Establishing an albedo scale as a means to measure a formations brightness is an important tool to assist you in your study if Lunar Transient Phenomena (L.T.P.). It has been found that many of the reported L.T.P. that been reported as brightening might in fact be the normal appearance of the formation. It can be determined what the albedo ranges that a formation goes through by making measurement of the formation during the Lunation. This study will help you determine what the normal behavior of the features is during long lunar month.

There are a number of different techniques and instruments that can be used to assist in determining an albedo measurement. The British Astronomical Society Lunar Section has a device called the crater extinction device or (CED). It consists of two filter wheels with varied ranges of neutral density filter in addition to red and blue filters. The plans for building this instrument are located in the Guide to Observing the Moon by the British Astronomical Society, Chapter 9. Other observers have used black & white film developed to various degrees of darkness for extinction purposes. Others have establish a gray scale going to different gradients from pitch black to bright white, with different gray scales in between. This would then be held up to one eye while the other eye is peering through the telescope. I found an excellent example at the U.A.I. Lunar Section web site who are involved in LTP research as well. Albedo Gray Scale I have found the best way that worked for me to determine an albedo was using a variable density-polarizing filter. I purchase this filter through the Orion Telescope Center.

What ever you use, it’s important that you calibrate the device during full Moon. This is to be standardized against the features listed in Elgers albedo scale. The exciting part of this kind of study, happens when a lunar formation you been monitoring suddenly jumps up in albedo brightness for no apparent reason.

I calibrated my variable density polarizing filter by putting a blank address label on the outside of the cell and cut the label down in size so it neatly fits. I then place fine graduation marks onto the label before sticking it on the filter cell. This gave me the reference marks needed, when I began my calibration. At the next Full Moon I then selected the telescope and eyepiece I would always use when making my albedo estimates. I put the variable density-polarizing filter over the eyepiece and would rotated it until the selected feature was extinguished. At this point I would make a note of where the graduations lines were in relationship to each other and write this down on my calibration table.

This process took time and practice and I did not get a calibrated scale completed until several observing sessions at different Full Moons. After I was satisfied that the graduations reflected the true albedo I permanently etched the lines onto the cell with a scriber.

I did find that the variable polarizing filter does have certain limitations. It does not perform well when dealing with dark albedo features. You have to make the estimate of the darker features by comparing them to blackness of dark shadow. You have to use a combination of visual estimation as well as calibration of a standardized filter. Speaking with individuals who have used the BAA crater extinction device runs into the same limitations.

After constructing the chart practice using it, during your observations. This will help you in time become very knowledable about the features appearance over the lunation and make you comfortable with making albedo estimates. You will find by doing this you will no longer look at the Moon surface as you did before.

For each feature you need to choose the cardinal points on the crater. These locations are the north, south, east, west rim or wall, the central peak ,if there is one; and the crater floor. Also a nearby plain located north or south of the feature. It is important to continue to use the same nearby plain location for each albedo measurement. When making your measurement always use the same chosen points. When examining the lunar feature take two albedo reading 5 minutes apart during the observing session, then average the two reading together. When you make these measurement you can submit them on the LTP Observing form provided in chapter 10.

10.0 = central peak of Aristarchus.