Focal Plane Imaging

Focal plane imaging is one of the hardest ways to photograph (or image) the things we see when we look back in time.  It’s also one of the more rewarding.  First, the short version about how Time Machines work.

When you hold a piece of paper at arms length and try to read the writing on it, you may or may not be able to do so comfortably, depending on how good your eyes are.  But let’s say that you have problems reading the fine print on a car loan contract.  What do you do?  You bring the paper closer to your eyes.  This shortens the distance (or focal length), from your eye to the paper, and makes the image look bigger.  That’s basically what our Time Machine does.  It takes the image that you see with your eyes of, say, the moon, and optically moves you closer to it so that it appears larger.

Two things work against us here, though.  The first is light gathering ability (or rather, the lack of it), and magnification.  There is always a second lens, called the eyepiece on a Time Machine that you peer through.  This lets you focus on the image that the Time Machine has created, and also allows you to further magnify it with the lenses in the eyepiece.  The smaller the focal length of the eyepiece, the greater the magnification.

But there’s a trade off.  More magnification means a smaller field of view and less light reaching your eye because the exit pupil, or the diameter of the circle of light the eyepiece allows through, shrinks.  In addition to this light loss, the Time Machine has only a limited amount of light that it can gather, based essentially on how wide its opening is.  A 4″ wide Time Machine can capture a lot of light – about 12.5 square inches worth.  An 8″ wide Time Machine can capture more than 50 square inches of light, or 4 times as much.  A 12″ wide opening can capture over 452 square inches and a 24″ Time Machine can capture a huge 1800 square inches of light.  That’s 144 times the amount of the 4″ Time Machine!

The more light we start with, the brighter the image, the more we can magnify, and the better things are for imaging what we see through the eyepiece.  The problem is that with focal plane imaging, we replace the eyepiece with the camera so that we lose the magnification ability of the eyepiece.  We’re stuck with whatever intrinsic magnification the telescope can provide us with, based on how far away from it the camera is placed.

Most focal plane imaging is done with DSLR cameras or CCD devices.  Each is a little different, but fundamentally the same:  You put an adapter on your camera (or CCD imaging device) called a T adapter.  This has threads on it that you can thread to a T mount adapter that screws onto the back of the Time Machine.  You now have a camera (or CCD imager) sticking out the back of your Time Machine.  The farther away you can get the camera, the more magnification you’ll get, but you’ll also get some light loss.  This is accomplished by adding tubes of various widths to the T mount adapter so you can sometimes see quite a long tube behind the Time Machine before you see the camera or CCD.

Once you’ve done this, the camera or CCD becomes your eye.  You focus the Time Machine, image the thing you want to see with the camera or CCD, and generally do it again and again and again, adjusting focus, exposure, perhaps different colored filters, and perhaps different sections of the sky if the object is bigger than the field of view of the camera or CCD.

This is all fine in the dark, where everything glows and is relatively easy to see.  But do this in daylight, while the Time Machine is aimed at the sun, taking images that show the light that left the sun 8 minutes and 14 seconds ago, and it can be quite hard to see the camera screen, tell if you’re in focus, and ensure that you’re getting what you want.  Luckily, digital imaging means almost infinite storage, so we can take hundreds and piece together the best pieces of each one with computer software.

More on software processing later.