Re: AP900 DEC sawtooth guiding

Roland Christen

Still no information to work with. need to know the calibration parameter numbers after you do a calibration run.
Example: If pixel scale is 1 arc sec per pixel, then Dec movement would be 15 pixels per second. This parameter (15) is then used to determine the amount of move command that is sent to the mount. Therefore if the error is 1 pixel, the mount would be commanded to move 1/15 sec at 1x guide rate. This also means that the loop gain is 100%, which means that a 1 pixel error is compensated by a 1 pixel move command.
Knowing the pixel scale and the calibration result (parameter number obtained after calibration run) allow you to know if the loop gain is 100% or not.
If calibration number is wrong, your resultant loop gain might be more than 100%. If the parameter number obtained after a cal. run is only 7.5 rather than 15 in the above example, then for every 1 arc second position error the mount gets a 2 arc second (1/7.5 second) move command. Loop gain is now 200%. So, the net result is overshoot after every error with the axis constantly bobbling back and forth. The smaller the number relative to the correct number, the larger the overshoot.

-----Original Message-----
From: anismo@... [ap-gto]
To: ap-gto
Sent: Thu, Sep 22, 2016 12:30 pm
Subject: [ap-gto] Re: AP900 DEC sawtooth guiding

Hi Rolando

I dont have keypad connected to the system. It is only connected to the computer. ASCOM v2 and SGP

The PHD2 settings are
1. Camera pixel size - 8.62 micron (Lodestar x2)
2. Focal length - 382mm (Connected via OAG , so FSQ106 with 0,73 reducer)

I did calibration few times to be sure with calibration steps of 500ms (as well as let PHD2 calculate based on DEC). The calibration always completed without any issue

I also tried with Hystersis as well as resist switch DEC algorithm, as well as with different aggressiveness (all the way to lowest possible aggressiveness).  

Every one of these times, it will guide properly if I start the guiding but once it receives the dither pulse, the star will start jumping between two points and never settle. (almost same amplitude)

Here is one screenshot that I took whe n it was running ok:

The DEC always stayed below the 0 line.. not sure if I have PA misalignment (I did PA using polemaster and was able to get it calibrated pretty well as per that software).


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