Focal Length Lesson

Here’s my first astrophotography telescope, assembly in progress.

• The big white tube aimed up and right is an Askar- 71F f/6.9 Flat Field Quadruplet APO (Apochromatic) Optical Tube Assembly w 490 mm focal length telescope. Quite a mouthful, don’t you think?
• Smaller black tube to left on top thereof - An Askar 32mm f4 Guide Scope w 128 mm focal length, the guide ‘scope.
• Beneath those and sitting at an angle atop the tripod is a blue box, a JUWEI-17 Harmonic Equatorial Mount. It contains the motors and electronics that, according to the computer’s dictates, will move the two ‘scopes to keep them aimed at the object of interest while pictures are taken.

How’s It Work

To simplify, the computer “looks” through the guide scope and tells the mount how to move to keep all the stars in the exact same place on the guide ‘scope’s camera sensor. And because the guide scope is mechanically locked to the big ‘scope, they move together and the object of interest stays in the same place through the big ‘scope as we rotate around on the Earth’s rotisserie, one revolution per day.

And if the computer is guiding on one star, then all stars will remain in the same position on the camera sensors. The distances are so vast that, for millenia, all the stars have maintained the same relative positions. (Our prehistoric ancestors saw the same arrangement of stars–constellations–as we do.)

Planets are a minor exception–the word means wanderer–but for the sake of a single night’s picture taking, they are almost motionless relative to the stars. The moon is even less so, but for this discussion I’m going to ignore the differences between siderial (star) motion, planetary motion, and lunar motion. (There’s also solar motion to be considered if you’re going to photograph the sun–but you’ll want significant eye protection and some serious filters on the telescope for that.)

Boxes of Parts

Sitting on the floor in this same picture are three unopened boxes. Top to bottom, they contain the “AAF” (Astro-Automatic Focuser), the (small) camera for the black guide ‘scope, and the main (larger) camera for the main ‘scope.

The AAF (focuser) replaces the human (me) to focus the main telescope. It contains a small motor that runs the main telescope’s focus in and out according to computer commands. As it does so, the computer “looks” at the image on the main camera to find the point where the dots (stars) are the smallest.

The two cameras attach to the respective telescopes, main and guide. In their descriptions above, I included each instrument’s focal length. That’s an important, and for astrophography, an essential number to know.

Focal Length

By definition, the focal length is the distance from the lens to where a crisp, in-focus image will appear.

If you have a magnifying glass in a desk drawer, take it out and hold it a couple of inches from your computer screen, and then hold up a piece of paper even farther away but in-line from screen to glass to paper. Now, move the paper in and out until you see, on the paper, the image of your computer screen. The distance from the magnifying glass to the paper is that lens’s focal length. (Your magnifying glass/piece of paper measurement will be a little off from the optically-defined focal length because the computer screen is not infinitely far away, but you’ll get the idea.)

For the big, white telescope, that distance is 490 millimeters. If, like me, you think in inches and feet, that’s a little over a foot and a half.

Try This

Hold your hands in front of you about a foot and a half apart, 19.2" if you want to be precise, then move your eyes back to the picture of the white ‘scope. Its lens is almost at the right end of that tube, and so the focus will be 19.2" back from there. (In the picture, it’s at the left end of the tube.) That’s where the main cameras sensor has to be positioned.

Making Things Fit

The telescope comes with multiple adapters for that image-end of the ‘scope. Some are for DSLR cameras, others are for observing through an eyepiece, and some are for use with the telescope camera. In mounting that camera, I had to figure out which adapters put the image plane at the right distance, 490 mm, from the lense.

Similarly, for the guide scope’s 128 mm focal length, its smaller camera–the imaging sensor, actually–needs to be 128 mm back from that smaller ‘scope’s lens. And as with the big scope, the little one also came with multiple adjusters for optical versus camera use.

Metal Expands and Contracts

Finally, each ‘scope’s focus adjustment changes that distance because not everything we see, in fact most things we see, are not infinitely far away. And when things are not infinitely far away, the focus (where objects appear most clear) is not at infinity. Instead, they’re closer, and so the lens-to-sensor (or eyeball) distance has to be changed. That’s what the focus ring on your camera does. And in the main ‘scope here, the “AAF” motor and computer will adjust that distance therein. And in the guide ‘scope, its focuser will have a similar job.

Ballpark Adjustment

When the computer tries to adjust the focus of the main ‘scope (via the AAF), it assumes things should already be fairly close. It will only tweak the focus a little, and if it doesn’t see a significant improvement, it’ll conclude that something is amiss and give up.

Because of that, it’s up to me to get the focus “pretty close” before turning it over to the computer. Fortunately, everything in space is far enough away that, any adjustment needed because the metal tube has expanded or contracted from a change in temperature, will be small. Once set, the focus will be close-enough that the computer can make the small adjustments needed through the night’s temperature changes.

And for the guide ‘scope, the focus doesn’t need to be as exact. When tracking, the computer simply watches the bright dots (or small white balls) move across the camera sensor, and then tell the mount how to move to get them back where they were a moment ago.

First (Terrestrial) Photograph

So, here’s my first picture taken through the guide ‘scope after manually adjusting its focus. It’s out the window of my office to a desert ridge on Moon Mountain a little less than a mile away.

Picture quality is, to put it mildly, lousy.

But remember this is an astronomical instrument. It is intended to be used with things that are millions of miles (and much, much farther) away. As that ridge was less than one mile away, the little ‘scope’s focus was at its absolute “close-up” limit.

And the objects the computer will be “seeing” through it will be millions of times dimmer than those saguaro cacti or the blue sky beyond, so I had to change the exposure time from several seconds to well-under one-thousandth of one second to see anything except a total white-out.

Lesson Learned

So, today’s key lesson was that focal length is not just a meaningless number. It is a distance that must be complied with when positioning the camera’s sensor.

And with that lesson now applied to the hardware at hand, my main ‘scope and the guide ‘scope can now focus on deep-space objects (or saguaro cacti a mile away).

What’s Next

My next steps include wiring and configuring everything, and then letting the computer take over the pointing, focusing, and picture taking. That’ll be done by a piece of software known as N.I.N.A. - Nighttime Imaging ‘N’ Astronomy. (I’ll need to learn how to use that software soon.)

Once that’s underway, I’ll go to bed.

In the morning, I’ll awake to dozens, maybe even a hundred or more, images to be stacked and processed on my main (Mac) computer with PixInsight. (More software to learn.)

Learning is Fun

One of my life-lessons is that learning can be fun. And, trust me, I’m having to learn a lot to make this astrophotography-thing work. And yes, it’s both frustrating at first, but then fun when it not only works, but I understand why it works.

Science and math actually work together.

Isn’t that incredible?