Mercury!

As an avid planetary imager, I've had my fair share of accomplishments, but Mercury has always eluded me. One of my goals for 2025 was to end that frustration and finally capture an image of the smallest planet in our solar system. This current mid-to-late August apparition offered the best opportunity of the year. Fall morning apparitions provide the most favorable geometry, with the planet's solar separation translating to vertical height above the horizon due to the inclination of the ecliptic in the autumnal pre-dawn sky.

Due to the trees (which need trimming this winter), Mercury must reach about 40° altitude to clear them, necessitating a daytime observation. That’s not too difficult for Venus, but it posed a bigger challenge for Mercury, which isn’t visible in the finder scope. Fortunately, the ALPO webinar last spring on daytime planetary imaging provided valuable tips and encouragement, confirming that I had the equipment and skills needed for success.

The weather from spring through much of summer this year was dismal. Clouds were the norm, and even when the skies cleared, seeing conditions were often below average or marred by smoke from the Canadian wildfires. So when a forecast for below-average transparency but average seeing presented itself on August 17th during the favorable morning apparition, I decided to commit to an imaging run.

I stepped out at 4 a.m. to align my telescope, only to be greeted by a strong haze that limited visibility to 2nd magnitude stars. By 9 a.m., the clouds had worsened, making it difficult to even spot the fat crescent Moon. However, I managed to bring Mercury into the camera’s field of view and run some captures. The resulting image showed a chubby crescent phase with little to no discernible albedo features - a technical success, but not truly satisfying.

With a forecast for better transparency and above-average seeing on the morning of Saturday, August 23rd, I decided to give it another shot, with Mercury just past dichotomy. I set up my scope to cool and went to bed Friday evening, again waking at 4 a.m. to align the CGX-L mount. The alignment was good enough to bring targets into the low-power view. After aligning, I did a couple of lucky imaging runs on Saturn in color under pretty good seeing conditions. I then moved on to Jupiter for more practice and captured an IR image as dawn approached.

Then, despite being plugged in, my laptop died unexpectedly. Clearly, the recently purchased AC adapter will need to be returned. Frustrated, I took it back upstairs to charge while waiting for Mercury to clear the tree line at 8 a.m.

Returning to the driveway, I was pleased to see clear blue skies with only minor clouds. The morning breeze was gentle, and transparency and seeing conditions seemed promising. I started by slewing to Venus to practice my hand controller motions, ensuring that one short press to the right and 2–3 presses up would bring the slewed object into range.

Finally, I slewed to Mercury for the moment of truth—and the planet popped into view! Initially, it looked turbulent, but after adjusting the focus manually and using SharpCap’s Fourier Detail Detection routine, I was able to achieve a good focus. The strong surface brightness of the innermost planet allowed for high frame rates, and I adjusted the exposure, gain, and region of interest to achieve over 100 frames per second.

The planet danced wildly due to the rising Sun and atmospheric currents, with only fleeting moments where the half-moon phase was discernible. I captured a series of three 3-minute videos at 3ms exposure and a gain of 180, followed by experimentation with higher gain and lower exposure to increase the frame rate. In total, I captured about 20 minutes of video.

Processing the data later that day revealed good-quality images, with sufficient seeing to produce a reasonable stack. I whittled down nearly 64,000 frames from the 3ms videos to create a nice image. I also experimented with AstroSurface for the first time to sharpen the result and was pleased with the tool.

While this face of Mercury lacked high-contrast albedo features, my image showed a mottled surface, reflecting large-scale features. After all, at only 6.6 arcseconds in size, I’m targeting something roughly the size of a tomato seed viewed from across a football field! With this success under my belt, I’m looking forward to making Mercury imaging an annual event—weather permitting!

Transit Terminus

This past Veterans Day the tiny planet Mercury was scheduled to cross the face of the Sun in an event visible in its entirety from my vantage point in North America. Such alignments are infrequent yet not as rare as those when we get to see Venus interposed between us and the Sun; we get two of those within a decade and then the interlude is over a century before the next transit pair. For Mercury we get to witness roughly 13 such events in a century. Like any celestial phenomena that takes place in under 12 hours, not every point on the globe gets to see the transit - in some locales the entire event happens while the Sun is below the horizon (a.k.a. night time).

