Polar aligning a Dob.

[BAZ]

Even I didn't think that I would even utter the above sentence. But stick with me on this one.

 

When at Kelling, both Rusty Strings and myself were experiencing errors with the setting circles and Wixey method. As the scope is only roughly aligned to Polaris, this means that there is an error introduced to the Alt & Az coordinates. This isn't too bad when it is pointing towards the North, but this becomes quite a significant error when moved West and East and is at it's worst when pointing South.

 

So while struggling to fall asleep the other night, I thought it wouldn't be too long before some numpty put a polar scope on a Dob.

 

And that numpty is me!

 

Here's the theory. If a star is centered in a high magnification eyepiece and a polar scope is then aligned so the cross hair is on the same said star, then I thought I would then be able to actually polar align it correctly. Even if it's just close, it will be significantly more accurate than previous attempts.

 

As I won't need this other than for setting up and maybe checking if things go out of kilter, I thought it would be easier to mount it on the finder shoe, so it can be whipped on and off at will.

Also as Rusty Strings and Doc have done, I have made a plate for the Wixey to sit on and this can be calibrated so that that is a lot more accurate. This will be attached to the scope with heavy duty Velcro.

 

What do you reckon, hero or zero? I won't be able to confirm if it works until I have tried it. Tonight doesn't look good.

 

 

 

 

 

 

 

 

 

 

 

[Doc]

I used to use a eyepiece reticule so this enabled me to put Polaris in the centre of a 10mm eyepiece, you can then with a little more accuracy set your wixey and circle.

 

It does work better the more care you take but it is still a little out when facing south, to compensate for this I used to pick a star in the South and follow the above procedure, this if done correct does make it a little more accurate.

 

To be honest I cannot see much advantage in using a polar scope but then I have never tried.

[BAZ]

If it doesn't work, then I have only wasted some time, but if it does work then it should place the alignment correctly, both Alt & Az.

 

I suppose the proof is in the pudding.

[tuckstar]

Martyn, is your setting circle straight on to the Base or separate like mine and Daz's? I can't remember. But I just crank up the mag on polaris and alter the circle to the correct degrees. Spin it to something about 180°, same again then clamp, back to polaris and double check before another clamp. This seems to take the slop out of the gap between the setting circle and lazy susan, and stops me knocking the circle. Takes some faffing to get spot on but worth it when it is.

But that's no good if your circle is fixed. Be interested to see how you get on with it.

[BAZ]

Yeah, it's just stuck straight to the base. I have always had errors, but if I can get it to be accurate to 1°AZ, the Wixey should be accurate to 1/10th of a degree on ALT.

 

I am just mucking about really, but I would be real happy if it worked bang on.

[Doc]

You do really need a movable circle so you can check and adjust in both North and South directions. It's very similar to go-to on an EQ mount the more stars you claibrate on the more accurate it becomes.

 

Give it a go Martyn and I'm really interested to hear your findings.

[Daz Type-R]

Not that I'm blowing my own trumpet here but I very rarely have issues in finding any thing.

All I do is use my highest mag EP, spend time getting it bang in the middle of the FOV, check my phone to get the exact coordinates of Polaris ( it alters by half a degree or so) alter the setting circle to read the same as the phone and then off we go.

What I have found in the past that throws me off (other than catching and moving my setting circle) is if my dob base is not level.

I suppose having a 100 degree field of view helps as that allows for some slop in the measurements but generally I find an object almost immediately.

Interested though to see if this method pans out, look forward to the results.

[Tweedledee]

Nice idea if it works Martyn and it looks a really neat job .

 

I would have thought that it might introduce more errors and complications. It will be interesting to see how you get on with it though.

 

As Mick says above, I would have thought that the easiest method to improve accuracy would be to make the azimuth circle movable.

[BAZ]

Making the az circle move ain't happening now, it is stuck down, but I can slide it around five degrees either way on the steel leveling pads.