If you have an interest in astronomy, chances are that you’ve heard of the upcoming North America Eclipse, in August 2017. And if you have a conventional telescope, you know the problem is these telescopes are all made for nighttime use so, to use them for solar and solar eclipse viewing, you need special filters to protect your instrument and (more importantly) your eyes. Luckily, we here at B&H have you covered, with filters to adapt your telescope for viewing the sun in general, and the solar eclipse, in particular.
First Things First
Before we worry about protecting your telescope, let’s start with protecting your eyes. The first piece of gear you’re going to want to pick up is a pair of solar glasses. Why? Because at some point you’re going to need to look at where the sun is so you can start observing, so having a pair of protective glasses will save you from the inevitable glance right into the sun and the possible blindness that can accompany it. Whether you’re setting up, aligning, or just doing general prep work, having reliable safety gear to protect your eyes is probably the most important item you’re going to have.
All things being equal, if you plan on using them more than once or twice, I’d recommend solar glasses since they will be more durable and hold up to multiple uses better than the cardboard frames of the shades. Personally, I have these from Celestron, and they came with a white-light filter for my DSLR lens and a 2017 solar eclipse viewing guide as a bonus.
Also, you’ll want to replace your standard finderscope with one designed for solar viewing. You can grab this Coronado Sol Ranger that will fit into 1" finderscope holder brackets, or a Tele Vue Sol-Searcher. Both use a projection system to show when the Sun is centered so you don’t have to worry about hurting your eyes, trying to align your scope for viewing.
White Light or Narrow Band?
The next thing you need to ask yourself is what kind of solar viewing you want to do. Are you going with White Light or Narrow-Band (Targeted) viewing. For specifics about the difference, feel free to click on over to my B&H Explora article, Solar and Solar Eclipse Viewing 101. Suffice it to say that White Light viewing is probably the most popular and easiest to do. It essentially entails using a filter that filters more than 99.99% of the visible light from the sun (and all harmful IR and UV radiation) so it’s safe to look at. This differs from Narrow-Band viewing, which filters all light except a very narrow wavelength of the spectrum to reveal extraordinary details that White Light filters can’t resolve.
White Light Filters
A basic White-Light filter simply fits over the front objective of your optical tube assembly (OTA). Fitting the filter is easy: Measure the diameter of the dew shield or front retaining plate and then choose the filter that most closely matches, without being the same size or smaller. Most of these filters will fit a range of sizes and secure in place using thumbscrews so a perfect fit isn’t critical—but ensuring it’s not too big is. It’s also important to note that you need to take any screw or bolt heads into account in the initial measurement, since you need to ensure the filter housing will completely fit in place, so you’ll need to clear the screws that might hold the retaining ring in place on a Cassegrain or reflector-style OTA.
MrStarGuy offers four versions of filters to fit OTAs (or spotting scopes) from a small 66mm to massive 155mm (2.598 - 6.102"). The filter ring is made of a lightweight aluminum with a non-reflective matte-black finish, and the filter itself is a rip-resistant film that delivers high-contrast images of the Sun with a neutral color balance revealing the Sun in white, as opposed to the more common red or blue seen when viewing through other solar filters. A coating is applied to each side of the film to prevent rips and pinholes to ensure safe viewing.
While being known mostly for narrow-band viewing filters, Daystar Filters offers film White-Light solar filters in 50mm, 70mm, and 90mm to fit an array of objective lenses. The designers also understand that people may be using binoculars to view the eclipse, so each size is offered in a choice of one- or two-pack. Size the filter the same as described above.
A slightly different design of filter is offered by Lunt Solar Systems. Instead of fitting on the front of the OTA, they fit into the focuser draw-tube and the eyepiece goes into the filter for viewing. Available in 1.25 and 2" versions, the former is recommended for use in refractors up to 4" (101mm) and the latter in refractors up to 6" (152mm). If you’re going to use an imaging system with this filter, use it as-is, but if you’re planning on observing the sun visually, Lunt recommends also using one of its polarizing filters to cut the intensity a bit more for added safety. You could also pick up an ERF (energy rejection filter) for the front of the telescope, to reduce the heat load on the optical components, to avoid damage due to overheating. The Lunt products linked in there are just the filters, without any kind of mounting hardware, so consider those only if you’re experienced and can handle fabricating your own filter ring to attach it to your OTA—otherwise grab the Daystar ones, and fit them as we’ve previously discussed above.
Narrow-Band Filters
The veteran solar-viewing company, Coronado, offers a line of SolarMax II H-alpha filter sets that will fit on various refractors. The sets each include a front etalon filter and a rear blocking filter that fit into 1.25" drawtubes. The front filter isolates the H-alpha wavelength, dissipates the heat, and has a tuning system built into it, while the blocking filter further filters the H-alpha and reduces the brightness to a safe level. If you go with this set, it’s imperative that you use them together since they will only offer protection when used together. You have a choice of front filters, from 40mm to 90mm with different blocking filters, based on the focal length of the OTA on which you’re mounting the set.
Similar sets are offered by Lunt, but they give you some options that Coronado doesn’t, such as a choice of either 1.25 or 2" drawtube options and a wider range of front apertures (up to 100mm) and blocking filters (6mm to 34mm). As with the Coronado filters, you need to use the two parts together for proper and safe filtration, and unless you’re using them on a Lunt H-a solar scope (for a double-stacked etalon configuration) or one of the company’s conventional refractors, you’ll need to pick up adapters to fit your particular OTA separately, so bear that in mind when ordering your set.
Narrow-band viewing is where Daystar Filters really comes into its own. The Quark line of filters is offered in multiple bandpasses: Hydrogen-alpha (two versions: one for Chromosphere viewing and another for Prominences), Sodium Na D-line, and Calcium Ca H-line; with a combination option to get the H-alpha versions with a blocking filter for use on larger Cassegrain OTAs. They all share the same digital temperature tuning that allows you to tune the filter precisely on-band in tricky atmospheric conditions. It should be noted here that Daystar recommends an ERF—just make sure you measure correctly and if you have a Cassegrain you’ll want to get an off-axis version to avoid the secondary mirror obstruction. Since these are temperature-tuned and digitally regulated, you’re going to need to have access to some battery power to run the filter, so take that into account. The company offers this 5v, 30Ah battery pack, which will run a Quark for up to eight hours, but a similarly powerful pack will also do the job.
Similar in form and function to the Quark is the Quantum series. What sets this series apart is the precision with which the components are made, to allow extremely targeted viewing to very specific bands, plus they are offered in standard grade for everyday people or research grade for top-tier university work. In addition to the digitally controlled temperature tuning, they are also outfitted with an RS-232 port that allows remote control via a computer connection, which means that you can be in a different room or a different state and still tune the Quantum. Be aware that you’ll need to get an ERF as noted above, and control cables, drawtube snouts, eyepiece holders, or other adapters are all offered separately to allow you to build your system à la carte.
If you have a competent nighttime scope and reliable mount, simple add-on filters for white-light or narrow-band solar viewing can be a simple and easy way to start viewing the sun in preparation for the August 21, 2017 North American Solar Eclipse. So, click over to B&H, browse through your options, and get outside and look (safely) at the sun… you know you’ve always wanted to.
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