ISS Lunar Transit Mesquite Texas: Composite Image

As I shared in my last post, I was up in the very early hours this morning to catch a unique event, the International Space Station (ISS) passing in front of the Moon, a lunar transit. What I didn't share in that post is why I used the video mode on my Nikon D750 instead of taking individual photos. 

The D750 is a great camera with some powerful capabilities. It has a top capture speed of 6.5 frames per second. Unfortunately, it can't sustain that speed for long. After about a dozen shots, the buffer fills up and the camera slows to closer to 1 frame per second or even 1 every two seconds. With an event that only lasts about 6 tenths of a second, since the camera only shoots at 6.5 frames per second for about 2 seconds, it would take perfect timing to be able to capture any part of the transit. If I started shooting 4 or 5 seconds too soon or too late, I might miss it all together. 

However, with a video mode of 60 frames per second, I just had to start recording well before the start of the transit and then let it run long enough to be sure the transit was over before stopping the camera. Thankfully, when I was scrambling to finish focusing this morning and realized it was almost time for the transit, starting the video and letting it record for a few minutes was enough to capture the transit and capture far more frames with the ISS than I could have even if the D750 could capture images at 6.5 frames per second indefinitely. Using video has its drawbacks, e.g., things in motion (and the ISS moves incredibly fast) inherently have a little blur, even with a really fast shutter speed, but capturing more images with the ISS in them and having a bit more flexibility in starting/stopping the capture was worth it. Perhaps someday I'll get another chance to do this and will try capturing it with images instead of video. 

To create the image above, I went through the video with Lightroom, selecting each frame with the ISS and exporting it as a JPEG file. I then took these files and opened them in Photoshop as levels. By using auto-align and auto-blend, Photoshop produced a single composite image showing the track the ISS took across the Moon. I decided that the first draft was a bit busy so I did it over again just using every other ISS frame to produce this photo. I also applied some intelligent sharpening to make up for the camera being a little out of focus. It's not great but I am satisfied with it. 

Click on the image to see enlarged. Enjoy!

 

ISS Lunar Transit Mesquite Texas

I have captured images of the International Space Station (ISS) passing overhead before but I've never captured an ISS transit. Until now!

Every few weeks I check transit-finder.com to see whether there are solar or lunar transits happening nearby. Earlier this week I found that there would be one early AM this morning. 

Lots of things have to come together just right to see the ISS cross in front of the Sun or the Moon. 

• A location on the Earth where the ISS passes directly in between that location and the Sun or Moon.
• The ISS has to be close enough to the Earth for its angular size (how big it appears in the sky) to stand out. My sources indicate it's best when the ISS has an angular size of at least 42 arc seconds. 
• A time of day or night when there will be enough contrast for the ISS to stand out against the given object. For the Sun, this is pretty straightforward but a transit in front of the Moon when it's up during daylight makes for a more challenging capture. 
• And, of course, you have to be able to see the event so that means little or no clouds. 

The last few times there's been one in the area, the weather didn't cooperate but this time it was pretty clear. The location was within 15 minutes of home and the ISS was forecast to have an angular size of 59.72 arc seconds. 

A little before midnight, I packed up my equipment and Luna in the car and headed to the area where Linda, Lexie and I used to take walks when we lived in Mesquite. With a tripod, mount, telescope, camera and other gear, I needed a spot where I could set up next to the car so I had to scout a few spots looking for a place without too much bright light, on public property, and with a clear view of Polaris (to align the mount) and the Moon. I finally settled on the parking lot at the Palos Verdes Lake Park. Unfortunately, that chewed up enough time that I was still trying to get a sharp focus on the Moon when I realized it was time for the transit. I went ahead and started video rolling, let it go for a few minutes and then packed things up and headed home. 

Today, sitting down with the video I was thrilled to find I had captured it! The focus is not great but it is clear enough to be able to see the ISS moving across the face of the Moon. The entire event was just over a half a second so shooting at 60 fps I caught about 30 frames of the ISS. 

I'm sharing the video for now. Later, I'll work on putting together a photo that captures the entire event in one image.

Be sure to watch full-screen for the best view of the transit. 

In the meantime, here are the technical details:

SkyWatcher Evostar 72ED

0.85 Focal Reducer/Flattener

2X Teleconverter

Nikon D750 (DX mode)

Deep Sky Dad AF3 focusing motor

60fps video at ISO 3200 1/4000th second shutter speed

SkyWatcher Star Adventurer GTi mount

Radian Carbon Fiber tripod

Edited in DaVinci Resolve 

Event details:

See transit-finder.com screenshot below. The centerline crossed the trail just north of Northwest Drive. I captured this video from the park parking lot (not shown in this diagram but it's at the northwest tip of the lake).