Archive for the ‘Sun’ Category


Observing report: the transit of Mercury from western Colorado

May 19, 2016

Mercury transit 9 May 2016 - telescope setup

I was in Utah from May 4 to May 15, chasing dinosaurs with Mike Taylor, a colleague of mine from England. I took a telescope along in hopes of getting some dark-sky time, and to hopefully catch the transit of Mercury on May 9.

Things did not look promising at dawn on the 9th. I was in Fruita, Colorado, and when I got out of bed, the sky was completely overcast. Mike and I decided to head out west of town to visit Rabbit Valley, where a nearly complete skeleton of the long-necked dinosaur Camarasaurus is visible in a hard sandstone ledge. (Why is no-one excavating this dinosaur? Because we already have many nice specimens of Camarasaurus, and the sandstone around this one is like concrete. It would be a mountain of work for very little payoff.)

We spent about two hours measuring and photographing the skeleton, and as we did so, the clouds started to break up a bit. By the time we got back to Fruita, a little after 11:00 AM, the sky was clear except for a few scattered wisps of cloud. I set up my telescope in front of the Dinosaur Journey museum and started watching and photographing the transit.

Mercury transit 9 May 2016 - Mercury crossing the sun

I was using the same setup as in the last post: my Celestron C80ED refractor, a Celestron 8-24mm zoom eyepiece, and a GoSky full aperture solar film filter. For photography, I used a Nikon Coolpix 4500 for still photos and my iPhone 5c for video.

I caught about the last hour of the transit, and I got to share the view with about a dozen museum staff and passersby. A few light clouds drifted through the field of view, which looked pretty cool and didn’t obscure the view at all.

At 12:42 Mercury finished exiting the disk of the sun. The next Mercury transit will be in 2019 – I hope I’m as lucky then as I was this time.


Getting ready for Mercury

April 18, 2016


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.


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!


A tour of Big Bear Solar Observatory

October 13, 2015

BBSO from up high

The gleaming white domes of the Big Bear Solar Observatory sit at the end of a causeway that projects from the north shore of Big Bear Lake – they draw the eye from almost any point in Big Bear Valley. And as I mentioned in my last post, the Pomona Valley Amateur Astronomers got to visit the BBSO on Friday, October 9.

BBSO causeway

We were greeted at the gate by Claude Plymate, Chief Observer and Telescope Engineer at BBSO, and Teresa Bippert-Plymate, who is not only a professional solar astronomer but also the president of the Big Bear Valley Astronomical Society. As pros who are also enthusiastic amateur observers, Claude and Teresa did a great job of pitching the tour with just the right balance of necessary background, technical detail, and the hands-on practicality of managing big scopes and the complicated hardware and software necessary to run them.


The first thing you come to on the causeway is a big white storage container with a coelostat (sun-tracking mirror) – this is one of the six Global Oscillations Network Group (GONG) installations spaced roughly equally around the world. The GONG telescopes track the sun around the clock for helioseismology research, mapping the acoustic pressure waves that propagate around and through the sun.

PVAA group outside BBSO domes

The smaller dome just short of the end of the causeway holds two telescopes on a common mount. One is a 10cm full-disc hydrogen-alpha solar telescope, the other is a second smallish refractor for Project Earthshine, which tracks the Earth’s albedo by measuring the intensity of the earthshine that falls on the moon’s unlit side.

London with BBSO New Solar Telescope

The observatory’s ‘big gun’ is the 1.6-meter New Solar Telescope, an off-axis Gregorian. One-point-six meters is 63 inches, which means this scope has a slightly larger aperture than the famous 60-inch reflector on Mount Wilson (which I’ve been fortunate to visit – see here and here). Here’s the light path of the NST (an unmodified version of this image is at the bottom of the post):

BBSO New Solar Telescope light path

And here’s a view on the right side of the scope showing the mask that rejects the light from most of the sun (which bounces onto the back wall of the dome, landing at about the same intensity as natural sunlight). The mask has a small hole which allows light from a small part of the sun to pass through to the chain of lenses and mirrors that bounce the beam to the research instruments on the next floor down.

