Re: Notified!😍 #mach2gto #Mach2GTO

Roland Christen

I wonder how much error results from using a tripod vs. a pier,  and then much variation I'll see throughout the night.    
You should be able to get polar alignment good enough and keep it so with a solid tripod. Once aligned, I have not seen a tripod change the polar alignment.
Good polar alignment for imaging should result in Dec drift of 1 arc sec per 10 minutes at the meridian. If you also drift align the RA at the zenith, you should then also get 1 arc sec or better drift in 10 minutes for up to 2 hours on either side of the zenith. In PHD2 you can monitor your drift using the Trend Lines. You can then make tiny adjustments to the Alt and Az while watching the trend Lines and eventually zero in on perfect PA. Do the Azimuth first until the Dec line becomes flat - ignore the RA. Then tweak the Altitude until the RA line becomes flat. Do it on an overhead star. You might have to iterate a couple of times to get both Trend lines flat for at least 5 minutes. 
-----Original Message-----
From: Terri Zittritsch <theresamarie11@...>
To: main <>
Sent: Tue, Mar 10, 2020 2:17 am
Subject: Re: [ap-gto] Notified!😍 #mach2gto #mach2gto

On Tue, Mar 10, 2020 at 01:59 AM, uncarollo2 <chris1011@...> wrote:
The RA and Dec drift rates indicate fairly large Polar Alignment error. It means your Polemaster is giving you a significant PA error for whatever reason. That's why I don't rely on electronic devices for polar alignment. If you want the drift rate to be zero, then you have to do drift alignment where you twiddle with the Alt and Az adjusters until the drift rates are zero (which is the definition and goal of drift alignment). Ray Graylack's Pempro does that without muss or fuss in surprisingly short time. Once I have drift alignment zeroed in, I simply attach my right angle Polar scope, place Polaris at the correct point on the dial via the push-pull adjustment and I'm good to go from then on using only the Polar scope. Of course with guiding the drift doesn't matter, but for best performance it is good to have essentially zero drift. Yerkes observatory did it in the late 1800's and they had some pretty crude methods with that 40" refractor mount. But it was good enough to allow unguided imaging for considerable time exposures.
Every generation of astro-imagers has to go thru this learning process, and the fundamentals never change. The 2.5 arc sec P-P excursions are almost exactly what the seeing was for that night, so that will be the best you will get under those conditions. No way anything less than a $1million active guiding system along with high power laser for forming an artificial star will do any better.
By the way, your Vermont skies are not too bad. In the last week I had 1.2 arc sec FWHM resolution with no guiding (simple model in the keypad only) for 2 nights, then the seeing morphed into 7.5 arc sec FWHM stars when the trade winds picked up. Sky was crystal clear but stars looked like small shaggy dogs.
Thank you again Roland for your personal time here.    I think what you're telling me is that by drift aligning, and then adjusting your polar scope to put polaris in the correct position based on the drift alignment, that you're calibrating your polar scope for refraction effects at your location.   I guess you're actually moving your reticle to match where polaris is after drift aligning.   I know Polemaster is supposed to be able to account for some refractive effect, but likely not super-accurate and just some kind of calculated estimate.    Are you also saying that for a particular location, refractive effects are constant?

If the offset due to refractive effects is constant, then maybe I can estimate in polemaster.    Right now I have no idea what this might look like (how much).   I guess maybe a drift align one night with polemaster monitor running will give me my answer.     The other complication is that I have no pier or 100% stable setup.   I wonder how much error results from using a tripod vs. a pier,  and then much variation I'll see throughout the night.    


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