Venus (27 March 2007)

Date: 27 March 2007 U.T.
Time: 1:30 - 1:50 U.T.
Telescope: 80mm refractor
Magnifications: 20x, 40x, 67x
Filters: None, #56
Seeing: Fair
NELM: Bright Twilight
Temperature: +53 F (+12 C)

Tonight's observation was made using a relatively simple, 80mm, f/5 achromatic refractor. Such telescopes are occasionally used by individuals just starting out in amateur astronomy. More experienced amateurs sometimes use such instruments as quick-look, grab-and-go telescopes or even as finders for larger telescopes.

I chose this particular telescope this evening for the following reasons: Predictions called for bad seeing, wind, and increasing clouds. I wanted to see what Venus looked like through a small and relatively low quality telescope on a relatively poor night.

At 20x Venus looked like a tiny blob of bright light surrounded by a substantial, mostly purple glow. Yellow-green light was occasionally also seen around the planet. These colors were due primarily to the telescope's achromatic objective lens. Such objectives fail to bring all colors to the same focus; but often, particularly when looking at much less brilliant objects, this defect can go unnoticed. At 20x the planet's disk could not be made out with any degree of certainty.

At 40x the planet's disk was clearly visible. It was difficult, but possible to discern the planet's gibbous phase. False color surrounded the planet just as it did at 20x. The planet's normally white disk appeared somewhat yellowish due to the 80mm achromat's chromatic aberration.

At 67x the planet's disk was clearly visible; and its gibbous phase was clearly discernable. The wind was strong enough to shake the telescope a little; but it wasn't strong enough to compromise the observation. The false color extended outward from the planet in all directions a distance of at least three planetary diameters. The disk of the planet was only mildly discolored (like it was at 40x).

At 67x with a #56 Light Green filter the planet looked much better. Green was the only color visible with the filter. For all practical purposes the filter effectively eliminated the achromat's false color. The purple glow around the planet was gone and replaced by a much weaker greenish glow. The seeing conditions caused the planet to jiggle a little as if it were made of jello; but the jiggling was quite mild. The planet's gibbous phase was well defined and obvious.