See the transit of Venus in Claremont

If no clouds come to spoil the fun, I will be in downtown Claremont this afternoon (Tuesday, June 5) with a scope set up for free public viewing of the transit of Venus. The transit starts at 3:06 PM, PDT, and will still be in progress when the sun sets at 7:59. I plan to be there for all of it. If all goes well, from about 2:50 onward I will be in the public square in front of the theater, on the northeast corner of First Street and Indian Hill Boulevard. Whenever the sun gets low enough to go behind the theater, I’ll head up to the top of the parking garage across the street, to watch the sun set with Venus still crossing the solar disk. You, whoever you are, are welcome to join me.

If by some freak chance it is cloudy this afternoon, I’m going to throw my gear in the car and run up to Big Bear, which gets more sunny days than almost anywhere else in SoCal (that’s why the solar observatory is there). In which case, you’re still welcome to join me, if you can find me. Try the Discovery Center on the north shore, if it’s sunny…or the nearest pub if it’s not.

Fingers firmly crossed for clear, sunny skies!

More low-cost solar observing

In preparation for the transit of Venus tomorrow, I did a little hacking and tinkering late this afternoon. Although the sun funnel worked well enough for watching the eclipse, as we’ll see below it is not perfect for photographing the sun in any detail. My full-aperture solar filter still hasn’t arrived, but I got to thinking about how to make a safe direct viewing setup.

I recently acquired a Celestron Travel Scope 70, a little 70mm (2.75 inch) f/5.7 achromatic refractor. Like a lot of small refractors, the dust cap for the objective lens has a smaller removable cap in the middle, in case you want to stop down the scope for more pleasant viewing of bright targets like the full moon. The diameter of the small hole in the middle of the big cap is 40mm, so with big cap on but the small cap off, the scope functions as a 40mm f/10.

I don’t have any loose solar film to make a 70mm solar filter or even a 40mm solar filter. But I do have a stack of eclipse glasses, each of which has two 1×1.5 inch eye holes covered with solar film. So I cut one of the eclipse glasses in half, made a round 25mm aperture in a square piece of cardboard, and mounted the eclipse glasses ‘lens’ (solar film still surrounded by two sheets of thin cardboard) and the 25mm aperture stop on the back side of the big dust cap. I didn’t think to take any pictures of the inside of the dust cap to show how it all goes together, but hopefully the general idea is clear enough. With the big dust cap on and the small dust cap off, the scope admits a 25mm beam of fully solar-filtered light to the objective, turning the scope into a 25mm f/16 solar refractor. And because the solar filter is on the inside of the big dust cap and protected by the small dust cap (in front) and the second piece of cardboard with the 25mm aperture stop (behind), I can leave it in all the time. Take the big dust cap off, the scope functions normally. Take only the small one off, I’ve got a 1-inch solar scope.

Two other design decisions to note. First, the finder–and I use the term advisedly–that came with this scope is without doubt the worst finder I have ever seen on a commercial scope from a brand name manufacturer. It looks like a 5×20 straight-through magnifying finder. However, right behind the (single, plastic) objective lens is an aperture stop with only a 1-cm hole in the middle. So in fact it’s a 5×10 finder with a plastic singlet objective. The immense irony is that the scope doesn’t need a finder at all; throw in a 32mm Plossl and you get 12.5x and 4-degree true field of view, so the scope effectively functions as its own superfinder. So I unscrewed both ends of the finder and dumped out all the plastic optics, turning it into a hollow sight tube. Why is this important right now? Because it’s really dumb to leave a magnifying finder on a telescope being used for solar observing; it’s too easy to forget what you’re doing and accidentally looking through the unfiltered finder and cause serious eye damage or blindness. There’s a good reason that every commercial telescope comes with a “don’t point the scope at the sun, dummy” tag or sticker or both. This is not something to mess around with. If you’re going to observe the sun with a telescope, cultivate the same habits of awareness and deliberate action that you would use around loaded firearms and power saws.

Oh, the included 45-degree prism diagonal is also rubbish and the light tripod looks pretty dodgy. Today I used my standard small-scope setup–an AstroTech 90-degree dielectric star diagonal and a Universal Astronomics DwarfStar alt-az head on a Bogen/Manfrotto tripod–and I’ll doubtless do the same in the future.

