Building a Solar Funnel

This afternoon I went to the library to read the recent issue of Sky and Telescope. Lo and behold, there is an article on how to build a solar funnel. 

Coincidentally, I happened to have built one earlier this morning!

First, you need to get a funnel from the auto supply store.  Most funnels won't work.  The neck needs to be wide enough to accomodate an eyepiece from your telescope.  A Blitz Super Funnel (#05034) works perfectly.  I couldn't find one at any of the auto shops, but found one at KMart.

You also need a couple of hose clamps - a small one (1.5 inch) and a large one (5 inch). 

The one thing you will need to order is the rear-surface projection screen.  I ordered a Da-Lite High-Contrast Da-Tex rear-surface screen #95774.  I bought two pieces, each one-square foot in size, but you only need one (I assume problems will happen, hence the duplication). 

Step one, saw the funnel neck -- about ten inches from the large end, and seven inches from the small end.

Sand the rough areas.

Cut into the small end, about an inch or two deep in two places - this will allow a hose clamp to sqeeze this end and secure the eyepiece.

Insert the eyepiece of your choice - I used a 12.5 mm.

Use the small hose clamp to secure the eyepiece in place.

Wrap the rear-projection screen fabric on the large end and secure it with the larger hose clamp.  Either side of the fabric is as good as the other and it doesn't matter which side faces in or out.

It is best to use the solar funnel with a refractor, but I don't have one - so I used a reflector.  The problem with using a reflector to look at the sun is that the mirrors can be damaged.  Therefore, one has to reduce the amount of light going into the scope.  I drilled a 1.5 inch hole into the end cap used in storing the telescope, but this seemed to reduce too much of the light.  I ended up lifting the cap and holding it slightly ajar to allow more light to enter.

I didn't read the instructions in Sky and Telescope, but I assume they are sufficient. 

It may not be visible in this image, but the projection does so sunspots. 

Is it worth building one of these?  It is not as good as a direct view I get with the Questar or the Coronado PST, but it is sufficient for public viewing where two or more are gathered together.

Review of the Solarscope

I purchased a Solarscope in preparation for the Venus Transit, but also to use with elementary students at my wife's school.

These products are sold at www.solarscope.com.  The devices project the image of a slightly magnified sun onto a screen within a box, shaded from excess interfering light.

There are four editions, with the price ranging from $89 to $249.  The one that I really wanted was the "traveler's edition" which can break down into a smaller package for transport - but I did not think I needed to spend the $249 on this edition.  I wanted something more durable than the less expensive ones, so I bought the "wooden edition" for $189. 

As expected, it came shipped in a flat box and assembly was required.

The instructions could have been written with greater clarity.  Instead of making separate assembly instructions for each product, they are combined so that the reader has to be attentive as to whether a particular step applies to the Traveler or the Wooden Edition.

The instructions are also incomplete.  On the last page, Step 14 refers the reader to Step 5 in the Instruction Manual, and Step 10 refers to Step 15 in the Instruction Manual.  What is missing is any reference to completing the project are references to Steps 16, 17 and 18 of the Instruction Manual. It is not difficult to figure out how to complete the project, but it was disconcerting since one of these steps says, "Caution, once the screw cap is fastened, the lens cannot be dismantled!"

The terminology in the instructions are also inconsistent.  The instructions refers the reader to "The Instruction Manual," but among the documents, there is no such document.  The document in question is actually entitled "User's Guide."    The parts are also inconsistently labeled.  In the parts list there is a "lens tube" but in the text it is simply referred to as "tube." 

The instructions are not specific and each step must be carefully thought out during construction.  One of the panels is white on one side and tan on the other.  The instructions do not state this, but the white needs to face in, not out, in the construction.

Extra care should be taken in placing the panels into the grooves on the sides.

You'll need a hammer to gently tap the panels into place.

Be sure to check the intersection of the panels and sides to be sure there are no gaps, as is the case above.  This is easily corrected if done before the glue dries.

This tube-like stopper secures the mirror in place.  The problem is - which direction to insert it?

On one side it is solid, and the other side it is hollow.  My suggestion is to have the solid end opposite from the mirror.  That way it is easier to press the stopper all the way against the mirror, as I did with a screwdriver head as seen below. 

Once constructed, the Solarscope does everything it advertises. It is very easy to align for viewing.  I did find that it is easier to manage when the sun is lower in the sky, rather than at "high noon." 

Focusing is easy, but I did find one problem here.  I found was that there was too much play in the lens tube to suit me.  Maintaining focus was not easy because the tube tended to slide on its own. To resolve this problem, I took some parts I found in my garage. I took a plastic ring that was wider than the tube, drilled a hole in the side, and used a wing screw to build a device to secure the lens tube in place.

Don't expect the image of the Sun to be very large.  There is not much magnification power in the Solarscope.  It is sufficient to see the sunspots, but not enough to see detail of the spots.  It is perfect for eclipse viewing.  I am confident that it will be sufficient to see a transit of Venus.  I am not sure that it would be sufficient for a transit of Mercury - but this statement is purely speculation. The solarscope's web page says it is good for viewing transits of the smaller planet, so I would assume that is true - but for me the jury will be out on this one until 2016, when I hope to next see a Mercury transit. 

It will be excellent for use with school groups, and I look forward to the next time I spend a day at my wife's school with her students.

All in all, I rate this product very highly.  I would like to see them work on securing the focusing system, and would definitely like to see them revise their assembly instructions.  All in all, well worth the money.