Polarfinder calibration.

[parsecundo]

Can someone explain please something which has been puzzling me? Today I have been calibrating my polar finder scope using a distant TV aerial. I had no difficulty placing a datum on the aerial directly on the cross hairs of the NCP reticule and  after adjustment of the reticule and rotating the RA axis through an 180 degree arc the aerial datum remained directly on the cross hairs. However during the rotation of the axis the cross hairs were seen to move slightly away from the datum   and I wondered why this should be the case. Surely if the optical and the mechanical axis of the mount are aligned the cross hairs should remain on the datum over the full 180 degrees rotation of the RA arc?

Thank you for any replies.

" I've  seen things you people wouldn't believe, attack ships on fire off the shoulder of Orion........"    Roy Batty.

[parsecundo]

I think this movement that I am seeing whilst the RA axis is transitting it's 180 degree movement can only be due to a tripod which is slightly off level. Further investigation is needed.

[jambouk]

I think this is a common finding, it certainly happens with my polar scope too:

http://stargazerslounge.com/topic/209502-question-re-polarscope-fitting-from-a-newbie/#entry2234448

Jd

[parsecundo]

Many thanks for your comments jambouk but I believe I have found the cause of this. After much thought and lying awake last night I took pains to level the mount exactly as one would do with a theodolite

ie in two axes at right angles to each other. I then re collimated the polar scope and lo and behold the NCP cross hairs maintain their position over the full 180 degree arc of the RA axis.

I confess to feeling considerable relief as I like to have things exactly right. The first law of polar scope collimation......Make sure the tripod is dead level.

Yippee!!