Barlows and Powermates

Most of the world's top imagers use Barlow or Powermate lenses to stretch their telescope focal lengths to the appropriate f/20, 30, or 40 focal ratios. Because of the renowned quality of the devices made by the U.S.'s TeleVue Corporation, founded by Al Nagler, most leading amateurs use their systems for this purpose. In fact, the trade name Powermate is reserved by TeleVue. Any optical system in the telescope light path can degrade the image, so it is a good idea to use a quality component, especially at short telescope f-ratios where aberrations are more common. TeleVue's Barlow lenses and Powermates come in a variety of f-ratio enlarging values. At the time of writing, 2 x and 3 x Barlows were available as well as 2 x, 2.5 x, 4 x, and 5 x Powermates. For narrow-field planetary work there is little difference in the performance of the 2 x Barlow and the 2 x Powermate, however, the Powermates dominate at the high-power (4 and 5 x) range. The 5 x Powermate is especially popular with Newtonian users who wish to extend their relatively modest f-ratios. In essence, all a basic Barlow lens needs to be is a negative, concave (diverging) lens, like the sort you would find in a pair of short-sighted user's spectacles. (Perhaps confusingly, when used in that application, a negative lens actually makes objects look smaller!) However, in practice a well-made Barlow lens will not add any color aberration to the image and will be multi-coated for maximum light transmission. The Powermates go one step further by featuring an additional field lens to reduce vignetting and allow much higher f-ratio enlargement.

Confusion often arises when using Barlows and Powermates as, on inspection, you often find that the f-ratio you get is a bit larger than you expected. Fortunately, TeleVue have some very useful diagrams on their web pages that show how the enlargement factor for all their lenses increases with the projection distance from lens to CCD chip. When you put accessories like filter wheels in the light path, between Barlow/Powermate and the webcam chip it is surprising just how much the f-ratio can increase beyond the nominal 2, 3, 4, or 5 x. There is another source of focal length enlargement, too. When a compound instrument like a Schmidt-Cassegrain is employed, if the instrument is focused by moving the primary or secondary mirrors, the focal length of the instrument varies. Predicting exactly what f-ratio your system will end up being can be an almost impossible calculation, but if you assume it will be some 20 to 30% more than you would expect, you will not be surprised. For example, I have a 250-mm f/6.3 Newtonian and a 5 x Powermate, so I would expect an f-ratio of around 31.5. In practice, with no intervening filters, I get f/38, a 20% increase. Knowing this can actually save you some money. For example, you may feel that your system requires a 4 x Powermate, which, at the time of writing, is only available in a large (50.8-mm eyepiece diameter) barrel size. However, if you take a 30% enlargement into account, you may find that a 3 x Barlow will suffice, as it may give nearly 4 x magnification in actual use. At current prices that will save you 60%. In extreme cases, a 3 x Barlow projecting 100 mm beyond the end of the tube will give 5 x enlargement. For a 5 x Powermate, the extra projection distance yields a whopping 8x magnification.

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