For Sale: 4-inch f/6 refractor OTA
The objective is a military surplus 4-inch diameter Bausch & Lomb Aero Tessar, with a 610 mm focal length (f/6). It is in a brass cell, and the glass has a single quarter-inch long scratch that will not impair performance. The eyepiece is military surplus, probably an Erfle, and its diameter at the knurled part is 3.1 inches. It has a focal length of about 36 mm, providing 17 power wide field views. The field of view is 4 degrees, and the apparent field is about 68 degrees. The eyepiece is rotated to focus. The OTA has an integral elliptical front surface diagonal mirror. The optics are in a robust tube of 7.3-inch diameter. The tube interior is flat black, and the outside is glossy black. The whole OTA is 26.38 inches long, and it weighs 20 pounds. The objective has a lot of glass in it, so the OTA balances toward the objective end. The OTA shows excellent workmanship. It gives great wide-field views of the moon, star clusters, and the Milky Way, as well as terrestrial views (the Edmund Astroscan gives similar views, but at twice the price!). Chromatic aberration is very slight. Price is $100, plus shipping costs.