views might suffer. I've been using a 96% enhanced reflectivity mirror diagonal for many years.

The reason I dislike making a sketch using a mirror image reversed view is that almost all published images and sketches are oriented scientifically correct with N at the top and W to the right. It's also very easy to confuse the cardinal points with a standard view diagonal, when making a sketch or noting a faint feature of the object being observed.


One thing I should mention. A correct image prism diagonal such as this Orion, also known as an Amici: When observing a very bright star with any correct image diagonal will show a very thin thread of light crossing from edge to edge. This is not a defect, but is inherent to this type of diagonal. I only noticed it when observing Sirius, and found it not to be objectionable. I purchased this diagonal, not to observe the brightest of stars, but to observe the faintest of deep-sky objects possible with a 102 mm refractor.

I plan on much more observing, notes and sketching with my trusty 17-year-old Orion 102mm refractor. Again...I love a correct image view, and sketching with my refractor will never be the same again. I'm very happy with my new purchase.

While looking through the Orion Telescopes & Binoculars catalog last week, I noticed a 1.25-inch, correct image diagonal. After many years of wondering how this type of diagonal would perform, I picked up the phone and ordered one. After all, Orion has a fabulous return policy, should it not meet my satisfaction. I had nothing to lose...

Within just a few days, my diagonal arrived. I could hardly wait to give it a try. On March 5th, 2014, I set up my Orion 102mm refractor for the big test. I started with a very high magnification of 200x, to examine the Trapezium stars and see how the view would compare with my current enhanced mirror diagonal. The stars were beautiful in both, and even the "E" star could be glimpsed. First test: passed. I then went to my favorite galaxy pair, M81 and M82 at 57x. I immediately loved the non-reversed and correct image view of these two beautiful galaxies using my new diagonal. I really couldn't see any difference between the qualities of view. The next test would be Jupiter, and again both diagonals presented fabulous views. The cloud bands appeared very sharp with an incredible amount of detail visible.

It is my opinion, the correct-image Orion image diagonal, catalog #8787 seemingly passed all tests with flying colors. At this point, I put my beautiful enhanced solid aluminum mirror image diagonal in my eyepiece box, where it will probably stay. For the remainder of the evening I enjoyed using only my new "correct image" diagonal.

I can hardly wait to start sketching with my new refractor accessory. I just wish that I had made this purchase many years ago.

Read more of Roger's astronomy blogs and see his sketches at

Date Taken: 03/11/2014
Author: Roger Ivester
Category: Equipment

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