BBSO New Solar Telescope right side optics

It took me a while to wrap my head around how this works. If the mask rejects most of the sun’s light, doesn’t that mean that most of the telescope’s 1.6-meter aperture is wasted? The answer is no – the mask functions as a field stop, not an aperture stop. If I put a mask across the front of my 10″ Dob and let only a 4″ beam of light through, that’s an aperture stop – it effectively turns a 10″ f/4.7 obstructed system into a 4″ f/12 unobstructed system (which may be desirable for sharp planetary and lunar views, where light-gathering is not so important). But imagine I left the front of the scope uncovered and instead masked down the field stop at the bottom of one of my eyepieces, so that I could only see a tiny hole in the center. If I put the scope on Jupiter, I’d see Jupiter in the center of the field but nothing else – I’d be getting the full benefit of the 10″ mirror’s light-gathering and resolution on Jupiter, but rejecting the light from the surrounding starfield, which would reflect off the mask at the bottom of the eyepiece. That’s more or less what happens with the New Solar Telescope, only “the rest of the field” is the rest of the sun, and the small area that the scope focuses on is not a planet but a small patch of the sun’s surface. But that patch can be imaged with the full benefit of the 1.6-meter primary mirror’s angular resolution.

BBSO burnt light shield

Now, a 1.6-meter mirror focusing the light from the full disc of the sun onto an area about 3cm across is a hell of a lot of energy. That beam could fry electronics, melt metal, and start fires if it got off-course. There are multiple redundant systems to prevent that from happening – the dome can close, the primary mirror has a cover that can activate quickly, and if all else fails a 1/16″ steel plate slides into position in front of the field stop. A few years ago – before Claude’s tenure as Chief Observer! – there were not so many safeguards in place. The software that allows the telescope to track the sun briefly got confused by some passing clouds, and the scope stopped tracking properly. That allowed the concentrated beam of sunlight to slide off-target. The steel plate did its job and slid into place, and the scope melted two holes in it in the space of about 30 seconds. The folks at the observatory keep the melted metal plate as a visible reminder that they are in a very real sense playing with fire.

BBSO sunspot image

This sunspot is a bit larger than our planet.

Our last stop on the tour was the telescope control room, where another professional astronomer was driving the scope and taking data. There was a minor mechanical hiccup at one point and Claude had to swing into action, running back and forth from the control room to the instrument room to get everything back on track. It was amazing to see live images coming in in real time. I’ve been fortunate to tour a lot of observatories but never while they were working. At one point Claude and the other astronomer put the scope on a sunspot group which was just swimming in atmospheric distortion. Once the computer had enough data to engage the adaptive optics, they switched on the AO and the view instantly settled down to nearly rock-solid, like it was painted on the monitor.

BBSO New Solar Telescope

The NST is currently the largest, best-equipped solar telescope in the history of humankind, and it is producing the sharpest images of the sun ever taken. BBSO joins Mount Wilson and Palomar in continuing the long, proud history of world-class astronomy in southern California. And it’s 65 miles from my house. Many thanks to Claude and Teresa for being such gracious hosts and letting us see their beautiful machines in action.


Observing Report: Partial solar eclipse on Oct. 23, 2014

October 27, 2014


Last Thursday afternoon I went to London’s school to show the eclipse to the students. I was rolling with the Astroscan-plus-Sun-Funnel combo, veteran of the 2012 annular eclipse and transit of Venus, and the GalileoScope that David DeLano built for me, now sporting a Baader solar film filter from


The eclipse started here at 2:11 PM, Pacific Daylight Time.

2014-10-23 eclipse in filtered scope

I’m still struggling to get good digiscoping photos with the iPhone. This one, shot through the filtered GalileoScope, is the least wretched of the lot. The immense sunspot group is AR 2192, the largest seen in 24 years. At nearly the size of Jupiter, It was easily naked-eye visible with eclipse glasses. There’s a nice video of it from before the eclipse at APOD.


Oh, I also passed out a lot of eclipse glasses. The best deal I have found on them is this pack of 30 for $33 from Amazon. Of that 30-pack, two got mailed off to relatives (along with our entire previous stash of eight), London and I each brought home a pair (London promptly disassembled his to see how they were put together–that’s my boy!), and the other 26 went home with other excited kids.

