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Disclaimer: All the how-to directions below are for information only. They are not intended to be sold or used for unlawful purposes. The author takes no responsibility if they cause the reader bodily harm, loss of life, or cause the damage of any property, person, dog, cat, or any other animal. They are presented for information only and all liability is on the part of the reader.
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Discussions on White Light filtering
Materials needed:
A telescope with an aperture from 40mm to 120mm
A white light filter (see my "How to build a white light filter" in the previous how-to area) made with Baader ND 3.8 film
A green #58 or Baader Continuum filter
Baader U/V Ir cut filter
Baader K-line filter
Violet #47 filter
Neutral Density 3.0, 4.0, .96 filters
A good tracking tripod, preferably an Equatorial Mount tripod
A solar finder (see my "How to build a better Solar finder" in how to section)
How to: This project is a one sided discussion :) on shooting white light images. I'm not an astronomer, just a regular guy that shoots the Sun so my discussion is completely biased toward my equipment and my practices. I am definitely not here to tell you this is the one true way of shooting the Sun or the best way for that matter, or the safest, but it is my way. Starting out I wanted to get the best possible shots of disk detail and spot detail. I searched the internet for the various types of filters out there and found several for viewing the Sun. I also lucked out and read an article in a popular telescope magazine that did a comparison on the various types too. It had good images representing the images seen from the telescope but I found that it was pretty lacking. No real definitive data just alot of Tongue Fu to play nice with all the vendors. Through all the searching two things stood out, Baader solar film and the Herschel Wedge. Below is my solar filter material rating. Take it with a gram of salt for what is worth:
Thousand Oaks Black Poly film: available on Ebay is a good "starter" film. Its safe because its thick and very durable. There is a shinny side and dull side so you can tell which goes toward the sun. (shiny side goes to the Sun). The image that it produces is yellow orange and it looks nice. My experience with this film included using it on a Nexstar 11, Orion 80ED, and a couple of other scopes. This film works best with aperture sizes below 4 inches. It does not produce good details on small spots. I would not recommend this for photographic use because of the lack of visual details that cannot be seen. From 1 to 10 I would rate the visual use as a 6. For photographic use I would give it a 5. Its definitely better than nothing and makes great blocking for a finder but lacks the punch needed for serious solar photography.
Thousand Oaks R6 film: I purchased the photographic spec version of this film which appears like silver mylar but has a metallic base. It presents a yellow orange view of the sun. Its thin and lightweight and is pretty easy to cut but I found no real value in it over the black poly film I got from Ebay. From 1 to 10 on visual use I would give it a 7.5. For photographic use I would give it a 6. Again it would be better than nothing and would make a good finder cover but it not produce good results when I tested it for photographic use. With the addition of a couple ND 1.0 filters, which brought the filter to ND 5, it performed pretty good visually and the orange yellow image was soothing to say the least. Overall it gives good visual views of the Sun in natural color. For those that like to sketch or draw the Sun, the visual version would be great.
Baader ND 5.0 or turbo film: as it is called is the best for visual use out of all the films I tried. It present a whitish blue image and also shows details at high mag. The details can be further enhanced with a Green #56 filter and Baader UV/IR filter. It is very thin and somewhat difficult to cut without wrinkling. My how-to section contains write ups on making solar filters with this type of material. It is really good for visual use and light photographic use. It is not so good for extreme closeup use because of the ND (Neutral density) 5.0 rating. This is the rating of how much light is transmitted. The smaller the number or ND the more light is transmitted. ND 5.0 is safe for visual use when used with the Sun. From 1 to 10 for visual use I would give it an 8.5. For photographic use I would give it a 7. Its good, it provides detailed views and if your mildly into photography then its a perfect compromise. Here's the deal with this film, if you use it photographically you'll have to use a slower shutter speed than if you use the baader ND 3.8 film. Slow shutter speed equals more susceptibility to atmospheric turbulence and more chance for motion blur. It will work but it will not work as efficiently as the ND 3.8.
