Archive for March 2014

A New Diagonal: Correct Image and The Advantages

March 6, 2014

I really like observing with my 102mm refractor, but don’t like the reversed mirror image view, due to the 90 degree diagonal.  On many occasions I choose my Newtonian, as I want my sketches to be correctly oriented.  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.  

Why do I dislike a mirror reversed view?  When making a sketch, it does not match most published photographs which are oriented scientifically correct, with N at the top and W to the right.  I also like to compare my sketches with photographs.  While observing, it can also be easy to confuse the cardinal points when using a standard diagonal, due to the mirror reversed image.    

While looking through an Orion Telescope and Binocular catalog last week, I noticed a 1.25-inch, correct image diagonal for only $49.95.  After many years of wondering how this type of diagonal would perform, I made the decision to order one, and give it a try. 

Last night (March 5th, 2014) I set up my refractor for the big test.  I started with a high magnification of 200x.  I examined the Trapezium stars in the Orion Nebula to see how the view would compare with my current enhanced mirror diagonal.  The stars were beautiful and sharp 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 liked the non-reversed and correct image view, and couldn’t really see any difference between the two diagonals.   The next test would be Jupiter, and again, both diagonals performed comparably with the cloud bands appearing very sharp in both.   

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