This info may be elsewhere, but consulting my records from Bob Mylar and Steve Smith, here is the range of bit sizes they used for for 12-16 inch star globe/cylinders, along with a link table

magnitude 0-1 .076 - .089 48 - 45
magnitude 1-0 .046 - .063 57 - 55
magnitude 2-1 .040 - .033 68 - 60
magnitude 3-2 .028 - .025 71 - 70
magnitude 4-3 .0225 - .020 76 - 72
magnitdue 5 .0125 80




http://www.mytoolstore.com/irwin/numbers.html

There have been so many numbers giving different hole sizes for different stars that I thought it might be worth a formula for hole sizes based on the actual brightness as per magnitude. I first started thinking about f-stops and the like but magnitude is based on a 2.5 times difference which is more like 1 1/4 stops. Seeing the need for a formula that would provide hole diameters for any size of star ball and the decision to use lenses or not, I through up my hands and called my brother (PhD in physics). He mumbled a bit and came up with the perfect one.

D = Do(E.4605)(Mo - M)

D ~ Diameter of hole.

Do ~ Diameter of hole of Mo.

E.4605 ~ Natural logarithm to the .4605 power. The number should be raised a bit but I can't seem to do it here. (This might appear strange but remember we have a magnitude scale that is not base 10).

Mo ~ Magnitude of star drilled with diameter of drill at Do.

M ~ Magnitude of star we wish to find drill size for.

If your eyes are glazing over, I am assigning a # 79 drill for 5.0th magnitude which appears to be the Spitz A3P design.

Magnitude ~ Hole in mm ~ Nearest drill

Sirius ~~~~~ 7.011 ~~~~~~ "J"
-1.0 ~~~~~~ 5.832 ~~~~~~ #1
-0.5 ~~~~~~ 4.632 ~~~~~~ #14
0 ~~~~~~~~ 3.680 ~~~~~~ #27
0.5 ~~~~~~~ 2.923 ~~~~~~ #32
1.0 ~~~~~~~ 2.322 ~~~~~~ #42
1.5 ~~~~~~~ 1.844 ~~~~~~ #49
2.0 ~~~~~~~ 1.465 ~~~~~~ #53
2.5 ~~~~~~~ 1.164 ~~~~~~ #56
3.0 ~~~~~~~ 0.924 ~~~~~~ #64
3.5 ~~~~~~~ 0.734 ~~~~~~ #69
4.0 ~~~~~~~ 0.583 ~~~~~~ #74
4.5 ~~~~~~~ 0.463 ~~~~~~ #77
5.0 ~~~~~~~ 0.386 ~~~~~~ #79
5.15 ~~~~~~ 0.343 ~~~~~~ #80

Spitz added lenses to all of the stars of magnitude 2.5 and brighter. Basically any star hole larger then 1mm has a lens. Each one of these I checked appears to hold to the above table. If you did not want to use lenses but still wanted to limit the size of a projected star to a 1mm hole, then unless you found some smaller drills (which is doubtful both practically and optically) you would be limited 3rd to 4th magnitude.

With the above formula you can assign any diameter to any magnitude and then easily calculate any other magnitude star hole diameter.

The above chart can be used on any size star ball. Just remember that the smaller the ball the larger the stars will look projected. Spitz used the 18 inch diameter ball to hold the diameter on any star to 3/16's of an inch on a 30 foot dome. Thus for most viewers all of the stars would appear as points of light rather then circles.

you are writing the book on this subject ron!

i wonder now how many gauge needles are available, since i'm experimenting with needling vinyl rather than trying to drill metal (although this may change once i get owens BIG metal globe.

gare

Personally I'd go for the big metal globe.

I did find a chart for hypodermic needles at:

http://en.wikipedia.org/wiki/Needle_gauge

If Steve is out there perhaps he has a chart on the diameters of various types of sewing needles.

My thinking is that you might still want to use standard drills to bore the star holes. It would be easier (I would think) then punching with a needle and would also (I believe) make a cleaner hole.

Hi Ron,

I am getting aperture fever (big ball fever?) and planning to do my own star globe. I am having trouble getting your equation to work. I basically was using your example of the 5th mag star being a #79 drill and trying to recreate your table of drill sizes. Here is what I am doing:

Do = .386
Mo = 5
M = 2 (trying to replicate your entry for a 2.0 magnitude star)
e^.4605 = 1.5849

Therefore D = (.386)(1.5849)(5-2)

I come up with D = 1.8353. Your table says 1.465.

Am I doing something wrong or did the great internet swallow a factor in the equation?

Thanks!

Lee