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.

Collimation procedure:

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.

IMG_0955

here is a picture of the aperture on the polar guide itself, ensure that this does not move too much.

img_8292b

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~ by Gareth Hogarth on September 14, 2009.

7 Responses to “Collimating the AstroTrac Polarscope”

  1. I purchased a polar scope from an American dealer. The condition of the scope when it arrived seemed OK, and it needed collimating. Out of curiosity I removed the optical tube and found the thread to be stripped in places. The hex keys adjacent to the reticule illuminator switchwere round, the bottom screw was in good condition.

    The first step was to point the scope at a blank wall and focus the reticule with the eyepiece and then focus the scope by rotating the optical tube, which was so loose that any attempt to focus will inevitably throw the collimation out.

    The scope is a good bit-of-kit, rear of the optical tube, but the tube really lets it down.

    The solution was to fit a collar around the optical tube so that it holds the tube firmly in the seat ahead of the focus screw thread. I don’t know if this part was missing, but the scope is useless without it. The single grub screw that locks the tube inevitably cocks it out of alignment.

    Collimated following your instructions, except that I rotated the tube 120 degrees adjusting both uppermost screws simultaneously – rotate and so on. Not sure it makes a difference. I’m happy to say that the collimation is near perfect.

    Aside from being supplied with what seems to be a used scope, I was very disappointed with the quality of the tube assembly. This can be improved.

    • Glad you found the post useful, I do agree though the polar scope is not built to the same standard as the AstroTrac.

  2. Good write up, helpful! Did you find that rotating the PS also shows the misalignment of the rear optical tube assembly? as when I rotate mine the whole field of view wobbles slightly, i have tried to combat this with extra holes and grub screws but its not exact, only slightly better. I also found the sighting something half a mile away was not a good idea and discovered excellent results collimating using a target object about 4-5 miles away.. Managed a 300mm unguided track for about 3-5 minutes before star elongation set in.. just need better lenses now :) Like many others I find the tube assembly cheap, I have a EQ5 PS and that is solid, it uses a threaded ring instead of a grub screw to hold it in focus / place, unfortunately this tube does not fit in my Astrotrac PS.. If someone could mill a replacement tube and ring in small volumes im sure they could charge an excess as a solid replacement for the cheap plastic one we all have right now.

  3. Hello and thanks for the write up & pictures. I have a basic question for you about the illuminator part in the 2nd picture above – does it simply unscrew to reveal that aperture, or do I need to do something else like adjust the “fourth” hex screw (in your picture above) to get it to unscrew?

  4. Hi Fred, I absolutely agree, the PS scope is certainly not made with durability in mind. I also had an issue with plastic tube not being centric to the reticule holder and the tube not positioned at a right angle to the reticule holder.
    I found that using a ‘little’ bit of plumbers – PTFE tape on the plastic thread helped reduce the tube from being distorted by the grub screws.

    An aluminium collar positioned around the tube which makes contact with the reticule holder would certainly reduce allot of issues with the PS scope.

    The collar doesn’t necessarily need to be round, you can buy aluminium plate 15mm thick off ebay, cut a piece 1.5″ x 1.5″ square (just estimating here) then drill a hole the same diameter as the pastic tube.

    You could aslo drill smaller holes and use some plastic m6 or m8 bolts to lock the collar into positon.

  5. Thanks, this was one of the best explanations i got :)
    it’s quite easy once you know how.
    the only frustration i had was that the screws are so tiny but i ordered some thumbscrews to replace them, much easier seems to me.

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