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AstroTrac Polar Scope Collimation

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Ok, some of you know that I have an AstroTrac (hereto referred to as AT) which I plan to use as a more portable, easier to set up, imaging rig. What you might not know is that, whilst the AT is a superb piece of engineering, like many other astro items on the market, there is always room for fine tuning.

With the AT this is particularly true of the polarscope alignment, or collimation. This is something which I am aiming to undertake over the next week or so and thought it might benefit the members of SGL if this process were documented.

Firstly, Id like to thank StuartJPP who intially brought the need for collimation to my attention in this thread.

For further reading I would highly recommend this:

Rotating Polar Scope Arm Messes Up Alignment - Yahoo Discussion

and this:

Fred Miranda Polar Scope Collimation PDF

So how do you begin to collimate the polar scope? The answer is two fold. The first step is to ensure that the polar scope is collimated within the holder i.e. about its own axis. The second step is to ensure that the holder is centred about the AT 'polar' axis.

Step One - Polarscope Reticule Centring

 

The initial step is to ensure that the polar scope is centred about its own axis.

 

To do this you need to first set up your AT and aim the polarscope on a distant object such as a pylon, telegraph pole or any other distant, stationary object

 

Once you have chosen your object align your polar scope so that the centre (the intersection of all markings) is aligned to the top, or other definable point, of your object. Once aligned you can rotate your polarscope through 360 degrees. A correctly collimated or centred polar scope will keep its place when rotated. If your polar scope is badly collimated you will see the centre move around and off your definable point as you rotate.

 

I chose a distant radio mast with a conveniently placed light ontop. The following set of photos show how the light is covered, but then emerges as I rotate the polar scope around through 360 degrees:

 

ReticuleComparison_zps9d520ef9.jpg

This clearly shows that the polar scope is in need of collimation.

Here is where the fun begins!

To collimate the scope you need to adjust the 3no grub screws that are located around the barrel near where the illuminator sits:

P1000413_zps69dd2fd1.jpg

To make this easier, the Fred Miranda paper suggest replacing these with 3mm thumbscrews.

I have currently ordered some thumbscrews from here:

Kustom PCs - Black Metric Thumbscrews

 

You can also order Thumbscrews from 365 Astronomy, they are labelled eyepiece fixings. Once they arrive I can see if they are long enough.

 

 

 

And here is where the story, temporarily ends........

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Looking forward to the rest of this im sure some of it is equally valid for any polarscope.

I do find that TV ariels/masts  and the like are the best targets the staight lines and divided sections give a certain built in scale makes things easier i think.

Alan

Edited by Alien 13

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Hopefully people will find it useful.

I think the radio mast with a light on are perfect for the first step. The one I can see is far enough away that the light is completely obscured when the initial centring is done. This makes and movement when rotating clearly visible.

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Well the KustomPC thumbscrews turned up, a little under 24hrs after ordering - very impressed from Scotland, unfortunately they are not long enough :-(

They measure about the same length as the grubscrews (approx 4mm) but can only screw in a certain distance before you are upto the head, therefore cannot be screwed in enough to hold the reticule.

Still I have some nice thumbscrews for my next PC build!

Take two now, just ordered some 10mm long M3 Thumb Screws from here, which probably would have been the sensible option in the first place!

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OK, now its getting serious!

The eyepiece thumb screws arrived from 356 Astronomy yesterday. Today I replaced the grub screws with them in preparation for collimation:

P1000414_zps24edd467.jpg

P1000418_zps98aaf646.jpg

Having these thumb screws will (did) make the whole collimation process easy so I would highly recommend the mod.

Now to collimate. As you will have noticed from my initial post the polar scope is out of alignment as it is rotated through 360 degrees and, in replacing the grub screws collimation was knocked out further. So, to collimate:

1. Set up your AT and polarscope pointing to a distant object. I used the same mast and light as in my first post.

2. Using the illuminator as a point of reference align the polar scope. I started with the illiuminator at the 12:00 posiiton i.e. pointing to the ceiling and aligned the centre onto the light.

3. Rotate the polarscope through 180 degrees, i.e. so the illuminator is pointing to the floor. You will note that the centre has moved 'off target'

4. The key here is to adjust the polarscope only half the distance back to centre.

5. Once adjusted rotate back to the start position i.e. illuminator pointing to the ceiling, and re-centre the polarscope using the controls on your geared head not the thumb screws.

6. Repeat steps 3-5 - you should notice that the distance of adjustment gets smaller and smaller each time until the polarscope is perfectly aligned.

Now you have a perfectly collimated polarscope:

ReticuleComparison-Collimated_zpscdbb646

But this is only half the battle.......

Once the polarscope is collimated we need to make sure the arm that holds it is centred. Using the thumb screws make polarscope collimation easy, but there is now easy way to perform the second step......watch this space!

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Step Two - Polar Scope Arm Alignment

Last night I tried aligning the polar scope arm directly after to collimation process but ended up getting frustrated thus decided to leave it until a good nights sleep had passed.

After the polarscope is collimated we need to make sure that it sits in the arm in a position which is aligned with the centre of rotation. To check this we need to align the polarscope with our distant marker. An aligned polarscope arm will keep this marker in place when the arm is rotated through 180 degrees.

