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Sunday, May 4, 2014

Mars Observing Hints


If your astronomical juices get flowing every time Mars comes near Earth, then perhaps it's time you learned a few Mars observing tips. After all, you wait over 2 years between Mars oppositions, and Mars only stays near max apparent size for a scant few months each time. So get ready.

Go Long

First suggestion: "Go Long!" 

Say what?

I mean pick an instrument out of your telescope arsenal that has a long effective focal length. The longer, the better. Why?

For one, it's to get the most magnification out of your eyepiece selection. Mars isn't a gas giant after all. At best Mars gets as close as around 35 million miles to Earth, and that's not often. Mars is only a tad over 4000 miles in diameter, about half the size of Earth. And when at closest approach, it doesn't stay around long. Earth laps Mars in their respective orbits and rapidly pulls ahead.
http://www.cafepress.com/keendesigns/7955109
Mars, when on the opposite side of the Sun from Earth, is only about 4 arc-seconds in apparent size. That's about the angular size of a 4 mile diameter crater on the moon. That's small. Don't expect to see much detail when Mars appears that small.

At best, on the infrequent very close approaches, Mars can get to about 25 arc-seconds in size, a size that though small, can yield some tantalizing details to the determined observer. Usually, Mars ranges from around 12 arc-seconds to perhaps 20 arc-seconds at opposition. At the time of this writing, 2014, Mars presented a disc of only about 15 arc-seconds at opposition. So, you'll need magnification, at least 150 times. 200 to 350 times even better.  

Another thing about long focus telescope is that optics of longer focal lengths tend to have less optical anomalies, mostly because long focus optics are easier to make accurately. Longer focus optics in refractors have less color distortion, and in both reflectors and refractors long focus optics have less curvature of field. The depth of field of long focal length telescopes makes it easier to achieve optimal focus since focusing isn't as sensitive. And as it happens, longer focal length optics, because of the ease of construction, tend to be cheaper than short focal length optics made well enough to perform at high magnification.

One thing in your favor is that Mars is quite bright. It's not that much further from the Sun than Earth, so it gets lit up pretty well. So you can push even a small telescope to at least 200 times and still see a nice image. I recently spent an evening looking at a 15 arc-second sized Mars image with a 60x1000 mm refractor, and was able to push it to 200 times without difficulty. The image was amazingly good. I was able to easily spot Syrtis Major, and some hints of other features near the opposite Mars pole.


What are some good telescope options?  A long focus refractor will do nicely. It can be as small as 60mm, like my Long Focus Carton Refractor that I constructed from parts. Most of today's 60mm scopes, while good in optics, come with pretty shaky mounts, so shop carefully or consider making your own mount. You may prefer something a bit bigger than a 60 mm telescope,  say the Orion 9024 AstroView 90mm Equatorial Refractor. On it's equatorial mount, it might be a more capable long term choice.

Long focus Newtonians make excellent and inexpensive planetary telescopes. A six inch f/8 DOB, like the Orion 8944
SkyQuest XT6 Classic Dobsonian Telescope, makes a great all around performer, and performs on the Moon and Planets very well. Even a 4.25 inch DOB, like the Star Gazer Steve 4 1/4 Inch Planetary DOB will do a respectable job on planets.

Maksutov Cassegrains like my ETX 90 work well on Mars and other planets as well. A bit bigger Maksutov that's attractively priced is the Celestron NexStar 127SLT Mak Computerized Telescope. I know, I said go long, and the NexStar 127 or its cousin the Schmidt Cassegrain NexStar 125 appear to be very short. But because of their Cassegrain designs, these telescopes have very long effective focal lengths, great for high magnification. It's only the optical tubes of the Cassegrain designs that are short.

Go High

If possible, observe Mars when it's significantly higher than the horizon, at least 30 degrees. The higher Mars is in elevation, the less atmosphere you are trying to observe through, and the better chance the image will be stable. Take a look at the Seeing And Transparency tutorial to see an animation that shows what poor seeing looks like. Transparency, or how dim of objects you can see, is not particularly important in Mars observing. Sometimes good images occur even when transparency is poor. But you definitely need the good seeing of a stable atmosphere.

