Observing the Sun in H-alpha
Gregory Gross

Observing the Sun in H-alpha

WARNING: When observing the Sun, be sure to use only equipment designed specifically for that purpose and produced by reputable manufacturers. Follow their directions closely. Do not improvise your own filter material for solar observing. If you are careless, you risk instant and permanent injury and/or vision loss.

The Sun as it appeared in a double-stacked Lunt LS50THα 50mm solar telescope on September 5, 2018
The Sun as it appeared in a double-stacked Lunt LS50THα 50mm solar telescope on September 5, 2018. I captured this rather poorly resolved image by aiming a point-and-shoot camera into the eyepiece.

The Sun is the ultimate source of life and energy on Earth. Without the Sun, our world would be a frozen, lifeless wasteland.

I get a thrill from knowing that I can not only observe something as forbidding as the Sun but also that, by taking all necessary precautions, I can do so safely. Being able to observe something as powerful as the Sun instills a sense of awe and wonder that nighttime astronomy can’t match.

Watching the chromosphere of our nearest star change day to day and even minute to minute is a particular source of fascination for me. There’s always something new to see. Even during periods of minimum solar activity in which one can go days without seeing a sunspot, the chromosphere teems with activity every day.

And for an aficionado of precision instruments, I love using and appreciating gear for safely observing the Sun at that narrow band of the spectrum centered around the hydrogen-alpha (H-alpha) line where the chromosphere is visible.

My Journey Down the H-alpha Observing Road

50mm Lunt

I started my journey down the H-alpha solar observing road in October 2017, when I acquired a Lunt LS50THα 50mm solar telescope. At the time, I was new to H-alpha solar observing, and I was reluctant to plow a lot of money into an area that is notoriously expensive and that I was unfamiliar with. I took it easy at first: for those first few months with it, I used my LS50THα just as it came out of the box.

During that time, I remember being mildly impressed with the view of the Sun when it was headed towards the depths of minimum activity in its eleven-year cycle. Prominences, filaments, and surface granulation were visible, but I found that I almost had to train my eye to see them. Still, I was increasingly intrigued.

Lunt LS50THα 50mm solar scope, LS50C double-stack module, and case
Lunt LS50THα 50mm solar scope, LS50C double-stack module, and case.

As my excitement for H-alpha solar observing grew, I added various upgrades including a better focuser and a Lunt zoom eyepiece. But adding a double-stack module made the biggest difference. Upon first glimpse with that added etalon attached to the scope, I was blown away by the increase in performance. Surface details appeared with far better contrast, and prominences, while somewhat dimmer, didn’t suffer too much. It was like I had an entirely different piece of gear.

Overall, my double-stacked 50mm Lunt gave me very satisfying views of the Sun in a compact, lightweight package. Each upgrade step that I took with that scope triggered a new level of enthusiasm for this little niche within a niche in amateur astronomy.

But there was no getting around the fact that I had an entry-level instrument. To be sure, that little scope punched way above its weight, and it offered a very satisfying observing experience at the eyepiece. But the more time I spent with it, the more it was obvious to me that my 50mm Lunt’s etalon produced a well-tuned sweet spot that was just a bit smaller in diameter than the total width of the Sun’s disk. There came a point when I realized that I had grown very serious about solar astronomy and that I had outgrown my entry-level solar scope. As a strictly visual observer, I wanted to go to that next level.

Upgrade Decisions

In the spring of 2019, I was lucky enough to have an opportunity to visit the good folks at Lunt Solar Systems in Tucson and try out some upgrade options in person. While I was there, I knew that I had been completely sold on the value of double stacking, and knew I wouldn’t consider going back to single-stacked mode. As a result, I was intent on testing double-stacked scopes during my visit. I limited myself to trying two options: the 60mm LS60THα and the 80mm LS80THα.

I was really impressed with the LS80THα even in single-stacked mode. The sweet spot seemed to cover almost the entire field of view, and the contrast was excellent. But there’s no going back once you double stack, so I knew I wanted to keep that going with whatever upgrade path I took. Once it was added, I saw for myself how much the internal double-stack module creates some significant glare issues that a number of observers have noted in a variety of posts on Cloudy Nights. The addition of a high contrast filter addressed those issues, but it also dimmed the view substantially.

