Easy drift alignment

One of the problems facing beginners is how do they align their mount with the celestial pole. You can use the polar scope that is built into your mount (if it has one!), but if like me you’re in the southern hemisphere you might find it to be pretty difficult to use.

The method used here is called Drift Alignment by Robert Vice, or D.A.R.V for short.

Things you’ll need

  • String
  • A bit of wood with a 1M dowel screwed into it, 90° to the flat surface
  • Your equatorial mount
  • An inclinometer
  • A laptop
  • A DSLR/CCD
  • Your telescope
  • GPS (A phone with GPS will do fine)

What to do

The first thing you need to know is which way is South/North. You can use a compass here to get the rough direction of the pole nearest you. A better way to work out the north-south line would be during the day, get a stick and mount it 90° to the ground. Look up the Solar Noon for your location and free up that time. You’ll want something at this point to mark the north-south line – I used string. When you reach the time you found out to be solar noon for your location, mark out using string/tape/whatever you have along the line of the shadow cast by your stick. This is the north-south line that you will need to roughly get your mount aligned to the celestial pole.

You can start setting yourself up before sunset, there’s no point in wasting good imaging time setting up the telescope and mount! The first thing you need to do is get the tripod set up where you want it, pointing roughly at the celestial pole. You should be able to eyeball it by looking down the tripod leg and comparing it to the line you now have indicating the north-south line. It doesn’t have to be absolutely perfect since we’re going to be doing further refinement later on using the mounts built in adjustments. That said, the closer you get it now the less adjustment you’ll have to do later. Next, set your inclinometer on top of the tripod. At first set it east-west and try to get it level by adjusting the tripod legs. Once you have it sitting absolutely level, turn it so it’s aligned with the north-south line and adjust the north/south tripod leg to level it out. You should now be seeing 0° no matter how you orientate the inclinometer. Make sure the legs are settled, if they can be pushed out further you will have to re-level the tripod! When you attach the accessory tray it will push out the legs a little. Unless you are on a very uneven surface or didn’t make sure the legs are fully pushed out, this shouldn’t affect the inclination of the mount.

Mount the equatorial head on top of the tripod, and screw in the primary locking shaft and accessory tray. When the locking knob is wound up completely it will push against the tripod legs, locking them into place. At this point you should work out your latitude from your phone or other GPS device. We’re going to need it to set the correct latitude on the equatorial head. Mount the inclinometer to the dovetail mount and switch it on. Use the latitude adjustment screws on the equatorial head until the inclinometer reads the same latitude as your GPS. We’ll refine this further later on.

Remove the inclinometer and set up the counter weights on your mount. Mount the telescope and camera, and balance everything. Hook up the power and any other cables (eg, USB to the camera) and fire up your laptop. Turn everything on and follow the prompts on your mounts computer to configure the right lat/lon, time, timezone, date, daylight savings etc. Skip the star alignment for now, and just go into settings and get the mount moving at sidereal rate. Slew the mount to point it towards a bright-ish star and get your focus roughly right. It doesn’t need to be perfect for this method to work but it will work better with a sharper image.

telescope-setup

Using your hand controller, slew the mount to point north if you’re in the southern hemisphere, around 0° Dec.  Fire up BackyardEOS on your laptop and switch on the camera if you haven’t already. Just use the frame&focus mode for now. Set the ISO level to something low – 200 usually works well but you can use 400 if there are no brighter stars visible. Set the exposure time to 130 seconds – the longer you expose the more accurate your alignment will be. Set the slew rate on your telescope to 1x, and start the capture. Wait until 10 seconds have passed and then press the Left/West slew button on your hand controller for 60  seconds. At 70 seconds of exposure time, press the Right/East button on your hand controller.

When the exposure finishes, you’ll see a number of trails with a bright spot at one end. If you’re extremely lucky you’ll just have single straight lines, which means no adjustment is required! It’s far more likely you’ll see something that looks like a < symbol. In this case, we need to make some azimuth adjustments to the mount. There’ll be two knobs on the celestial pole side of the mount (the front I guess?). Adjust those a few turns in one direction, lock them off, and try the exposure again. You’ll get an idea for which way to adjust the knobs for your setup. For me it’s clockwise if the star appears to move up, counter-clockwise if the star appears to move down.

Screenshot (8)Picking a reasonable bright star.Screenshot (9)Pointing north, drifting down. The corrected needed here is counter-clockwise. Screenshot (10)Same star as above after the initial adjustment, still a bit more counter-clockwise needed.Screenshot (11) Whoops! Too far! Slight adjustment clockwise.Screenshot (12)Perfect!

Once you’re satisfied with the azimuth alignment of your mount, slew it over to the east or west, again around 0° Dec. As before, slew speed 1x, 130 second exposure, and wait 10 seconds before pressing the Left/West button. Hold down the West button for 60 seconds and then at 70 seconds, press and hold down the East button. Since we set up the latitude using our inclinometer before, it’s far less likely to need any huge adjustments. If you’re always imaging from the same location, you can leave it set at the same latitude. As before, experiment with the latitude adjustment screws until you get a single straight line and not a <.

You can repeat the above steps as many times as you like, but you’ll see diminishing returns. It’s a good idea to go back and repeat the north/south check before you go any further. You should now perform a 2 or 3 star alignment if you want to use the go-to functionality of your mount.