Rapid Transit

I was finally able to get a shot I have been after for about 4 years now - the International Space Station (ISS) passing in front of the Moon.

For any given location it's a fairly rare event. If you're willing to drive a bit, it becomes less rare but then there are a number of other factors you need to account for in order to take a decent shot. Cloud cover, phase of moon, altitude of transit and time of day can all make the shot either more difficult or impossible (cloud cover is usually my biggest obstacle). For example, there was other recent transit that would've been viewable from only about 10 miles from my home but it was around 2am on a work week night. If the pass happens at an altitude lower than 40 degrees or so (angle of the transit from the horizon), the ISS probably won't look that great as the solar panels will probably be edge-on. Another factor that a lot of people don't think about is that for most transits, you're probably not going to see the ISS move across the sky until it's in front of the Moon or Sun. The ISS is bright when you catch a flyover during the hour / hour and a half window after sunset or before sunrise, but most of the time it's flying over either in daylight, where it's too dim to see, or it's in the shadow of the Earth. And finally one of the most difficult aspects of shooting a transit is that the ISS is pretty fast...17,500mph. So most transits happen in less than a second.

Thankfully for me, the conditions perfectly aligned for this transit. It happened at a fairly early time at night, 10:05pm. Skies were clear as a cold front had moved through earlier in the day. The altitude of the transit was greater than 55 degrees. The Moon was nearly full - just a day before the penumbral lunar eclipse. The only issue is that I had to drive around 80 miles to Orlando in order to get the shot.

Planning is KEY for getting shots like this. Yes, having the right camera equipment is also important, but having the correct data on the transit is by far the most essential step. For this, I use a couple of different sites: Calsky & Transit Finder. Below is a screenshot of the map from Calsky with the areas on the ground where the space station appears to cross the center of the Moon during last week's transit.

Calsky is a powerful website, if a bit cumbersome, that gives users data on a variety of astronomical events such as eclipses, star maps, deep space objects, meteor showers, satellite flyovers and transits. Unfortunately, the interface can be a bit tricky to use and tends to crash on me from time to time. Regardless, it's a great tool for amateur astronomers and I used data from it to plan my trip. In order to fine-tune the location, I used the more-focused Transit Finder., which has maps that are more detailed than the ones provided by Calsky.

I used two different camera setups in order to capture the transit. My Nikon D500, Nikkor 300mm f/4 VR PF lens and AF-S TC-20e Teleconverter was used for the composite of the transit. In order to make it slightly easier for me, I also put this setup on the SkyWatcher Star Adventurer motorized mount that tracks objects once you align it with Polaris. This allows me to center the Moon in the frame earlier in the evening and not having to worry about re-positioning it closer to transit time. I also have a wireless remote shutter release so I don't have to hold the shutter during the event. I had the D500 set to continuous mode, which allows for 20 seconds of exposures at 10 frames per second. The exposure was set to f/8, 1/3200sec at 800 ISO, which exposes the Moon with pretty good detail and will also freeze any motion from the fast-moving ISS. I started shooting about 10 seconds before the transit, which was happening at between 10:05:38 and 10:05:40 according to the times listed on the sites I used. I was able to get six frames with this setup. Below is the transit in gif format.

The other camera setup I used was with my Nikon D750 and Orion 10" DSE Dobsonian telescope with a 2x Barlow lens. Getting a shot with this was going to be trickier as the field of view is fairly small - you can't fit the entire Moon in frame so I actually had to plan out where I thought the ISS was going to cross. This camera is also slower and has a much smaller memory buffer than my D500. I can only take 6.5 frames per second for about 2 seconds when shooting in RAW or about 5 seconds in JPEG. I decided to use JPEG to give me a little more room in case I don't get the timing correct. I could've gotten away with my original plan to shoot RAW though as I only managed to get one frame with the ISS and it was the first shot in the series...cutting it pretty close with this one. Exposure was set to 1/2000sec at 1600 ISO. The aperture of the telescope with Barlow is probably around f/11 but I'm not 100% certain on that (otherwise I would've liked to have shot it at 800 ISO). I'm going to try to create a high resolution version of this image eventually by combining all of the shots I took of the Moon along with the frame with the ISS. I'll try to upload it here later this week. Here's the one frame I managed.

Hopefully this info is useful to anyone who is hoping to get a shot of an ISS transit. If you really want to see some impressive transit photos, check out Thierry Legault's site. His work is incredible. Oh and here's another transit shot I took in December...another photo I've been after for years. I've gotten other plane/moon shots from a profile view, but I wanted it to be closer and head-on.