I ended up going with the LS60THα that I tried at Lunt. I also got the bigger 60mm double-stack module as opposed to the still very satisfying 50mm double-stack module. The smaller module performed much better than I expected. But since more aperture results in better resolution, I opted for the full 60mm double-stack module. And since I wanted the best experience that a 60mm Lunt could offer, I also added the larger 12mm blocking filter and a two-speed Feather Touch focuser.

60mm Lunt

Lunt LS60THα 60mm solar scope and LS60FHa double-stack module
Lunt LS60THα 60mm solar scope and LS60FHa double-stack module in action.

For the kind of full-disk solar observing I like doing best, my Lunt LS60THα 60mm solar telescope with 60mm double-stack module offers me the best balance among such factors as performance, size, convenience, and weight.

The width of the sweet spot on the LS60THα extends a bit past the Sun’s disk. To my eye, that sweet spot is maybe 120% the width of the Sun’s disk, meaning that one gets the full benefit of a well-tuned H-alpha view across the whole Sun.

In single-stack mode, prominences are visible as are more obvious filaments. A mild orange-peel texture covers the disk of the Sun. On the downside, a good amount of reddish glare surrounds the disk of the Sun and extends out past its limb and into the field of view for a length equivalent to perhaps three-quarters of the Sun’s apparent diameter.

But as is the case with any other H-alpha solar scope, the addition of the 60mm double-stack module completely and utterly transforms the experience at the eyepiece. The double-stack module largely but not totally eliminates the reddish glare that is plainly visible around the Sun in single-stacked mode. As a result, prominences take on a bolder appearance with fainter ones popping into view where they weren’t visible at all in single-stack mode. When they are present, “filaproms,” or filaments that appear near the limb of the Sun and that extend beyond it, are more obvious and appear with better definition. Such structures may not even be visible in single-stacked mode.

Lunt LS60FHa 60mm double-stack module
Lunt LS60FHa 60mm double-stack module.

The second etalon makes a huge difference with enhancing surface detail. It does dim the Sun somewhat, but that dimming is not at all objectionable. Surface granulation appears with far better contrast and with much finer detail, the difference in fineness being comparable to coarser white table sugar versus powdered sugar. That granulation also presents itself with somewhat of a scaly web-like appearance. With the improvement in surface contrast, subtle active areas become much more visible, and robust ones are often swimming with a mixture of bright spots and fibrils.

In all, the addition of a double-stack module transforms a 60mm Lunt into a wholly different scope, one that was far more capable in showing fine and subtle chromospheric detail.

To my eye, the images that area available on the GONG H-Alpha Network Monitor closely resemble what I see at the eyepiece in my double-stacked 60mm Lunt. Whatever prominences, filaments, and active areas that are visible on those images are generally visible to me in the eyepiece.

The Sun as it appeared in a double-stacked Lunt LS60THα 60mm solar telescope on August 12, 2022
The Sun as it appeared in a double-stacked Lunt LS60THα 60mm solar telescope on August 12, 2022. I used a Canon M200 mirrorless camera to capture the image, and I did my postprocessing using the raw file.

It’s important to bear in mind that every etalon has subtle differences, so achieving optimal tuning involves a bit of trial and error. This is compounded when one is working with two etalons in the case of a double-stacked solar scope. When I first was settling in with my 60mm Lunt, I tried rotating the module itself to obtain optimum tuning. If I fully screwed the module on to the scope and then backed it off by just a half-turn, the view seemed more pleasing in the eyepiece. But later I found that the looseness of the module rotated out a half-turn introduced some negative side effects, so I went back to threading the module on to the scope all the way. With the module now attached squarely and tightly to the scope, I worked the tuning wheel to shift the reflection of the Sun on either side of the main image and found what appeared to my eye to be the most evenly tuned view of the Sun. At bottom, patient experimentation was necessary for me to settle on the best tuning.

Lunt zoom eyepiece with neutral density filter
Lunt zoom eyepiece with neutral density filter.

