This author goes to great lengths to bring you information of interest -- indeed, at this very moment he runs the risk of missing the final episode of 'Frasier' to insure the correctness of the following...ahem...

Everyone's been hooing and hahing about the upcoming Venus Transit, and how they'll watch it, and what solar filters are best, and no one makes one to fit my scope, and it costs so much, and boo hoo yadda yadda...horsefeathers. You can make your own with readily available materials, and the prices won't break you, either.

There are two popular sources for homemade solar filter material. Baader AstroSolar™ material is available from Astro-Physics. It's a silver mylar-style film material. Thousand Oaks Black Polymer is available from Thousand Oaks Optical, and is a black plastic film-appearing material.

How easy are they to work with? Very simple. Over the last few weeks I've been making filters for all my various oculars, working with nothing more exotic than cardboard, construction paper, binder clips, and double-sided tape.


The filters are easily constructed if you follow the instructions. Trust me, if a ten-thumbed compute-o-geek like me can whip these out, anyone can. Let's look a little closer...

The monoculars are little items I've hauled around for ages that are unfortunately no longer carried by the original source. They may be found on ebay. They're 8x21 monocular refractors (if you get lucky, you might find the one with two objectives, giving you a 6x21 power also). The objective lens shells are about 1" across, making it easy to build a filter. There's a cylinder of construction paper inside and outside the pie-wedge cutouts in a piece of cardboard, that slips over the end of the lens. Two thin 2" square pieces of cardboard (like shirt cardboard) have holes cut just a bit less than the lens diameter, so that the lens shell will not touch the filter material. The filter material is secured between the cardboard pieces, and the pieces are taped together. They are clipped to the piece that slips tightly over the lens. Depending on my preference of the day, I can swap filter material in seconds. I carry one of these in my ready bag that goes wherever I do (I'm a ham radio operator affiliated with the Red Cross), and the other one stays in my bag of astronomy books that I take to show'n'tells...it may not seem like it, but magnifications/apertures as low as 6x21mm and 8x21mm are more than enough to see sunspots.

The filters for the binoculars are constructed the same way. These are my 30+year old Tasco 10x50 binoculars from my days in the South Pacific.

As you can see, I had more than enough filter material to make doubles for most of my oculars. The Thousand Oaks was purchased off eBay, and the Baader film was actually recycled from a homemade filter for a 9" scope -- we found it while cleaning up at the club observatory. They were gonna pitch it, but yr obdt svt saw the potential and snagged it in a heartbeat...

Filters for larger scopes are easy, too.

Here's my first scope, the red 3" Tasco reflector, and the 'bowling ball with a smokestack', a Bushnell Voyager 105mm reflector. As you can see, I've made both types of filter fot them -- the Tasco is wearing the Thousand Oaks, and the Voyager has the Baader.

Construction methods were extremely similar, with only minor modifications when scaled up.

Here, you can see the frame that fits over the scope's aperture, with the two filters clipped to it for reference. The cardboard for the frame is X-cut only partly through to allow the wedges to slide tightly over the aperture edges. The filter of choice is binder-clipped to the frame. The frame looks a bit cocked only because of the weight of the filters pulling it off-center for
this picture. When correctly clipped on, the filter material plate lies flat against the edges of the aperture (again, the holes are cut smaller than the clear aperture so that the filter material is never touched).

It may surprise you, but the frame for this setup will fit more than tightly enough to securely stay on the scope if you measure and cut carefully. A couple dry runs during construction wouldn't hurt -- after all, cardboard boxes are cheap.

Now, what's the difference between the filter materials? Let's start with color.

I took my garden variety drug store 35mm camera and took a picture of the sun by holding each filter in front of the lens. When I got the film developed, I scanned the pics at max resolution and cropped them so that only the Sun was visible, then blew the pictures up in Photoshop so the solar image was a decent visible size. There were absolutely no color adjustments made to the pictures. As you can see, there is a definite difference. The Baader material is on the left, and gives a whiter more intense image. The Thousand Oaks is on the right, and is more yellow and a bit easier on the eye in this writer's opinion. Sunspots were plainly visible equally well through both filters.This is a case of personal preference, which one gives the more pleasing view. Neither one has the capability to show prominences, so don't expect these to perform like H-alpha filters...just ain't gonna happen. In my opinion, while neither filter is as rugged as your standard double-wall lawn'n'leaf bag, the Thousand Oaks material does seem to be a bit more stout. Check each manufacturer's website for respective handling cautions and material guarantees. I've checked with both manufacturers -- both make a version that's safe for photography but not for eyeball use. The denser visual materials (they cut the visible light down to 1 part in 100,000) are suitable for photography also if appropriate allowances are made in exposure times.

We've got about three weeks to go until the Venus Transit, and from information I have, both manufacturers are running about a two-week delivery time on sheets of material. That's just enough to mull this over for a few days, place an order, and stock up on cardboard boxes.

(OBLIGATORY DISCLAIMER: the author has no connection to Astro-Physics or Thousand Oaks Optical, other than that of a satisfied customer. The author has neither asked for nor received any remuneration of any kind from Astro-Physics or Thousand Oaks Optical. This review was written and submitted in the interest of public information only, and is solely the
opinion of the author.)

The preceding article is published simultaneously (except as noted) in the following places:

Phil Plait's Bad Astronomy Bulletin Board
http://www.badastronomy.com for the site in general
http://www.badastronomy.com/phpBB/index.php for the Board (see the General Astronomy forum)

The James Randi Educational Foundation Bulletin Board
http://www.randi.org for the site in general
http://www.randi.org/vbulletin/ for the Board (see the Science, Mathematics, Medicine, and Technology forum)

The Miami Valley Astronomical Society message board
http://www.mvas.org for the Society's site in general
http://www.mvas.org/discus/messages/board-topics.html for the Board (the General Astronomy forum)

and depending on space limitations, will also be printed in the MVAS's monthly newsletter The Amateur Astronomer
http://www.mvas.org/aa.html (past issues available on the MVAS site in Adobe .pdf format).

__________________
"If a tree is cut down in the rainforest, and is used to make paper to print a book, and the book is really bad, and there's nobody that will read it, do you still hear a sucking sound?"
Charlie in Dayton, A.AsC.

<grins'n'giggles bump>

__________________
"If a tree is cut down in the rainforest, and is used to make paper to print a book, and the book is really bad, and there's nobody that will read it, do you still hear a sucking sound?"
Charlie in Dayton, A.AsC.