Archive for the ‘Edmund Astroscan’ Category


Observing report: the transit of Venus in Claremont

July 4, 2012

Here, just one day shy of being one month overdue, is my post on the transit of Venus on June 5. As promised, I took scopes downtown and did some sidewalk astronomy, and eventually some rooftop astronomy. As with the solar eclipse on May 20, the primary instrument was my son’s Astroscan with a homemade sun funnel, and once again it performed beautifully.

My partner in this enterprise was fellow Claremont paleontologist Matt Benoit. He was there for the whole thing, and helped keep scopes on target and help people understand what they were seeing. We hit a grocery store beforehand for soda and snacks and basically made an extended party out of the event.

I wanted to see the transit, period, but I was especially keen to watch the entry of Venus onto the solar disk to see if I could spot the “black drop effect” that bedeviled transit-timers in previous centuries. Sure enough, as Venus started to pull away from the limb of the sun there was a persistent dark blob or zone that seemed to connect the planet to the black space beyond, like surface tension keeping a drop of water from falling off the faucet. The black drop effect was once thought to be an effect of the atmosphere of Venus, but it’s not, because airless Mercury shows the same effect during its transits (the next of which is coming up in 2016, by the way). It’s nothing to do with our visual perception, either, since it’s easily recorded photographically, as you can see above. It’s now understood to be an effect of diffraction when a vanishingly thin line of light separates two darker spaces or silhouettes. You can see it by holding your finger and thumb up to the light and bringing them together–just before they touch, the black drop effect will seem to bridge them.

Along with the Astroscan and sun funnel, we had along the Celestron Travel Scope 70 with the aperture mask and solar filter described in this post, for direct viewing. Here’s my friend Marcy, who was there with friends for about half of the transit, getting her first look.

Although we both put in time on both scopes, for the most part I drove the Astroscan while Matt minded the Travel Scope. He also helped people get some photos through the eyepiece, as he did here with Marcy’s DSLR.

The view through the filtered scope was not as detailed as in the sun funnel, but the warm yellow color was more aesthetically pleasing, and many of our visitors appreciated both views.

Like the eclipse, the whole effect of the transit was a little unreal. In addition to the scopes, we also had eclipse glasses and a piece of welder’s glass. Every few minutes we would look up with our naked eyes and see a little black dot on the sun, and know that it was a whole world. And not just a world, but a twin of Earth. Someone on Mars watching a transit of Earth would see something very similar–our whole planet, all our evolutionary and human history, everything we’ve done or built (except for the handful of tiny things we’ve sent away)–all shrunk to a point, no larger, to the naked eye, than the period at the end of this sentence.

We had a steady stream of visitors downtown until a little after 6:00, when the theater blocked the view of the sun from the public square. So we decamped to the top of the parking garage across the street. Some people followed us over from downtown, and some found us up there on their own. One guy said that he found us because he had Googled for Venus transit events in Claremont and found my morning-of invitation post, which is nice, because that’s exactly why I put it up. In all, about 85 people saw at least some part of the transit through one of our scopes.

My son, London, watching the very tail end of the transit with the welder’s glass.

Venus was still crossing the face of the sun when they set together. As with the eclipse, I managed to get a shot right when the world crossing the sun touched Earth’s horizon. A moment later, it was gone, and the last transit of Venus until 2117 was over. I’m glad I got to see it, and to share it. I hope you had the opportunity to do the same.


More low-cost solar observing

June 4, 2012

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…


Banner photos, part 1: the little boy with the telescope

March 10, 2012

London with Astroscan

Once when I was a kid Mom and Dad and my brothers and I sat out in lawn chairs until midnight or maybe later to watch a total eclipse of the moon. We only did it once, but it’s always stuck in my mind as Reason #4,769,341 why My Parents Are Awesome (that’s actually a pretty high rank, considering how many museums, zoos, turtles, cats, chickens, model rockets, hikes, skinned knees, dirt clod fights, movies, puzzles, trips to the bookstore, vacations, homeworks, county fairs, kolaches, and giant pans of lasagna went into our upbringing).

