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Imaging the Enigmatic Mercury


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nrothschild Neil is an expert in several areas, including camera support Registered since 25th Jul 2004
Thu 23-May-13 01:36 PM | edited Sun 26-May-13 07:14 AM by nrothschild

Of all five naked eye planets, I find Mercury the most interesting. Compared to Mercury the other planets are quite staid and predictable. They just go on doing what they do, and don't appear to vary much as they follow their respective orbits.

Mercury is different, and why I find it so interesting. While it is predictable if you understand the mechanics, it can be very confusing to those that do not understand those details. I'm going to provide a long story and a short story

The Short Story

If all you want to know is how to best image Mercury, just do the following

1. Determine the date of Greatest Elongation (greatest distance from the sun) in the current or future appearance of Mercury. Try to shoot it as close to that date as possible. Within about a week each side is best, and the week leading up to Greatest Elongation is superior to the week after.

The next several Greatest Elongations:

6/12/2013 Evening
7/30/2013 Morning
10/9/2013 Evening
11/18/2013 Morning
1/31/2014 Evening
3/14/2014 Morning
5/25/2014 Evening
7/12/2014 Morning
9/21/2014 Evening
11/1/2014 Morning

2. In the Northern Hemisphere, favor January through March for evening appearances and July-September for morning appearances. In the Southern Hemisphere, reverse that- July-September for evening appearances and January-March for morning appearances. But don't be too picky or you may never find the perfect date, weather and location opportunity. Use favorable apparitions to prioritize things.

3. Favor longish focal lengths. On FX, 70mm and up, 200mm is very good. If you ignore this, and you can see Mercury but not find it in your final output size image, go longer next time. But be quick- it's called Mercury for a reason!

If your primary objective is to image Mercury then I think a longer focal length composition is almost always better anyway, in order to better highlight the planet. Otherwise you end up with a tiny faint dot near the horizon.

The Long Story

Mercury orbits the sun in 88 days. That is the length of a year for an observer positioned on Mercury.

Edit: the length of a day on Mercury is equal to two orbits around the sun. Just defining precisely what a year is, or what a year means on Mercury is an interesting discussion .

Of more concern to us is the Synodic Period. For the two inner planets, that is the length of time required for Mercury (or Venus) to go from Greatest Elongation (separation from the sun) at evening dusk to the successive Greatest Elongation at evening dusk. Or morning dawn Greatest Elongation to the successive morning dawn Greatest Elongation.

Mercury's Synodic Period is 116 days. If you miss this upcoming Greatest Elongation on June 11, 2013, you must wait 116 days for it to happen again.

The Synodic Period is essentially one orbit, from our Earth based perspective, relative to its position to the sun.

Greatest Elongation
This is the angular distance between the object and the sun, measured in degrees. For the outer planets, they all reach opposition once each Synodic Period and they are, by definition, separated from the sun by almost exactly 180°.

The two inner planets (Mercury and Venus) are different. They travel away from the sun for a certain angular distance and then turn around and head back into the sun, where they repeat the process on the other side - giving us alternating dawn and dusk appearances.

Mercury never gets very far from the sun. Due to its highly eccentric elliptical orbit, its Greatest Elongation varies considerably, from about 18° to 27°. The greater the elongation the further the planet is from the glare of the sun (the dusk or dawn glow).

That dawn/dusk sky glow is both your friend, as a landscape photographer, and your enemy as an astrophotographer. Each time you shoot Mercury you will get a unique image, balancing those opposing interests in different ways. When everything works, cherish the image(s). It may be years before you equal or better it. That is the magic of Mercury.

All else equal (it never is!) greater Greatest Elongations are better than shorter/lesser elongations.


Seasonal Factors
If you've read my posts about chasing new moons then you know that there are, on average, better and worse annual seasons for shooting them. The typical 24 hour moon has an elongation of about 11°. A 48 hour moon, about 22°.

