Astromart Reviews - William Optics FLT110mm f7 TMB designed ...

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Astromart Reviews - William Optics FLT110mm f7 TMB designed triplet and matching field flattener By now it is time to mount the telescope on a mount. The image below shows the FLT110 on my AP600 GTO mount, with the dedicated field flattener and DSLR adapter in place. The FLT110 is light enough a telescope (6 to 7 kgs depending on accessories) to not require a heavy equatorial mount. It will fit easily on a Vixen Sphinx or GP-DX mount, or on Losmandy GM8 equatorial system. So portability will hardly be an issue, not even for people who need to drive to an observing site. Visual observations, in depth comparisons Even though I am a photographer more than a visual observer, I decided to first have a good visual observation run with the telescope before star testing it. This avoids getting biased upfront. The telescope certainly does not disappoint visually. Compared with TMB 80mm and Takahashi TOA130F I observed some deep sky objects with Nagler eyepieces and the stars to me are sharp as needle points. My TOA130 still does a better job in rendering the stars perfect, but this scope is not in the same price league! The moon is quite spectacular in the 110mm telescope and views are gorgeous with plenty of contrast and resolution. Saturn pulled up a nice show, even at 277x (5mm TMB eyepiece and 1.8x TMB ED Barlow lens) it was still very good. Higher magnifications could be possible, but some slight image softness started to become noticeable at 277x, so I did not push it beyond that limit that night. I compared the FLT110 with my 1/10th wave TMB 80mm (made in Russia) and found the in and out focus images of the 80mm to be in a somewhat higher league than the FLT110. While the in and out focus images of the little TMB are virtually textbook perfect, much like the TOA130F I have (which again is a bit better than the TMB - how perfect perfect can be I wonder), I do see slight residual spherical aberration left in the FLT110 telescope and a tiny bit of a zone. Not much, but it is not a perfect star tester. The patterns are not as well defined as with the 80mm TMB or the TOA130. http://www.astromart.com/articles/article.asp?article_id=523 (4 of 19)1/6/2007 10:54:28 AM
Astromart Reviews - William Optics FLT110mm f7 TMB designed triplet and matching field flattener Now, how serious is this slight residual spherical aberration and little zone? Not that serious I would be worried about it. It will take superb seeing conditions, excellent eyepieces and a keen visual eye to detect real differences on a planet I thought that night. Unfortunately William Optics does not give absolute quality numbers for these refractors, so it is difficult to judge if the one I received is representative. If judged by the simple criterion etter than diffraction limited? it most certainly passes the visual test. For curiosity sake, on another night I pushed the magnification on a distant spider web (illuminated by a streetlight) to 385x and the image remained sharp. I perform this spider test with all my telescopes, and the WO obviously did very well. Comparison with the TOA130 on the same spider web target definitely gave a resolution and brightness edge to the TOA (with 2 cm more aperture) which was to be expected. The image and contrast quality was very close, with the TOA still giving an advantage in sharpness and contrast (which does not surprise me as the Takahashi TOA is a superb piece of optic). The image below shows the test setup with the FLT110 and TMB 80 on my AP1200 GTO mount. Please note there is a blue sky each time I take an image of the equipment. Usually this blue sky disappears as soon as I want to do observing with the instruments. Compared with TMB 105mm We (Philippe Vercoutter, Jorg Versluys and myself) also compared the TMB 105mm f6 with the FLT110 under the dark skies of Grandpre, France. Given the star test reveals even tiny imperfections, we started with that test this time. We tried to evaluate the in and out focus patterns as good as seeing would allow us and the results were very good visually, but not as symmetrical as the patterns in the TMB 105. We also tried recording the impressions on video. Ideally, such video tests should be done in-house in lab conditions we soon realized. Seeing was not really good enough to allow the desired highly magnified star patterns on video and still keep enough resolution in the resulting image stack. But ok, the low magnification test does give an idea on what to expect. The human eye still has an edge on adaptive optics it seems ;) Perhaps I will try to do the test with video later on to show a higher magnified version. Given the fact it is cloudy most of the time, that could be in a very distant future though! The left side of the image below shows the infocus pattern, the right side the outfocus pattern, while the middle is a star in focus with the first diffraction ring visible. The outfocus image as usual suffers more from seeing turbulence. http://www.astromart.com/articles/article.asp?article_id=523 (5 of 19)1/6/2007 10:54:28 AM
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