The other design thing was the sun shield. At first I tried going without but look into a dark eyepiece to catch a filtered (= comparatively dim) view of the filtered sun while unfiltered sunlight was hitting the top of my head and my upper eyelid had me squinting and developing a minor headache almost immediately. The plastic dewshield on this scope pulls right off, so I got a handy piece of cardboard (part of the packaging of a picture frame), cut a hole just big enough to admit the front end of the scope without the dewshield, slid the cardboard sunshield on and used the plastic dew shield (and dust cap with solar filter) to hold it in place. I also cut a second, smaller hole to let light in to my sight tube sun finder.

If you do something similar, make sure that the sun shield can’t get blown off and take the solar filter with it. In my case, the dewshield slides on a long way and grips both the sun shield and telescope tube firmly; a strong enough breeze might upend the whole setup, but it couldn’t blow off just the shield and filter. Again, eye safety is paramount; don’t take any chances.

Okay, so how did it work in practice? Pretty darned well. I had already aligned the sight tube with the telescope, so all I had to do was rotate the sun shield a bit to make sure the second, smaller hole lined up with the sight tube. Then I could point the scope roughly at the sun and pan around until a perfectly round beam of sunlight (projected on my hand) emerged from the sight tube. That always put the sun in the field of view of a 25mm Plossl (16x, 3 degree true field of view). The view of the sun at the eyepiece was reasonably bright–for an astronomical object, not compared to the unfiltered sunlight streaming down all around–and razor-sharp. The sunspots with their umbrae (dark centers) and penumbrae (lighter borders) were striking, like they’d been etched on stained glass.

Happily, the filtered scope yielded nice, even light all over the surface of the sun, no matter where it roamed in the field of the view. My one beef with the sun funnel is that it can be hard to get really good photos because of the inherent granularity of the screen material. Inevitably some part of the projected sun is brighter than another, and if you manage to get the light perfectly centered, it can easily wipe out the sunspots. The best way I’ve found to avoid this flashlight-beam effect is to photograph the sun from a bit to the side, out of the direct path of the projected light (that’s how I got this very sharp photo), but then the sun is out of round–not ideal if you’re hoping to combine images into a composite or movie, or even get a nice, square-on shot of a circular sun.

For example, in the photo above the sunspots on the left are sharp enough–the big one even shows the umbra and penumbra clearly–but the dimmer two on the right are lost in the flashlight glow of the sun lighting up the screen material from behind. And in this view the sun is already way out of round.

Also note that this image is flipped horizontally compared with the image from the refractor. In fact, this image is correctly oriented. Normally Newtonian reflectors show things rotated by 180 degrees, but projecting the image on the screen undoes that and gets everything back to normal. The solar filter on the refractor just cuts down the intensity of the light, it does nothing to reorient the image, so the image at the eyepiece is right side up but, because of the 90-degree mirror, flipped left-to-right.

I didn’t go to all of this trouble just for the transit of Venus. I mean, I happily would have, had the transit been the only game in town. But it’s not–the Astronomical League has a Sunspotters observing program, and now that I have the gear for solar observing, I might as well start logging. I’ll keep you posted on that.

Now, I should point out that the flashlight-beam effect washing out the sunspots in the sun funnel is mostly a photographic concern. For visual appreciation, even solo, I think the sun funnel still wins. A 4-inch image scale and the ability to put your head and eyes wherever you want–and even wear polarized sunglasses to observe–can’t be beat. But for photography, I prefer the filtered direct view–even in a one-inch scope.

Fortunately I’ll be rolling with both tomorrow. Now if the weather just cooperates…

Observing report: the 2012 annular eclipse

Today rocked. It would have rocked a lot less if things had gone the way I wanted them to. I put off ordering a solar filter for my telescope until last week, and of course everyone was sold out and even the manufacturers were backordered. One is on its way to me, hopefully, but it didn’t arrive in time for our eclipse trip, so I fell back on the sun funnel I built a couple of months ago, and my son’s Astroscan. This turned out to be the perfect combo. In the photo above I was testing the sun funnel in the hotel room, after our long drive from SoCal to Page, Arizona (spread out over 2.5 days, so very civilized and enjoyable, but still a lot of miles).

For the eclipse we set up on the lawn of the Courtyard Inn here in Page. Here’s first contact, when the moon first starts crossing the solar disk. Click for the big version and look at the sunspots–this is the sharpest sunspot photo I got all day.