We can see from the above that a poor Greatest Elongation of 18° will put Mercury, on the best day, slightly closer to the sun than a two day crescent moon. And if you are clouded out or have a Hot Date, and have to shoot it many days later or earlier, it only goes downhill from there.

On a good apparition the elongation will get to 24-27°. In that case it is similar to a full 2 day moon or a bit better.

Both the Moon and Mercury roughly follow the Ecliptic, the imaginary line in the sky of our sun's path. That line of the ecliptic is tilted relative to the horizon. For observers near the equator it always rises nearly straight up and they are blessed with a very favorable ecliptic in all seasons.

As you move up in latitude to the mid latitudes of 40-50° where people tend to congregate, the ecliptic takes on a generally southern tilt in the Northern Hemisphere and a northern tilt in the Southern Hemisphere.

That tilt varies by season. In the northern mid-latitudes, at evening dusk, the tilt is most favorable in January-February when it is near vertical. In the following months the tilt rolls back over and becomes poor again. December, March and April, or so, is also reasonably decent. It is poorest in and around August when it may be impossible to get a usable shot or even a decent visual observation.

In the north(ern hemisphere), at dawn, that time of favorable ecliptic tilt is 6 months later/earlier, around July-August.

Even though you might have a 20° elongation on a given day, in the worst season (August at evening dusk) you may have Mercury only 10° or less in altitude, at sundown.

In this case Mercury is likely never seen during the entire apparition because it always sets before it is dark enough to stand out from the dawn/dusk sky glow near the sun. As a very general rule you will not see Mercury until the sun is 6-10° below the horizon.

It is actually possible to observe Mercury in full daylight with a high powered telescope. So the idea of when you can first see Mercury at dawn/dusk is a murky one, depending on the optical aid. Here my context is naked eye viewing, and imaging with short to moderate focal lengths (roughly 200-300mm and wider).

In the Southern Hemisphere mid-latitudes, such as Sydney, Australia, those dates are "reversed" by 6 months. August is the most favorable time for evening dusk and January-February most favorable for morning dawn.

Orbital Tilt

Although only half as tilted as the Moon, Mercury wanders about 2.3° to each side of the ecliptic. In the Northern Hemisphere, with a usually tilted ecliptic, that can cost a couple of degrees of altitude (or provide a 2° bonus when it is north of the ecliptic).

Edit: I'm having trouble getting a number that makes sense. In looking at some time series models, I see deviations from the ecliptic of up to 5°, and it could go as high as 7°?.

As I have discussed in my new moon posts, the orbital tilt can be even more important that the seasonal tilt of the ecliptic. This is why I depend on astronomical mapping programs to help sort things out.

Magnitude (brightness)

As Mercury leaves the morning dawn sky and passes the sun, it is on the far side of the sun. The distance to Mercury is then equal to the distance from Earth to sun plus the distance from the sun to Mercury.

You would think that Mercury would then be relatively dim, but it is not. As it passes the sun it is as bright as it will get.

Both inner planets display phases to us, similar to the moon. When Mercury passes the sun on the way up into the evening sky it shows us a full phase and is technically brightest. Except we cannot see it because it is in the glare of the sun.

When Mercury reaches Greatest Elongation on the far side of the sun it has a gibbous phase, making it still fairly bright.

After Mercury then passes the evening dusk Greatest Elongation and starts its slow decent to the sun it is increasingly closer to us than the sun. As a result we see a mostly "back lit" view, and a surprisingly thin crescent in a high powered telescope. That thin crescent is relatively dimmer than the gibbous phases, similar to the crescent moon within a few days past new.

For that reason Mercury is best viewed before and during the evening Greatest Elongation, not on the way back to the sun and then the morning dawn sky.

For dawn apparitions, the situation is reversed. Mercury is dimmest as it first rises toward dawn Greatest Elongation (again on the near side of the sun and a back lit crescent), and then brightens as it approaches greatest elongation. It continues to brighten after the dawn Greatest Elongation but it is sinking further into the dawn glare.