Incidentally, my favorite view of the eclipse was through the glasses, with no magnification. There is something awesome and terrible about watching another world come between you and sun, even partly.


I wanted to do an activity with the kids so I brought a pack of index cards and had them make pinhole projectors. That succeeded beyond my wildest dreams. The kids were completely occupied for a solid 20 minutes, and we could do the projections indoors and save our UV exposure for the scopes (which I brought inside, of course–you don’t leave a solar scope set up and unattended).


London’s school is Oakmont Elementary and ‘BLAST’ stands for Best Learning After School Time.


We also looked at pinhole projections of the eclipse cast by trees.


Just a bit after max eclipse, which was at 3:30.


The last of the wine, at 4:40. Unless I get really rich in the next couple of years, rich enough to go on eclipse cruises, my next solar eclipse will be in August of 2017. A total solar eclipse will cut a path from the Pacific Northwest to the Southeast US. My tentative plan right now is to fly to Oklahoma, see the relatives, and then drive up to northern Kansas for the event. Kansas in August should be hot and sunny, and on the Great Plains you can usually see bad weather coming hundreds of miles off, which will let us adjust our targeting on the fly.

Eclipse story in Claremont Courier

A guy from the Claremont Courier came out to interview me and some teachers, parents, and kids. Thanks to the paper’s paywall, I haven’t seen any more of the story than this web preview, which at least features two of London’s best friends. If anyone out there has a hardcopy they’d be willing to scan or pass along, I’d be very grateful. Update Oct. 31: Whoops! The story wasn’t paywalled; it was unavailable because it wasn’t done. Here’s the full story, and here’s a post with a couple more eclipse shots.

All in all, I think about 90 people got to see the eclipse through my scopes. The kids were mesmerized–so were the adults, actually–and I was very, very happy. Can’t wait until the next one!


Observing Report: a semi-cloudy night at Joshua Tree

October 8, 2012

My scope at Joshua Tree Saturday night. Clockwise around the scope are the bright star Capella just in front, the constellation Perseus (12:00), the Pleiades (2:00), the Hyades (V-shaped arrow of stars directly oppose Capella), and Jupiter (4:00). Photo by Kevin Zhao.

Saturday evening I was at Joshua Tree. My summer anatomy students invited London and me along to the Indian Cove campground. I didn’t have room in the car for the big gun so I took my 5” Mak, which is what it’s for—times when I need a decent amount of aperture in a small package. That was no loss: the sky was striped with high, thin clouds all night and never really cleared out. We got decent views of a few things, but the 10” would have been wasted. We used the Mak to look at the Double Cluster and Jupiter. In moments of steady seeing there were quite a few cloud belts showing, and all four Galilean moons were lined up on one side of the planet, which was pretty cool. London brought along his AstroScan and we used it to look at extended objects like the Pleiades and the Andromeda galaxy.

iPhone panorama by Chad Claus. Click for the big version!

The clouds might have made for lousy telescopic views but they made for gorgeous naked-eye skywatching. At sunset the whole sky was striped with light from one horizon to the other.

Here’s another view, actually taken by me for a change. This is the unprocessed raw image, direct from my Coolpix 4500.

Moon halo photo by Kevin Zhao. Jupiter is inside the ring at 1:00, and the Pleiades are outside at about the same angle.

When the moon rose around 11:30, it was surrounded by a ring of faint light. I thought it was a moonbow, but that’s something different. The ring we saw around the moon is called a 22-degree halo and apparently has no other or more poetic name. That’s a shame. In the early morning, when the moon had gotten well above the horizon, it was surrounded by a complete circular halo with radiating clouds on either side. That was worth the clouds. I’ve been under wonderfully clear desert skies many times, but I’ve never seen a moon halo quite like that. For once, I think the clouds were worth it.

Update: There wasn’t just a moon halo, there was also a sun halo Saturday afternoon. Agnes Kwon captured it in pixels. Witness:

Many thanks to Agnes, Chad, and Kevin for letting me illustrate my post with their awesome photos!