Before the Nectaris Basin, There Was Rosenberger and Vlacq

This sketch was made on an exceptionally steady night of seeing and centers on two old craters in the highlands of the southeastern quadrant near the approaching sunset terminator. At times of higher sun the dominating feature here is a uniform scattering of similar looking 30-50 km. craters that dot this highland plain. It is something of a nightmare to identify small impactors in this local. This region looks and is ancient. Here you will find two large sketching targets side by side, the craters Rosenberger (96km.) and Vlacq (89 km.).
Both of these craters pre-date the impact that formed the Nectaris basin some 3.9 billion years ago. As obviously ancient as these craters appear at high sun with the worn out rims, lack of large central peaks, lack of ejecta blanket rays, etc. At a low sun angle this pair takes on a more youthful appearance. The sharp shadows hide some tell tale signs of age. The smaller of the two, namely Vlacq is noticeably more youthful, with its larger, central, double massif, lower floor crater count and slight bulge inward onto Rosenberger.
It was a pleasure under near perfect weather conditions to observe and sketch this pair again at the eyepiece after 3 years hiatus.

Sketching:

For this sketch I used: black Strathmore 400 Artagain paper, 9”x 12”, white and black Conte’pastel pencils and blending stumps, gum eraser, brush. Brightness was slightly decreased with the scanner.

Telescope: 10 inch f/ 5.7 Dobsonian and 6 mm eyepiece 241 x
Date: 8-28-2010, 6:30-8:20 UT
Temperature: 22° C (72° F)
clear, calm
Seeing: Antoniadi II
Colongitude: 133.2 °
Lunation: 18.1 days
Illumination: 88.8% Waning Gibbous

Frank McCabe