#### Re: PEMPro and accurate polar alignment

Roland Christen

Here's what I do at the zenith. I link to the mount via MaximDL, acquire a guide star and start a guide exposure with aggressiveness set to zero. I open the tracking Error Graph and watch the RA drift. After a few minutes it is apparent that the drift is either + or -. In my case I know that with my camera orientation a minus drift means I need to raise the altitude axis, but in your case you need to make that determination by trial and error.

How much to lower or raise? Usually from a PEMPro drift method to the zenith adjustment the difference is about 1/4 rotation of the altitude adjustment wheel. In my case, after this adjustment I get less than 1 arc sec drift in a 15 minute time period on either side of the zenith up to about +-1 hour. Beyond those boundaries that, the drift begins to increase, but even though there is slightly more drift I can still do 5 to 10 minutes with no ovality showing up in the stars.

For those wishing to wade thru more rigorous methods of polar alignment and a handy calculator, here are two websites:
http://canburytech.net/DriftAlign/DriftAlign_3.html
http://www.bbastrodesigns.com/equatTrackingRatesCalc.html

Rolando

-----Original Message-----
From: Hemant Hariyani hemanthariyani@... [ap-gto]
To: ap-gto
Sent: Mon, Sep 25, 2017 12:54 pm
Subject: Re: [ap-gto] Re: PEMPro and accurate polar alignment

I tried measuring drift using PEMPro at zenith and am a bit surprised by the result.

I first tried to change position of altitude drift measurement in PEMPro slew dialogue but I was not able to select a degree offset less than 30. So I slewed my mount to zenith using my latitude as declination and a close to zero degree offset from meridian. I chose to be on the west of meridian.

I then started measurements in PEMPro to start with, it showed a huge offset and asked me to lower my altitude by 60 odd arc minutes. This reduced over a period of time and by the end of 5 minutes run, it was around 15 arc minutes. Without re-positioning the mount to zenith I let it continue for more time, restarting measurements and after about 20 minutes, this was down to 2.8 arc minutes. I tried this again and the results were fairly consistent. I have no idea on what is going on here. I did not affect this significant a change in measurements in such a short period of time. Any guesses on what can be going on? I have encoders enabled.

Regards
Hemant

On Mon, Sep 25, 2017 at 10:01 AM, chris1011@... [ap-gto] wrote:

PEMPro tries to align to the non-refracted pole to minimize field rotation over the night.

Roland's technique of minimizing RA drift is great for short-term unguided tracking in a local area of the sky. The technique eliminates RA drift caused by polar alignment error, refraction, telescope component flexure, and worm wheel eccentricity, all of which can cause slight variations in RA speed.

But, I do not think polar alignment using RA is likely going to be as accurate as using the (non-moving) Declination axis unless there is an encoder on the RA axis to compensate for the worm wheel's (very slight) eccentricity, possible tooth-tooth variations in the worm wheel teeth, and residual periodic error.

Roland, please correct me if I am overlooking something here?
Yes Ray, you are absolutely correct. The issue is whether you want zero drift around the zenith, which is useful for unguided imaging, or you want zero Dec drift (and variable RA drift) over larger parts of the sky. You can achieve one or the other. Pointing will be more accurate with the PEMPro drift alignment method, but you will get more RA drift overhead.

Rolando

-----Original Message-----
From: 'Ray Gralak (Groups)' groups3@... [ap-gto] <ap-gto@...>
To: ap-gto <ap-gto@...>
Sent: Sun, Sep 24, 2017 11:26 am
Subject: RE: [ap-gto] Re: PEMPro and accurate polar alignment

> Then what is the value of PEMPro, if it can't get this right?

PEMPro tries to align to the non-refracted pole to minimize field rotation over the night.

Roland's technique of minimizing RA drift is great for short-term unguided tracking in a local area of the sky. The technique eliminates RA drift caused by polar alignment error, refraction, telescope component flexure, and worm wheel eccentricity, all of which can cause slight variations in RA speed.

But, I do not think polar alignment using RA is likely going to be as accurate as using the (non-moving) Declination axis unless there is an encoder on the RA axis to compensate for the worm wheel's (very slight) eccentricity, possible tooth-tooth variations in the worm wheel teeth, and residual periodic error.

Roland, please correct me if I am overlooking something here?

Best regards,

-Ray Gralak
Author of APCC (Astro-Physics Command Center): http://www.astro-physics.com/index.htm?products/accessories/software/apcc/apcc
Author of PEMPro: http://www.ccdware.com
Author of Astro-Physics V2 ASCOM Driver: http://www.gralak.com/apdriver
Author of PulseGuide: http://www.pulseguide.com
Author of Sigma: http://www.gralak.com/sigma

> -----Original Message-----
> From: ap-gto@... [mailto:ap-gto@...]
> Sent: Sunday, September 24, 2017 8:14 AM
> To: ap-gto@...
> Subject: Re: [ap-gto] Re: PEMPro and accurate polar alignment
>
>
>
> Then what is the value of PEMPro, if it can't get this right?
>
>

------------------------------------
Posted by: "Ray Gralak \(Groups\)" <groups3@...>
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