polar alignment - another angle

[Andrew C]

I am curious just how accurate SCP alignment for an equatorial mount should be to avoid the need to carry out drift alignment. I am fairly newly using a 6" f5 scope on a CG5 mount with an astro video camera and small monitor for visual only use, and so far have not got around to practising drift aligning (which sounds quite cumbersome). It appears that so long as due care is taken with locating the SCP, and optical axis is well aligned with mount RA axis, there does not seem to be much movement over say up to a minute of frame integration to warrant going to the extra trouble of drift alignment.

Am I right on this or can anyone say from experience that is there significant benefit in sharpness in going the extra step? I guess video is intermediate between true visual and imaging in this regard.

Also, can anyone tell me roughly how close in arcmin one should aim to get to the SCP? I tend to use the method of aiming for the empty corner of the equilateral triangle formed with the bright pair of the '7' star pattern a degree or so south of sigma octans, which should achieve about +-10 arcmin.

[iceman (Mike)]

I guess it just depends how long you want your exposures to be. Ideally, the more accurate the better. 1 minute exposures might be ok for an integrating video camera??

For a DSLR or astro CCD, 5-10+ minute exposures are common, so any drift is unacceptable and drift aligning is critical. Not to mention guiding. Guiding will hide polar alignment inaccuracies in single exposures (sometimes) but you'll get field rotation over the course of your session.