Hello, everyone out there on the Internet and in print. This is the beginning of a (hopefully) regular series of articles on astronomy-related items that can be found on the web. The series will be posted on:
Phil Plait's Bad Astronomy Bulletin Board
www.badastronomy.com for the site in general
www.badastronomy.com/phpBB/index.php for the Board
The James Randi Educational Foundation Bulletin Board
www.randi.org for the site in general
www.randi.org/vbulletin/ for the Board
The Miami Valley Astronomical Society message board
www.mvas.org for the Society's site in general
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).
A short while back, I was at the MVAS dark sky site near Yellow Springs OH, enjoying a rare clear and steady Ohio summer night. The new scope (130mm Newtonian) was up and running, and Mars was the object of most everyone's attention as we were only a week or so away from the Mars opposition and close Earth approach. A discussion was being held with the club president about filters, and which one would do what to the view. El Presidente reached in his pocket, and tossed me something. It was a sample book from a theatrical supply house.
The samples were 1 1/2" X 4" swatches of gel filter material for theatrical spotlights. The filters are thin sheets of heat resistant polyester film, dyed with precisely formulated colors. I raised an eyebrow at this handful of colored plastic -- but since it was the middle of the night, my Mr. Spockian expression went unnoticed, and I had to ask what this was about.
The explanation was that these samples performed the same function as the colored eyepiece filters for a telescope. Use of the swatches isn't quite the same as screwing a filter into the eyepiece, but the effect is very close. Just lay the selected swatch over the eyepiece and look through it. The tough part is figuring out which filter does what.
There is a set of standards for telescope eyepiece filters that were taken from photographic filters. They are known as Wratten filters (Wratten is a registered trademark of Eastman Kodak), and specific numbers have been assigned to particular colors and light transmittal qualities. You can go here ( http://www.geocities.com/thombell/curves.html ) and see the various colors, the Wratten numbers, and a spectroscopic graph and short description of what wavelengths will be transmitted through the filter.
Now, what does all this have to do with these filter swatch books I got? Stay tuned...
The Wratten filters are used to block certain light wavelengths coming through from the Sun and nature to fall on film. The gel filters are designed to block certain wavelengths coming through from a theatrical spotlight to fall on a stage, and then to be seen with the human eye. The light sources used by each are of different color temperatures (refers to amounts of blue in the light), and are different distances from the light source. The purposes are generally the same, but specifically different. Therefore as far as I can tell, there is no correlation chart between these two types of filters, especially since the different gel filter manufacturers are free to make their own color codes and names for their products. There is a loose 'standard' set of colors, but nothing as tight and specific as the Wratten series. But all is not lost.
I emailed two manufacturers and told them specifically what I was interested in and why (I told them also that there could be an Internet article about this in the works). I contacted two sources:
Lee Filters USA home page http://www.leefiltersusa.com/index.htm
Lee Filters USA swatch book page (swatch books free of charge)
GAM products home page http://www.gamonline.com/
GAM Color specifications and color theory (interesting read!) http://www.gamonline.com/catalog/gamcolor/index.php
(GAM's catalog says that the standard GamColor swatchbook goes for $4.00, but they sent me one free...try, and see what happens)
Each manufacturer sent me a sample book with dozens and dozens of sample filter swatches. Everything was bound into a handy litle book (1 1/2" X 4") for easy access. Each sample had an accompanying paper page with manufacturer's info and a spectroscopic graph of its transmittal qualities (in the front of each book is a color representation of the visible
spectrum with light wavelength information, for reference with the individual graphs).
A little searching on the Internet led me to a couple of pages on filters for telescope viewing:
Observing the Planets with Color Filters, by Jeff Beish (former ALPO Senior Mars Recorder)
(this page gives the basics of 'why filters, and what do they do?')
Kodak Wratten Gelatin Filter Transmission Curves
(this page gives Wratten filter colors, numbers, and spectroscopic graphs of each filter listed)
So, seeing as how for now there is no manufacturer who's correlated their product numbers with the Wratten numbers, those who choose to use this "poor man's eyepiece filter" method is going to wind up sitting down for awhile and comparing general colors and specrographs to make his own little list of what sample matches what Wratten filter.
Again, it's not exactly like screwing a filter in the end of an eyepiece. This takes an extra hand (lessee...focusing, RA, Dec, holding the gel book, holding your beverage...yep, five hands total oughtta do it), but with the much wider range of filter colors available, your odds of finding THE filter for your viewing pleasure are greatly increased.