In addition to the sun funnel I brought a piece of #14 welder’s glass for naked-eye viewing. The eclipse glasses London is wearing were supplied by the hotel when we checked in–I thought that was an awesome thing for them to do, and I told them so.

The middle group of sunspots is getting devoured by the moon.

Almost to second contact, when the trailing limb of the moon crosses the edge of the sun. I like the meta-ness of this photo of a photo-in-progress via a projected image of a projected image.

Just after second contact–we have annularity!

Totality. This was incredible. I would write more about it, but words fail me.

And here’s why it’s a good thing that my solar filter didn’t arrive on time. If it had, I would have brought my 5″ Mak and left the sun funnel at home. And when the tour bus pulled up 50 feet away and disgorged all these people 5 minutes before totality, they would have missed the eclipse. Thanks to the sun funnel, we had a nearly constant stream of visitors coming by during the first half of the eclipse, and we made some new friends. There’s no way all those folks would have had time to see the eclipse at the eyepiece if I’d been rolling with a solar filter. So from here on out, I’m a sun funnel man. Oh, I will probably also set up a filtered telescope nearby for observing at the eyepiece, but the sun funnel is a key piece of gear, and I don’t intend to voluntarily be without it for future solar events (like the transit of Venus coming up in two weeks).

Third contact–the leading edge of the moon hits the far edge of the sun. See the little points of light between the ‘horns’ of the moon sun? Those are Bailly’s beads, the last rays of sunlight shining through valleys on the limb of the moon. They’re visible at second contact, too, I just failed to capture them in pixels.

I was afraid that the second half of the eclipse would be boring–like the first half run in reverse. It turned out to be a blast. Precisely because we’d all seen it all before (or thought we had–keep reading), we felt free to goof around a bit. Here I removed the sun funnel and put in a regular eyepiece to project the eclipse on my t-shirt. This is a hairy operation–you don’t want to be the projectee and the one pointing the scope at the sun, or you’ll be tempted to glance down into–what? Oh, that’s right, the blindingly intense beam of concentrated sunlight shining out of the telescope. Fortunately I had the presence of mind not to do that, but after this shot, we didn’t let anyone get on the eyepiece side of the scope without eclipse glasses on. This led to some modest hilarity of trying to guide the effectively blind subject to kneel just so beside the scope.

More second-half fun: the sun goes behind an antenna on the next ridge over, maybe a mile away. I suppose a purist might not want anything man-made screwing up the eclipse, but we all thought this was super-cool.

People farther west got to see the entire eclipse, but here the eclipse was still in progress when the sun started to set. Again, some folks might have been bummed but we thought it was crazy-cool to see the sun blocked by both the moon and the Earth. Check out the electrical towers on the distant horizon, much farther away than the antenna in the previous pic. All three sunspot groups are still visible, too.

Moonset at sunset. I don’t even know what you call this…third-and-a-half contact, maybe? Whatever the actual name, we all thought it was the highlight of the second half of the eclipse.

The last sip of sunlight. Good times.

Stay tuned, we’ll do it all over again in a fortnight, only with a much smaller (in apparent size, anyway) object blocking the sun. There’s still time to build a sun funnel and scare up a cheap scope if you’re so inclined. Clear skies!

Guest post: Sun Funnel built and tested

Here’s something new–my first guest post on this blog. It’s by frequent commenter David DeLano, whose DIY astro gear I have featured here once before. David puts a lot of thought into equipping and fine-tuning his scopes to get the best performance possible, and you can see that attention to detail in his sun funnel writeup. He sent this as an email message with permission to post. It’s pretty much as sent, with just one or two additions to clarify things that were already familiar to me from our email conversations. The sun funnel parts list and instructions are here (or in PDF form with background math here), and I previously mentioned it on the blog here. Enjoy!

First off, this is one of the coolest projects I have done lately.  It was easier than I thought, and the results were better than I would have expected.  Thanks for pointing it out!! [I didn't put him up to that, promise!--MW]

Here are some experience points worth noting.

