# Has anyone seen an actual spectrum of a satellite made by an amateur photographer?

If you use a shallow wedge prism or very low dispersion diffraction grating (e.g. circa 100 lines/mm) front of a camera, you can photograph the spectra of stars with a modern DSLR camera with a fast, wide field lens fairly easily I believe.

If one prepared for a bright satellite pass and oriented the dispersion perpendicular to the expected path, one could capture the evolving spectrum of the satellite as it tumbled or passed in or out of Earth's shadow. You'd have to do a little processing/averaging of the image to get a spectrum.

Has anyone seen an image like this published somewhere, or has made one of them oneself?

One example of the kind of hardware I'm talking about his this, but it would be better to use something with very low dispersion but without blocking the full aperture of the lens.

I don't mean to advocate a commercial product, this is a handy example, and Tom Field is a contributing editor to Sky and Telescope magazine.

above: from here.

above: from here

• This kind of basic setup works fine for stars, especially bright ones. I've done it with a film camera for Sirius, and it wasn't difficult. The hardest part was aiming at the m=1 fringe, but IIRC the fringe was visible through the viewfinder. But a typical satellite (not the ISS) is a relatively dim object that is moving fast. If the motion is perpendicular to the dispersion provided by the grating, you'll get a very dim spectrum spread out over a wide area of the image -- probably too dim to see. If parallel, the spectral lines will be completely washed out. – Ben Crowell Jun 9 '17 at 21:14
• So I think you'd need a computer-controlled mount. You'd need to program it to follow the satellite in real time from your location, and also to aim off to the side so as to get the diffracted light. After all this trouble, I'm not sure what you'd get that would be super exciting. Some sort of average of the spectrum of the satellite's surfaces, which might look like a solar spectrum multiplied by the reflection spectrum of any paint, solar panels, ... – Ben Crowell Jun 9 '17 at 21:16
• @BenCrowell yikes I did a back-of-the-envelope calculation. For an 80 degree diagonal wide angle lens, f/1.4 on an average 16 Mpix DSLR, a +3 magnitude satellite in LEO would leave a trail of about 120 $e^-$ (photoelectrons) per pixel, because it only spends ~20 ms at each pixel. That would certainly be visible at an effective ISO of 1600, but once dispersed by more than a few dozen pixels it would be in the noise. Looks like this would be pretty tough without some changes; maybe a particularly bright satellite and extremely low dispersion, or tracking like you've mentioned. Thanks! – uhoh Jun 10 '17 at 4:42
• @BenCrowell however this is not at all impossible, and I think someone who is interested and motivated to make a low resolution spectrum of a satellite with amateur equipment could do so. The numbers are there, but it takes a little work and thought. One could of course start with a predicted Iridium Flare of course, then push down from there to the ISS for example. – uhoh Jun 10 '17 at 4:56
• @BenCrowell This technique may work for a -5 or -8 magnitude Iridium flare. Gotta go for it soon, I think they will start disappearing soon youtube.com/watch?v=MTGVuPr9Epg – uhoh Jun 21 '17 at 14:57