I've been fortunate enough to catch a couple of prior Mercury transits. My first was on Saturday May 9, 1970, just about two months following the amazing total Solar Eclipse for the east coast that took place on March 7th. My notes have long since been lost on that event but I do recall seeing the tiny dot of our smallest planet against the Sun using my RV-6 Newtonian using a solar filter by the same company. The next transit that I have a clear memory of (and some notes) was Monday May 9, 2016. It's not just coincidence that both are on May 9th - the orbital mechanics dictates that a transit of Mercury can only occur within a couple days of May 8th or November 10th when Mercury's orbit intersects Earth's plane. If Mercury happens to be undergoing inferior conjunction at that time we have ourselves a transit.

Looking at the predictions for Mercury transits beyond 2019 showed that while there was one set for 2032 and another in 2039 neither would be visible from my side of the planet. That makes the next candidate May 7, 2049 - putting me well into my nineties. So for me the reality was that the November 11th event would be my swan song for any planetary transits. In a year that brought several reminders of my own mortality here was the universe offering yet another commentary upon it.

I decided to try to make this last Mercury transit the best yet. The prediction for 1st contact was 7:35 a.m., which meant the Sun would be quite low in the sky. If I could find a suitable location I might be able to witness that 1st contact and maybe even the "black teardrop" effect. But a more realistic goal was to try to obtain some nice photographic captures of the tiny orb as it marched its way across old Sol. I planned to take the RV-6 on the Atlas mount with me and the Canon Rebel t6s camera and the DFK21AU042 planetary video camera. I gathered everything together the evening before and then fashioned a solar filter using some Baader solar film, sandwiching it between stiff cardboard. The weather forecast was iffy - most prognostications had high clouds moving in as the morning wore on. I set out the equipment so that when I got up it would be easier to load it into the Rogue and set off in search of a venue with good eastern horizon.

The pre-dawn was not encouraging. While portions of the sky were clear the eastern direction had a buildup of high altitude clouds - the Sun was clearly not going to penetrate it. Regardless I set off looking for a location. I toyed with the idea of heading to the parking lot at Leidos where I work. It would be pretty empty on Veterans Day (a government holiday) and the horizon is pretty good. But the commute could be 30 minutes or more meaning the transit would likely be underway by the time I made my own transit and got set up. In the end I set up on the western edge of the ball field by the BYKOTA senior center in Towson, trying to position myself to catch the Sun soon after it rose. I had used the spot before in 2004 to catch the Venus transit soon after sunrise.

Setup outside BYKOTA, waiting for clouds to disperse

The 1st and 2nd contacts came and went without my witnessing them, a combination of clouds and obstructing building. Finally by about 8 a.m. the clouds thinned just enough that I could target the Sun using the shadow technique. I popped in the 16mm Plossl and focused. It took a moment but then there it was - a tiny black silhouette of planet Mercury against a solar disk completely devoid of sunspots. Yay! I could at least say I had successfully witnessed my third transit of Mercury!

Fighting clouds & buildings at the start

My initial attempt to record the phenomena was with the DFK21AU042 video imager, but it proved a frustration. With the Sun over my shoulder making it hard to see the screen and focus at the same time I was not getting any worthwhile results. I switched to the 35mm camera adapter for eyepiece projection using the 16mm and the camera's live view mode,  working to bring the solar limb into sharp focus. Even though I finally got to what seemed to be a good sharpness it was challenging to see if the view contained the tiny dot of planet Mercury. Without any sunspots that could serve as landmarks to guide me to position the best I could do was approximate, My approach evolved into taking a shot, bringing it up for review and enlarging it on the tablet to see if I by chance had caught the messenger planet. If not I used the RA/Dec control pad to slew to another part of the Sun to take another shot. It took a little while but finally ended up with a pretty decent image of the transit.

