I woke up on the morning of Sunday, July 13 for the express purpose of viewing Mars. I had only seen it once before during this apparition (due to both bad timing and bad weather); the only feature that I could definitely discern then was the southern polar cap. Of course, that was almost a month earlier and Mars appeared far smaller then.
In addition to my set of filters, I was armed with a recently purchased Baader Super Contrast filter. I was quite impressed by it during a demonstration in which it enhanced Jupiter’s belts without changing any of the planet’s colors.
After setting my equipment (a Celestron NextStar 8) and finding my target, I tried viewing Mars through the Baader filter, along with my red, light red, orange and yellow-green filters as well. (The later four are designated W25, W23A, W21 and W11, respectively.) Although each of these filters improved the image to some degree, the only feature that I could make out was a dark band along the edge of the polar cap (and even that was somewhat indistinct).
I wondered if there was any way of improving the contrast. Then again, a dust storm had begun about a week ago. Was I yet again a victim of bad timing?
I then had a maybe-bizarre idea: what if I stacked the filters? I had no clue as to the bandwidths of the various filters, but I figured that I should use two that were similar in color so as not to completely darken the image.
I attached the W25 to my 6mm eyepiece and then the W23A to the W25. It seemed that the darker features seemed to be more distinct. As a lark I added the Baader filter to the stack; heck, it wasn’t going to affect the color at all.
The final result: far more surface detail than before. I could see a large circular red region with a large dark arc to the south.
Since I hadn’t had time to align my scope before observing, I had to constantly slew the telescope in order to keep Mars in the field of view. (With the 6mm eyepiece, the magnification was at 339x.) This made any attempt to create sketch futile. Nevertheless, I kept a mental picture of the view and looked up the longitude in the July issue of Sky & Telescope. At 4:00 that morning, the Martian central longitude was about 85 degrees, which runs through the Tharsis volcano region. Although I could not see any of the volcanoes, the large dark arc resembled the portion of Mars south of Tharsis.
The reason that I inserted the question mark above is that I am not completely certain that the filter stack was the cause of this wondrous effect. First of all, as Mark Deprest has pointed out previously, the more you look at an object, the more detail you will see. Additionally, I noticed that the full moon had finally set below a nearby building shortly before the end of my observing run. It is quite possible that the reduction of scattered light could also have been a major factor.
Has anyone heard of this method from another source? If you have, or if you try it yourself and it works, please let me know.