The great annular eclipse of 2023 has come and gone. The internet and social media are flooded with amazing shots of the Sun. Most are red or orange, and there are a few white light images taken with white light filters or solar wedges. “The Ring of Fire” was not in the cards for me this year, and I had to be content to observe and image the event from my driveway in Central Florida. I shared the event with neighbors and gave away some solar glasses from DayStar filters. No eyepieces though, I had a camera on the back of the telescope (oh, did this require some “explaining” and warnings), and a black and white image of the Sun was displayed on my laptop, cleverly shielded from the hot bright sun by a cardboard moving box.
My processed images though are Purple. Whaaat?!
I used a Daystar Calcium Quark, specifically the Calcium-H line. Some of you know this, but many people do not know that much of what we know about the Universe and what things that are far away are made of, is done by studying the light we receive from them. Here’s a great article on the Hubble Space Telescope web site that explains some of how this works: What is Spectroscopy.
So, back to the Wright Earth Telescope(s). The filter I used on the eclipse is what we call a “narrowband” or “Line” filter. It only let’s through a very specific wavelength of light, and this filter is tuned for the H line of Calcium (396.9nm), which if you had looked at it through an eyepiece, would have been a deep purple! I once had a similar dedicated telescope that was for the Calcium-K band, which also appears Purple, but is so deep that many people cannot actually see the details on the Sun’s surface. I’ve heard various explanations about genetics, and “old people”. My own experience is that I could see it years ago, and now when I first look all I see is a smooth purple disk. Then as I fish around, my eye will suddenly focus and I can see it for a few seconds, and then it’s lost. I don’t think it’s so much “detecting” the wavelength (which is very far towards the violet), as your eye loses the ability to focus on it. Maybe that’s the same thing. We’ll see if in another few years if I can ever catch the surface details any longer.
But this is why the Calcium-H line is very popular for visual solar observing. It reveals a very similarly detailed image and can be more easily seen visually in an eyepiece. Since it’s monochromatic light (just a single wavelength), typically what imagers do is use a more sensitive monochrome camera, and then colorize the image after they are done processing the image for sharpness and contrast.
The Gear
I used a Sky-Watcher AZ-EQ6 mount in Alt-AZ mode. That was more mount than needed for my tiny Takahashi FS60-CB telescope. It has a fluorite lens, and many people worry about damaging fluorite with solar observing. It is true, fluorite lenses can be damaged by sudden temperature shifts, but here’s the thing about glass lenses… light passes through them. They don’t really absorb much heat. If your optic however gets too far off the Sun, and the concentrated light starts hitting your baffles or the edge of the tube, you can superheat the inside of the OTA, and bad things can happen. Bad things. I do understand some older oil space lenses can be problematic for solar though, so check with your manufacturer before you start tinkering with hot sunlight.
Inside the tube, I had an IR/UV filter, which reflects a great deal of the Sun’s invisible, but heat bearing wavelengths right back out the front of the telescope. Behind that was an Astrophysics 2X barlow. I love this barlow because it also acts as a flattener. If you are doing full disk solar work, or even high resolution solar or lunar work, it annoys me terribly when I use a large sensor and only the middle of the image is really in focus. The solar filter was the Daystar Calcium-H Quark and the camera was Player One Ares-M (IMX 533 monochrome) that I ran cooled to zero degrees C. This was a lot of fun to explain to the neighbors. Concentrated sunlight is going through a filter heated to a specific temperature, that then reaches a camera that is cooled and kept at 0 degrees Celsius. I’m just your average mad scientist working in his driveway…
Clear skies, day or night friends!