Am I doing polar alignment right please?

Started 6 months ago | Questions thread
hha1 Junior Member • Posts: 27
Re: Am I doing polar alignment right please?

EricTheAstroJunkie wrote:

hha wrote:

cheddarman wrote:

Having checked the calibration of my Sky watcher Star Adventurer Mount (Appendix ll) it is spot on, no movement when I rotate it 180º.

Is this an adequate alignment or do I also then have to fiddle with the Time Meridian/ Date Graduation circles at the rear of the mount?

I had a similar concern: What is an adequate alignment accuracy? You want any trails during one exposure to be less than two pixels. At least this is my definition of adequate, given that I use software to register the individual pictures.

This is a difficult thing to measure in a single exposure. I take a series of exposures of 30 seconds each. Ten shots in 10*30=300 seconds is typically good enough to see trails.

Now add the shots without registering the stars, or step through the shots in a picture viewer at 100% zoom. You will probably see short trails. If the trails in 10 shots is 20 pixels long or less, your pole axis is not well aligned, but you are OK, since the star registration program will take out the alignment error. If you see sudden shift, you have a problem unrelated to pole axis alignment.

You can measure your pole misalignment this way as so many arcminutes E or S of the true pole, but who cares. If the trails are less than 2 pixels per exposure, you are good enough, no matter how you did the alignment.

Try it out in you backyard.

hha

This may be entirely impossible depending on the pixel scale of your imaging system, even at wider focal lengths and larger pixels it's likely stars themselves will occupy more than 2 pixels.

I agree, that you can't tell anything with two pixels looking at a bright star. With the roughest alignment your pole axis pointing may be one degree off  the NP. This means you get 0.26 arcsec/sec trails. i.e. 45 arcsec in 180  seconds. If you are using a 200mm lens and and a FF 24 Mp camera, the pixels are 6 microns, and one pixel will subtend 5 arcsec. With a 180 second exposure you will get 9 pixel trails, which should be obvious looking at faint stars. Obviously a 1 degree error in the alignment is not good enough. If you want to limit the trails to 3 pixels with a 200 mm lens and 180 second exposures,you need to be aligned within 0.25 degree of the pole.

Depending on what I'm imaging I'm ok with eccentricity values up to 0.6ish, 0.4 is perfectly circular stars by human vision. I'm currently working on a 20 panel mosaic of the Sadr Region, 1 hour of each narrowband per panel, 3 minutes per exposure, I'm accepting slightly worse stars knowing that the resolution of the image will be so high that I can make stars virtually imperceptibly small and remove them via star size reduction even with some amount of elongation.

Making a 300 second test exposure to verify that you pole alignment is good enough is a good investment when you do an hours long mosaic.

Best of luck with you Sadr region mosaic, it will soon get too low in the sky.

hha

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