Observer:	Jeff Barbour
E-mail address:	barbour@ihwy.com
Web site:	www.ihwy.com/~barbour/jeff

UPGRADED STAR DIAGONAL TESTING:
Friday, December 8, 2000

The mirrored star diagonal originally received with my scope has always been suspect. Early on I had to "shim" the mirror into position within the diagonal holder to get a reasonable on-axis light bounce from the secondary into the eyepiece. In addition, the diagonal appears to diminish the brightness of common objects viewed through it (statically). A certain amount of edge distortion is also present. (This shows up when viewing star fields and making checks by statically viewing grid lines -- such as Venetian blinds). The diagonal may also contribute to "spurious glow" around stars (8th magnitude and brighter). And can contribute to contrast issues associated with planetary detail (especially Jupiter). All in all, it made sense that some of these problems could be overcome by replacing this item with something of superior quality.

NOTE: Star diagonals are commonly used on MCTs (like mine), SCTs and refractors. Even Newtonians, however, can have analogous problems with their secondary mirrors.

Two weeks ago I put in an order with Orion Binocular and Telescope for a replacement unit. As it turned out what I wanted was not available on site so they back-ordered the part (PN 8778 a 50 dollar 1.25" unit manufactured by the Vixen Telescope Company of Japan). Notice came by phone earlier this week that the part had arrived. I was also told that an optical technician had inspected the diagonal and confirmed its quality. 

Friday afternoon I drove down to Watsonville to pick up the new diagonal. While on the premises, I made a quick visual inspection. The aluminized mirror surface looked flawless. I peered through it at some Venetian blinds (an impromptu "ronchi" test). The lines looked straight (except, to my eye, near the periphery of the mirror). Images reflecting off the mirror didn't appear appreciably "darker" than those seen natively. The unit was well crafted and much heavier than the older diagonal. With the exception of a slight distortion seen at parallel lines near the edge of the mirror, I was pleased.

On getting home, I mounted and assembled the scope. (Due to mechanical deficiencies with the old diagonal, I had to re-collimate the secondary.) With everything in place, and the light cone falling squarely into the eyepiece, I set everything aside -- and hoped the weather would prove cooperative later in the evening.

I had no illusions about being able to put the new diagonal through a full observation series. Currently the moon dominates the night sky, and lately high, thin clouds have been regular visitors to -- or should I say interrupters of -- my nights out. However, around 9:00pm I returned from an evening out and set up the scope for planet views.

Jupiter:

In observing Jupiter, first thing I noticed was that belt to body contrast seemed better. (This despite the fact that high thin clouds were present in the atmosphere above me.) Jupiter's NTB was clearly discernable and showed variations in texture along its length (something I had never noticed before). In fact, the north polar region hinted at texture -- rather than the normal uniform darkening to the polar limb. The twinning ("split") of the NEB was quite obvious. For the first time, I noticed some hint of a very narrow, low contrast "equatorial belt" running straight across the waist of the planet.

NOTE: For completeness, I should mention that the SEB appeared a single unit this evening -- there was no visible split between the broader, higher-contrast southern and thinner, lower-contrast equatorial component. (There did appear to be a slight "brightening" of the region between the two components however.) The STB was not visible, and unlike the north polar region, there was no hint of texture at the opposite pole.

In viewing the Galilean's, I was pleased to see that all four moons showed "clean" disks at 360 and 540X. (Yes, stability was that good.) In addition, the amount of glow surrounding each disk was appreciably less than seen in the past. The glow itself was also more concentric and "airy" in nature. (A comet-like glow associated with Jupiter's moons was one of the things that recently alerted me to ongoing failings with the old diagonal.) It was also clear that each Galilean shows a different size disk, of varying surface brightness and hue. (I definitely need to spend more time exploring these characteristics in later viewing sessions.) 

I re-installed the original star diagonal (briefly) to compare views. Other than differences in the Galileans, I wasn't able to see enough change to make any bold claims that the new diagonal was a "huge" improvement over the old one except...

... except that later, after viewing Saturn and just before putting away the scope, I tried both diagonals with the sky clouded over. With Jupiter barely perceptible to the unaided eye through the clouds, I could see considerably more detail on its surface using the new diagonal than the older one.

Jupiter viewing continued for about an hour at various magnifications -- 210, 360 and 540X (using 25/15 & 10mm ultrascopics through a 3X ultrascopic barlow). I also pulled out two filters (blue and green). (As usual, the filters revealed nothing I couldn't see directly.) Views at 360 and 540X were required to see textures associated with the NTB & NPR. Throughout all this, I was pleased with the overall contrast of detail. (As mentioned, I've had problems with image contrast in the past.) 

In reading the above, you'll note that I saw some new (for me) "features" of Jupiter's atmosphere and moons. In viewing Saturn, I also noticed some new stuff -- although I spent less time viewing "The Ringed Wonder" than Jupiter.

In past Saturn tours, I had never really taken the time to see if I could trace Cassini all the way around the ring system. Tonight, I decided to see if this was possible. As it turned out, I was only able to do so (using direct vision) at 540X. (It could be done at 360X but required averted vision.) While inspecting the ring, I noticed its shadow falling on the planet's limb. (Previous sessions showed the shadow of the planet on the ring behind the globe, and the shading of the planetary equator by the dusky ring, but I'd never seen the shadow of the ring on the globe itself.)

Even through the high thin clouds I could make out 4 of Saturn's moons. The dimmest occasionally disappeared as the clouds thickened. The largest/brightest (Titan) hinted at a disk -- but only at 540X.

NOTE: Again for completeness: Saturn's equatorial belt was visible this evening. (Although it seemed to shade into the polar region.) Meanwhile, detail in Ring A (the Encke Gap?) was more plainly visible than previously (and visible at both ring ansae). The ring system is clearly beginning to recede from its peak presentation angle (of about a month ago). I did not see the shadow of the planet on the posterior ring surface. Ring C was easily picked out blending into interstitial space (in the direction of Saturn's globe). Anterior Ring C clearly darkened the body of the planet behind it. The planet showed definite darkening at limb edges. Perhaps my only disappointment in viewing Saturn is that I am as yet unable to make out any detail or texturing on the body of the planet or its one visible belt. 

In Sum:

Without the use of high grade optical equipment, it's hard to objectively tell how much of a performance boost is had by upgrading any particular component in the optical path. So far, the biggest improvement I've seen in my scope's performance has come about through proper collimation. (Getting the on-axis light cone to converge into the very center of the eyepiece is critical.) Based on preliminary observations done this evening, I believe that the Vixen star diagonal was a solid investment.

Improvement in the definition of the Galileans (along with a reduction in spurious image brightness, and size) is perhaps the most definitive fact I can point to. Keep in mind, however, that such improvements may be the result of better mechanical qualities (impacting collimation) than that of the new diagonal's mirror. Finally, I would note that several new features of Jupiter's moons, its atmosphere, Saturn's moon Titan, and Saturn's ring system were noted this evening. It is possible that the new star diagonal (whether through it's mirror surface and/or mechanical properties) contributed significantly to revealing these new features.

More testing is needed. As I do so I hope to answer the following questions: Will my extra-focal star test clean up the "mushiness" I have seen to date? Will I finally see the Crescent Nebula? (And not just a bunch of field star haze.) Will I be able to regularly make out the 13th magnitude field star next to M57 during less than ideal viewing conditions? Will the compact 6.5 magnitude globular cluster M15 prove susceptible to resolution without extreme averted vision? Will I ever again feel the need to get new stuff for my 150mm Maksutov Cassegrain Telescope?

Stay tuned!