Solar Observing With a Smart Telescope, Safely
Several smart telescopes ship with a solar filter and a solar mode. The filter is what makes it safe, and there is exactly one rule about it that matters more than everything else combined.
Solar observing is the one part of this hobby where a mistake causes permanent injury rather than a wasted evening. It is also entirely safe when done correctly, and smart telescopes with proper solar filters have made it more accessible than it has ever been.
The safety rules are short, absolute, and worth reading in full before the first attempt.
The rule
The filter goes in front of the telescope, over the aperture, before the telescope points anywhere near the sun. Not at the eyepiece. Not partially. Not a filter you improvised.
The reason front-mounted filters are the only acceptable design: a full-aperture filter rejects the sunlight before it enters the optics and before any of it gets concentrated. An eyepiece-end filter sits behind the point where the telescope has already focused the sun's energy, and such filters have been known to crack from the heat, without warning, while in use.[1]
Smart telescopes with solar capability supply a front-mounted filter for exactly this reason. Use the one supplied for your model.
Before every session
- Inspect the filter against a bright light, looking through it at a lamp. Any pinhole, scratch, crease or separation of the film means the filter is finished. Do not use it.
- Fit it securely and check it cannot be knocked off. Wind is the realistic hazard. Several filters use friction fits that are adequate indoors and marginal in a breeze; tape it if there is any doubt.
- Cap or remove any finder scope. A finder is a small telescope with no filter on it, pointed at the sun, at eye height.
- Point away from the sun while setting up, then approach the sun only once the filter is confirmed on.
What never to use
Improvised filters cause injuries every eclipse. None of the following are safe, regardless of how dark they look:
- Sunglasses, welding glass below shade 14, exposed photographic film
- Smoked glass, CDs, mylar food packaging, space blankets
- Neutral density or polarising photographic filters, which pass infrared
- Solar eclipse glasses held over a telescope aperture, which are designed for naked-eye use only
The specific danger with several of these is that they dim visible light while passing infrared, so the view looks comfortable while damage occurs painlessly. The retina has no pain receptors, so there is no warning at the time.
What you can actually see
With a standard white-light solar filter, which is what smart telescopes supply, you see the photosphere: sunspots, their penumbral structure, and limb darkening. During an eclipse you see the moon's silhouette crossing that disc.
Sunspots change day to day, which makes solar observing one of the few areas where a daily record is genuinely rewarding. Photographing the same active region over a week as it rotates across the disc is a small, satisfying project available in daylight and in winter.
What you do not see with a white-light filter are prominences and flares, which need a dedicated hydrogen-alpha solar telescope. That is a separate instrument, not an accessory.
Practical notes
Seeing is usually poor during the day. Ground heating creates turbulence, and detail suffers. Early morning, before the ground has warmed, is generally the best time.
Watch the instrument's own temperature. Sitting in direct sun for an hour is thermally harder on the electronics than a night session. Shade the body if you can.
Store the filter carefully. It is the safety-critical component and its film is fragile. A crushed filter in the bottom of a bag is the most likely route to a pinhole.
Notes & sources
- Safe solar filter practice, including full-aperture front-mounted filters, inspection before use, and the hazards of eyepiece-end filters and improvised materials. American Astronomical Society, solar eclipse eye safety ↩