I hardly ever do visual, but would only look at a triplet myself, perhaps not so important with mono as can focus each colour independently?? Great visual performance comes cheaper in a doublet and good photographic color correction tends to be more affordable in a triplet. It would be almost impossible to see any chromatic defocus even on the brightest targets when used visually. The SW120ED f/7.5 with fpl-53 glass sells for $1549. Perhaps just bad luck even though I am careful with my astro gear. There are a number of ED or FL doublets that are for all practical purposes true visual APO's at least where color is concerned and are for all practical purposes almost visually indistinguishable from a high end triplet. A very modest improvement - hard to see it as worthwhile, at least in terms of reducing secondary color. The invention of the camera in the early 19th century led to an array of lens designs intended for photography.The problems of photographic lens design, creating a lens for a task that would cover a large, flat image plane, were well known even before the invention of photography due to the development of lenses to work with the focal plane of the camera obscura. IME for Planets the difference is negligible, thus the Doublet will be lighter and acclimate faster. Aperture isn't so important either, as a top class figure on a 100mm scope has more impact on the planetary view than the extra aperture of a not so well figured 130mm scope. If you want a 100mm class scope that is an Apo, not so expensive, and visually fairly colorless, the 100mm Sky-Watcher f/9 Doublet is the way to go. Because I'm an imager. With 3 pieces of glass, it is a little easier to correct for chromatic aberration than with 2 pieces. See our #5 pick above, which is a fantastic high-end 102mm option. Reflectors are CA free, as the light does not pass through a lens, but bounce off mirrors. Hint: you’ll need a field flattener. I'd prefer a triplet. The Triplet is a slightly bigger aperture, but the cost is only a few 100s in difference. The next type would be an FPL 53 doublet. 130mm Triplet APO Refractor, Ohara FPL51 > 9mm Image Size <.65a Bandpass (Single Stack) <.45a Bandpass (Double Stack) Use with: CaK and White Light Filters; 910mm Focal Length; 34” Overall Length; Lead Time: 7-10 days Lead Time For visual use at 4", an FPL51 triplet is sensibly perfect. I'm looking at the AT125 Doublet with the FCD100/Lanthanum or the AT130  Triplet with the ED glass. The Sky-Watcher HEQ5 is a GoTo equatorial telescope mount capable of deep-sky astrophotography. I'm just interested where people come down on this. I have a 4" Vixen doublet with FPL53 but the mating element is unknown though rumor has it's lanthanum. The reason would be cost. Tips and tricks But under optimum conditions and given enough time to cool down, the very best triplets will do very well because of their better color correction. I appreciate your help. An FCD100/F2 doublet gives nice performance, but and FCD100/LAK8 (or similar Lanthanum glass) doublet at 125mm and F/7.8 can theoretically be nearly perfect. But (130/125)^2 = 8% difference in aperture? But just for optical quality, it is an excellent choice. A camera lens (also known as photographic lens or photographic objective) is an optical lens or assembly of lenses used in conjunction with a camera body and mechanism to make images of objects either on photographic film or on other media capable of storing an image chemically or electronically.. I see that the Lanthanum glasses can have lower Abbe numbers than BK7 (good), but some don't match the partial dispersions (e.g., PF,e) of FPL-53 or FCD100 as well. Everybody should have one of these for not only great view but for knocking around without a lot of worry! It was one of the first mounts I had ever used for astrophotography, and one I still use to this day. (Fortunately for modern designers, and today's observers and astro-imagers, they not only have a large dispersion difference from normal crowns, they also closely match the Relative Partial Dispersion of normal crowns.). This is a very exotic (and expensive) kind of glass with better dispersion characteristics than FPL 51 (or equitivlent glasses). An FCD100/F2 doublet gives nice performance, but and FCD100/LAK8 (or similar Lanthanum glass) doublet at 125mm and F/7.8 can theoretically be nearly perfect. There will be a pale violet non the surface of the moon and Jupiter, and you may see a heavy fringe n the limb of the moon. All scopes loose contrast as the angular size of the detail decreases.. As far as the Orion 110mm F/6 with the FPL-51 based objective.. Thanks! So strictly speaking a refractor that is only a doublet is not an APO and never can be regardless of how good the glass is. So not cheap when it is not consistent IMO. It features a compact design with features custom-suited for wide-field, deep-sky astrophotography. Should be almost indistinguishable from an APO triplet.,,,not just for visual but for imaging as well. Dramatically better. It wasn't even close, the 90T clearly was noticeably sharper and showed more contrast and detail than the 102ED. a Doublet), Apochromat brings 3 wavelengths to focus (i.e. I would have been more interested in the SW 100ED if it had a sliding dew shield and a different paint job. I doubt it ever want for $2G's. If both are excellent, then from a visual planetary observers point of view I'd personally prefer the faster cool down of a doublet. For example, a perfect 6" will have lost 50% contrast at about .4 times its limiting frequency, while the 6" f/8 achromat will have lost about 70% contrast and for detail this size, will not be that much better than the best 4" Apos..

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