Making and using a manual barn door star tracker

posted: 1644 days ago, on Tuesday, 2013 Mar 26 at 15:29
tags: astronomy, observing equipment, astrophotography, Ed Foster.

The interwebs are full of plans for building barn door tracking platform for astrophotography. Most of them require motors, gears, Arduinos, C programmers (Level 4), arsenals of power tools (+2 or better) and Level 7 in Woodwork and/or Metalwork. After downloading the entire interweb and paging through scads of circuit diagrams and soldering instructions, I gave up.

The urge to capture starlight, however, was strong, so Ed and I called in some favours and made vague threats, and soon we had plans and instructions, courtesy of Hannes Pieterse and Chris "Televangelist" Stewart. From my lips to yours.

What it looks like

[left] The completed manual barn door star tracking platform gadget. The square turny bit allows quarter-turns to be made. The piece of white plastic glued to the bottom of the top board shouldn't be forgotten. [right-top] Below the hinge, a supporting plank holds two suitably sized snug-fitting terry clips into which a green lazer is clipped. Activating the lazer looks cool and keeps curious members of the public at bay. [right-bottom-left] The all-important hinge, with plastic inbetween-bits for extra verve. Note the terry clips. [right-bottom-right] Near the top, see the egg-shaped dome nut, threaded onto the curved rod, running in a T-nut, bashed into the wood. Near the bottom, a spare washer is held in place with a bolt.

How to build it

Construction plan schematic diagram. Essentially, the parts list consists of two pieces of wood about the size of a small plank, various wood screws, cold glue, two T-nuts, two dome nuts (in case you drop one - these buggers roll!), a decent hinge, two terry clips, a wafer of polyethylene (a.k.a. a bit of plastic milk bottle), a threaded rod, a bolt to attach your camera, and another bolt to attach it all to a tripod and in the darkness bind them. Measurements are in mm and inches. The following dimensions are not used in the construction: 24mm, 1.75-inches, 5'2, and a furlong.

How to use it - Part I

Place your tripod on a level patch of ground. If necessary, a grader or light bulldozer can be employed beforehand. Clip your green lazer into the terry clips, making sure the lazer is snuggly aligned with the hinge barrel, and that the end from which the light comes, is pointed upward. Twist and turn your tripod about until the lazer points directly at the South Celestial Pole (SCP). Note: This will not work in the northern hemisphere! (Generally, astrophotography in the northern hemisphere is not recommended.)

I rest a finder-scope on the top plank (not shown) and wiggle it about until I find the lazer beam, then nudge the apparatus until the beam is on-target. The little diagram shows the SCP, which is located a few centimetres from the dim star sigma Octantis. [You can download the diagram by doing the right-click-save-as routine or you can get it as part of the ConCards (free download from DOCdb.net) set of star charts, on the "Octans" page.]

Sigma Octantis and the four other stars that make up the trapezium shape should be easy to see in a small finderscope. Then look for the little cerise triangle that has sigma Oct at one edge - the SCP lies near two equally-bright stars at the apex of a sharp triangle, as the diagram shows.

How to use it - Part II

Once the apparatus has been aligned, turn the twirly thing once a minute, and your camera will follow the movement of the Earth's rotation. Even better, since you made the twirly thing from a square piece of wood, you can easily give it a quarter-turn every quarter-minute, i.e. 15 seconds. This is a far superior tactic as waiting a full minute is, phenomonologically, a very long time and you are are liable to fall asleep and risk bumping your head sharply against the apparatus, thereby ruining your exposure.

Take care to turn the twirly thing clock-wise! The second image shows how silly things look when you get the rotation direction wrong.

Despite the Silly Look, turning it the wrong way on purpose is a great diagnostic tool: the dots should be evenly spaced along an arc. If they aren't, you're stupid and should die by being force-fed a printed copy of the First Amendment to the United States Constitution, set in 30-pt Comic Sans.

Typical results

The maximum untracked exposure time depends on the focal length of the lens you have on the camera, the declination of the area you are imaging, and how close to empty your camera battery is. For a 50-mm lens on a DSLR, near the celestial equator your max-time is about 8 seconds, while near the celestial pole it is upwards of a minute. But then, who wants to image the celestial pole?

Using a wide-angle lens you can, of course, exposure for longer before star trailing becomes offensive, but even then, you will probably only be able to image for about one minute. With a barn door, you can get good results with 6 or more minutes of exposure time, which extends your space penetration powers to at least the 12th Level.

Other results

10-mm lens at f/4.0, ISO 640, a single 240-second exposure.

50-mm lens at f/5.0, ISO 640, a tight crop from a single 185-second exposure.

nothing more to see. please move along.