Baader ND 3.8 photographic film: is the best film I have tried hands down, feet up. There does appear to be some variation in quality though. I have bought 6 sheets at different times. And had different results from each. I have tried it as marketed by Kendrick and also by Baader Planetarium. The stuff bought from Kendrick seems to vary and appears to have been cut from a larger sheet and rewrapped in an optical tissue. The Baader Planetarium stuff comes in a plastic sheath between two cardboard pieces lined with optical tissue. Both work great and both are relatively in expensive. This film is very thin and is difficult to cut because of its thinness. I found it easiest to use a razor or sharpened knife. It is very wrinkle-able. The wrinkles do not appear to affect the performance though. I have found it performs best with a UV/IR filter and Green #58 filter. I discovered this one day while experimenting with my Orion filters. Who'd of thought that that useless green #58 would ever come in handy. I had had it for 6 years and never really found a use for it until I used it with the Baader film. I shoot mostly in B&W so I do not really see the green color. It appear to suppress the UV/IR on its own and enhances the contrast greatly. You can image details at the smallest level. This configuration simply works! It provides a narrow band image around the Iron absorption line on the Solar Light spectrum and allows you to shoot details that cannot be seen with other films. The ND 3.8 rating is just right for close ups as well as full disk images of the Sun. I have found that it works best with aperture sizes from 7 inches to 3 inches so it covers a wide range. For visual use I would rank it as a 1 simply because it was not intended for this and because you would have to use secondary filters to make it safe. There are warning labels on the films containers that state it is not for visual use unless you want to burn your eye out. Ok I made up the eye burning thing but its clearly marked as not safe for visual use. For photographic use I would rate it at an 8.5 mainly due to the variation in quality between stocks and because it requires extra filtration to get the maximum benefits. Its good photographic stuff. Baader also makes a filter called a solar continuum filter and it can be used with this film. It provides about the same punch as the green #58 but is more expensive. I got one thinking it would be better than my Orion #58 but it was not. I got just as good an image by using the #58 as I did with the continuum. So what's the deal with the continuum? It looks cool that's what the deal is. Its silver on side side and light green on the other. It is what is commonly known as an interference filter. It rejects all the light except for a specific band centered at a specific point on the light spectrum. That point happens to be extremely close to the same one that the Green #58 is at. Because the Green #58 is so dark it effectively suppresses the other wavelengths of light just like the continuum does. When coupled with a UV/IR filter in front of it, it becomes even more efficient. The continuum filter has a green look to it and appears to be based on a green #56 filter with a metallic coating on one side. Either filter will get you to where you want to go but the green#58 does it with old school style while the continuum has a futuristic space age feel about it. I use both on occasion, it really depends on my mood.
Herschel Wedge filters: I'll discuss more of this type of filter in my "How-to build a simple Herschel Wedge" section of the website. But basically there are fewer filters involved with this type of system and its considered to be the most pure form of visualizing the Sun in its natural color. The wedge was first used in the 1800's by John Herschel and since that time has not gone through too many modifications. It works by passing 95% of the sunlight through the prism and reflecting 5% off the surface. The 5% is what you look at when using a Herschel Wedge system. You still have to filter it using a UV/IR filter and an ND 3.0 & Polarizer or ND 5 filter but the views are more detailed than with any film. Films tend to add distortion to the image. The wedge does not. This is one of the more exotic types of methods. Because it is not mainstream there are many that do not understand how it works or how to properly use it safely. The 95% exit beam is nothing to fool with. Its hot enough to cook an ant with and contains all the radiation of the sun, concentrated. Part of the reluctance of many to use this type of system is because its more expensive and there are plenty of people out there that have gotten burned by one because they were careless. The bottom line is that you can pay 20$ for Baader photographic film and get an excellent image or pay 600$ for a wedge system and get a slightly better image. Personally I've gotten images with the Baader film ND3.8 that rival wedge images. Its good stuff and its cheap but, that being said, the wedge still has a coolness to it that feels Old World and it performs best of all the filters. If your a do-it-yourselfer you can put a wedge together much cheaper than 600$. Or you can buy one that I have built for 150$. I'd be glad to sell one of the ones I have made.
Filtering: Understanding where the filtering falls on the solar light spectrum is important. It helps me understand what is happening inside the scope. To some degree what we are doing is putting together an illumination recipe. From the chart below we can see the main absorption lines for many of the popular solar imaging filters. For the purpose of this discussion we will be focused on mainly the Iron line. There are actually more than one Iron line in the real Solar light spectrum but for simplification I am just showing the one I like.
To be continued........................
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