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COUNTING SUNSPOTS WITH A $10 OPTICAL TUBE ASSEMBLY
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WITH A $10 OPTICAL TUBE ASSEMBLY
Copyright 2022, Boris Starosta
The seed for this project was sown by an article in the March 2021 Reflector (p.24 by John Briggs, "A Funny Thing Happened in Solar Physics"), which led me to Solar Observing Project: Re-calibrating Historical Sunspot Observations in the Journal of the American Association of Variable Star Observers, here: <https://www.aavso.org/solar-observing-project>.
Because of my interest in "real science" accomplished by way of amateur astronomy, I was keenly attracted to this sunspot observing program. But with various other projects of mine in an unfinished state (and many continue on that way;-), it took me over a year to get around to building the needed "vintage" solar telescope. But finally I got going on it, and this is just the beginning of the story.
Executive Summary: With a minimal outlay of expenditure, you can do "real science" solar astronomy. All it takes is building a very simple telescope, and thereafter an occasional 15 minute observing session in sunny weather. The work is relaxing and makes for a nice break in the middle of a busy day (or the morning, or the afternoon, whenever the sun shows itself!).
Background: The work is coordinated by Leif Svalgaard of Stanford University. You can see a nice resume of his ongoing projects on this page <https://svalgaard.leif.org/research/>, where several of the links/URLs relate to the sunspot counting research. The aim of this particular project, is to improve the utility of sunspot counts that were done in the 17th and 18th centuries by various astronomers.
Typical Staudach drawing of sunspots. The red circles are modern - highlighting the difficulty of not knowing how Staudach defined sunspot groups.
The largest database was created by Staudach, who observed and counted sunspots for some 50 years in the second half of the 18th century. To make this old data useful, one has to calibrate it, so that it can be compared to present day sunspot counts. Here's a nice review article on research that relies on such historical data. https://oxfordre.com/physics/view/10.1093/acrefore/9780190871994.001.0001/acrefore-9780190871994-e-9.
The red lines are ancient counts, with most-recent calibrations to the modern data by Svalgaard.
Today's telescopes enable the detection of many more sunspots than was possible three hundred years ago. So if you want to compare that ancient data with modern results, you need modern data acquired with ancient telescopes and methods. This is the point of the Svalgaard sunspot counting project, which he has "crowdsourced" out to the amateur community. And by "crowd" we're talking about less than a dozen observers worldwide, most of whom cannot observe every sunny day. (As it happens, presently there is only one member on the team, who is observing on a "daily" basis, and they are in the not so often sunny Netherlands. With my own efforts, I hope to make that two "daily" observers.) Thus, I am sure Svalgaard would be happy to find a few more participants - as amateurs into astro-imaging know, more data means less noise in the final results.
This chart shows recent modern telescope sunspot counts (smaller, closed symbols) compared to Svalgaard's concurrent ancient telescope sunspot counts (open symbols).
BUILDING THE TELESCOPE
My total out of pocket expense in this project is about $10, spent on a 50mm diameter bi-convex lens of 100cm focal length that I ordered off eBay. This was the most expensive component in the project. To approximate a typical 17th century scope, I actually want only 3/4" aperture with the 100cm focal length, so the lens from some reading glasses of 1-diopter strength would have worked, but likely would have cost more! (...and wasted half an eyeglass, which goes against my philosophy).
My first (and only) purchase: an "Optically True" double convex lens. Cost: £7 postpaid from England.
As it happened, this lens was the perfect size for a press-fit into a 2" shipping tube, of which I have several. While the press fit was probably good enough, I wanted to secure the lens against possible bumps and impacts in use, so I put glue inside the upper edge of this tube, then pushed the lens back in, up against the glue. I'm building this scope solid, for the ages!
To ensure good collimation, I rested the tube on my workbench, lens side down, while the glue cured. If the lens was going to settle in position, this way it would settle flush with the end of the tube, presumably normal the the axis of the tube.
Anyway, as part of this effort to simulate the performance of a 17th Century telescope, I figured if the collimation was off a bit, it could only enhance the intended functionality.
After a day, the lens looked lovely and secured. You can see the glue overlapped the edge a little bit behind the lens, and also seeped through to the front edge in a few places. Perfect.
The shipping tube was known to be too short (only about 70cm). A second tube would be added, and to keep the two tubes stable after joining, I figured a piece of wood would do the trick. I did not buy a piece of wood, I found a scrap piece about the right size lying about in the basement, and secured the first tube (with the lens) to the wood with tape. Admittedly, I also had a nice Orion EQ3 mount just lying about in the basement, having recently been retired from holding my maksutov.
To get the plank into the mount, I modified a smaller piece of wood (2x2?), making it into a "dovetail" to fit into my Orion-type mount clamp. This 2x2 piece wood is permanently screwed onto the "spine" 1x4 plank of wood. On the side where the clamping screw comes in, I put a metal plate to take/distribute the force of the clamping screw. Otherwise this would end up splitting the wood.
