Safe Solar Observing
by Andrew Wood
Following a period of unusually low solar activity (the appearance on the Sun’s disc of sunspots), followers of Monty Leventhal’s monthly presentations and articles will be aware that the Sun is starting to become more active. Monty’s equipment is sophisticated, allowing views of the Sun’s granular surface as well as solar flares and sunspots. Without this specialised equipment, however, ordinary telescopes fitted with solar filters can be used to observe -and photograph- the appearance and progression of sunspots.
Full-aperture Solar Filters
Solar filters are not a highly expensive observing accessory. They can be purchased ready-made or, even cheaper, sheets of solar film from which you can fashion your own filters can be purchased. The -very- important point, though, is that they are used as full-aperture solar filters. That is, don’t allow unfiltered sunlight to enter an instrument. The filter must be deployed before any sunlight enters a telescope, binoculars; or your eyes.
The simplest solar viewing equipment is a pair of filters that cover your eyes like reading or sunglasses (Note: don’t look at the Sun through sunglasses). See Figure 1. Commercially made (often marketed as “eclipse shades”), they are quite safe, as long as you never use them in conjunction with binoculars or a telescope (or a camera). Sometimes sunspots can be large enough to see this way, and a pair of these is a quick way to see if any large sunspots are present.
Filtered binoculars (see Figures 2 and 3) are also good for a quick check to see if any sunspots are present.
The best views, of course, will be through a telescope with a full-aperture solar filter. The finder scope should also be filtered. See Figures 4 and 5.
Using these safe methods, although we can’t see any detail on the Sun’s surface or solar flares, we can follow the appearance and disappearance of sunspots throughout the Solar Cycle.
Don't trust old eyepiece filters
Many years ago, solar filters that screwed into eyepieces were sold. (See Figure 6). If you ever come across one, don’t use it. Firstly, it will be old, and the coatings likely deteriorated. But most importantly, allowing the Sun’s energy to enter a telescope and become concentrated on a filter may cause it to fail and allow unfiltered and magnified sunlight into your eye. Not worth the risk!
Another method for viewing the Sun is to project its light through a telescope (or possibly binoculars) onto a white surface. See Figure 7, when I did this a very long time ago (I was in high school, and I’m now 60) for a partial solar eclipse. The telescope I owned at the time was a tiny 40mm refractor. For the purposes of this article, I tried it through my 120mm refractor. I could feel a lot of heat coming through the eyepiece of the telescope. And aiming a telescope at the Sun without a filter is not easy. This is an outdated method considering we now have safe full-aperture (not sorry to keep emphasising it) solar filters. It can, however, be used to observe sunspots. But don't allow yourself or anyone else to look through the eyepiece.
Solar filters can also be used with modern digital cameras to photograph sunspots. A DSLR with the correct fittings can be fitted to a filtered telescope at the Prime Focus (see Figure 8). Much easier is a camera with a telephoto lens. See Figure 9. This is what I used to photograph a group of sunspots on September 8 this year (Figure 10).
This photo was taken through a Panasonic Lumix FZ300 camera fitted with a full-aperture solar filter. The camera focal length is 600m fully extended. The ISO was 100, f4.5, at 1/4000th second. With this fast shutter speed hand-held shots are possible, though a tripod is better; especially for focussing. You need to use manual focus and focus until the sunspots are visible. The photo in Figure 10 has been cropped and Adobe Lightroom was used to make some adjustments.
With the sun apparently becoming more active, we will hopefully see larger and more numerous sunspots in the coming months and years.
Observing and photographing them safely is possible with FULL APERTURE SOLAR FILTERS!