DIY Barn Door Astro Tracker

MadMarco

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For anyone wanting to give astro photography a try, you really don't need to spend a fortune on equipment (this comes later once you are hooked). For a start you can use a tripod and ball-head and the "rule of 500" as a rough guide.

Maximum exposure without star trails = 500/focal length

So for a 28mm effective focal length you have about 16 seconds to play with, combine this with image stacking (DeepSkyStacker) and you're in business. For longer focal lengths and longer exposures you will run into issues with star trails which you will need a tracking mount to resolve. There are some great solutions out there, I've got an Astrotrac which is a superb piece of kit. It takes payloads up to 15kg and tracks as accurately as your polar alignment will allow. There is also the iOptron SkyTracker which is very portable and an excellent solution for "only" £300. Alternatively if you've got a few pieces of wood, a door hinge, some threaded bar and a few nuts and bolts then you can try make one yourself.

You will need:
  • 2 pieces of 3/4" ply wood
  • A large door hinge or 2 (good quality ones will be better)
  • M5 threaded brass bar
  • M5 nuts (5 should be enough)
  • A piece of carboard
  • Superglue
Some maths to help get the holes in the right place:

M5 threaded bar information
Drill Diameter - 4.2mm (for if you need to tap your own thread)
Thread Pitch - 0.8mm
1 rpm = 0.8mm/minute = 48mm/hour
2 rpm = 1.6mm/minute = 96mm/hour
You will need to bend the threaded bar to the approximate radius from the calculations below.

Tracker Calculations
Sidereal day = 86164 seconds = 1436.066 minutes

Radius = Circumference /2xPi

If we drive our tracker at 1 turn per minute with 0.8 turns per mm thread
Radius = 1436 / (2 * PI * (1/0.8)) = 182.837mm

So the threaded bar is 183mm from the centre of the hinge. You can make the tracker as big or as small as you like, although smaller/lighter is usually better to take the strain off the tripod. Mine cost about £40 and that included having to buy a second ball head for the tripod/camera.The circular cardboard disk is divided into 12 segments and glued to an M5 nut. To operate you start by lining up the hinge with the pole star Polaris and then turn the disk 1 full turn every 60 seconds or one segment every 5 seconds. I promise that this works if you are very steady and use a wide lens!

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Once the concept of the manual tracker was proved, I then added a geared motor which rotates at 2 RPM with reduction gearing of 2:1 for a final drive of 1 RPM. The larger of the 2 gear wheels was threaded by running an M5 tap down the middle of it, this makes the tracker move up and down as it rotates. The motor is powered by a variable voltage regulator that can be adjusted to calibrate the motor speed. A great upgrade for anyone with the skills would be to use a stepper motor which could be controlled with greater accuracy.

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The following image of the Andromeda Galaxy M31 was taken using the motorised tracker; 20 exposures @ 10 seconds 141mm focal length (211mm effective). Stacked in DeepSkyStacker and then some image manipulation applied. There was definitely more to come from the tracker, but by this time I was bitten by the bug and bought a SkyTracker.

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RobWatson

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Nifty trick for "upgrading" the barn door is to glue two washers (one on each end) to the hinge and use then as a sight guide to align the hinge axis to the celestial pole (north star is usually close enough but hard to find in the southern hemi).

Barn door trackers are great. Use the isosceles drive method as a first pass. Or, go whole hog with a corrector cam for hours of perfect tracking!
 

MadMarco

Mu-43 Veteran
Joined
Oct 30, 2014
Messages
437
Location
Guildford, England
Nifty trick for "upgrading" the barn door is to glue two washers (one on each end) to the hinge and use then as a sight guide to align the hinge axis to the celestial pole (north star is usually close enough but hard to find in the southern hemi).

Barn door trackers are great. Use the isosceles drive method as a first pass. Or, go whole hog with a corrector cam for hours of perfect tracking!
That's a great tip. I eventually bought the cheapest finder-scope that money could buy off eBay, removed the mount and just held it against the hinge for polar alignment which worked really well.

The ultimate upgrade for astronomical work is always in the mount. The Manfrotto 410 Junior Geared Head is an awesome piece of kit, I actually use one with my William Optic telescope. It makes aligning the telescope or tracker a piece of cake.

http://www.manfrotto.co.uk/410-junior-geared-head
 
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