The night of February 20, 2008, there was a total eclipse of the moon. It started at 5:45 PM and ended a little after 9:00 PM. We were living in Merced at the time, and I’d only been into amateur astronomy for a few months.

2008-02-20 Eclipse 03

London and I watched the eclipse from the back porch, and Vicki came out a few times to look, too. The “baby red telescope” (a used Edmund Scientifics Astroscan) was my Valentine’s present from Vicki that year. London loves it and at the time he was very possessive about it, but we talked about it and he decided to share. He was just three when these pictures were taken; he’s seven and a half now.

I passed this Astroscan on to a friend, but I got another just for London. They are nearly indestructible scopes. The newest versions from Edmund Scientifics have red-dot sights, but the old ones have a metal peepsight, so there is typically zero user maintenance. That doesn’t sit well with everyone–for one thing, there’s no provision for collimating the mirrors so if they slip out of alignment, you’re basically stuck. Unless you want to disassemble the telescope, that is. For the brave, Gary Seronik posted instructions on how to disassemble, collimate, and reassemble the Astroscan.

2008-02-20 Eclipse end

The tank-like build of the scope is a big selling point. We often take London’s Astroscan when we go camping, because we can always find enough room for it in the car and we don’t worry about it getting banged up. I have read about people who leave their Astroscans in the trunks of their cars on a semipermanent basis so they’re never without a scope, and about Astroscans that have tumbled off tables and tripods and been none the worse for the wear.

One of my favorite observing sessions with the Astroscan was in Yosemite Valley with some ecology students on a field trip. A group of us drove out into the middle of the valley to get out from under the trees, set the Astroscan on the hood of the truck, and spent a couple of hours just cruising the skies. One of the students had never seen Saturn before with his own eyes. It was tiny in the Astroscan–the short focal length limits the magnification–but very sharp, and he stayed hunched over the telescope after everyone else had gone off for some naked-eye stargazing. I will never forget that night; I was so glad I had a scope along and could show people a little of what’s up there (this was about a year before I started doing sidewalk astronomy).

If you want to read up on the Astroscan, the review at dansdata is funny, Tony Darnell has an inspiring writeup here, and Ed Ting’s review is both thorough and charming.

2008-02-20 Eclipse with London 02

If you actually want to buy an Astroscan, you have a choice of paths. The new Astroscan version with the red-dot sight is called the Astroscan Plus, and Edmund Scientifics will sell you one with two eyepieces and a shoulder strap for about $250. For an additional $180, the Astroscan Deluxe package includes a third eyepiece, a sun-projection screen, a tote bag for the scope, and a full-size tripod for getting it up off the ground.

However, you can probably have an Astroscan for a lot less dough if you want. The scope’s basic indestructibility and absence of things for users to fiddle with means that, apart from the switch from the metal peep sight to the red-dot finder, 20-year-old Astroscans are about the same as the ones rolling off the assembly lines today. And the scope has been in production since about the time I was born, so there are a lot of used ones out there. I’ve bought two used Astroscans, the one I passed on and the one London still has, and I found them the same way: put ‘Astroscan Craiglist’ into Google and see what comes up. You might have to do this on a regular basis over the course of a few weeks to find one that is (a) in your area and (b) in your price range, but they do turn up with surprising regularity. You can also find them on eBay sometimes, but usually being sold more dearly. Expect to pay $100-150 for a used Astroscan, although I have seen the occasional one pop up at $75 or even $50 if someone finds one in the back of the closet and just wants it out of the house.

In terms of price and aperture, the Orion StarBlast 4.5 ($180-200)and SkyScanner 100 ($110) are clearly intended to compete with the AstroScan, and if you’re just looking for a reasonably sturdy, portable, 4-inch widefield reflector, either of those would do nicely. But the Astroscan is more maintenance-free (in that it has no provision for user maintenance–whether this is a bug or a feature is up to you); solidly built to the point of near invulnerability; and has a distinctive charm all its own. I suspect that at this point most people don’t end up owning an Astroscan because they wanted a small telescope, but because they wanted an Astroscan specifically. If that’s you, go nuts. It’s an awesome little scope.