Collimating the AstroTrac Polarscope
Collimating the AstroTrac polarscope
(Note: The AstroTrac polar guides have changed, the new version has the “AstroTrac” logo, the older type does not.)
Owners of the Astrotrac may not be aware that the polarscope needs to be collimated before use, I say this because both of the polar scopes I have received needed to be collimated to achieve good polar alignment.
The AstroTrac polarscope has three grub screws located on the outside of the aluminium chamber. These grub screws can be adjusted to collimate the polarscope.
I have come across a few methods, and by far the easiest is as follows:
Take a small piece of aluminium sheet +/- 1mm thick and cut a strip from it about an inch wide.
Wrap the aluminium around the polarscope barrel (forming a sort of semi-closed “U” shape) allowing polar scope to rotate inside the aluminium strip. Take one ordinary bench vice and situate it so that it has a clear view of an outside area. (You don’t have to use aluminium trip, some folks use a piece of PVC pipe a little larger than the polarscope barrel)
You will want to find a street light or object to collimate against. The object must be stationary and approximately 50m away.
Please take note when adjusting the polar scope grub screws:
If you remove the illuminator from the polar scope you will notice a small aperture, this allows the red LED to illuminate the polar guide.
Ensure that when adjusting the grub screws that the aperture does not move too much as it will reduce the effectiveness of the illuminator.
1. Locate a street light with the polar scope, position the polarscope so that the adjustable illuminator is pointing upwards.
Where the lines in the polar guide intersect this will be the centre point. Take a mental note of where this point is.
2. Rotate the polar scope 90 degrees clockwise. If the centre point has moved, use a small allen key to adjust the grub screws re-positioning the polar guide back to your centre point on the terrestrial object.
3. Rotate the polar scope 90 degrees clockwise again, (the illuminator should now be pointing down). If the centre point has moved, re-adjust the polar guide (using your allen key, tighten or loosen the grub screws.)
4. Repeat the procedure until the centre point no longer moves.
Note: don’t over tighten the grub screws, I find it easier if I make tiny adjustments to the grub screws.
The AstroTrac is a great bit of kit, I hope you find this post useful. If you have any questions please comment.
When you have achieved good collimation, the centre point (where all the lines intersect) should no longer deviate when the polarscope is rotated.
here is a picture of the aperture on the polar guide itself, ensure that this does not move too much.