Getting proper polar allignment with a fork mount and wedge!

Hi All,
This is a long one but I think is well worth the read if you have a fork mount and are trying to polar allign it.
I was thinking hard about polar alignment during the weekend and didnt like the description that meade has in the manual nor did I fancy the thought of spending hours getting the drift method right, with the wedge been the way it is, it's not a trivial procedure.!

The reason why I wanted to get it right is because in my setup, the gotos seem to completely fail in polar mode and the tracking was completely out.

So, after alot of brain storming I came up with the following routine. After using it, I can now do a goto from Mars to Capella and catch both objects in the FOV of the toucam without adjustment, which I didnt think was even possible :shock: :shock:

So,
First of all make sure the telescope is collimated as best as possible, the accuracy of the steps below are only as good as your collimation.
Use a reticle/astrometric eyepiece in the main scope, helps centering, if you dont have one, use atleast a 15mm eyepiece and try higher powers later on.

Place the scope on the wedge and point the scope at a bright star like capella and center the finder and main scope as best as possible, i.e. make sure that the finder and scope are pointing exactly at the same place in the sky.

If your scope is a goto/motorised scope then turn it off and keep it off!

Now, set the declination axis to 90 degrees (should be pointing near polaris), then use the latitude and left/right adjusters on your wedge to center polaris in the finder as best as you can.

Now for the tricky bit.
Once its centered, rotate the RA axis on the scope itself fully east and then west, try and get as much rotation as possible, keep looking in the finder to while you do this, if polaris moves then it will draw a circle, this means that that the declination axis is not parallel with the fork. Adjust the declination of the scope by using the declination slow motion controls on the scope, not the wedge trying to center the stars circular path in the eyepiece/finder, and rotate the RA axis again and see if this has improved or worsened the movement of the star. Now re-center polaris in the scopes FOV using the wedge control only.

Repeat the above paragraph until you are happy there is absolutely no movement (see "problems" below if you cannot get it)
Once you have achieved this, you now have the scope perfectly parallel with the fork, this is vital for good polar allignment.
Do not trust the 90 degree point on the declination setting circle, its not always accurate.

Now open starry night or skymap or some other planitarium program or if you have a good star atlas, go to the polar section and you'll see that the triangle of the 3 stars, polaris, lambda ursae majoris and hip7283 easily fit into any 8x50 finder, and the pole is easily visible from these referance stars. Using polaris as the pole is not good enough.

Once you have these stars and know where the pole is relative to them, center the scope on the pole position using the latitude and left/right controls of the wedge, do not use the scopes fork controls for this.

You now have accurate polar alignment of the scope, accurate enough for most applications.

PROBLEMS,
Mirror flop will throw off the results, if you can lock the primary mirror, all the better.

Are the optical and mechanial axis the same???, if they are very different, then I dont think this technique will work for you.

A well known problem that also degrades the goto funtion of the scope in altaz and polar modes is that the tube is not sitting squarely in the fork, if this is so with your scope, you will never be able to get a stationary star in the FOV, it will always rotate, for this, I'd recommend you visit the MAPUG arcieves to see how you re-align the tube, Its not a trivial operation, but well worth it.
www.mapug-astronomy.net/AstroDesigns/MAP...Alignment/OTA-1.html


The only real unknown left using this technique is that you're not gaurenteed that the base of the wedge is level, this I leave up to you.

Let me know what you think, or if you can add to this to help make it more accurate.

Nice description Dave, this also works with german equatorials as well. I use this method on my GEM CG5 mount when its late and I dont wanna wake up the neighbourhood with noisy slews that come with the built in polar alignment routine.

I suppose the only thing I can add to your description is that when you rotate the RA axis and see polaris descibing a circle then note where the center of the circle is (thats the point your scope is rotating about and using the dec slo motion (not the wedge/mount) try to center that point. This makes the process of aligning the dec axis with the rotational axis much quicker (and less guesswork). Also, when I get down to star hopping to the pole I use a mid range eyepiece 15mm to do the starhopping.

Thanks Dave for the feedback, I've adjusted the description above accordingly.
The method above might sound obvious, but if the only options open to you are the procedure in the instructions and the drift method, then the above method should be a great help.

That's very useful Dave. Certainly seems less painful than the drift alignment method. Thaks for putting thisinfo on the boards. It will certainly become useful once I start using my wedge.

Let us know how you get on with it.

Ok Dave. I tried this out tonight. I got it more or less sorted in my finder first and then I moved onto my 30mm eyepiece. It's slow work, especially when trying to adjust the declination of the wedge. However, I'm making progress, slowely but surely. More work to do though. I found the advice above very useful - thanks!