As an example, consider the current May-June evening apparition of Mercury:

On May 12 Mercury passed behind the sun and achieved magnitude -2.3.

On June 5, a week before maximum elongation, Mercury will be Magnitude 0.

On June 12, at maximum elongation, magnitude 0.5

On June 19, a week after maximum elongation, magnitude 1.1

In the two weeks straddling Greatest Elongation Mercury will drop 1.1 magnitudes in brightness, or about 1 2/3 stops. Considering all the difficulties of observing and imaging Mercury in the dusk glow, that 1 2/3 stops can be the difference between success and failure.

And of course, Mercury is incredibly bright when we cannot see it .

Using a sky mapping program

All the above may or may not make much sense, but it should impress upon you that a good sky mapping program is essential for planning Mercury observations and imaging. A good app like Sky Map Pro or the freeware Cartes Du Ciel will show you how high Mercury will be at any time, including the critical window after sunset or before sunrise, the slope of the ecliptic, and most importantly the altitude and magnitude. If you know altitude and magnitude and can relate to those numbers then you have a good idea of your probability of success.

My Experience

To date I have imaged Mercury 7 times, during 4 different apparitions.

On 6 of 7 occasions I imaged it within a few days of Greatest Elongation (19-20° elongation) and in at least one case on the very day. None of those apparitions were particularly favorable, with Greatest Elongation between 19.3° and 21.5° of a possible 27° or so.

In each case I was successful imaging it, with an easily visible "star" even on a web size final output, usually around 70mm (earlier on DX, later on FX).

On April 26, 2009 I imaged it at 50mm DX (20° elongation) with reasonably good success. It recorded as what I consider the minimum useful brightness for an uncropped web output size image.

My least successful shoot was on March 27, 2010, 12 days prior to Greatest Elongation, at 13.4° elongation. I was with about 10 other people and they were more interested in going to dinner than imaging Mercury. I shot one frame at 70mm FX but it was not enough. Mercury is there, but not apparent in an uncropped web size image without moderate contrast stretching. I distinctly recall at the time regretting my use of the 24-70/2.8. If I had had the time I would have retrieved my 70-200 and shot it as close to 200mm as possible.

I shot that apparition again on April 6, this time by myself . Two days before Greatest Elongation, the elongation was 19.1°. I got a fairly decent image at 44mm FX. I was trying out a new site, convenient to my home. The image would have been improved if shot around 100mm or more but I needed about 44mm for the composition I wanted.

Based on careful study of hundreds of images shot on those 7 occasions, I arrived at a rule of thumb that I want 20° elongation and at least 70mm FX. I can violate that, but with much lesser success.

High Power Telecopic Imaging

Outside the scope of this post, which deals mainly with "landscape" type short focal length imaging. I will only say that the severe issues of poor seeing when imaging at great magnification near the horizon dictates that it be shot within only a few days of Greatest Elongation, and on a favorable opposition more in the range of 24° elongation or more if possible. That might be a once in a decade opportunity, or so.

The few visual observations I have done at high power resulted in such a wavering, quivering mess that I did not even consider shooting it. I should, though, just to do it .


An interesting pair of upcoming Greatest Elongations.

September 4, 2015

This, despite being a most favorable possible 27° elongation, happens at the wrong time of the year for the Northern Hemisphere mid latitudes. If it can be observed or imaged at all, it will be within a few degrees of the horizon. It is also several degrees south of the ecliptic, losing another couple of degrees altitude.

It will be very favorable in the Southern Hemisphere, though.

Click on image to view larger version

January 31, 2014

Here is a contrary example of a very poor Greatest Elongation of only 18° but at the most favorable time of year. Mercury will be about 10° altitude when likely to be visible. And note that one day moon! Mark your calendars for this one.

Click on image to view larger version

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