Full Io Orbit How-To, Part 2 -- Photography

Actually taking the photos was, of course, the most rewarding part of this whole process.  In addition to looking around at the rest of the night sky and appreciating just how light-polluted Los Angeles is, I got to see the Galilean Moons actually dance their orbital dance, both minute-by-minute and night-by-night.  Capturing what I saw on camera was the least I could do to illuminate this celestial beauty. Here are some of the nuts and bolts details of how I did it.

How Often to Shoot

I decided rather arbitrarily to take a picture every three minutes.  This worked out well for a few reasons.  First, that amounted to about 850 frames.  At 24 fps, the video would be about 35s long, a nice length.  Further, getting at least one picture every half-degree of Io's orbit seemed like another nice metric to hit.  In hindsight, it was overkill, as Io's orbit only traverses about 120-150 pixels of the image with my camera's setup.  However, what's nice about overkill is things aren't ruined by having one bad frame.  Dropping a single frame when taking them every half-degree is pretty innocuous.  Most importantly for me, however, 3 minutes was about how quickly I could take a picture, open it up on my laptop, zoom in, review and take another picture if I didn't like what I saw.  Now that I've done it 850 times, I could probably get this down to 1 minute (though doing so would be stressful).

Camera Settings

Most cameras come with various automatic settings, things like portrait, landscape, action, etc.  At least with my camera (though I suspect this to be true of most cameras), none of these was useful for this endeavor.  The amount of light assumed by all of these presets far exceeds what was in my typical frame.  I decided to go full manual, which most DSLR's will allow.  There are really only three settings that matter: exposure time, aperture and sensitivity (aka "ISO").  I'll go through each of these separately.

Exposure Time

The longer the exposure, the more light enters the camera and the brighter the image.  Without a tracking mount, there is a maximum exposure time before the rotation of the Earth leads to blurring in the image.  At 300mm focal length, I found this maximum time to be about 1/3 second.  Luckily, Jupiter itself is quite bright, so very short exposures can still capture it.  However, to get the Galilean Moons, longer exposures are required.  They're bright, but not that bright.

Aperture

Aperture is the size of the hole that light is allowed in through the camera to the imager.  For the purposes of focusing at infinity, the only thing aperture affects is how much light gets in.  The larger the aperture (low f-stop), the more light hits the imager.  F-stop affects other things in terrestrial photography, most notably the depth of field.  If you don't care about this, you can skip the next paragraph.

For reasons I'm still not terribly clear on, aperture is specified not by the diameter of the aperture, but by an "f-stop" or "f-number."  The f-stop is the fraction of a focal length that the diameter of the aperture is.  So, in my setup (300mm focal length), an f-stop of f/8 means the aperture is 37.5mm (= 300mm / 8).  In addition to allowing more light, lower f-stops reduce the range of distance from the camera in which things are in focus (known as a "depth of field").  Any given lens will usually be specified by its minimum f-stop, which is the largest the aperture can be.  So a 300mm, f/5.6 lens can open its aperture up to 53.6 mm (= 300mm / 5.6).

Sensitivity

Back when cameras still used film, the ISO number referred to how quickly the chemicals on the film would react to light.  Lower ISO numbers meant a slower reacting film.  This setting on a digital camera is intended to mimic old photochemical film.  For astrophotography, getting brighter pictures is almost always good, so using a high ISO is usually preferred.  However, it should be noted that a high ISO also leads to higher noise in the photograph, as things that appear black to the eye don't to the camera.

Final Settings

After some experimentation, I settled on 1/4s exposures at f/6.3 and 6400 ISO.  These were the settings I used on very clear nights.  A discerning eye will notice a lot of pictures taken on not-so-clear nights.  For those, I would adjust the exposure or sensitivity.

Time

While putting together a video of Io's full orbit was quite both challenging and fun, more than either of these, it was a commitment of time.  Io's orbit is 42.5 hours.  There's simply no way around spending that amount of time taking pictures.  Further, overlap with portions of the orbit that have already been photographed keep increasing.  At first, just going out when convenient to take some pictures was unlikely to find Io in the same place in its orbit around Jupiter as it was in photographs already taken.  Towards the end, it was just the opposite and it became harder and harder to find good times for observing.  Missing an opportunity means waiting 42.5 hours for it to come around again.  Even the willingness to just drop everything when that time comes may be thwarted by the rising and setting of the Sun or Jupiter.  This is all to say nothing of a ground-based astrophotographer's absolute worst enemy, clouds.

Finding good times to observe portions of the orbit that haven't already been photographed was crucial.  There are some nice sites devoted to showing the Galilean Moons' placement in the sky (I used this one a lot).  I found their principal usefulness to be qualitative.  When I really needed to drill down to when the next 3-minute window was, I cooked up my own simple Python script.  I will eventually post it to github and update with a link here.