For some unknown reason I am not able to get an OAG to work reliably and I am sure it is not a mismatched hardware issue.
I have tried several different OAGs, guide cameras and scopes without much success; the calibration looks very similar every time but the results vary. Just for clarity, the calibration step was adjusted every time to give a reasonable number of steps - last try at 1650 mm and 825 mm focal length, the calibration step size was 200-250
Here is the list of components I have tried:
OAG: ZWO OAG-L, Celestron OAG, QHY OAG, ZWO 2600MC DUO
Guide Camera: 174MM mini, 290MM mini, 220 MM Mini and a 0.5 focal reducer
Scopes: Celestron C9.25, Sharpstar 150HNT
Mount: Sharpstar MK III
Every calibration looks quite the same: the mount will take something like 8-12 steps in one direction (West) then reverse and take only a couple steps back (sometimes moving so much that the star will be lost and reacquired).
After clearing backlash (the mount is harmonic) and starting on North-South the behaviour will be similar with the same uneven number of steps (North = 12 steps, South = 4 steps).
The calibration is said to be completed, guiding is all over with the total error in the 1-2 arcseconds and the stars are (very) oval, getting worse at longer exposures.
Same behaviour when using the 2600MC DUO with its integrated guiding chip.
Polar alignment is always within a few arcminutes (smiley face on PA page) and the scope is roughly balanced even if that is not a requirement with harmonic mounts. Everything works great with guidescopes but terrible with OAG.
Here is a snippet from the PHD log showing the calibration with 174mm mini and I will try to attach the whole log (tried the "Upload" option and got an error, the log is about 3.5 MB in size).
I hope somebody can give me some insight here because honestly, I don't know what I can do more. Thank you.
Calibration Begins at 2024-01-05 23:52:48
Equipment Profile =
Camera = ZWO ASI174MM Mini
Exposure = 1000 ms
Pixel scale = 2.93 arc-sec/px, Binning = 2, Focal length = 825 mm
Mount = ZWO OnStep, Calibration Step = 250 ms, Assume orthogonal axes = no
Dec = 0.2 deg, Hour angle = -0.12 hr, Pier side = West, Rotator pos = N/A
Lock position = 561.028, 368.581, Star position = 561.031, 368.504, HFD = 2.29 px
Direction,Step,dx,dy,x,y,Dist
West,0,0.000,0.000,561.277,368.642,0.000
West,1,-0.380,-0.618,561.657,369.260,0.726
West,2,-1.905,-2.407,563.182,371.049,3.069
West,3,-3.643,-3.610,564.920,372.252,5.129
West,4,-5.183,-5.301,566.461,373.944,7.414
West,5,-7.138,-6.550,568.415,375.193,9.688
West,6,-8.893,-7.901,570.170,376.544,11.896
West,7,-10.333,-9.715,571.610,378.357,14.182
West,8,-11.722,-11.495,572.999,380.137,16.417
West,9,-13.713,-12.439,574.990,381.081,18.514
West,10,-15.156,-14.136,576.433,382.778,20.725
West,11,-17.325,-16.335,578.603,384.977,23.812
West,12,-18.631,-17.572,579.908,386.214,25.610
West calibration complete. Angle = -136.7 deg, Rate = 8.537 px/sec, Parity = Even
East,3,-18.631,-17.572,579.908,386.214,25.610
East,2,-12.396,-11.504,573.673,380.146,16.911
East,1,-5.783,-5.637,567.060,374.279,8.076
East,0,1.511,1.021,559.766,367.621,1.824
Backlash,0,0.000,0.000,559.766,367.621,0.000
Backlash,1,0.505,-0.670,559.261,368.291,0.839
Backlash,2,1.268,-1.219,558.498,368.840,1.759
Backlash,3,1.292,-1.845,558.474,369.466,2.252
Backlash,4,2.648,-3.122,557.118,370.743,4.094
Backlash,5,3.861,-4.268,555.905,371.889,5.756
Backlash,6,5.350,-5.598,554.416,373.219,7.743
North,0,0.000,0.000,555.905,371.889,0.000
North,1,1.488,-1.330,554.416,373.219,1.996
North,2,3.201,-3.254,552.703,375.143,4.564
North,3,4.842,-4.582,551.063,376.471,6.666
North,4,6.291,-6.362,549.614,378.251,8.947
North,5,8.335,-8.135,547.570,380.025,11.647
North,6,9.485,-9.547,546.419,381.436,13.458
North,7,10.539,-10.543,545.366,382.432,14.907
North,8,12.547,-12.387,543.357,384.276,17.632
North,9,13.828,-14.149,542.077,386.038,19.783
North,10,15.331,-15.500,540.574,387.389,21.801
North,11,16.584,-17.260,539.320,389.149,23.936
North,12,18.065,-18.702,537.839,390.592,26.002
North calibration complete. Angle = 134.0 deg, Rate = 8.667 px/sec, Parity = Even
South,4,18.065,-18.702,537.839,390.592,26.002
South,3,16.579,-16.658,539.326,388.547,23.502
South,2,12.346,-10.663,543.559,382.552,16.313
South,1,6.280,-4.264,549.624,376.154,7.591
South,0,5.815,-3.340,550.089,375.230,6.706
Calibration complete, mount = ZWO OnStep.
