Thoughts on Moon/Spica occultation during the day
11/29/2013 11:33 AM
A few days ago I wrote about the Moon occulting Spica... but during daytime hours. I knew that stars like Sirius could be seen during the day - but Spica is also 2.5 magnitudes dimmer - or 9 times fainter. During nighttime, that may not be much, but during the day, it could mean the difference between seeing something, or not seeing it. The sky was going to be clear, so I was determined to give it a shot.
Originally I had planned on using a 10" telescope, because I wanted maximum light gathering. But that wouldn't help other people with smaller scopes know if they could see things like this in the future. So initially, I decided to bring out my 4.5" Orion Starblast, and if I couldn't see Spica near the Moon with that, then I'd go to increasingly larger scopes. Because I started looking a good 30 minutes before the occultation, with the star within a quarter of a degree of the Moon, I knew I'd at least have it in the field.
Low magnification? Eh, maybe not.
I started with low magnification, around 18x, with a 25mm eyepiece. At night, the low power isn't so bad, but during the day, one can start to 'see' the secondary mirror in a scope this fast. It's not clearly there, but shows up as an annoying smude in the middle of the field, that moves with every movement of your head. I found I could SEE the star, but it took some time to find it, which brings me to my next point:
Take your time.
Viewing a star during the day isn't like viewing one at night. Though I knew where Spica should be, that didn't make it easy to spot. I knew I needed really good focus, and the only way I could achieve that was with the Moon there. The Moon wasn't the largest crescent either, so it was difficult, since the contrast on it was so washed out compared to the sky background.
Amazingly, it didn't take much in the way of clouds to block the star. Mind you, Spica is 0.95 magnitude (effectively a first magnitude star), and in the night sky is fairly bright. Not so during the day. And there was a narrow band of high, thin cirrus clouds wafting RIGHT THERE where the Moon and star were in the sky. There were times the star wasn't visible at all. Other times, it popped out and was obvious. Sky transparency matters. That said...
Use more magnification
There's a well known idea for night time observing that says use more magnification on objects if you can so that the sky background will darken. In light polluted areas, this effectively appears to enhance the contrast between the sky background and the object. It doesn't actually change the contrast, but a faint grey smudge on a slightly darker faint grey background is harder to see than a faint grey smudge on a black background.
The same is true of stars during the day. A faint, point-like object is a lot easier to see against a darker blue background than a lighter blue background - especially when that star is a blue-white star to begin with! I found that as I bumped up magnification to 50x, 75x, and 150x, the star was MUCH easier to see. Now, if I'd had a tracking mount, I might have left the 3mm eyepiece in at 150x, because that was by far the darkest background, and easiest star image to observe. But I knocked it back down to 75x just before the occultation so I could manually track the two objects as the Earth rotated.
Eye issues are more obvious
Every view the Moon or Jupiter at night and see those "floaties" in the view? Those are in your eye. Other issues can be there too. Well, during the day, while looking at a bright sky, those are going to be an issue too. I actually had trouble seeing Spica initially at lower power because of problems seeing past the floaties and other lens issues in my eye. Ah... the joys of aging... NOT. Oh well, c'est la vie. But something for others to bear in mind might cause issues. Higher magnification made it easier to see the star.
It IS possible to see stars during the day. The Moon being nearby REALLY helps in finding focus. Clarity of the sky matters, BUT, even with the clouds, more magnification helped despite the clouds. And it is possible to see a first magnitude star during the day with just a 4.5" / 114mm telescope.
Give it try sometime - just be careful of the Sun. I blocked direct sunlight from hitting my telescope by placing a tree between the Sun and telescope.
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