Making a screw-in Solar Filter

[hollis_f]

What with this year being the last opportunity for a transit of Venus, an annular eclipse for some Americans and the approaching solar maximum producing a lot of sunspots, it's likely that more people will be interested in shooting the Sun.

First off - Safety is paramount. Looking at the Sun with the naked eye can be dangerous. Using magnifying optics can be very dangerous. Do not be tempted to try ND filters, smoked glass, exposed film, Mylar, crisp (potato chip) packets or any of the myriad other items that I've seen recommended. Even if they block most of the visible light there's a good chance they'll let all the retina-frying infrared through. Use something designed for the job.

Now, there are a few specialised filters that cost several hundred Pounds/Dollars/Euros. Luckily there is also Baader AstroSolar Film. This is a very thin plastic/metal film that does a great job of blocking all the nasty radiation from the Sun. And an A4 sheet of it costs just £20. That's enough to make a couple of 77mm filters and one big one to fit a super-tele lens or a telescope. Today I want to describe how I make the 77mm filter. Later in the year I'll show how I make a large filter for my 300 f2.8 (these big ones aren't easy to store, so I'll wait until a week or so before the transit in June).

[hollis_f]

To make a 77mm Solar Filter I got a sheet of the AstroSolar film and two 77mm UV/'protective' filters (I found two Hoya Pro1 second-hand for a good price). First, in one corner trace around the outside of one of the filters - like this...


(The larger circle is for my 300mm lens).

Then cut out the circle of AstroSolar film. You're going to want some overlap, so make sure you cut outside of the circle you've traced earlier. Now, make sure the surfaces of the filters are clean (you'll not be able to clean the interior surfaces later).



Place one filter, thread down on a table and carefull lay the film circle on top - making sure there is some overlap all around. Now, carefully lower the second filter, thread-side down, onto the first and screw them together. Don't worry if the film becomes wrinkled. You should end up with a filter-film-filter sandwich, with bits of film sticking out all around.



OK, it's time to test it out.

[hollis_f]

Set your camera up on a tripod looking outside at a fairly bright scene, without the filter, and take a shot. This was ISO200, f4, 1/2000s



Now, the filter should reduce the exposure by around 15 stops. This was ISO200, f4, 30s which is around 16 stops.



What we're looking for here is any sign of leakage, where an ill-fitting filter, or a hole in the film, is letting light through. This looks good so it's time for the final test.

Attach the filter then set the camera up on a tripod on a bright sunny day. Or, if you're in the UK, when the cloud is thin enough for you to be able to see the Sun. If your camera has liveview then use it. Sweep about until the Sun appears on the LCD. It'll be a grey circle, small or very small depending on your lens in a jet-black background. Now you're ready to Shoot the Sun.

You'll want to focus manually (liveview with zoom can really help) preferably on any sunspots that are present. Don't use automatic exposure, as with the Moon it'll be horribly overexposed.

Here's my effort from the morning with my 100-400. ISO400, f11, 1/1000s. This is a crop from the centre of the frame. You'll need around 1500mm on APS-C for a full-frame shot.

[SteveInNZ]

Good idea to post something about how to do it safely.
Another method is to use the adapter ring for a Cokin style filter holder. Put glue on the flat surface and place it down on the film so you end up with flat film and no gaps (which are a pain). Then just screw it in like a normal filter. Don't bother trying to make a Cokin style (square) filter though. You need the light-tight seal around the edge.
You can make a simple solar finder with a cardboard tube mounted in the hotshoe. Watch the shadow on the ground until you can see the hole and you're pointed at the sun.

[j4cub]

Very nice work. I'm going to order a piece of the film and give it a try.
Tim

[ConEd]

Perfect Idea.
If you allow i will copy that
I'm also looking for such an solution, but here in Austria the Venus Transit is not visible in its full lengh.
And i don't know if the weather is fine on this day.

[Celestron]

I agree with SteveInNZ , also i might add that the Baadar film should be wrinkled to a certain amount anyway cause that allows for expansion of the film as it gets hot and it does get hot ! If it's tight the heat expansion can cause it to tear while viewing and that second can cause permanent eye damage and possible blindness that is not reversible ! So take caution no matter what !

[Jay-Bird]

Bump for a thank you on how to make this! Why not just sandwich the filter between the lens and 1 UV filter? Is it strong enough to scratch the lens?

[hollis_f]

Originally Posted by Jay-Bird

Bump for a thank you on how to make this! Why not just sandwich the filter between the lens and 1 UV filter? Is it strong enough to scratch the lens?

The Astrofilm is a very thin film. If you were to sandwich it between the lens and a filter I'm doubtful that it would be reusable after it was removed. Using the two filters means you can keep the whole thing assembled for re-use.

[Jay-Bird]

Originally Posted by hollis_f

The Astrofilm is a very thin film. If you were to sandwich it between the lens and a filter I'm doubtful that it would be reusable after it was removed. Using the two filters means you can keep the whole thing assembled for re-use.

Of course, that makes too much sense for me to figure out! I am going order me up some film! Thank you!

[rburbank]

While looking for help in obtaining a solar filter, I came across this thread. So I ordered the film, purchased some used UV filters and just finished a successful test shot of the sun (I followed all the instructions as written). I live in the Seattle, WA and will head down to the Oregon/California border next weekend for the annular eclipse. Now that I have the tools, I'm really excited at the prospect of getting some hopefully good images. Thanks to Hollis_F for the great instructions; I'll let you know how it all goes.

[Celestron]

Originally Posted by rburbank

While looking for help in obtaining a solar filter, I came across this thread. So I ordered the film, purchased some used UV filters and just finished a successful test shot of the sun (I followed all the instructions as written). I live in the Seattle, WA and will head down to the Oregon/California border next weekend for the annular eclipse. Now that I have the tools, I'm really excited at the prospect of getting some hopefully good images. Thanks to Hollis_F for the great instructions; I'll let you know how it all goes.

Just keep it protected so it doesn't get damaged .

[vandarix]

The solar filter sheets are in the mail, will pick up some filters to make this with and will test it tomorrow hopefully. Cheers.

[Regulus12]

Thanks for posting this DIY. I see that they have 2 film options for the Baader astroSolar. A visual and a photo only. Am I correct in assuming that using the photo type for observation is dangerous, but the observational film, while not ideal because of the reduced light transmission, can be used for photography? Have you used the denser film for photographic work?

[PM01]

Are you sandwiching the solar film between two pieces of glass? Or even a single piece of the UV glass?

I went away from using the solar film - too many pinholes over a short time. Went with the Baader Ceramic Hershel wedge instead on my TEC160FL.