Idaho Falls

Dramatis Personae:

This finely tuned optical instrument is aligned by twisting wires. Next time, add ballast and small sail to front for weight and wind balance, perhaps an old Tarheel yearbook from gracious host. Details below.

click images for larger view:


Keith. Dan


Keith, Dan


Keith, lens,
chewing gum
Sun image


Char, Keith


Near Totality

Details - Building This Was Half the Fun !

While Dan and I tested this after we assembled it the day before the eclipse, we pessimistically assumed that the round white image projected was merely the unfocused light passing through the tube. However, there were black speckles in the middle, that sharpened as we turned the projector lens focus knob. Were the speckles artifacts? We rotated the tube and lenses; the spots did not turn with the lenses. Houston - we have sunspots!

Since we could see an array of sunspots, smaller than 0.5% of the Sun's projected disk, we were hoping we could see lunar topography the next day. However, the cheap objective lens, or perhaps the mis-use of the projector lens, created significant chromatic aberration, obvious along the edges of the sun images pictured above. Red dark adaptation goggles would filter out chromatic aberration and help with pre-eclipse dark adaptation - but who sells them???

Alternately, some narrow-pass red theater gel in front of the objective lens can accomplish the same goal. Perhaps blue theater gel would be better for visual resolution, and it would keep the infrared out of the optics, but it would provide less dark adaptation the main event - totality.

Why not use a my cheap garage-sale reflector telescope? Transporting it both ways to Idaho would have cost $70, and a lot of hassle. I could carry the junkbox lenses to Idaho in a small box; the rest of the parts were easily scrounged. Further, a telescope (without a prefilter) would focus a lot of solar infrared heat on the secondary mirror, perhaps damaging it. A huge advantage of a using a DLP (Digital Light Projector) lens assembly as a secondary lens is that it is designed to handle very high intensity light. If you buy a lot of second-hand DLP projectors (I find them for $50, sometimes with less than 100 hours on them), you end up with a lot of broken projectors, a great source of Fun Optical Gadgets. The DLP lens also has focus and zoom, very useful for tweaking the best projected image.

Some claim "all you need is a simple pinhole camera". That's only good for making a fuzzy crescent, not fine detail (like sunspots, or lunar limb detail). If the projection surface is a meter from the pinhole, then a 0.5 millimeter pinhole creates only a 10 mm diameter, 20 pixel diameter image, illuminated at 0.3% Sun intensity. A meter-sized box will be buffeted by a modest breeze; the image will bounce around like a hyperactive child.

Our box was about 40 centimeters deep, and suffered less from wind buffeting than a larger box would have. It produced a focusable 500 pixel image at 20% Sun intensity. Yes, pinhole boxes are easy to explain to children, and to adults who have learned nothing since childhood. However, some of us aspire to more (and did, even as children), and thrill to walk in the same footsteps as Galileo and Kepler.

The great theorists were also competent experimentalists, and understood the nature of the data they turned into theories. Even Einstein built apparatus, and patented a refrigeration mechanism with Szilard. Most of the raw data produced by a measurement instrument is measurements of the instrument itself. Nulling out instrumental error requires understanding the instrument. Theorists who merely interpret data gathered by others do not know how to weight and evaluate empirical evidence, and elevate experimental errors into theoretical nonsense.

Eclipse2017 (last edited 2017-11-17 00:52:16 by KeithLofstrom)