After several years in the doldrums, solar observers once again can rejoice: the Sun is active.
AFTER AN UNEXPECTEDLY LONG snooze as solar cycle 23 wound down and cycle 24 began, the Sun is once again an exciting target for observers. While following the daily evolution of sunspots with a telescope equipped with a safe white-light lter is a fascinating way to keep tabs on solar activity, the real excitement comes from observing with a special lter that transmits a nar- row slice of the spectrum centered on the red wavelength of hydrogen-alpha light at 656.3 nanometers. At this wave- length the Sun reveals constantly changing prominences dancing along the solar limb, some of which can extend tens of thousands of kilometers into space. And if the lter transmits a slice of the spectrum less than an ang- strom (1 angstrom = 0.1 nanometer) wide (the narrower the better), the Sun’s disk can appear lled with intricate detail around active regions on the solar surface.
In late 2011 I followed solar activity for several months with two Coronado telescopes that we borrowed from Meade Instruments for this review. The SolarMax II 60 and SolarMax II 90 are, respectively, 60-mm f/6.6 and 90-mm f/8.8 refractors made exclusively for hydrogen- alpha, or H-alpha for short, solar observing.
Both SolarMax II scopes isolate the H-alpha wave- length with an internal etalon and a separate blocking lter located near the eyepiece. The blocking lter for the 60-mm scope is permanently mounted in a 90° diagonal, making the diagonal a required part of the setup. Although the 90 mm scope is also available in this con guration, the version I tested is made for “straight through” imaging with a larger blocking lter mounted in the focuser draw- tube, so it can be used with or without a diagonal.
The basic SolarMax II telescopes are speci ed as having a bandpass of less that 0.7 angstrom. They are also avail- able with a so-called double-stacked option that narrows the bandpass to less than 0.5 angstrom, providing even greater visual contrast in the detail seen on the solar disk. The double-stacked scopes have a second etalon mounted in front of the telescope objective. The scopes I tested were double-stacked models, and I tried them with and without the second etalon. More about this in a moment.
Each SolarMax II telescope is supplied with a clam- shell mounting ring tted with a projection-type solar nder. Although the double-stacked scopes are relatively compact, they are also rather heavy for their apertures; the SolarMax II 60 tipping the scales at 7.6 pounds (3.4 kg), and the 90 at 21.6 pounds (9.8 kg). The 60 can be used with a heavy-duty photographic tripod, but the 90 needs a much more substantial mount. Although I was satis ed using the 90 on my aging Vixen Great Polaris DX mount, I consider it about the minimum mount suitable for serious observing with this scope. To get the best experience with either scope, you’ll want a tracking mount with slow- motion controls.
Coronado SolarMax II Hydrogen-Alpha Telescopes
U.S. prices: SolarMax II 60 starting at $1,499
SolarMax II 90 starting at $3,599
Meade Instruments, 27 Hubble, Irvine, CA
92618 949-451-1450: www.meade.com
Even if you’re new to H-alpha solar observing, the SolarMax II scopes are easy to use. With the single-etalon models, you simply focus the Sun in the eyepiece and adjust a lever on the side of the telescope tube until solar features display the greatest image contrast. The “sweet spot” showing the greatest contrast moves across the
eld of view as you adjust the lever. While I could nd a setting that showed the overall solar disk to advantage, I generally adjusted the lever so that the point of maximum contrast was centered in the eyepiece eld and then used my mount’s slow motions to move the portion of the solar disk of interest to this point.
The double-stacked scopes work the same way as the single-etalon models, except that you also have to tune the objective-mounted etalon after adjusting the internal one. Since the double-stacked images are a little dimmer, it really helps to block as much stray light as possible from entering your eye.
When the double-stacked scopes were properly tuned, the views were nothing short of spectacular. Even a rather minimally spotted Sun often showed sinuous dark la- ments crossing the solar disk and dramatic prominences, some of which visibly changed in a matter of minutes. (If you want to see how the Sun looks right now in H-alpha light, check out the GONG Network at http://gong.nso. edu, where a global system of telescopes keeps constant vigil on the Sun with images and movie loops that are updated every minute.)
The Coronado SolarMax II telescopes, pictured here in the double-stacked con guration explained in the accompanying text, o er outstanding views of solar activity. They are very easy to operate even for those new to hydrogen-alpha observing.
SolarMax II scopes have helical focusers. While these o er precise adjustment, especially for photography, it took some time for me to adapt to them for visual observ- ing, since I’m used to pinpointing the sharpest image by quickly “racking” back and forth through the focus point. This is especially true at the H-alpha wavelength, which is near the eye’s limit of visibility. The relatively slow action of the helical focuser, coupled with a small amount of backlash in the units I tested, meant that it often took some nessing to get the visual focus set right.
Last November 22nd the author used the SolarMax II 60 and a DMK video camera to image a solar lament straddling the Sun’s limb. It appears dark against the Sun’s surface, but becomes a bright prominences when seen against the sky
I had very good results using both scopes in the double-stacked con guration with a DMK video camera. But my colleague Sean Walker, who has considerable experience imaging with H-alpha scopes, notes that equally good results are possible with the single-etalon con guration since the image will be brighter and you can use image processing to achieve the boost in image contrast that visually occurs with the double-stacked setup.
In the world of telescopes, aperture is king, and at the outset of my review I fully expected to spend more of my “leisure” time observing with the SolarMax II 90. But that ended up not being the case. The grab-and-go portability and more-relaxed mounting requirements of the smaller SolarMax II 60 were two reasons that I often chose it, but there was another reason that’s less obvious. You tune the second etalon on the double-stacked scopes by turning a ring mounted on the front of the etalon. With the smaller scope I could easily reach up and do this while comfort- ably looking into the eyepiece. But with the larger scope I’d need arms like an orangutan. It’s really a two-person job to ne tune the 90’s front-mounted etalon.
Overall, I really enjoyed the SolarMax II scopes. Each clear day as the rising Sun crested the trees bordering our o ce parking lot, I’d check the GONG Network to see what was happening. If something looked interest- ing, I’d grab the 60 and head outside for a rsthand look. It emphasized just how easy it is to use these scopes for H-alpha observing on a moment’s notice.
Senior editor Dennis di Cicco has been patiently waiting several years for the Sun to wake up and solar activity to return to exciting levels.
WHAT WE LIKE:
- Crisp, detailed hydrogen-alpha views
- Easy to operate
WHAT WE DON'T LIKE:
- Relatively heavy for their size