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Chicagoland Astronomy Club Message Board › Solar Filters Question

Solar Filters Question

user 11660017
Group Organizer
Glenview, IL
Post #: 15
Here is a question that was forwarded to me. Paulie, you know a lot about solar filters, can you share some info?

I would like to purchase or fashion a safe, good, solar filter for my telescope in order to view the sun. I am particularly interested in doing this in preparation for the Venus transit on June 5th this year. Can anyone share some advice on how to obtain a safe, economical solar filter?
A former member
Post #: 38
I can maybe help a little, but I don't have the time right now to go into detail. I'll try to give a better answer tomorrow, but this thread might be a helpful start.­
A former member
Post #: 20
You can get filters on-line at a lot of places. The thin film filters are fine for viewing sunspots and transits.

Here's an example: http://www.telescopes...­

There are a lot of do-it-yourself web pages for building a solar filter using special filter material (usually from a company called "Baader").

Here's an example: http://www.backyard-a...­ .

Orion ( and other companies offer large, glass mounted filters that provide the same white-light view of sunspots and transits. Personally, I've never seen any reason to spend the extra money on the glass filters. The film filters work fine and withstand normal use. Even if you get a small cut in the filter material, you can just fix it with opaque tape and you won't notice.

A former member
Post #: 39
As with any other observational astronomy, there are many options for solar observing. Each option has it's advantages and trade-offs. The main choices are full aperture filter, or off-axis scaled down filter, then glass, Baader solar film, or hydrogen alpha filter.

Obviously, when observing the night sky, big aperture is desirable to gather more photons. When viewing the Sun, though, you're filtering out 99.99% of the Sun's visible, infrared, and ultraviolet light, so large aperture isn't necessary. Many people with large aperture telescopes buy or make solar filters of only 2" or 3", offset to the side of the secondary mirror on reflectors. This drops the cost of the filter, but doesn't take advantage of the resolution that a full aperture solar filter will give.

Full aperture solar filters will be more expensive, but by taking advantage of the full primary mirror, allow for better resolution and higher magnification. The arguments against this is that higher magnification will also magnify the imperfections in seeing conditions, or atmospheric turbulence. This is always the case, but the daytime atmosphere, heated by the Sun tends to be more turbulent than at night.

Baader film solar filters, sometimes called Mylar, are the most cost effective solar filters. A 9" or 10" square sheet costs around $50, and pre-made full aperture and off-axis filters can be purchased for less than the same aperture glass filter. Although the Sun's visible light output peaks in green wavelengths, the total output is white light. Some Baader films give a bluish color to the Sun. Mine gives a slight bluish-purple tinge to the image, but at a glance, it looks very white. Some people feel that blue gives the highest contrast solar views. Baader film , and really all solar filters, need to be stored properly so that they aren't scratched or torn. Check for pinholes before each use. Tiny pinholes can be repaired by black marker to the back of the hole.

Glass white light solar filters are more expensive than Baader film, but are more durable. Most tend to show the solar disk as orange. We tend to think of the Sun as yellow or orange, so these give a more "natural" look to the Sun. It's not true, but it matches our preconceived notions.

Hydrogen alpha, or H Alpha filters show a very narrow wavelength at the extreme red end of the solar spectrum. While white light filters show only sunspot activity, H Alpha filters show granulation, prominences, and filaments as well as sunspots. Even large aperture white light filters can be found for less than $200, and average less than $150. H Alpha filters are much more expensive. I don't know about just the filters, but H Alpha scopes like Coronados and Lunts run about $1500 for a 60mm scope. That's way out of my range, but the views are very good. If you have the money, and plan to do some serious solar observing outside of the transit, I highly recommend an H Alpha solar scope.

If you're looking for a solar filter solely for the Venus transit, I would recommend a white light filter over H Alpha. The biggest advantage to be gained for the transit with H Alpha is that Venus will be observable against the outer solar atmosphere before first contact. I think the greater detail of an H Alpha scope would be distracting, although it would make a far prettier picture.

In November I made about a 2" offset Baader film filter for my 4.5" reflector. It gives pretty good views, but I think I'm going to use the rest of the material to make a full aperture filter for my 6" Dob. Really, it all comes down to preference, and what your budget will allow. Hurry though, because I've heard that solar filters are going out of stock in some places in preparation for the transit.

A former member
Post #: 41
This is the Baader solar film I bought.­

Here is the solar filter for my 4.5" Newtonian, and how I made it.­
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