For special sky events such as solar eclipses and transits, I always choose my words very carefully when advising people on how to safely view the Sun. I want people to enjoy the spectacle of these beautiful and rare astronomical alignments. But on the other hand, there’s a natural tendency to be overly cautious. After all, I don’t want anyone to suffer eye damage.
The problem with observing the Sun is obvious: it’s so bright that prolonged, direct exposure can cause permanent damage to the retina, leading to loss of vision or blindness. To observe the Sun safely, you need to filter out more than 99% of the Sun’s light before it reaches your eyes.
Given these caveats, here’s some practical advice on how to safely observe the June 5th/6th transit of Venus. This article covers a wide range of options:
- Viewing the Transit Directly
- Projecting an Image of the Sun
- Using a Telescope or Binoculars
- High-End Filters for Telescopes
Viewing the Transit Directly
Venus's disk will be large enough to seen without optical aid during the transit. To view it safely, get a pair of solar observing glasses or a #14 arcwelders glass. Babak Tafreshi
There are numerous ways you can observe the beauty of the Sun with complete confidence that nothing bad will happen to your eyes. If you’re observing the Sun with your naked eyes, all you really need are low-cost solar observing glasses from companies such as Rainbow Symphony. You can often order such glasses in bulk quantities at dirt-cheap per-unit prices. I’d order as soon as possible, in case companies start running out of glasses prior to the transit.
Alternatively, you can go to a welding-supply store and buy a piece of #14 arcwelder’s glass, which reduces sunlight enough for safe direct naked-eye viewing. And it’s important to remember that Venus’s disk will be large enough to see without optical aid during the transit, although I would certainly recommend binoculars or telescopes for the most interesting periods at the beginning (ingress) and end (egress).
But no matter what, do not use “filters” such as smoked glass, stacked sunglasses, polarized filters, camera filters, candy wrappers, or compact discs. They might reduce the Sun’s glare, but enough harmful radiation can sneak through to damage your eyes. Only use materials specifically manufactured for safe solar viewing, or #14 arcwelders glass.
Projecting the Sun
If you’re still queasy about using filters, or if you want to show the Sun to many people at the same time, you can use a small telescope or binoculars to project an image of the Sun onto a screen or white sheet of paper (almost any flat surface will suffice). A big telescope lets in a lot of sunlight, which poses the risk of overheating internal components. So I recommend either using a telescope with an aperture no larger than 4 inches, or using a mask with a 3- to 4-inch-wide hole to block the opening of any telescope with an aperture greater than 4 inches.
Remove the finderscope and place an eyepiece at the telescope’s focuser. Aim the telescope in the general direction of the Sun (without looking at the Sun through the telescope!) and move it around until sunlight streams out of the eyepiece. Believe me, you’ll know when you hit the sweet spot. You can also use binoculars mounted on a camera tripod, but make sure to cover one lens. A sunshade that blocks ambient light from falling on the projection surface will improve your view.
Learn more about solar projection.
Using a Telescope or Binoculars
If you want to observe the transit through a telescope, there are many options. Because binoculars and telescopes concentrate the Sun’s blazing light, it’s even more crucial to use safe filters. Make sure to avoid any filter that is placed at the eyepiece end of the scope. The concentrated sunlight will probably destroy such a filter, followed shortly thereafter by your vision.
The easiest and least expensive option is solar filter material from a company such as Baader, whose material has been highly rated in an S&T Test Report. Note that this material might bedifficult to find at present — some suppliers, such as Astro-Physics, are temporarily out of stock due to high demand.
Make sure to place the filter material at the front end of your telescope, and to cover the entire opening. If you plan to use a large telescope, no problem — simply create a mask with a 3- or 4-inch-wide hole and cover the hole with your filter material.
Filtered telescopes can capture fantastic detail around sunspots such as granulation, the small cells resulting from the boiling motion of gas on the surface of the Sun.
S&T: Sean Walker
Many different companies sell safe solar filters (often made of glass, plastic, or Mylar) that go on the front end of scopes, where they block more than 99% of sunlight before it ever enters the telescope tube. These filters allow you to gaze at the Sun for hours with no risk whatsoever, and are basically showing you the Sun’s visible surface (the photosphere) in white light. Besides seeing Venus’s silhouette, you will probably see scads of sunspots as well, which are fascinating in their own right.
Make sure your filter is securely attached to the front of the scope, so there is zero possibility that it will come off while viewing. And to avoid damaging your finderscope, either remove it or place a cap or solar filter at its front end. Also note that safe solar filters work equally well with binoculars.
High-End Filters for Telescopes
This full image of an active Sun was made through a 40mm Coronado Personal Solar Telescope using an H-alpha filter. S&T: Sean Walker
Last but certainly not least, many amateurs are currently using specialized solar equipment that allows them to observe the Sun at very narrow wavelengths, particularly the hydrogen-alpha line at 656.3 nanometers or the calcium-K line at 393.3 nanometers. Such filters are rather costly, but they allow you to see different layers of the Sun, and they can provide spectacular views of prominences and filaments that you can’t see in white light. You’ll want to use these filters frequently for solar viewing even when the Sun is not being eclipsed or transited.
I will be traveling to Hawaii to see the transit of Venus with an S&T tour group. I am planning to bring my Coronado Personal Solar Telescope, a small and easily portable 40-millimeter refractor (with a built-in hydrogen-alpha filter) that provides stunning views of the Sun for a relatively modest price.
Learn how to photograph the Venus transit.
I wish you safe viewing of the upcoming transit, and most of all, clear skies! After all, even sunlight can’t poke through heavy clouds in Earth’s atmosphere --- the ultimate natural solar filter.