Thoughts on Polar Alignment

[Daveskywill]

Hi IISpacers:

Some scopes invert the view. Mine in my Meade both scope and finder invert the view. So it can make it hard to polar align the wedge. I thought a laser pointer (in the sky attached to my scope) would help. ScopeStuff, on the net, has cool, neat gizmo gadgets that can help. And they have laser pointer holders, with double stick tape, plus adjusting screws to center to same plane as scope (you know like the type on rings).

I wish there was an equation to tell me how close to align (between Polaris and that bright star in Ursa Minors cup, by 3/4 degree is True North polar axis); but like how close is close enough for alignment for an exposure to not trial at equatorial region by so much and with a 10" F10 instrument for anything over 3 minutes (roughly). Does anyone know? Thank you

There is a book "The FX System of Exposure Determination" mostly meant for film. But does tell of how long an exposure can be (in seconds) until a trail of so many mm on a sensor or film at a Dec of so much using a certain focal length lens. The only thing is it doesn't seem to relate it to polar alignment (which is what I'm hoping for).

[mithrandir]

David, by do you mean Kocab? That is at 14h50'36.56" 77d6'23.8" and the 2nd brightest in Ursa Minor.
Polaris is at 2h49'28.66" 89d18'56.8"
A line joining them has PA=0 hence passes very close to the pole, so you can just use the declinations to get the ratio.
Kobab is 12.892556 deg from the pole. Polaris is 0.684223.
That's field width of about 14 deg and a ratio of around 19 to 1 so would be pretty hard to guesstimate.
In my experience it is harder to avoid trailing in images near the pole than near the equator. Look up drift aligning. Once you are in the ball park you can use that to iteratively refine your alignment until your stars don't trail a noticable amount.

[OzEclipse]

Very roughly, if the polar axis is offset 1 degree in azimuth and you start tracking with a star on the celestial equator and on the meridian, six hours later it will have trailed 1 degree in dec and obviously a little in RA. So it will trail approximately 1 arc sec per minute per degree offset. It is more complicated than this especially when the offset has altitude and azimuth components but it sounds like this is the sort of rough number you are looking for.

The plate scale of your scope is ~ 12 micron / arc sec so over 3 mins it will trail 36 microns. That's eight pixels of trail for a 4.5 micron/pixel camera.

At that scale an autoguider is pretty well essential.


Joe