July 31, 2014

Imaging Satellites: A Low-Tech Method.

Portable Satellite Tracking "Station". (All Photos by Author).
Portable Satellite Tracking “Station”. (All Photos by Author).

We here at Astroguyz have been working for some time on an interesting technique for capturing photographs of satellites, and by popular demand, we wanted to give a brief rundown at how we were ultimately successful. Go out star-gazing on any clear night, and it’s only a matter of minutes before you’ll notice a star or two that slide silently by. These are man-made satellites, objects in low-Earth orbit (LEO) that are shining because of sunlight glinting off of them. We wrote a recent post on satellite spotting and how you can predict the passage of these bodies in a sky near you; this week, we’ll go a step further and show you a cheap but effective method for obtaining detailed images. True, it’s fun and easy to just aim your camera on a tripod towards a passing satellite and let it streak by on a time exposure, but actual imaging will put you in with a totally different class of elite amateur astrophotographers!

A few years back, we first gained wind of the idea from various amateurs around the world that have successfully captured images of the largest and easiest target in LEO to image; the International Space Station (ISS). At an altitude of about 180 miles and a span of 356 feet, construction of the ISS will be complete this year. Visually, it subtends about the same angular size as Saturn with its rings, and hence, if you have an imaging platform for planetary photography, you can capture the detail on the ISS as well. Some years back, we constructed just such a device from an off the shelf webcam. If you own a laptop (you’re reading this, right?) a telescope, and a webcam, you can be imaging planets and satellites tonight, with the help of some online camera control & processing software.

There are three basic approaches to surmounting the two main problems that satellite photographers face; tracking and focusing. The ISS takes about five minutes to span your local horizon; a planetary camera has a field measured in perhaps dozens of arc minutes. One approach is to have sophisticated software that will allow you to program a computer to drive and track a satellite as it moves overhead; modern day amateur scopes equipped with GPS can now do this with a high degree of fidelity. But you’re here because, like the rest of unemployed blogging America, you’re poor, right? After all, the idea is to not have to sell the family car for a fancy new telescope…

Secret Weapon #1: The Telrad.

Secret Weapon #1: The Telrad.

A second method is to simply know when the ISS will pass in front of the Sun or Moon or near a bright target star or planet and simply aim and film at the appointed time. Sites such as CALsky will give you email alerts as to when this will occur for your selected area. Also crucial is time synchronization, such as WWV signals out of Fort Collins, Colorado. This has its advantages, as you can also catch the ISS when its not illuminated, and tracking isn’t necessary, as long as your scope is capable of at least sidereal (daily motion) compensation. A chief drawback, however, is that these sorts of transits may be infrequent from your particular backyard, and the ISS will move fast through the camera’s field of view; we were successful with this method once, and caught less than a half dozen frames that were only visible after careful review;

The ISS transiting the Moon caught via the stationary method.
The ISS transiting the Moon caught via the stationary method.

Which leads us to our third method; manual tracking. This is well suited to the Schmidt-Cassegrain style of telescope, and can be done with minimal modification. The idea is to catch some quality images in a stream of video; in a thousand frames, we typically snare perhaps 20 quality images. Key to this system is a Telrad finder, and a guiding pole fixed to the body of the scope. The pole should allow for smooth motion, and a pool or broom-stick will do. For our purposes, we used an IKEA pot grabber and simply bungeed it to the optical tube assembly. The longer the pole, the smoother the guiding, but remember, you’ll still have to be close enough to see the Telrad reticle, and you don’t want the pole hitting the mount as you track. Think of your role guiding the scope as that of one of the early motion picture camera operators, or a AAA gunner in an old World War II film. The key is to be set and ready for when the satellite pass occurs; you just want to be able to hit “record” and then simply concentrate on guiding. To obtain a good sharp focus, I would shoot a series of planetary images first to set the camera and the exposure times. I’ve found 1/500th shutter speed and about 15 frames a second works pretty well, although you could probably go a few 1/100ths either side of this. Remember; digital “film” is cheap; test this setup well before show time. Venus is a good candidate for stationary practice; it has a brightness similar to the ISS and is currently well placed in the dusk sky. If Venus isn’t visible, I would practice on another planet/bright star/or portion of the Moon, in that order. Ready to practice on a moving target? A plane (the higher, the better) will move very similar to the ISS, and provides a good “dry run”.  For camera control, we use K3CCDtools, which used to be free but is now available for a small purchase. Trust me, its worth it!

K3CCD Tools.

K3CCD Tools Screenshot.

In the field tracking is also vital to satellite imaging success. There are many sites that will tell you when a satellite will pass over your locale, but by far we prefer the stand-alone program Orbitron. This will work in the field sans Internet connection, and will chime when the selected satellite clears the horizon. Just make sure you refresh those TLE’s occasionally, as satellite orbits do evolve and decay over time.

Secret Weapon #2: The "Bungee Guider".

Secret Weapon #2: The “Bungee Guider”.

I was thrilled with our final results; I could tell early on as the image bounced around that some detail had been caught. Further processing with Photoshop even revealed a then-docked Atlantis on its final flight! The ISS also changes appearance as modules are added, ships dock and undock, and amateurs have even caught spacewalkers in action!

ISS III

Success!

Further modifications? I’ve wondered if a zoomed in video camera might provide a better targeting device, or perhaps a mounted laser (note: don’t aim this rig at aircraft, unless you want the guys from Homeland Security paying you a visit!) By the way, make sure any finder or pointer is properly boresighted and the batteries are fresh before beginning. I’m also looking forward to catching other targets in orbit such as the Hubble Space Telescope or the shuttle itself. Do let us hear of any results out there, and remember, you can do this sort of imaging with simple, cobbled together equipment!

Telrad view.

The view through the Telrad.

Comments

  1. RayGil says:

    Great video, packed with useful information, Thank you :-)
    Lets have more how to video’s David.

  2. Bob says:

    Great idea with the pool que. I just ordered a Telrad scope and I am going to try to get some photos of Phobos-Grunt next week. I have a Celstron Neximage CCD… do you think that type of camera will work? You mentioned 1/500 shutter speed, that seems pretty fast. Will I be able to see anything? What kind of camera do you use?

  3. David Dickinson says:

    That should work; if it can image planets, it should get detail on satellites OK; the ISS now sustends about the apparent angular size of Saturn including rings. Smaller satellites like Phobos-Grunt will be tougher, but not impossible. I practiced on planets first, then aircraft. 1/500 is pretty fast, but I found that slower shutter speeds were smearing and blurring the image. I use a Logitech 4000 webcam converted to do planetary work, but its starting to have technical issues, I may trade up. Thanks for posting; let us know of any tips/success!

    Thanks,

    Dave Dickinson

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