Polar star height and observing site altitude

[cesco]

Hi all,

I observe from an elevated site at 400m altitude, 41° latitude. After leveling the mount and centering Polar in the polar scope, I expect to read  41° (approximately) on the polar head height scale. But the reading is out, about 1.5° higher. Of course I would tolerate a slight deviation, for mechanical matters, but not that much. Could this discrepancy be due to the altitude the scope is placed at? Is there any correction to apply to the Polar height, due to the observing site altitude?

Similarly, when I level both the tube and the counterweight bar, I assume I am aiming at the horizon, due south. But, as the scope is levelled at 400m, is that point the same I would aiming at at sea level? When I convert azimuthal coordinates of this point (180° azimuth, 0° height) to equatorial, should I apply some height correction?

Francesco

[AstroKeith]

Your altitude won't affect the poor alignment angle. Something else must be slightly off. Align with the polar scope (assuming it is itself aligned!)

Similarly when aiming at the horizon, you can normally ignore your altitude. The Horizon is many kilometres away and any error will not be significant.

[Laurieast]

6 minutes ago, cesco said:

I expect to read  41° (approximately) on the polar head height scale.

What mount is this? Those scales are usually only very approximate.

[vlaiv]

It has to do with leveling - your mount should be level and altitude scale should be precise enough.

Actual elevation above sea level has zero impact

Atmospheric refraction can also have impact on this, but effect is rather small:

(up to half a degree - but it really falls down to just few arc minutes at 40° altitude).

 

 

[ScouseSpaceCadet]

3 hours ago, Laurieast said:

What mount is this? Those scales are usually only very approximate.

Mine is *very* approximate! My latitude is 53.4. to get Polaris roughly centred in the AVX polarscope, the scale sits at 55. Gotos and tracking are fine so I don't worry.

 

[cesco]

 Thank you all!

My mount is a manual Losmandy G11 dating to late nineties. I never attempted to upgrade it to Goto, and will not, given the amount of malfunctioning reports I read every day in the Gemini website. I stick to manual and star hopping…..Nonetheless, I usually get very good polar alignment by use of a PoleMaster, and good autoguiding by use of PHD2. I agree with poor precision of the height scale, and long since ignore the issue I raised just for curiosity.