It was cool to have an EQ mount, even as light as this one is.  I had to do some tweaking on it, and it took several brain sessions to get it aligned properly, but one I got it configured, it helped to be able to tweak one knob to follow the sun.  It moves fairly quickly.  You get maybe a minute to look at any specific thing before you need to adjust the view.  I also had to tighten the Alt down most of the way.  This setup is very heavy on the EP end.  I had to slide the scope a bit back to clear the Alt micro stem, which made it even worse.  I need to find a counter weight to put on the front of the tube.

When you mount the screen on the funnel, make sure the back, inside, is as clean as you can get it.  I had an annoying spot on mine, and I ended up taking it apart, cleaning it, and putting it back together.  One of the directions says to put the shiny side inside the funnel.  I agree.  This side is easier to clean, as it’s slicker.  Try to keep the clamp as close to the top as you can, as it will want to slip down as you tighten.  After a while, though, the grabbiness of the cloth will keep it in place.  Pull the cloth down from under the clamp as you go, and try to eliminate any creases.  After a while, the sliding down will just tighten.  I tightened mine until I could ping the top like a drum.

I don’t have a shade yet, but what I’d suggest more is a shade umbrella or tent.  A beach umbrella clamped to the mount, or even a chair would be nice.  It gets hot observing the sun.  In fact, this is probably conducive to sitting in a chair to observe anyway, since you don’t have to be hunched over the EP.  The only part of the scope that got noticeably hot was the front of the funnel, from the outside, not the inside.

The Solar Scope

Pardon the leaves…that’s another project for this week.  This is a Bushnell 50mm F12.5.  The ideal EP is around 17.5mm, but the closest I have is 15mm (OWL), and it works fine.  Note that this diag is either 90* or 45*.  I didn’t try 45*, but that might be nice for sitting in a chair.

The Solar Finder

This is a nifty and very inexpensive solar finder that David came up with and described in a couple of previous emails. He wrote:

Something finally worked and was darn easy.

I took a clear-ish film canister, with a grey lid, and punched a hole in the center.  This was the hardest part.  I used my centering punch to fight the little nib and get a hole a close to the center as possible.  (If you don’t have one of these, get one.  They are cheap, and you can use them for all sorts of things.  You put the point where you want to mark and press down. It’s spring loaded and will “pop” when enough pressure is applied.  You are under control up until the pop, so you can’t miss making a small indent exactly where you want it.  Pop it several times to make a deeper dent.  I use this for starting screws, getting a drill bit to start where I want, etc.)  I ended up with a bit if a smile hole, but in testing it worked.  I did go back up and clean up the hole with a 1/16″ bit.

I bought two nylon spacers, zip tied them together, punched holes in the bottom of the finder, and put the spacer set on with a zip tie.  I zip tied the assembly to the end of the scope.

  

Notice the dot of light on the back of the canister in the second picture.

The film canister finder works very well.  I has some movement, but it really easy to align.  It didn’t need to be shimmed, only twisted by a minute amount.  This is actually the second one I made, and I tucked the first one away and am not quite sure where.  The hole is just punched.  It looks like it’s overkill to drill the hole out.

The Solar Funnel Eyepiece

    

I keep a cap on the end of the EP when not in use.  I’m not sure where/how I’m going to store this, as it’s relatively large.

The View

Note that this looks like you are looking inside something.  But, it’s a flat surface.  You can make out some sunspot activity about halfway out from the center to the left.  I’ve looked through solar scopes before, and this setup shows most of the details that I’ve seen in those scopes, though the scopes can zoom.  The surface of the sun even roils in this view.  One note – don’t try to focus on the edge of the sun, as it’s moving.  Try to pick an interior detail if possible.  Now, some close-ups so you can see the real deal.

This is a big picture–click to thermonucleate!

The dark fleck about an inch up from the bottom is a speck on the back of the screen.  You can easily see three sunspots, and in fact, I could see six.  At one point I thought I saw a brief flare, but it didn’t last very long.  There is also a nice sunspot on the right edge, that you can see better in the next pictures.  I left the fleck out of the picture so that it isn’t confused as solar detail.

  

Here is a cut of the sunspots that are on the left side of the picture.

One question that might be asked…how do you know the other spots aren’t on the screen.  Well, for one, they move with the sun as it goes across the screen, and second, they can be focused.  Anything on the back side of the screen is always in the same spot, and always in focus.

Many thanks, David, for a very clear and compelling writeup, and for permission to share it here. Now I want to build one more than ever.