meade astrometric and celestron microguide

[ving (David)]

just a quick question about the microguide and astrmetric EPs.... obviously measurements made with these EP are going to be different depending on what fl telescope is used right?

1200mm fl @ 120 times.
2000mm fl @ 166 times.

the measurements in arc seconds are going to show as different for each of the scopes... or am i wrong?
and if i am right are these EPs designed to work at a specific focal length?

[Rodstar (Rod)]

Ving, the position angle measure will be the same, regardless of magnification, but I think you are right that the separation scale will need to be calculated depending on the magnification achieved with your particular scope. My understanding is the the separation scale is not usually expressed in specific units (such as arc seconds) on the reticle, but rather as regular notches, onto which you impose the scale relevant to your scope.

I have been pondering another question, which relates to working out position angle (hereafter "PA"). Obviously this needs to be measured by reference to north. The books I have read about taking PA speak of letting the stars drift across the FOV (ie untracked), with that movement always being from east to west. Once that is established, North can be inferred, and from there, you can take the PA measure.

What I want to know is whether, once you have done the drift test once, and set the reticle up with north at the top, whether you can leave the reticle in that same position, and take the rest of the measures for the session assuming the north in the reticle to be correct. Or, would this only work if you are polar aligned? Do you need to be polar aligned at all?

Maybe I should just get a jolly reticle, and find out by trial and error!?

[ving (David)]

lol, rod. best to get a jolly one too i'd say!

I am guessing that if you have an Eq mount then once you find north you can just leave it there as long as you are polar aligned, but on an alt az you would have to check it each time... i doubt it would take much to do tho.

is there an easy formular to convert notches to arc secs do you think? something like divede the FOV in " by the number of notches to find out how big a notch is?