Calibrating a polar scope.

[BEYOND EARTH]

I have a cg-5 mount and bought the polar scope not to long ago. I've tried to calibrate the scope but cant seem to get it down, so I looked here and found this bit of info. "Centre the crosshair on a distant object...preferably a fixed one, not a moving TV aerial for example! Rotate axis 180 degrees. Adjust grubscrews to position crosshair half way back. Realign crosshair on target using alt/azi bolts. Rotate 180 degrees and start again. For ages I didn't realign the target on the cross hair after tweaking the grub screws....that just meant I was going back and forth." After reading this I understood the process a little more. I just cant seem to wrap my head around the part about moving half way back.. I know it sounds sad but does any one have a visual representation of this part that they could link me, or even post here? Any help would be great!

Thank you!

Be.

[JamesF]

I don't have any images to help I'm afraid, but what it means (I think it's AstroBaby's guide?) is as follows:

1. Centre the polar scope on your object using the alt/az adjusters

2. Rotate the RA axis 180 degrees

3. If the polar scope is still centred on the object, you're done.

4. If the polar scope crosshairs have moved off the object, imagine a line drawn between where they are now and where they ought to be (the point you originally centred on)

5. Using the grub screws, move the crosshairs of the polar scope halfway back along the line to the object.

6. Rotate the RA axis 180 degrees back to its original position

7. start again at 1.

Does that help?

James

[Bizibilder]

Have a look at this video - it explains it very clearly: Astronomy Shed UK Astronomy Forum • View topic - Polarscope Reticule Centering & Mods

[BEYOND EARTH]

Yes! thank you so much guys!

[Photohound]

Hey Bizibilder - thanks for the link.

[Aenima]

Hi there sorry for jumping on thread here but the title is just what I was going to use so didnt think it warranted a new thread....

I have recently suspected my polarscope is out by a small amount and so I tried to test it out on a stationary object in daylight - ie a roof corner tv aerial - and found that the actual TV aerial moved as I rotated indicating that the polarscope was off center and I'm not sure it should be causing the image to move like this when rotating the RA axis, I expected the crosshair to move a little but it seems like the whole polarscope is badly position in the mount.

Can anyone tell me if this is normal? I dont know what needs doing - the grub screws can give small adjustments but for the roof TV aerial to wobble as I turn the RA 360' it means a bigger adjustment is needed, yes?

I looked into the polarscope from the front and it doesnt seem any other way it can be adjusted, so is it all in the little allen screws or are there other things I need to do?#

Any advice would be highly welcomed.

Regards

Aenima

[glowjet]

This, as it happens, has cropped up before. In manufacture when the hole has been threaded for the polar scope, it has not been done as accurately in the machining as it should have been, resulting in the finished thread being ever so slightly off the central axis of the hole, the result being that when you screw in the polar scope the field of view revolves in a cone, or that the scope is not seating properly when screwed home. This should not cause any undue problem, so long as you can adjust the cross hairs to stay fixed on your chosen target when you revolve the mount through 180°, this will mean that your polar axis is now in line with your mounts axis, which is what you were trying to achieve in the first place. HTH

John.

[Aenima]

This, as it happens, has cropped up before. In manufacture when the hole has been threaded for the polar scope, it has not been done as accurately in the machining as it should have been, resulting in the finished thread being ever so slightly off the central axis of the hole, the result being that when you screw in the polar scope the field of view revolves in a cone, or that the scope is not seating properly when screwed home. This should not cause any undue problem, so long as you can adjust the cross hairs to stay fixed on your chosen target when you revolve the mount through 180°, this will mean that your polar axis is now in line with your mounts axis, which is what you were trying to achieve in the first place. HTH

John.

If what you said holds true it would be an amazing help, at least something I can fix or allow for an error rather than totally unreliable polar alignment.

And its cool the way you explained it makes sense.

Regards

Aenima