A Tale of Two Filters

Like a lot of photographers in North America, I’ve been making preparations for the total eclipse coming up in slightly over a month now from the time of this writing. I’m planning to take an arsenal of solar gear and spend a couple of days doing solar imaging with a variety of gear and techniques, and fleshing out some software I’m working on for lucky imaging to boot. This means I’ve been doing a lot of solar viewing and imaging in my back yard, and cycling through gear to make sure I have all the right adapters, spacing, etc. There was a very active region on the Sun (AR 3590) on the clear afternoon of February 27th and I had a chance to image it in white light and then just a few minutes later in Hydrogen Alpha. The difference is quite stark and I thought very illustrative.

Two images of AR 3590
Active Region 3590 in Ha and white light.

The telescope was a Sky-Watcher Esprit 150 (still my favorite optic for just about anything), which I had on my Paramount MX+. The Paramount’s have the smoothest motion of any mount I’ve ever used for any kind of high resolution Solar or Lunar work, but back to topic. The top panel at right was taken with a 4x Powermate to get the image scale and focal ratio higher for the Daystar Quantum hydrogen alpha filter. I used a Player One ERF (Energy Rejection Filter) in front of the PowerMate. I don’t use a big ERF in front of the 6″ objective, but 6″ is about as large a refractor as anyone should dare use for solar without a larger ERF in front of the aperture. The camera on the back was Player One Apollo Max, and I shot about 5,000 frames, of which I used 20% for the final stack. The original image was monochrome, but I colorized it as most people (my wife especially) prefer to see a nice color image. The mad truth is the single wavelength of Ha is an electric pink, but the convention is to make these images yellow to conform with the public’s perception of what color the Sun should be.

Ha images are stunning. They show in stark relief how the gasses are flowing along magnetic field lines on the Sun’s surface. The darker areas are still quite hot and blindingly bright, but they are just ever so cooler than the surrounding areas, and the white areas are as you’d expect super hot and bright.

The bottom image was taken with a Starfield Herschel Wedge. A Herschel Wedge is a lot like a normal star diagonal you see on many telescopes, but it let’s the majority of the Sun’s light pass through it out the back. A tiny portion is reflected up towards an eyepiece or camera for viewing. It simply dims the Sun, so you see the Sun as it actually appears, just a lot less bright and hot. Not using a filter like this would blind you instantly, or melt your poor camera before you could hope to get a shot off.

I still used a 4x PowerMate and the monochrome Player One camera, but replaced the Player One ERF (which is tuned for Ha imaging) with a standard UV/IR filter. The Herschel Wedge does not need this normally for visual work, but when doing photography, it eliminates some of the stray light that would be out of focus. The result is a “White Light” image of the same region. You can still see some of the same overall structure of the region, but you are not seeing the effects of the magnetic field lines that are more pronounced in the hydrogen bandpass.

The true color of the bottom image is actually… white. The Sun appears white in space, and even on Earth when directly overhead. It’s hard to tell of course because well, looking at it long enough to study it’s color would of course blind you! Still, I chose to colorize it for aesthetic purposes to match the image above. Scientifically, the images represent the structures accurately (even the limb darkening at upper left is real), but the colors are for taste alone.

I hope you can forgive my creative license (or shameless conformity), and enjoy seeing the difference between these two common ways to filter and view or photograph the Sun.

Purple Haze

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.

Solar Eclipse in Calcium Light
Maximum Eclipse from Central Florida in Calcium-H light

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

Telescope pointed at the Sun
Driveway Solar Astronomy. Be sure and share with your neighbors

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!