This is the starting position which I used to align:

P1000424_zps04460abe.jpg

Polarscope centred on the mast light:

ArmTest-Start_zps7aee6b54.jpg

This is the test position 180 degrees the other side:

P1000425_zpsd60674be.jpg

No longer centred:

ArmTest-180Rotation_zps96590029.jpg

The process of aligning is similar to collimating the polarscope, i.e. make some adjustment return to start, re-centre and repeat. What makes this more challenging is where to place your strips of tape for adjustment. This is where trial and error comes in.

For the adjustment I used copper tape from B&Q.

Luckily I managed to do it in two goes following last nights frustration. The following photo shows the copper tape in place:

P1000429_zps2b6f8943.jpg

And thats it, all done. Now I just need some clear skies!

I hope the above is of use to other AT owners. Any questions please feel free to ask.

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Thanks for posting Lee. Extremely helpful.

I have just got to wait for my longer screws to turn up then I'm going to give it a go.

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Best of luck.

Remember to replace each grub screw one at a time. You also need to be careful not to lose the slot in the main barrel which is used for the illuminator - should not happen if follow the advice above.

Also, grab a chair when collimating. I found it really beneficial to be able to take a seat to collimate. Saves your back and with the thumbscrews makes the process easily manageable.

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Cheers Lee,

I will have to have a look at mine sometime...

I'm thinking that instead of the tape,  I might drill and tap holes three more holes 120 degrees apart on the same centres as the magnets and use grub screws to make the adjustments.... 

Peter....

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Funny you should mention that Peter, when I was facing with the tape I did notice that the magnets were ever so slightly proud and that by pinching the collar I could invoke some adjustment to assess where the tape needed to.

On that basis I think drilling and tapping some small thumb screws 'opposite' the magnet positions will work a treat and you an easy way to adjust without having to remove the polariscope each time like I did.

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Glad you finally got it sorted...it's been a while :)

I have done 2 AstroTracs and getting the polar scope arm right is a lot harder than collimating the scope and a lot more frustrating.

I also noticed that some of the magnets sit slightly proud of the ring which I am sure adds the problem.

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Cheers Stuart. It surprised me to note the date of my initial thread and your advice on collimation - which I would like to thank you for, again. Can't believe it took me so long!

Roll on some clear skies and no wind, I'm itching to get out.

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I also noticed that some of the magnets sit slightly proud of the ring which I am sure adds the problem.

I wonder, given the right level of skill, if you could replace these magnets with some sort of threaded, adjustable magnet?

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Hi Lee

Thanks for the useful post, could you expand a bit on

4. The key here is to adjust the polarscope only half the distance back to centre.

You slacken off 2 screws and tighten one to move in the direction you want?

Is it only necessary to do the adjustments in just the one, 180 degree rotated, position, rather than say at 90, 180

and 270.

Thanks

Ad

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Hi Ad and welcome to SGL.

Glad you found my post useful. Ill try and explain as clear as I can.....

Only adjusting the polarscope back halfway ensures a gradual collimation. You cannot adjust all the way back on point as when you rotate back to starting you will have essentially moved point on the polarscope that is out of collimation 180 degrees. Adjusting half way, rotating back and realigning allows for the error in collimation to be systimatically dialed out.

As for doing this at 180 degrees over 90, 180 and 270 degrees is a great question and unfortunately I dont know the answer. My gut feel is that you use 180 degrees because this puts you at the opposite side or rather puts the uncollimates scope at opposition to when it was aligned thus seeing the uncollimated polarscope at its worst. In other words a 90 degree rotation probably wont show the extent to which the collimation is out.

Not sure if that makes sense, hopefully someone more skilled and articulate can help.

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HEALTH WARNING

Now, this may be down to the fact that one of my copper tape strips sits over a magnet but......My polarscope is very sensitive to slightest knock, it fell out twice tonight when trying to set up and knocked my collimation out. I would have been more annoyed if it wasnt for the fact of a sudden onset of cloud and rain. So back indoors, 5mins later all collimated again - phew.

Time to source a slip washer or fathom some other method to secure the polarscope.

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Very good post, about to move up the food chain and attach a an ED80 of some sort to mine, so very timely. As for attaching and 'knocking off', i use pipe foam insulation, as mentioned, and for redundancy a ribbon from the Polar Scope to AT. I try and do it in a manner, where if it does get knocked out it will not hit anything.

Sounds heath robinson, but works.

Astrotrac

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Just taken delivery of an Astrotrac. Found the info in this thread very useful. The new tripod should be here in another day or two along with 3 thumb screws to do the polar scope mod.  As soon as I have everything I'll get the polar scope/arm collimation done. I have some foam to stop it dropping out and may get a slip washer at some point.  

Is there a reason the magnets are on the underside of the polar scope arm and it goes in from underneath ?

Seems like it sits quite nicely going in the other way (from the top) - but not having had it all set up yet I don't know if there is some potential issue with the camera/lens making contact. Also is there any reason why after aligning the polar scope can't just be removed and replaced if there is a need to realign!

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