Mars Opposition 2016
- Last Updated: 04 May 2016
By Dave Nakamoto
With the May 22nd Mars opposition coming up fast, lots have been written in the popular press and the Internet regarding it. However, I’m amazed that little has been written on the specifics of observing this event. I hope to clear up the air on this, and for that matter, any other planetary viewing event.
What Can You See?
The first topic is what you can expect to see.
This is the moon at full frame, with Jupiter to the right, and Mars below Jupiter:
In this simulated view, I tried to match the magnification you’d normally use in your “average” telescope with the types of magnifications visual observers use.
As you can see, Jupiter isn’t even as large as the largest craters on the Moon, and Mars is positively TINY. This puts a premium on several things, which I will address.
Your Telescope; More is Better (Up to a Point)
With regards to the telescope, the more focal length you have the better. More focal length means the telescope is capable of more magnification, and as the image above shows, magnification is NEEDED in order to see anything on the Martial disk.
More focal length usually comes with a larger objective lens or mirror, and this will help in resolving small details.
That being said, while this means larger telescopes, some perform better than others. Long refractors are the preferred scope, all other things being equal. The Garvey Ranch Observatory has a fine example:
Maksutovs can perform close to what refractors can do, and with a shorter tube, which reduces the headache of finding an adequate mount for a long, heavy tube. However, the large secondary can reduce the contrast between features on the Martian disk. However, I had good success with a 5-inch Mak from Orion telescopes during the last round of good Mars oppositions.
Cassegrain or Newtonian reflectors with a large focal ratio, typically more than f/8, can perform like Maksutovs and are also good telescopes for planet viewing, particularly Cassegrains with their short tubes compared to refractors with the same focal ratio.
Large Heavy Equatorial Mounts
But longer tubes and larger scopes need heavier mounts to maintain steadiness, and high magnifications require particularly steady mounts. Nothing is more useless than a large telescope operating on high mags on a mount too light to hold it steady.
Also due to the high magnifications, a true equatorial mount is almost a must, so Dobsonian mounts are not really suitable. In fact, except for computer controlled alt-azimuth mounts, no alt-azimuth mount is really adequate. Computer controlled mounts are fine. Any true equatorial mount is also fine.
But with any mount, electronic slow motion controls are needed. I’ve found that nothing is more frustrating than trying to center and maintain a small planet under high magnifications by moving the scope manually.
It is true that there are equatorial platforms for Dobs. These offer about an hour’s worth of tracking. However, there is still the problem of fine adjustments to the mount, and this is the reason even this solution is no solution.
Now things get a bit more complicated, but I’ll try and simplify the problems.
Obviously, you need as much magnification as you can get, but a couple of things put a limit on how much you can achieve. Two of the primary ones are atmospheric turbulence and your optics.
Turbulence in the air is called Seeing by astronomers. It can range from where you can’t see a disk at all (I’ve seen this) to no distortion of the view at all (I’ve also seen this). One way to measure it is to determine how much a star image spreads out. It can range from a few arc-minutes at worst, to zero, no spread. Most nights, the seeing is on the order of 5 to 50 arc seconds. Mars is, at its largest during this opposition, 18 arc-seconds wide, so seeing is critical.
Worse yet, it’s typically not the rolling motion, due to low frequency currents in the air, that is the biggest problem. For planet viewing, it’s the shimmering effect, which I call scintillation, that can ruin the view when you have to employ high magnifications.
Yes, often moments, perhaps lasting a fraction of a second or longer, do occur on most nights from time to time, allowing a brief glimpse of details on the planet’s surface. But the steadier the night, the more frequently this occurs and the better your chances of seeing faint details on planets.
Different locations offer different Seeing, so it can be vital, but predicting where Seeing is better or worse is not a science. However, there are some things you can do:
1. Avoid looking over paved surfaces, or even setting up on one.
2. Avoid looking over buildings.
3. Trees are your friend, as long as they’re not in the way, because they reduce ground heating and prevent it escaping off the ground at night.
4. Same with grass and other green living things.
5. Try not looking towards any source of direct lighting if possible. Turn off those lights if you can, and set up screens if you can’t.