On the opposite (passive) side of the clamp, I put two screws to take the forces into the wood.
The screws absorb and distribute the clamping force. On this side, the clamp "bites" into the dovetail with just two wedge-shaped teeth. This wood is too soft, so needs the screws to take the force.
Next I needed to know how long to make the second tube, so that an eyepiece held at the end of the joined tubes would produce an in-focus solar image. This length was going to be a little farther than the focal length of the lens, but I needed to know how much. My intent was to avoid the ost of a focusser, and make the tube a length suitable for one particular old eyepiece I had found in my junk box. I envisioned some minimal focussing capability by sliding the eyepiece back and forth within a simple holder.
Prototype scope out on the deck with only the objective tube attached, to measure focus distance.
I figured that counts as first light through this telescope, though the scope was only about half finished. After taking my measurements, I cut the second shipping tube to just the right length, so that the eyepiece would be held at best focus, when this tube was simply joined to the tube with the objective lens.
Just like my luck with the objective lens, I discovered that a 2" to 1.25" eyepiece adapter similarly perfectly press-fit into the shipping tube (I have yet to glue it). Admittedly, now this might be the most expensive part of the scope (they cost about $20), but this component is not absolutely necessary. It was just very convenient, obviating the need to kludge together an eyepiece holder.
After joining the two tubes, I took everything outside again to check on focus. This is the full aperture prototype. The only thing still needed is the camera obscura box at the back, and some kind of cap that will limit the aperture to 3/4 inch.
This is the camera obscura box I made using an Amazon shipping box of approximately the right dimensions: big enough to hold a 1/3 letter size sheet, but not too big or heavy. Using wood screws I attached this box to the 1x4 wooden "spine" of the scope. Note the cutout on the end of box, to allow the tube and spine into the box a ways.
My shipping tubes had come with plugs, and I decided that cutting a 3/4 inch hole into one of these caps was the easiest way to stop down the objective aperture. After cutting the hole with a box cutter - and not too neatly, either - I used silver HVAC tape to make the cap opaque. I was afraid that a translucent bright white cap might reduce contrast too much in the final image. Maybe I worried too much? No way to tell, but I knew that even in the 17th century, anyone making a telescope was not going to have a bright white ambient light source surrounding the objective lens (as seen from the eyepiece)!
After making the cap opaque with the tape, I spray painted the inside of the cap with black paint, to reduce possible internal reflections. That probably was going too far!
Stop cap added in front of objective lens, the scope is basically finished! The aperture looks off-center? Nothing gets in my way to reproduce the subtle flaws of ancient technology.
Soon after everything was put together, I got my first sunspot counting image sketched. Now, notice how uneven and mottled the solar disk appears? That's because of dirt inside the old eyepiece. When we got the first-light solar images using the full 50mm aperture of the objective lens, those smudges and blobs were not visible. But with the narrower ¾-inch aperture, now the dirt inside the eyepiece is casting very noticeable shadows, which interfere with identifying sunspots.
In this photo of my first sketch, you can see I penciled in the location of a few sunspots, onto some recycled paper I brought out. This was a prototype - actual routine observations are going on clean paper!
The above compares a more recent of my sketches to that of another team member in the Netherlands, both made on May 15. He is using a larger aperture, and the scope is on a very stable permanent mount, in a dome (It's the middle scope in the photo - a galilean optical design, 30mm aperture, beautifully built of machined tube and other custom parts)... ideal for careful sketching/drawing of features. He observes many more spots than I do. But compared to what I could find of ancient sunspot drawings (easy: google "staudach sunspots"), mine look about right in terms of level of detail. Although, the dirt in my eyepiece is getting in the way (literally!), and I will soon clean it (I'm waiting on a tool for taking it apart).
In the meantime, I've found that using a stereoscopic technique and digital contrast enhancement, facilitates seeing the sunspots and gets me an amazing amount of detail - but I'm not using these techniques to do my actual counts.
After an observing session, on any given day, I take a picture of my sketch and email it to Svalgaard. That's all there is to it. For my own future reference purposes, I also take a stereo photo of the solar image. That way, if ever there's a need for more data, it can be extracted from the photography, as shown here.
I encourage anyone with a minimum of technical facility to build their own vintage solar telescope, and join with me on the sunspot counting journey. Next to the study of Earth-grazing asteroids, I can think of no more important branch of astronomy. Aside its obvious ties to climate change science, solar astronomy is extremely important to our understanding of, and ability to predict flares and coronal mass ejections, which have the potential to devastate modern society (google "Carrington Event").
A technical illustrator and 3d photographer by trade, Boris Starosta shares his passion for astronomy with family and friends in Charlottesville, Virginia.
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