Used Tele Vue Genesis f/5 100mm on wooden surveyor-type EQ mount

Televue Genesis Fluorite f/5, Petzval design (two lenses in front and two in the back), 4 inch refractor, f = 500 mm. Given the short focal length and flat field, this scope provides stunning wide field views. You can easily get 4 degree fields of view with exit pupils of 4-7 mm and gorgeous views of the M31 complex, the entire Veil nebula, the North America nebula, the Lagoon and Trifid together, giant star clouds in the Milky Way, etc...

The Genesis is a quadruplet design with an ordinary doublet up front and a second doublet reducer-flattener just in front of the focuser to make four elements in total. The front doublet is held in a slim and simple cell attached to the tube by rivets or blind-headed screws. The collimation is set at the factory by carefully nudging it into place on an optical bench and then fixing it (no adjustment is possible) – sounds crude but works out fine. Multi-coatings are good, but not quite up to modern standards.

The rear Petzval lens allows greater freedom to correct monochromatic aberrations, like field curvature and coma, than most other designs, but it doesn’t correct chromatic aberration. Instead, the ‘trick’ is that it reduces the focal length by about half (down to a very fast 500mm, F5 in this Genesis). That means the front doublet is actually about F10. Even though the front doublet is an ordinary achromat, a focal length of F10 substantially reduces false color compared to F5, but doesn’t eliminate it.  

The Genesis is labelled “fluorite”, but here that is a little disingenuous. In a Takahashi, a fluorite element allows the doublet objective to be apochromatic. In the Genesis the magic mineral is in the Petzval element, but the doublet up front is an ordinary achromat (unlike the later NP-101 which has an ED-glass doublet up front, so it’s a proper apochromat).

One of the lenses is fluorite, and that keeps the CA to a minimum when in focus and allows nice high power views as well. 

The scope comes with all original parts: metal foam-padded case, clamshell, screw-on metal lens cap, screw-on dew shield, TV Everbrite diagonal, TV 26mm Plossl. 

The tube length is 24 inches plus 4.5 inches for the dew shield. The weight is 10 lbs.