Last column, I told you how much fun Paula and I were having with our new Coronado PST solar telescope and some of the tricks I've come up with to squeeze even more enjoyment out of it. Well, within a few days, I figured out that it was really easy to make solar photographs through the PST! I really do mean easy, not just "easy for Ctein."
You need a digital camera that has raw mode and manual focus and exposure settings; your camera is not likely to be able to focus on the monochromatic red light and relatively featureless disc of the sun, nor is it likely to accurately meter. Use an eyepiece with a rubberized eyecup, common on shorter focal length eyepieces, so that you can press the lens up against the eyecup and keep out stray light (Fig 1).
Set the camera on manual and focus the lens at infinity. ISO 800 and 1/125th sec seem to work well with my 5mm (shown in figure 1) and 3.2 mm eyepieces. Your mileage may differ. Leave the aperture wide open. If your camera has a burst mode, turn it on. Focus the telescope so the image looks sharp to you. Center the features you want to photograph in the eyepiece. Press the camera lens against the eyepiece and hold down the shutter release to fire off a burst of photographs; the Olympus Pen will do 10–12 raw photos before it starts to slow down.
Make a whole lot of photographs! You'll see why.
Okay, that was the easy part. Now it's a little more work. Don't try to evaluate the photographs in the screen on the back of the camera; the deep red light falls well outside the gamut of the display, and they're going to look featureless, a lot like figure 2, top. This is also how they'll look with the default settings in ACR (figure 3, left). Settings more like on figure 3, right, will give you an image like the one at the bottom of figure 2. Eliminating most of the color cast and increasing contrast lets you see a lot more of the detail in the prominences and what's on the surface of the sun.
Experiment! This column is not a tutorial and I'm not explaining exactly what the settings mean nor which ones will work best for you. Play around for yourself! I use different settings for bringing out detail in the prominences and in the surface of the sun. I usually kick the contrast way up to see solar detail, and that pushes prominences into black (figure 4). Not shown in this screenshot is that I also made the curves a lot steeper to increase contrast. Nice thing about raw; you can process photos any way you want.
Figure 4 shows one of the November 17 photographs I referred to in my comments to the previous column. After massaging the photo like this, I could see much, much more detail in the sun than I could through the telescope. This is a much better "view" that I could get with the naked eye. Notice all the "floaters! Almost all those dark spots and clouds are bits of dust and dirt and smudges in the eyepiece. You need to clean your eyepiece scrupulously or your photographs will be filled with these. Even Thierry's photographs aren't spotless. But, the narrow U-shaped cloud near the limb of the sun, lower right, is real. I could tell because from frame to frame it moved with the sun; floaters stay in the same place in the field.
So, why make a lot of photographs? Atmospheric turbulence! From frame to frame, the heat ripples in the air will make big changes in the sharpness of your photographs. 1/125th sec is enough to freeze those ripples, but it won't make them go away. Some frames will be extremely blurry; only a few will be maximally sharp. You will need to look through dozens of frames to find the best one.
Figure 5 shows one of the best frames on November 18. This was a very nice small prominence, but the seeing was really awful. I could barely make out the thin "rind" of darker spicules along the edge of the sun. Normally that rind would be very clear, even though this scope is too small to resolve individual spicules. I picked that frame out of about 200 photographs I made, and it's still not very sharp. Yes, searching is very tedious. Professional solar astronomers have the same problem—that's what students are for.
While you can expect photographs to show you a lot more detail than you can see in plages, granulation, and sunspot groups, you'll see less in prominence photographs. Our brains are really good at picking out fleeting fine detail in a wavering image; much better than the camera.
Can we do anything about that? Yes, but now it's no longer easy. You'll need a good image sharpening program, such as FocusFixer and a user-controllable noise reduction program such as Noise Ninja. (Again, I'm not going to tell you how to use these—not enough time, not enough space. I repeat, experiment!)
I took figure 5 and applied FocusFixer to it with a 16 pixel radius. This gave me the best sharpness with the fewest artifacts, but the photo was covered with "orange peel" texture; an artifact of the extreme enhancement. I used Noise Ninja with a manual selection of areas in the sky and the surface of the sun to eliminate most of the orange peel. Figure 6 shows me as much detail as I could ever hope to get under the really awful seeing conditions. Notice how ragged the limb of the sun is and the "rind" is almost invisible. But it's almost (not quite) as much detail as I could see with the naked eye.
I'm waiting for a day with good seeing and a great prominence. It'll happen; every day is different. That's also part and parcel of real solar astronomy.