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REVIEW: Skywatcher 70mm F900mm Refractor
SKYWATCHER 70mm, 900mm focal length refractor on AZ3 mount
I bought this telescope while I was out looking for an EQ2 mount, since I commandeered the mount that came with my 4.5" Newtonian to build a binocular mount. This scope was on the showroom floor mounted on the EQ2, and came with a diagonal, 2 eyepieces, small finderscope, and a barlow at a bargain price. It is shown on an AZ3 mount in this photo, as I routinely switch my smaller scopes between mounts. The plastic focusser only accepte 1.25" eyepieces.
This telescope is very light and portable. It's small 70mm objective lense is useless for any real deep sky observing, but it's long focal ratio (almost 13:1) makes it fantastic for high powered views of the Moon, as well as observing the brighter planets like Jupiter, Saturn, Mars, and Venus. If you have ever looked at the Moon or Venus in a short focal ratio achromatic refractor (5:1) without any filters, you will have seen the purple aura that surrounds brighter objects in such a scope.
This telescope is great for a first telescope if you want something you can just pick up with one hand, grab a few eyepieces in the other, and set up for some quick viewing- not bad for something I never really intended on buying. As I mentioned a lens this small will not give you good views of deep space objects like galaxies, nebulae, etc, but this telescope will show cloud bands on the planet Jupiter (as well as Jupiter's four largest moons), Saturn's ring system, the changing phases of Venus, and a lot of detail on the Moon.
I have noticed that Skywatcher makes other 70mm refractors. The refractor pictured above has a focal length of 900mm. Other 70mm telescopes with a shorter focal length (700mm or 500mm) will have 2 major differences. For one thing, the magnification (power) in a short focal length telescope will be less that it would in a telescope with a longer focal length using the same eyepiece. Secondly, if you have 2 entry level refractor telescopes with the same diameter lens BUT different focal lengths, the telescope with the longer focal length will usually come to a sharper focus. This is due to something known as Chromatic Aberation. The shorter the focal ratio of a telescope (focal length in mm. divided by the lens diameter in mm.), the more pronounced this effect will be. This is due to the fact that different colours react differently when they pass through a lens- they all don't focus at exactly the same point. The shorter the focal ratio is, the more pronounced the Chromatic Aberation will be.