100mm (4”) f/900 Fluorite ED Apochromatic Refractor Telescope  
Sky-Watcher supplied product information - 
Apochromatic Refractors, due to their unobstructed, text-book like diffraction patterns, produce the sharpest, highest quality optical imaging available, bar-none, aperture for aperture, of any telescope optical system.
These high-end telescopes all feature a doublet objective lens. One element is made from premium Japanese Ohara FPL-53 Extra-Low Dispersion (ED) glass, which virtually eliminates chromatic aberration to deliver true colour reproduction. Top quality Schott glass is utilized for the Crown element.
Each air-to-glass lens surface has exotic anti-reflection metallic coatings applied, to ensure optimum light through-put of approaching 99.5%!!
The proprietary Sky-Watcher “Metallic High-Transmission Coatings” (MHC) are the finest photon anti-rejection coatings in their class.
For a limited time only, this refractor is offered with a free 0.85X focal reducer, and lowered in price!

Our view

Refractors that use S-FPL53 as the low dispersion element usually have well corrected images for secondary spectrum. With short focal ratio S-FPL53 doublet refractors, (<f/8), it is still possible to see some residual colour in normal observing of bright objects. This 100mm f/9 refractor is free from axial secondary spectrum as well as can be expected from a doublet of this type. In normal use, images of the moon and planets will be as free of secondary spectrum as a good 100mm triplet refractor of shorter focal ratio. Considering the price, this instrument is the perfect aperture/focal ratio/weight for a premium refractor. It is extremely portable, will mount happily on any of the German equatorials of EQ5/CG5 and better. As a solar instrument in white light with Astrosolar filter over the aperture, the image quality is excellent. For astronomy at night, stars are pinpoint, deep-sky objects stand out well in contrast with the background sky, and planets will show all the detail a 100mm aperture can reveal. With the 0.85X reducer, it is difficult to recommend a better value for money refractor.

Tips

Try some of the 80o eyepieces with this scope, the f/9 focal ratio plus the off-axis corrections on the wide field eyepieces will result in spectacular  wide field views. Make a short dewshield to add onto the scope's dewshield, in order to lengthen it. The amount of dew we get in the UK requires a longer dewshield than is usually provided as part of the scope.