Venus (25 March 2007)

Date: 25 March 2007 U.T.
Time: 1:30 - 2:30 U.T.
Telescope: 30cm Newtonian
Magnifications: 60x, 200x, 250x, 395x
Filters: none, #47, #38A, #58, MSG
Seeing: Fair (most of the time)
NELM: Bright, then darker Twilight

The phase of the planet had changed by a small, but very noticeable amount since my last observation. The planet is slowly growing in apparent size. Its phase is gradually changing from a fat gibbous toward quarter. Much of the following experimentation with magnifications and filters was inspired by recent discussion on the newsgroup; sci.astro.amateur.

The above sketch shows the general appearance of the planet as observed on this evening. South is up. Preceding is toward the left.

A page full of notes was taken as I experimented with magnifications and filters. I started with a magnification of 60x without the use of any filters. The planet's phase was immediately evident. The disk was well defined. The planet appeared white, devoid of any false color.

Next up was a huge jump to 395x. Atmospheric turbulence became significantly more noticeable. The planet's phase remained easily visible. Some false color ranging from rust orange to blue to gray was introduced by the eyepiece optics in the sky immediately adjacent to the planet's limb. The planet itself remained essentially color-free.

I tried 250x next. The small amount of false color around the planet varied with eye placement. It was evident that this eyepiece had better color correction than the previously used eyepiece.

The ideal magnification for the telescope, object, and sky conditions seemed to be 200x. Once again, the small amount of false color in the sky immediately adjacent to the planet's limb varied with eye placement. The planet itself remained white with no false color. Up until this point in time no filters were used.

For most of the remaining of the session I maintained a magnification of 200x. A #47 Violet filter was tried. The image became steadier. The filter introduced a weak, violet, reflected image that overlapped the image of the planet when the planet was far from the center of the eyepiece's field of view. Otherwise that reflection was not seen.

I prefered the brighter image offered by a #38A Blue filter over the denser violet filter; but the seeing seemed to be a little better with the violet filter. I next combined the Moon-SkyGlow (MSG) filter with the blue filter. A very subtle improvement in the view of the planet was suspected over the blue filter alone.

The blue filter was replaced with a #58 Green filter. Some subtle albedo markings on the planet were suspected. The planet's terminator appeared to darken relative to the rest of the planet.

I removed the MSG filter. With the #58 Green filter alone the subtle detail seemed a little more difficult to detect. It would appear that the combination of the MSG and #58 Green filters may work best for my eyes, Venus, and the 30cm Newtonian.

The MSG filter was tried without any other filters. It imparted a very weak, purple (but almost white) color to the planet. The color was easier to notice with the planet placed out of focus.

By now Venus had dropped lower in my sky -- lower than I've previously observed the planet this year; but I remained curious about all the false color that other's have mentioned when observing this brilliant planet. I lowered the magnification to 60x to brighten the planet in the darkening sky in an attempt to exaggerate any false color. Venus now had a blue glow above its disk and a red glow below. Perhaps the planet had dropped low enough in my sky for Earth's atmosphere to refract the different colors differently enough to notice. Nevertheless, it remained very easy to see the planet's disk and phase despite the planet's much smaller apparent size and much greater apparent brilliance.

My attention was now drawn to the diffraction spikes created by the telescope's spider. In the darker sky it was possible to see that the spikes consisted of a rainbow pattern of color. At increasing distances from the planet, the color in the spikes changed from yellow to orange to red to blue and green. The pattern repeated further from the planet as the light in the diffraction spikes grew weaker. The planet itself remained free from such 'false' color.