Friday, December 16, 2005

Binoculars: Stars Near the Moon

Swarovski_ELs_100p.jpg Fujinon_10x70_100p.jpg

The sky is clear tonight and the nearly full Moon is rising above the buildings on Manhattan's East Side. I thought to check out how dim a star I could see near the Moon with two different pairs of binoculars. The key was not to know which stars are in the vicinity of the Moon. Binoculars used were:

• 8.5x42 Swarovski EL
• 10x70 Fujinon FMT-SX

The Swarovski's are my main birding binoculars and they fill this task extremely well. Unfortunately they are less capable when viewing the Moon. The reflections within the binoculars creates many bright ghosts which reduces the overall contrast. The sky is not black in the FOV when looking at the Moon.

The Fujinon 10x70 FMT-SX are my primary binoculars for nova hunting. When viewing a bright object like the Moon I'm very impressed with the way these binoculars deal with internal reflections. They are almost non-existent. The area surrounding tonight's nearly full Moon was black to within a half degree of the Moon's limb. There is a bit of a coronal effect but it is so uniform and thin that it doesn't hamper the view all that much. Viewing the Moon with these binoculars is an absolute pleasure.

At approximately 7:15p EST I studied the area surrounding the Moon with the Swarovski 8.5x42's. The two most obvious stars were Mebsuta (ε Gemini, mag. 3.1) to its right (west) and 28 Gemini (mag. 5.4) above and slightly to the left of the Moon. After looking around and not knowing what was around I came across three more stars forming a downward pointing triangle above and to the right of the Moon. The stars in this groups were mag. 5.3, 5.8 and 6.0.

The distance of the 6.0 star (54 Aurigae) was 2.5° from the center of the Moon. No other stars were noticed within 3 degrees of the Moon. I then took a look at ‘Planetarium for the Palm’ (PftP) and noticed a 6.5 mag star 2.1° from the Moon's center. When I studied the area where this star should be I was able to see it with the 8.5x42's.

With the Fujinon 10x70's all the stars mentioned above were easily seen and no others were noticed after scanning for a few minutes. Thinking no other stars would be visible in the 10x70's I checked PftP and noticed a 6.7 mag. star 1.1° from the Moon's center. After looking closely at the area where the star should be with the Fujinon's I spotted the star. It was steadily seen as long as I looked directly at it. This was clearly more difficult than the others. It was seen each time I made the attempt to view it.

Later on I noticed a 6.7 mag. had recently reappeared on the Moon's limb. I made an attempt at 8:20p EST while the star is a mere 12 arc minutes from the western limb of the Moon. I wasn't able to pull it out with the Fujinon 10x70's.

It's good know the ‘limits’ of your optics so you can tell which events are possible and what equipment is needed for a particular task. I'm always testing ... it's fun!