The 1974 Telescope Secondary Experiment

I wanted the most from my new 14.25 inch [36cm] Horseshoe mounted reflector.  Conventional wisdom of the era was that a reasonably sized secondary of 1/5 to 1/4 the diameter of the primary mirror was fine, that there was little benefit to going to a smaller secondary. Some people though were agitating that secondary size mattered a great deal, that I would only be getting the very best images if my secondary was small. Maybe there were onto something, maybe convention wisdom was wrong.


Worried that I might not achieve the very best images, I designed a nifty self-collimating quick-mount secondary holder so that I could swap back and forth between two diagonals: the smallest possible diagonal for planetary and lunar imaging and a larger diagonal for deep-sky use. What resulted was a surprise.

Here is the 1.83 inch diagonal in its self-collimating quick-mount holder. The large diagonal was a 2.60 inch. I built a motorized slide focuser with a single straight stalk diagonal holder. The 1.83 inch diagonal was 1/8 the diameter of the 14.25 inch primary and the 2.60 diagonal was 1/5 the diameter of the primary.



In the 1970’s, besides doing a great deal of visual observing, I was into eyepiece projection photography of the planets and cold-camera long exposure astrophotography.

I compared the two diagonals visually over many nights observing the planets with colored filters. I compared hundreds of highly magnified eyepiece projection negatives and slides. For deep-sky I visually counted stars in a small region of M13, the great Hercules Globular Cluster.

After a number of months I was forced to conclude that visually there was no noticeable difference on planetary images and no difference in star counts on M13. To my utter surprise, after months of photographing Jupiter and Saturn, I was forced to concede a very slight advantage to the 2.60 inch diagonal, the opposite of my expectation. How could this be?

Could the 2.60 inch diagonal be covering a bad center zone of the primary mirror? No, the mirror is without zones.

Could there be alignment or vignetting issues? No, I checked again and again.

Could there be a difference in optical quality of the diagonals? Perhaps the 1.83 diagonal had a slight edge problem. After all, only when a diagonal is absolutely minimally sized will its edge be used to form the image at the center of the field of view. I was forced to conclude that this was the most likely reason.

The real message though is that closely sized diagonals do not make a discernable difference in visual performance. Intrigued I set about making larger masks of 1/4, 1/3 and 1/2 the size of the primary. Over several nights I saw a consistent result: the 1/4 mask give nearly almost identical performance to the existing diagonals, but the 1/3 mask caused a modest degradation and the 1/2 sized mask gave sad images. I also realized that the best mask for star testing spherical aberration or overall correction of a telescope is a 1/3 mask. Well, how about them apples, as my father used to say.

I learned to avoid worrying about secondary size, and became aware of Parkinson's Law of Triviality. I also learned to ignore fear and concentrate on getting the most from all the factors involved.

Mel Bartels