Glen Deen 
Captures Martian Satellite

Deimos Photo

I took this photograph of Mars on the evening of August 19-20, 2001, and Deimos can be seen in the upper right-hand (NW) quadrant as indicated by the arrow.

The first clue that this faint image is Deimos is that it is round.  Since the telescope tracks Mars, stars appear to be elongated because Mars moves with respect to the stars.  With only a 2-minute exposure, the star elongations are small, but they are noticeable.  Even the faint “star 1” in the photo shows the same elongation as the other two bright stars.  Although Deimos orbits Mars, its period is about 30.3 hours, and since it was near maximum western elongation, it would show no transverse motion in a 2-minute exposure.

The next clue for identifying Deimos is its position with respect to Mars and the two bright field stars having magnitudes of 11.6 and 9.70.  For that we need an accurate star chart having modern orbital elements for both Mars and its moons.  (The orbital elements used to compute the Astronomical Almanac ephemerides for Deimos and Phobos are 90 years old).

The following chart was made with Guide 7.0 sold by Project Pluto.  One of the features of this powerful and accurate computer program is that it can superimpose the same field from the Digital Sky Survey (DSS) onto its charts.  This permits seeing stars that are not in the Guide Star Catalogue (GSC), because only GSC stars are plotted by the program.  In the chart below, there are three such stars: 116, 970, and 128 (left to right).

Those numbers are the visual magnitudes with the decimal points removed, and Guide 7.0 plots them beside their corresponding star symbols.  Inserting the decimal point causes these magnitudes to become 11.6, 9.70, and 12.8, respectively.  Notice that the black star symbols plotted by Guide 7.0 are superimposed on the white star images from the DSS field.  That was automatic.  All I did was specify that the chart and the DSS field be centered on the same star (SAO 185284) and that the field size be 15 arcminutes in both cases.  After I imported the DSS field into the Guide chart, I saved the composite image as a bitmap file, and I used a photo editor program to convert the file to a JPEG file and insert my yellow type annotation.

In this chart, the +D symbol is plotted at the position of Deimos, and the +P symbol is plotted at the position of Phobos.  The orange disk is Mars with its true diameter plotted to scale.  My photo shows that Deimos is just barely outside the edge of the glare of Mars, and so we can see from this chart that Phobos would be hopelessly lost inside the glare.

The DSS field shows a faint star between Phobos and Deimos, but that star is much fainter than “star 1”.  Since “star 1” is just barely visible in my photograph, it is clear that the image I am identifying as Deimos could not be a star.  Besides that, one can draw triangles between Deimos, the center of Mars, and either “star 1” or SAO 185284 on my photograph and on the star chart and see that corresponding triangles are similar.  In other words, Deimos is identified by its geometric position in the photograph from corresponding geometry in the chart.

Technique

I used a Nikon F1 camera at the prime focus of a 24-inch f/16 classical Cassegrain telescope loaded with unhypered (normal) Kodak T-Max P3200 black and white film push processed to ASA 6400.  I had the negative processed by a commercial photo lab for about $7, but I had the print made at my local Wallgreens’ one-hour photo lab.  I just asked the operator to center Mars (there is no exposed density in the negative to show the edges of the frame), pull the brightness up (under-expose the print) 6 f stops (the maximum), and make the color “battleship gray”.  He did not charge any extra for these custom services after I explained what was in the picture.

Glen W. Deen, AAAA
glen.deen@gte.net