Mercury Transit - ISO 400 at 1/15 sec using 6" f/8 with 16mm Plossl

As I worked to get some photos there was a gradual stream of dog walkers, joggers, and curious neighbors wondering what the heck was going on. I obliged most of them with a peek through the scope so that they could tell friends and family they had seen it. By a little before 9 a.m. I decided to pack up and head home to finish observing and capturing the transit. I was back in business on the driveway of 404 within about a half hour, setting up the Vixen 80mm as well so that I could take some whole disk shots of the Sun sporting Mercury. The clouds continued to threaten - in the short video below you can see them crossing the face of the Sun. There were a few times when the clouds won but surprisingly the bulk of the transit window the Sun was visible.

Around 10:30 a.m. I decided to start taking a set of bracketed shots of the whole disk every 15 minutes in the hopes of creating a final product that would document the tiny speck making its way towards 3rd contact. I got most of the shots but the 10:45 one was a bit out of focus. It was quite challenging to get a sharp focus on the Samsung tablet that was connected to the Canon due to the Sun making it hard to see the screen. I resorted to having a pillow case draped over my head every 15 minutes as I tried to recenter the Sun and check the focus.

Mercury marches across the Sun on Nov. 11, 2019 (times EST)

As we slid past noon the tablet was running low on battery, signaling that photography would soon be over. I took a few more looks at the tiny, silhouetted dot to appreciate the scale of things in our solar system. As the clouds began to thicken yet again I decided to conclude observations and began to breakdown the setup. I may have missed the ingress and egress but overall it was a fine and memorable event to mark my last planetary transit!

Mercury Transit - 2016

When I noticed earlier this year that the US east coast was in a good position for the May 9th transit of Mercury I made sure to put some placeholders into the work schedule - before work and around lunch time. One of the best perks of having a WFH ability is to take advantage of stepping away for an hour to run some errands - or catch a transit of the Solar System's innermost planet. A couple weeks prior I had purchased a filter for the Vixen (which has the added bonus of being yet another piece of paraphernalia in the tool chest for the August 2017 eclipse). However, we were in quite the rainy/overcast mode for weeks before so it seemed as though it'd be a missed opportunity.

The forecast indicated that the prospects would be best in the morning and then likely steadily deteriorate. The transit was due to start at about 7:10 a.m. with the Sun only about 10 degrees above the horizon. That would be challenging given the horizon.

I set up the Vixen and got the camera ready a little before 7. The sky had some clear sections but it looked like it was going to gradually worsen with high, thin clouds moving in. At first it seemed that a position just outside the front door would work, but the window of opportunity between trees proved to be inadequate. Finally the sunlight breaking at the end of the driveway guided me to a spot where I had a reasonable chance to observe for 20-30 minutes.

By the time I got the Sun targeted the transit was underway. And what a blank looking disk Sol exhibited - just one unimpressive sunspot formation to be seen. And there, at about the 11 o'clock position, was that unmistakeable circular pinprick of a shadow. I ran off multiple shots at a range of exposures, then spent a little time inspecting the minute ball using the 20mm eyepiece.

During the time I had multiple neighbors ask what I was up to, and the clouds carried out their threat to occlude the event so that by 8:30 I was back at my home office desk and ready to start the day. This is one of those events that is hardly spectacular yet for an astronomy wonk something you want to catch, highlighting the scale of our Sun compared to one of the smallest planets.

Mercury - tiny dot at about 11 o'clock

Date & Time: April 23, 2009 20:45 EDT
Location: Tuckahoe State Park, MD
Sky: Clear, light wind
Seeing: Bad (1/5) Transparency: Excellent
Equipment: 25cm Newtonian f/6 with 9.7mm Plossl

Notes: Springtime is always the best time to get a look at Mercury during an evening appearance. About an hour after sunset while waiting for the sky to darken at Star Gaze XV the messenger planet was easy to spot about 10 degrees above the horizon. As usual there was little more than a phase to be discerned in the eyepiece, the combination of turbulent seeing (Saturn barely showed any global detail when viewed) and low altitude wrecking havoc with the image. At the moments of semi-steady seeing it appeared to be at just past dichotomy, a very fat crescent.