Another instance of trial and error involved finding a solution to the reddish glare that I noted above. Having tried every low-power eyepiece I own in the 18-20mm range, I keep coming back to the Lunt zoom eyepiece I bought not long after I got stuck into H-alpha solar observing. I found that eyepiece’s adjustable eye cup works well to block out the surrounding glare of light (one uses these scopes on sunny days, after all), and it gives me the ability to zoom in when some feature or another warrants a closer look. But I find that eyepiece does introduce a certain reddish glare around the Sun’s disk. One very experienced amateur solar astronomer suggested I try a neutral density filter. I just so happened to have an unused one sitting in my spare parts bin. After I popped it on to the end of my Lunt zoom eyepiece, I was immediately surprised how much of a difference the addition of a simple ND 0.3 filter costing less than $11 had on the visual experience. That annoying was still present, but it was very much reduced to the point where it wasn’t bothersome anymore. The filter helped blacken the background behind the Sun and enhanced contrast of both surface details and subtle prominences. I keep that filter on the end of my Lunt zoom eyepiece permanently.

I find that using a simple, lightweight, manually-driven equatorial mount like my Orion AstroView gets me out far more often than a heavier and more cumbersome mount would allow. Weighing in a 9.2 pounds, my double-stacked 60mm Lunt is on the heavier end of what I’d ask that mount to handle.

The length of the Lunt LS60THα 60mm solar scope with LS60FHa double-stack module attached is about 19 1/2 inches.
Length of the Lunt LS60THα 60mm solar scope with LS60FHa double-stack module attached.

In terms of weight distribution, the scope is definitely tail heavy when it’s in single-stacked mode. Adding a double-stack module balances the weight across the length of the scope far better. It’s almost as if the scope wants to be double stacked. With that extra length added to the front of the scope and the blocking filter diagonal nose pulled out about an inch and a half from the focuser draw tube (one won’t achieve focus without doing this), the scope grows to 19 1/2" in length and looks formidable.

All in all, I’d go so far as to say that my 60mm Lunt solar scope seems incomplete without its double-stack module attached. My experience was the same when I had a 50mm Lunt scope, too.

It’s a financially painful experience to spend nearly as much for a double-stack module as what one spent on the scope itself, but it’s worth every penny. If I were faced with the choice between a single-stacked but larger aperture solar scope or a double-stacked but smaller aperture solar scope, I’d definitely opt for the latter.

Lessons Learned

Aperture Is Not King

For H-alpha solar observing, it’s important to set aside some of the lessons we learn from using telescopes at night, namely the notion that aperture is king. The optical system of an H-alpha solar scope is significantly more complex. To be sure, aperture is important, but it isn’t everything. The quality of the etalon and its ability to offer both the narrowest bandpass at that all-important part of the spectrum at 6562.8 Angstroms and the widest sweet spot or “Jacquinot spot” are very important and often overlooked characteristics. Remember that most of what one is paying for in terms of the price difference between a lower- and higher-end instrument is a better etalon, which is far and away the most expensive part of any H-alpha solar scope.

But Aperture Is Still Important

Still, aperture does play a role in determining the overall quality of the solar observing experience. While there’s plenty of light where the Sun is concerned—indeed, so much light that specialized gear is necessary to reduce the Sun’s light and observe it safely—aperture ultimately determines resolution especially at higher magnifications. That’s what one is ultimately going for in terms of aperture increases.

Internal vs External Etalons

Some H-alpha solar scopes situate their primary etalon at the front of the telescope while other has internally-mounted etalons. One of the reasons why internal etalons exist is because there is a cost savings to making the etalon smaller. Since internal etalons are placed further back along the cone of light, they can be made smaller and thus cheaper than larger front-mounted etalons. But front-mounted etalons have fewer optical elements and can offer better performance for various reasons.

Double Stacking

The best way to achieve a narrow bandpass and eliminate parasitic continuum from leaking through is double stacking, or adding a second etalon. Double stacking is definitely not cheap—nothing made with quality ever is. But doing so results in an utterly enhanced observing experience that is far and away better than single stacked observing especially during periods of minimum solar activity. Prominences can become somewhat dimmer, but not badly so. The rich contrast that the Sun’s disk appears with is breathtaking in double-stacked mode. Single-stacked scopes, especially in the small- to medium-range apertures, won’t show that richness in contrast that a double-stacked one will.

Double stacking any H-alpha scope is more than worth the expense of doing so for visual use. There is simply no comparison between the single- and double-stacked observing experience.

(This piece is based on earlier postings I made in the Cloudy Nights Solar Observing and Imaging forum on July 20, 2019, January 4, 2020, March 3, 2020, May 30, 2020, and August 1, 2020.)