6. However, do not shy away when the Moon is up, even if it’s full. In fact, the reduced perceived glare between the bright edge of a planet’s disk and the background can help your eye, and camera, to see details better.
7. This also pertains to clouds, if they’re not too thick.
What About Your Telescope?
As for your optics, they need to be near perfect. Mostly this means you need to have very good optics (no cheap scopes here!) and they must be correctly lined up, or collimated. There are many ways to determine this through procedures given through on-line sources so I won’t go into those here. With refractors this is moot; even if it is out of collimation, you’d need special tools and skills to correct it. With Maksutovs with the secondary provided by a mirrored spot on the primary lens, this is also moot. With SCTs, you can adjust the secondary, and again I’ll leave how this is done with on-line sources. With any reflector, you should learn how to collimate them since they tend to go out of collimation EVERY TIME YOU USE THEM.
How Much is Too Much Magnification?
There is a Rule of Thumb for how much magnification you can realistically expect given average Seeing and good optics. It is 20 to 30 times per inch of aperture. Obviously, this is a VERY general Rule of Thumb, given the tremendous variations in Seeing and optical designs, but it’s based on experience, and I’ve found it works.
For example, on the Garvey Ranch Observatory, 8-inches x 30x/inch = 240x. This means, most nights, a 10mm eyepiece or the equivalent is about all you can push.
On exceptionally steady nights, and depending on your optics, you can push it higher. For high quality refractors with f/ratios of f/15 or higher, this can mean up to 90x/inch of aperture! I managed to push it this high on Mars oppositions in the 1990s, on a 6-inch Yeagers refractor and using a 10mm eyepiece with a 2x Barlow, but this could only be achieved because the Seeing on those occasions was dead-steady Seeing and the refractor had excellent optics.
Be aware of too much magnification. Everyone can delude themselves that they see more, but you really can’t. If you spread the image out more than the telescope optics can perform at, you’ll see less. Sometimes the best views are at medium magnifications, where the Seeing would support it.
Is Video the Answer?
The only thing single shot images can give you is the chance to increased contrast and see more that way. Single shot images won’t see more detail due to Seeing, and cannot defeat Seeing unless you just happen to catch a moment where the atmosphere is steady.
Video is a little better. Seeing effects can be reduced through stacking, which tends to minimize, but not eliminate, noise and effects that mimic noise like Seeing. As an example of this, this is a single frame from one of my vidoes taken during the 2015 Jan 23rd triple shadow event on Jupiter. There are three shadows and two moons in front of Jupiter, but none of the frames could show it:
And here is what appeared after stacking a few hundred images:
But note the fuzziness of the image. Stacking can improve images to some degree, but it cannot completely eliminate Seeing and optics.
The Final Word
So, what can we conclude?
1. Seeing details on Mars will NOT be easy.
2. For your telescope, size does matter, in terms of aperture and focal length, in order to get the most magnification you can.
3. The mount does matter, especially in steadying the shaking, and having slow motion motors to make those pointing corrections possible.
4. The optics do matter, especially collimation or optical alignment.
5. Location matters to reduce bad Seeing.
6. Magnification does matter, but beware of using too much.
And I guess the final word is, JUST GO OUT THERE AND TRY ! No matter what scope you have, try and see as much as you can. And for this, you need to observe a long as you can. Get comfortable. Arrange to sit at the eyepiece or computer.
Visually, you’re waiting for moments of steady Seeing. Even in a small telescope, some details can be seen, albeit not too small.
With video, you need to take as much as you can, and for a minute or more in order to get hundreds of images for a good stack.
Seeing changes; it usually gets better as the night progresses. Be patient.
Mars changes, slowly. Its rotation rate is slightly longer than earth’s, so in time the features do change, night after night. It takes about a month to see all of it, if you observe at the same time each night.
The next time Mars will be at opposition is 2018, when it’ll be 24 arc-seconds wide, not 18. By then, who knows what technology and the market will bring.
Clear and Steady Nights !
--- David I. Nakamoto