----------------------------------- Mircea Pteancu 15 Aug 2009 22:07 ----------------------------------- zoth mai demult ti-ai exprimat dorinta de a afla noutati de la Thomas Jensen.Iata ca in aceste zile a facut o comparatie intre doua lunete Zeiss de 60mm (+) si o luneta Vixen diafragmata la 60mm (individu' asta e culmea,cum incalca ''legislatia'' romana).Cel mai scurt instrument este la F/13 iar cel mai lung la F/20. Rezultatele sunt interesante si pline de invataminte.De notat - ocularele folosite au fost ,cu o singura exceptie,orthoscopice japoneze Kokusai Kohki -paradoxul campului vizual mai mare al ocularului de 7mm -paradoxul Vixen Lectura placuta,Mircea .................................................................................................... Hi friends A few days ago, after some discussion about a 60mm f/20 over on CN, I decided to take some of my 60mm's out to play. I wanted to compare them to one another and see what I would find. I found both some things I had expected, as well as something totally unexpected. More about that later. First, we must meet the competitors: 1: My Zeiss Telemator, a 63/840mm cemented achromat of high quality from 1989. Fully coated with Zeiss coatings. Diagonal a standard 1.25" Vixen diagonal, coated. Telescope equatorially mounted, but motor not turned on. 2: My Zeiss "Reisefernrohr" (travelling telescope) with 60/850mm airspaced E-objective, all from 1910. Not coated, of course, and I used a Zeiss diagonal from the same era, which is also uncoated. Altaz mount. 3: My Vixen 80/1200 stopped to a 60/1200mm f/20. Objective fully coated with standard MgFl coatings. Diagonal, a 36.4mm Vixen thread-on, is also coated. Mounted on GP equatorial. No motor. I have enough eyepieces that I could get the scopes close to each other in magnification. This was very important to get a reasonably reliable result. I could use the following magnifications and eyepieces: Telemator: 12.5mm 1.25" KK ortho: 67.2x 9mm 1.25" KK ortho: 93.3x 7mm 1.25" KK ortho: 120x 5mm 1.25" KK ortho: 168x 4mm 1.25" KK ortho: 210x Zeiss Travel: 12.5mm 0.965" KK ortho: 68x 9mm 0.965" KK ortho: 94.4x 7mm 0.965" KK ortho: 121.4x 5mm 0.965" KK ortho: 170x 4mm 0.965" KK ortho: 212.5x Vixen: 18mm 1.25" KK ortho: 66.7x 12.5mm 1.25" KK ortho: 96x 10mm 0.965" Zeiss ortho: 120x 7mm 1.25" KK ortho: 171.4x 6mm 1.25" KK ortho: 200x. The images at high power were especially revealing, and most of the comparison was done at around 170x, in which case all the eyepieces used were KK orthos (Kokusai Kohki). So what did I expect to see? 1: Telemator: Brightest image, with the most pronounced chromatic abberation, but still very high contrast. 2: Zeiss traveller E60: Dimmest image, with second-best color correction and very good contrast. 3: Vixen: Close to no chromatic abberation and extremely sharp images. A little brighter than the Zeiss E60, but dimmer than the Telemator. And what did I see? I found all of the above to be true, but there was also some unexpected surprises, as I have already mentioned. Test object was the Moon, two days past full, in so-so seeing with good moments. A little wind and no clouds. Warm, but with surprisingly little dew. The Telemator was brightest by far, but had a noticeable (when you looked for it) blue halo beyond the edge of the Moon and upon close scrutiny, the Moon had a very pale yellow-green hue, but the images were still very close to a neutral grey. The hue was extremely pale, and when you just sat down and looked, you never noticed it. All as expected. Star test shows lots of chromatic abberation and some spherical abberation. This scope will show astigmatism during cooling, since the internal focuser tube works like a thermos bottle, so it takes a while to cool. The Zeiss E60 was quite dim at the high power of 170x, but didn't loose contrast or sharpness at that power. The image was almost yellow-brown, but there was almost no hint of chromatic abberation and the limb of the Moon was pure and crisp. The performance of this old scope is really something special. Essentially perfect star test with exceptionally crisp fresnel rings. Has trace amounts of astigmatism in-focus. Cools very fast. The Vixen, however, gave a startlingly clear and sharp image, even better than the Zeiss E60. There was basically not a hint of any false color and the image was extremely pleasing. It was slightly