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Getting ready for Mercury

April 18, 2016

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The planet Mercury will transit the sun on the morning of Monday, May 9. Mercury transits are not as rare as the more famous transits of Venus, but they still only come around once or twice a decade on average. The last Mercury transits before this one were in 2003 and 2006, and the next two after this year will be in 2019 and 2032. From southern California, the transit will already be underway when the sun rises at 5:57 AM, maximum transit (the point when Mercury is the furthest inside the sun’s disk as seen from Earth) will be at 7:58, and Mercury will exit the sun’s disk between 11:39 and 11:42 AM (all times in PDT).

For the transit of Venus in 2012, I used a simple homemade device called a “sun funnel” attached to a small reflecting telescope to project an image of the sun. You can read more about that here and here. The sun funnel worked well enough – I also used it for the annular eclipse in 2012 and the partial eclipse in 2014 – but the screen material degrades the resolution somewhat. Mercury is a lot smaller than Venus, and much closer to the sun, and both of those factors make it appear much smaller than Venus during a transit.

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I want maximum resolution for observing and photographing the upcoming transit, so I finally sprung for a full-aperture solar film filter for my 80mm telescope, which you can see set up at the top of this post. I got it out the other day for a test drive and got some decent photos of the current large sunspot AR2529, shown above. I’m pretty happy with the results – now if we can just get clear skies on the morning of May 9. If you’re curious, the filter I got is the GoSky Optics full-aperture filter with Baader solar film. There are several sizes available to fit all kinds of telescopes, and the filter attaches securely to your telescope tube or dewshield with three nylon-tipped screws. I got the filter for telescopes 81-113mm in diameter (outside tube or dewshield diameter, not optical diameter!), which is currently a little under $50 on Amazon.

This is my second GoSky product, after the universal cell phone adapter I picked up last fall, and I’ve been impressed with the solid construction and good fit-and-finish of both products. Some of the weird large-scale blotchiness in sun photos is probably either distortion from the iPhone’s tiny field lens, or gunk on the surface, and the uneven margin of the solar disc is from atmospheric turbulence. But I think the graininess across the surface of the sun is actual solar granulation. I couldn’t see it on the iPhone – not enough image scale. If I had, I’d have thrown in a shorter focal length eyepiece and tried some higher-magnification shots. They might not have turned out well even if I had taken them – the seeing was pretty awful – but it would have been worth a shot. Something to try next time.

The diameter of the sun is 109 times that of Earth. Here's how Earth would compare to the current large sunspot if they were side-by-side.

The diameter of the sun is 109 times that of Earth. Here’s how Earth would compare to the current large sunspot if they were side-by-side.

Unfortunately, I won’t be here in California to share the transit with my local friends and fellow observers. I’ll be in Utah chasing dinosaurs from May 4 to May 14, so I’ll have to catch the transit from there. I’m driving up and bringing my 80mm scope to take advantage of dark Utah skies in the evenings. If you want to plan your own transit observation, or just want to investigate how the transit will appear from various points on Earth’s surface, this interactive map is excellent. And if you need safe, inexpensive ways to observe the sun, check out my page on safe solar observing. Clear skies!

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4 comments

  1. Looking forward to doing outreach on the morning of the transit at my workplace. The plan is to set up my two small refractors with sun funnels at the main Westwood entrance to UCLA. I’m going to try to take off 2-3 hrs. from work that Monday morning.


  2. Great Post and exactly what I’m looking forward to. I recently got a Televue 85- never had a scope before so did a little research and settled on that. Now trying to figure out what filter to get and what cell phone adapter to do basically what you did. I would like a Hydrogen Alpha filter if possible.

    anyway thanks for a wonderful, informational post!

    Roger


  3. […] our host, Julia McHugh. She’d arranged to take us for lunch, but Matt was so obsessed with the transit of Mercury across the sun that he completely forgot, and sent me off to get salads from Subway instead. (Note that I am […]


  4. […] Stargazing for people who think they don't have time for stargazing. « Getting ready for Mercury […]



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