A laser polar alignment

Previously submitted to Popular Astronomy magazine by Martin Whillock.

Some readers may be interested in my latest slightly dodgy method of getting telescope polar alignment.

I use an old but trusty Celestron CG5 Equatorial telescope mount with two motors.  The built in polar scope is useless – too small with no illumination.  The following method could be adapted for other similar mounts.

I have made an adapter which fits into the mount polar axis casting, and holds a 10mw green laser pointer in the centre and parallel to the axis.   All castings are tapered to allow extraction from the mould and this one is no different, so the adapter was turned to fit the tapered hole.  A close fit hole was drilled into the adapter for the laser pointer, which happened to be a parallel sided straight tube.  An air release hole was drilled sideways through the adapter, to let the air out and in when the laser is fitted in or removed.

Here’s how it works :

  1. Remove polar scope.
  2. Accurately level the top of the tripod, with the mount peg / centre aligned with North using a compass. This is vital – don’t guess.
  3. Fit mount onto tripod and rotate the Dec’ axis as if using the polar scope.
  4. Fit adapter into polar axis & fit laser into adapter.
  5. Switch on laser. For this I made a collar to hold the button down to give hands free.
  6. Adjust the whole assembly so that the laser beam hits Polaris. This will include shuffling the tripod around a bit.
  7. Maybe the Altitude adjustment screws will have to be used – this need only be done once unless you take it to another Latitude.  Take great care to not bend or damage the two Altitude screws – try them out in daylight so that you fully understand how they work.  Have you replaced the manufacturer’s one which takes ALL the weight with a super strong one ?  This is a really good idea.

In conclusion I have found that this method is very good for visual observing as I get very good tracking.  I don’t do astrophotography, so I am not sure if it would be good for this.

The only dodgy part is my laser, which is a Class 3 X 10mw.  I have used it for years at star parties but am always very careful.  Just don’t let it get into your eyes !

From  http://www.lasersafetyfacts.com/3B/ :

Class 3B visible-light lasers are hazardous for eye exposure. They can cause burns to the retina. A person cannot turn away or blink fast enough to prevent retinal eye injury from a nearby Class 3B laser.

At the low end, around 5 to 50 milliwatts, a Class 3B laser poses a moderate risk of eye injury. It is unlikely that a handheld beam aimed from more than a few dozen feet away would cause injury — laser light could not stay on one spot on the retina long enough for heat to build up to injurious levels. However, the risk is increased if the beam is held steady or if the laser is relatively close to the eye.”

Caveat: Lasers can be seen at night because the beam lights up moisture and dust in the air, during really dry clear air conditions the beam may not be visible.