You can’t go to a star party these days without bumping into one of Coronado’s Hydrogen Alpha (Ha) Filters. All it takes is one look and your hooked. Seeing the solar chromosphere come alive in Ha light is thrilling. I have had the opportunity to look through and compare the 40mm, 60mm and 90mm versions as well as a stacked 40mm and a stacked 90mm. When it came time to purchase one, I bought the 60mm.

Description
The filter rating is listed at .7 Angstrom or less and has an operating temperature between 32 and 100 degrees Fahrenheit. It is wonderfully crafted and enclosed in a brass housing. The T-Max tuner was wrapped separately. A 1.25-inch blocking diagonal (BF10) is included along with a plastic, foamed lined carrying case. A small instruction card and plastic end caps are also included. The cost was $2600 from Astronomics and I received it within one week of ordering. The telescope to filter adaptor plate cost $160 and took about 8 weeks to arrive (I temporarily mounted the filter on an old dust cover until it arrived).

Performance
I mounted my filter on a William Optics Megrez 80 (f/5) refractor. A 25mm Plossl eyepiece gives a very detailed, full disk view of the sun. Prominences are easily observed along the limb. Filaments and active regions can be seen without effort and have good contrast. Spicula can be observed covering the entire solar disk.

A TeleVue 8 to 24mm zoom eyepiece works better. At 24mm, the image of the sun is smallest but contrast is at its best. Lighter active regions are much brighter than the surrounding areas. At 8mm the sun fills the eyepiece and allows you to magnify individual features. Contrast is decreased and the lighter and darker features tend to equal out into the same brightness of red. This is the setting to use on prominences. This eyepiece and telescope combination has quite a bit of reflected light. It is most noticeable around the sun and causes the blackness of space to be colored red. I don’t find it a problem, however.

Even though the temperature rating is between 32 and 100 degrees F, I have used the scope outside these parameters several times and still had good, on band views.

I rarely use the T-Max tuner. Occasionally I’ll try different settings if an eruptive prominence or strong flare has occurred. Ninety percent of the time I leave it alone.

I went with the 60mm version over the 40mm and 90mm based on what you see and how much they cost. The 40mm views are a little to small for me and the 90mm cost more that I wanted to spend at the time. If you have the money and a large enough scope, I’d recommend going with the 90mm. It offers the largest and brightest view I’ve seen through a Coronado filter. If you plan on photographing the sun, go with the SM90/BF30 combination.

Summary

For highly magnified, detailed views of the sun nothing beats an ultra-narrow band filter on a long focal length refractor but this cost a huge sum of money. At the other end of the scale, is the Coronado SolarMax 40.  The 40 is very acceptable but very low power and gives the smallest views I’ve seen in Ha. The Coronado 60mm Ha filter is a good compromise between cost and performance. The 60 will give you very satisfying views of the sun and not leave you looking for a larger filter a few months down the road.

Clear skies (day and night),

Greg