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The Orion Mars Filter

Posted by Robert Piekiel   09/20/2007 07:00AM


With the next closest approach of Mars coming in December, many of us are already turning our scopes to the red planet and getting our early views. For years, astronomers have used color filters, typically red, orange, or blue, to enhance contrast on planets, especially Mars and Jupiter. Eeking out the most contrast of either the polar caps or the dark surface areas was usually a matter of swapping in and out a light blue or orange filter. What about a hi-tech interference filter that passes both colors at the same time?

That’s essentially what Orion’s new “Mars Filter” does. Sold in a 1-1/4 inch size, you can buy the special filter by itself, or in a 3-filter set that also includes a standard blue and orange planetary filter. Having received my set long before Mars’ closest approach, I’ve been going out late to watch it every week, and gave the filter a substantial test. Because Mars is currently rising late, I decided to try this new filter on other objects as well.

For starters, the filter itself looks pale green, and when you look through it, it passes a pale purple or pink glow – sort of a combination of blue and red. It first tried it out on the moon, using my Orion 120mm f/5 refractor. I chose this combination because I didn’t really need high magnification, and I wanted to see if the filter would add or subtract any color fringing to that which is usually present with these scopes. As I suspected, it gave the moon an unnatural pink color. Maria were a LITTLE darker, but no other bright features were really all that enhanced. Comparing the views of the Mars filter with the standard blue and orange, the moon was more pleasing and natural with the blue filter. Easier to see, less eyestrain.

Next target: Jupiter. Here, I tired several scopes, ranging from an Orion 120mm f/5 refractor to a Celestron C16. Naturally, I had to crank up the magnification considerably on the Orion 120 in order to get a sizable disc on the planet, but no matter which scope I used, the filter performed about the same. The normally white disc was an unnatural pink. Fringing was not a problem, but surprisingly, the equatorial bands were noticeably darker and easier to see than either the view with the blue or orange filter by itself. The contrast was better, with no discernable drop in overall brightness. Subtle details stood out more with the Mars filter, if you didn’t mind the pink hue of the planet itself.

Now on to Mars itself. For this, I used the big guns, my Celestron C16 (which is currently the biggest of my permanently –mounted scopes that has a good view of Mars as it is coming up in the east). Here the Mars filter really begins to perform. Because the planet is orange-colored to begin with, the pink hue added by the filter doesn’t become all that noticeable or objectionable. With the Mars filter, I was able to clearly see dark surface features as well as a small white polar cap quite easily, all in the same view. Normally, I’d have to switch between a blue and red filter to enhance one or the other, but with this filter, both features were enhanced. I would go so far as to say that the Mars filter brought out details better than either of the other two test filters by themselves. All without having to swap filters around. Stacking the blue with the Mars darkened the view somewhat and further enhanced the dark features. Stacking the Mars with the orange filter did not yield any further increase.

Because Mars was still low in the sky when I made these observations, I was not able to use more than 300x. Even with a tiny disc, I can say that the Mars filter does what it is supposed to do – but really only on Mars. I did not find it highly beneficial to other solar system bodies, particularly those that are white in color. It may be helpful with Mercury, but I was unable to make any observations of it at this time. Even so, details on Mars are often difficult enough to see, and therefore every bit of help is welcome. For $49.95, it’s actually a good investment.

Bob Piekiel