A couple of weeks ago, Paula and I bought a small solar telescope. It just kinda happened.
Paula became very interested in getting a solar telescope several years ago when she had a chance to look through one and got to see solar prominences live and up close for the first time. I was less enthusiastic; many years ago I did solar astronomy professionally, so my standards for what constitutes a decent image are, well, a tad unrealistic. I doubted I'd be satisfied with low-cost gear. $10,000 or $20,000 will buy you research-grade equipment, but that's not exactly in my price range!
The Orion Telescope store in Cupertino announced an open house where they'd be showing off the Coronado line of solar telescopes. The technical specifications for low-cost scopes were not encouraging—small aperture and wide bandwidth are not conducive to serious viewing. Still, I was curious to see what the view was like through these low-end instruments. Then I could better figure out what level of unaffordable scope might make me happy.
To my surprise, I found I enjoyed the view through even the smallest and cheapest Coronado PST ("Personal Solar Telescope") (figure 1). On top of that, the price had dropped from $599 to $499. Paula and I looked at each other, shrugged, and decided we could always call it an early Xmas present (a blatantly transparent rationalization).
The PST has a 40mm aperture, a 400mm focal length, a 1 Ångstrom filter bandpass, and weighs 3 pounds. It's got a standard tripod socket on the base, so I just mounted it on my Bogen tilt-pan head trpod. For a scope this small, you don't need anything that fancy; pretty much any 'pod you've got lying around will do.
The PST comes with a 20mm eyepiece. That's entirely insufficient for solar viewing, except for the very largest (and rare) prominences and sunspot groups. A 10mm eyepiece is a much better choice as a standard. Normally I use a 7mm; if the seeing isn't good enough to support the 7mm eyepiece, it's pretty poor seeing. Which means that unless something really interesting is happening, I don't care. With good seeing I go down to 5mm, easy. With great seeing, 3mm. You don't need to buy expensive eyepieces: four-element Plössls are fine. With careful shopping, you should be able to pick up three for around $100.
There are some tricks to using a scope like this. The first is getting it pointed towards the sun. Swing the scope around until the shadow of the front hood or the adjustment ring is centered on the body of the scope (figure 2). Then tilt the scope up and down to align the sun's image in the bore sight (figure 3). The bore sight may not be perfectly aligned, and the proper position for the Sun image may not be dead center. When you find the proper position for the image, mark that spot on the ground glass of the sight with a sharp marking pen. Makes keeping the scope pointed a lot easier!
If you're using a tilt-pan head, tilt the scope so that the eyepiece is horizontal instead of vertical. The highly collimated, monochromatic light projected by the eypiece makes every damned floater in your eye clear as crystal. Keeping your head vertical keeps the floaters drifting a bit more and makes it easier to see the solar image. You'll still have to take breaks and move your head and eyes around to stir things up inside your eyeball.
What will you see? A deep crimson red sun, dappled with just-barely-visible granularity. The image will be dim. The telescope throws away more than 99.99% of the light! On almost any day, even in this period of low solar activity, you'll see small prominences around the rim. They change in a matter of hours or less. Solar astronomy like watching the weather. From minute to minute, the view will slowly change. We leave the scope set up in the living room and take it outdoors a couple of times a day, just to see if anything interesting is happening. Usually there is.
If you're lucky, there will be a sunspot group, which means there will be bright flares and dark filaments against the surface of the sun.
For reasons of space, I'm not going to explain how the H-Alpha hydrogen emission line filter in the PST works. See this URL instead. Normally, you'll want the tunable Fabry-Pérot etalon filter centered on the H-alpha line at 6562.8 Ångstroms. That produces the maximum contrast in solar surface detail. The PST has an adjustment ring (figure 4) for tuning the wavelength the etalon transmits. Adjusting this ring back and forth will bring out maximum detail in sunspot groups and maximum contrast in prominences along the edge of the sun.
This leads to a neat trick. With a scope like this, you are looking at such a narrow slice of the spectrum that modest velocities (by solar standards) will Doppler-shift the H-alpha line out of the bandpass of the filter! If you're looking at a sunspot group on the surface and you detune the filter to the long-wavelength side of H-alpha, you may start to see darker filaments and streamers appear against the surface of the sun. The reason they look dark is because their emissions are being Doppler shifted towards shorter wavelengths. That means this is stuff that's coming towards you! Similarly, you may see different parts of large prominences on the rim get brighter or darker relative to each other as you tune the etalon. That tells you the different parts of the prominences have different velocities towards or away from you. With a bit of practice and deduction, you can get some sense of what shape these "clouds" really are and how they're moving.
This, by the way, is one of the important tools that the big kids use for solar astronomy, which is why professionals want filters with bandpasses as narrow as 0.1 Å. Strange as it may seem, solar astronomers are always complaining about not having enough light.
So, am I getting $500 worth of fun out of this scope? Oh, you betcha!
More next time....
ADDENDUM: Dear folks, You want photographs? Okay, how about this one?
(The link is here.) I wasn't aware that this had even been an "APOD" photo* until tonight. I was Googling for some information on solar phenomena for my replies and stumbled across this. My first (and so far, only) APOD! This is a rather so-so scan of a rather brilliant print (if I do say so myself) of a rather spectacular prominence. For its time, circa 1971, it was considered a noteworthy achievement, being able to produce that much local contrast and detail enhancement without blowing out the highlights or the shadows (easy in Photoshop; not so easy in the darkroom). It ended up being on the cover of Science magazine in the early '70s. Not only my first APOD photo but my first magazine cover! (Well, nobody said how recent the photograph had to be...) If I can find my print of this photo, I'll try to make a better scan and post it. —Ctein
*Astronomy Picture of the Day
Featured Comment by David Anderson: "Perhaps just an idea to issue a warning that no-one should attempt to view the Sun through any optical instrument unless they are using suitable solar filters. Please don't try this with an ordinary telescope or binoculars, your eyesight will be permanently damaged."
Featured Comment by Animesh Ray: "Dear Ctein, Thanks for the article, but more for the photo. I actually saw the cover in early '70s and, amazingly, remember it! I was an undergraduate student then, back in Calcutta, and I chanced upon this image of solar eruption on the cover. It was just an amazing sensation to ponder the real magnitude of these eruptions. I still remember having 'pored over' this photo for hours and dreaming about it, in the USIS library on a dingy lane near College Street in Calcutta.
"Just to revisit the old times, I looked for and found the issue of Science with Ctein's photo on the cover. It is amusing to re-read Linus Pauling's acerbic letter to the editor in that same issue!"