Go Late

Mars: ETX 90
If you can, observe late. Of course, you have to observe when Mars is up. But early in an opposition when Mars is rising late, you'll likely get better views. After midnight, the atmosphere often settles down from the churning heat of the day. I obtained my best Mars photo, shown at left, by taking it past midnight at 3 in the morning.

In truth, I started much earlier in the morning than that, perhaps around 1 in the morning. But I was using Barlow projection into a Modified Webcam Astro-Camera, and the field of view of the combination was very small. It took me the two hours to finally get Mars visible on my computer screen so that I could take the photo. But it was worth it. Not a bad image for only a 90mm telescope, yes?

The photo does illustrate that you can definitely get decent results with modest optics, if they are of good quality and seeing is good.  On that 2003 opposition, Mars was near it's max possible 25 arc-seconds in apparent size. Thus the amazing detail for the small telescope.

Be Patient

To wring out the most planetary detail, be patient. For one, your telescope may take awhile to acclimate to the outside ambient temperature. For another, the atmosphere, sometimes even on an unsteady night, may have periods of clarity. So keep looking for those windows of opportunity. Also, once you've noticed the most obvious details, start looking for more subtle ones. You'll be amazed at what you eventually see that you didn't previously notice.

Use A Color Filter

This is not a necessity, but color filters, like those in the Celestron 94119-10 1.25 " Eyepiece Filter Set, can be a real help. Red is the color often recommended for Mars, as the red of the planet will go through the filter, but the darker areas, which can have a blueish tint, tend to get blocked. Thus the non-red areas look darker. On the subject of color filters, yellow is often recommended for observing Saturn and Jupiter.

Try Sketching 


I'm not a particularly good sketcher, though I've had my moments. The images above show one of my best efforts. On the left is a sketch I made of Mars during the very favorable 2003 opposition. I used my 6 Inch Planetary DOB to make the the observations for the sketch. The image on the right is the same as the previous Mars photo. It's the image taken with my ETX 90, and it was taken about 2 days after I did the sketch. You can see that I managed to sketch the major features fairly accurately, as confirmed by the ETX 90 photograph.

You might check out this Astro-Sketching tutorial for some hints about that particular skill. The sketcher in the tutorial took on Moon craters, a much more difficult topic for sketching.

So why sketch? For a couple of reasons. One, you document your session, and some years later you'll enjoy reviewing your sketches, as well as showing them off. Two, it will make you a more attentive observer, able to eke out those elusive details.

Try Photography

Planetary photography is not particularly easy, other than the occasional times you might be able to hold a digital camera up to the eyepiece and grab an image. Even that is hard enough, and doesn't give the best results. My best images have been obtained by using converted Webcams, both a homemade one and a commercial one like the Orion 52175 StarShoot Solar System Color Imaging Camera IV . I have an earlier design, the Celestron NexImage. With it I've managed to get some very nice Moon and Planetary pictures.

But planetary photography is meticulous and time consuming work. The main difficulty is getting the planetary image to show up on the small CCD of the camera. If you don't have the system in relatively good focus, you can have a planet go right through the field of view and not even see it. Once you get a planet into the field of view, obtaining images isn't so bad. My procedure is to take movies (avi files) of perhaps 20 seconds long. The camera takes 10 pictures per second, so that gives me about 200 images to work with from each avi file.

I use a Yorick Language program I created to process frames from my avi files. The program lets me find the highest correlated frames of the movie, then align them and average them. The final output can have contrast adjustments made to bring out the most detail. More time is spent at the computer working with the images than is spent taking them. A program that's available online for processing digital images is RegiStax. It is similar to my Yorick program. It has more features, but I enjoyed the challenge of writing the Yorick program.

The tedious aspect of taking and processing planetary images is another reason you may want to consider sketching. With sketching you get to spend your time observing, and not twiddling with equipment. Even if your sketches come out worse than you'd hoped, you'll still have the enjoyment of having had some great observing sessions.

Hopefully some of the suggestions will help you get the most out of your next Mars opposition. Many of the hints work for Saturn and Jupiter as well.  And the biggest suggestion I can make is to observe as often as you can. You never know when an exceptional night offers up marvelous observing, and it would be a shame to miss those rare opportunities.

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