John's Kitt Peak dial and Shadow Sharpeners

[Robert Hough]

To John Carmichael, and all who have followed the progress of John’s plan to “do a dial” out of the shadows from the McMath-Pierce solar telescope on Kitt Peak, and all of those who have offered him advise on shadow sharpeners, penumbral effects on large dials, and so on.   I just got back in the loop after an absence of a couple of weeks, and have been catching up on all the e-mails.  Wow, things have really been happening.   First of all, congratulations John for the accomplishment of approval, and I am looking forward to helping on the project.  I would like to visualize the situation first hand, just as John likes to really check things out first hand. I haven’t been up to Kitt Peak for a couple of years, so I am going to take a little trip up there and just look at the site and see if I can anticipate any problems that we might want to solve first.  John, if you would like to go along, than would be might be great. Any time is good for me, as I am retired.   I have been experimenting with the shadow sharpener idea and have learned a couple of practical things. I started out with punching holes in cardboard and found out that fuzzy-edged holes punched into thick material, like the cardboard, doesn’t work as well as clean-cut holes cut through thin material. Thick material with small holes requires that you “aim” the hole, back up over your shoulder (without looking at the sun), pointing toward the sun more accurately than with thin material. So I wound up drilling 4 or 5 different size holes in the bottom of a shallow tin (tuna) can. They range in size from 1/16 inch, about 1.6 mm, out to about 4 mm. The smaller ones require a closer position, of course, as shown in the formula D = 0.047 * SQRT f .     I also found out that it is a good idea to put cloth tape on the edges of the tuna can, because it gets hot when it lays in the sun, as I learned when we measured the Flandreau analemmatic dial and I picked up a black carpenter’s square that had been laying in the sun and promptly put it back down trying not to acknowledge to the spectators that I had just made a dumb mistake.   I also was having trouble with the piece of white cardboard on the ground.  The reflection was too bright, it made you want to put on sun glasses, which is counter productive to seeing the faint images.  But I found that a piece of “fiber board”, which is a tan color, works better.   John, I have gone over the list of “stuff to take” and think we should add “eclipse glasses” so we can look at the sun during sunrise and sunset. I have 3 or 4 pairs (I used them to watch the eclipse the other night, it was very nice). I have most of everything else, but we ought to get together and sort it out.   Robert Hough ShadowMaster Tucson AZ