questions before buying my first star tracker

Started 10 months ago | Discussions thread
hha1 Junior Member • Posts: 27
Re: questions before buying my first star tracker

Astrophotographer 10 wrote:

Start with shorter focal lengths. Get the counterbalance kit. Make sure its well balanced.

Get good at polar alignment. I haven't used the Skyguider Pro but use Vixen Polarie and Fornax Lightrack ii.

With a good polar alignment using the excellent Vixen Polar Scope but no counter balance I found about 100mm focal length was the limit. Perhaps with the balance kit it would be longer, I did not try that.

OK. I don't see your problem.

If your pole axis alignment is off by one degree, then you get 15*sin(1)=0.25 arcses/sec of trails. With a 180 mm lens  and 4 micron pixels in your camera ( D7200 and Z6) you have 5 arcsec per pixel. So you can expose for 5/0.25=20 seconds before the misalignment error exceeds one pixel. Few lenses resolve one pixel.

Assume you have Skytracker with a 26 arcsec worm error and a 176 tooth gear. Your worm period will be 24*3600/176=490 seconds. With 30 sec exposures each exposure will be shifted 26/2 * 30/490 = 0.8 arcsec. This is much smaller than the 5 arcsec resolved by a 180 mm lens. Sequator will track the stars and the worm error will become invisible (unless you have hot pixels where you can see them).

I got excellent results with 30 seconds and a 180mm f/2.8 on a Z6 on a Skytracker, 0.5 degree pole axis alignment error,  with 100  subs at 30 sec each, but with a good old Linhof tripod. The only rejects are airplane tracks.

hha

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