SharpCap Forums

This question may have been asked but I can't find it in the search. My observing site is at 2500 meters elevation above sea level, corresponding to about 750 mb of pressure. Using the formula for atmospheric refraction: P=pressure, T=temperature, E=site latitude

R (in degrees) = (0.00452 * P(in mb)) / ((273+T(in Celsius))* tan E (true latitude))

at my site 37.7 degrees latitude, given 10 degrees C temperature, I get a 1.26 arcminutes shift for the pole at sea level and 0.93 at my site. This seems significant enough to include the site elevation above sea level parameter in calculation in the polar alignment software. Keeping the atmospheric pressure at sea level, changing the temperature from 30 to -15 C, I get a shift of 1.17 and 1.38 arcminutes, respectively. While smaller, this correction should also be considered. I have a permanent observatory and want to lock down the mount once and forget it.
Last night was my first use of the SharpCap feature, and it is really good, much better than the iPolar device I have been using. Particularly useful are the "left/right, up/down" instructions, and the real time live view with on the fly adjustments as you move the mount. Well done on the GUI. Testing with my portable mount in my front yard driveway (elevation 1250 meters, temperature 20 C), I was able to align to where the drift in declination was less than one arc second in five minutes tracking an object near the meridian.

Hi,

Glad you liked the user-interface - that is the key thing I was trying to get right in the PA routine as there were previously already a number of PA tools, but most of them were hard to use.

SharpCap's correction for the refraction of the pole is just a basic calculation based on the latitude (altitude of the pole). There is no allowance for altitude or temperature. The question is 'is it really worth worrying about?'. Although you can dial the polar alignment error in SharpCap down close to zero if you have good alt/az adjustment mechanisms, for practical purposes that isn't necessary...

I did some calculations on this for a talk I gave a while back - if you have 30 arc second polar align accuracy then you should be able to do a 15 minute sub-exposure with only a max of 1 arc sec of polar alignment drift as long as you are at declinations between -80 and +80. If you are imaging near the NCP then oddly I think you want to turn refraction correction off and align on the refracted pole rather than the true pole - after all it's the refracted pole that is going to stay stationary in the sky as the earth rotates, with all the other stars appearing to rotate around that point.

cheers,

Robin