yellow in tone, but I am not sure why, since there was no chromatic abberation visible, but perhaps it was just the somewhat dim image that did it. It was a bit brighter than the Zeiss E60, but fainter than in the Telemator. Again, all as suspected. Perfect star test when stopped to 60mm f/20. Period. Cools very quickly, like the E60. Now for the unexpected! The image in the Telemator, to my great surprise, appeared somewhat "grainy" at 168x and 210x and this was also visible to a lesser degree in the E60. I am at a loss to explain it, since it had the brightest images, and it is very hard to describe. Especially since it never showed up in the Vixen, even at 200x! This is not to say that the E60 wasn't sharp, for it held its own against the Vixen and showed the same, difficult details. In the latter, however, the Moon appeared like in the Apollo shots - really - with softly rolling hills and with a smooth, realistic impression. It was distinctly different than the others, which gave a "harsher" view, especially the Telemator. Also, I couldn't find anything in either scope the others couldn't show, but there were differences here as well. The Telemator showed a lot because the images were so much brighter than the other two, that the eye had a much easier time seeing it, while the other two had superior contrast and ever so *slightly* crisper images, which in turn helped them show difficult detail. And every difficult detail showed up easier in the Vixen than in the other two. And once I realized that, I noticed something about the Vixen, that I hadn't before: It was much easier to look through and the images were much steadier. This intrigued me, since all three had lots of time to cool (hours) and all three were set up not a meter from each other, looking through the same air, yet the difference was very obvious. I jumped from scope to scope, but the result was always the same. Each time I got back to the Vixen, my eyes and brain sighed in relief and difficult detail I had strained to see through the others started to pop right out, though it never showed something the others couldn't. Also, the apparent field of the eyepiece appeared larger than in the others, and I was using the same eyepiece design in all of them! I did a comparison, by holding both the 7mm and 5mm KK orthos up to my eyes simultaneously, and the apparent fields were identical, but the 7mm just seemed somewhat larger, perhaps because the eye relief was 40% larger. But most interestingly, the seeing also appeared to be worse in the two Zeiss scopes than in the Vixen! Apparently, there must indeed be something about long focus telescopes giving steadier images. When a wave of turbulence rolled across the Moon, I could still see the detail rather well in the Vixen, but it was completely washed out in the two Zeiss scopes. The Vixen was also much easier to focus and had a focus spot several millimeters long, where I could fine-focus with my eyes without any trouble at all. I actually had to be careful to let my eyes rest at their most comfortable focus and not let them begin to focus on the image before I had found the best spot with the focuser. Once I had found the best spot, I could relax and spend many minutes just looking and there was never any eyestrain. It was very comfortable and completely different from observing with the two other scopes. It goes without saying, that I tried to obtain the same effect from the other two, by fine-tuning the focus, but I never succeeded. The focus on the two Zeiss scopes was more critical, but still rather easy to find, of course, since these are still f/13 and f/14 instruments, long by modern standards. Yet f/20 seems to be really loose and comfortable by comparison, as well as being *very* easy on the eyepieces. All in all, the shootout confirmed a lot of things, as well as revealing startling surprises. I would never have expected the f/20 to be so much easier on the eyes in actual use, compared to the others, though I have suspected it for some time. It is much harder to tell, if you haven't got the two scopes right next to one another. And I had not expected the difference in sensitivity to seeing to be so clearly visible. It was a highly interesting night, and I fully expect to do this experiment again, sometime this autumn, perhaps on more targets as well. Meanwhile, Clear skies! Thomas, Denmark