It's not the mount.
It's the OTA and it's dovetail bracket, rings, focuser, diagonal, adapters, flexure and collimation.
We are squaring the DEC axis with the OTA's optical axis.
1. Make sure everything about the OTA/focuser/rings/dovetail/etc. are tight and smooth. No flexure anywhere.
2. Do a careful collimation, as appropriate for OTA type.
3. Evaluate flexure as described in the AP manual.
4. Shim one of the ends of the OTA's rings/base/dovetail with appropriate material, such as cuttings from the side of an aluminum pop can.
5. Only one end will ever need to be shimmed, not both.
6. Go back to step 3 and repeat until error is minimized.
Note that future collimation changes will effect orthogonality.
Also note that when polar aligned and the OTA has no flexure, orthogonality-induced pointing error will only be in the E-W direction. This can help to find objects quickly in spite of suffering from orthogonality error.
I hope this helps.
Waikoloa, HI 96738
From: ap-gto@... [mailto:ap-gto@...]
Sent: Tuesday, March 24, 2015 7:10 PM
Subject: Re: [ap-gto] How to improve pointing
Yes, if I recal on the other side of the meridian, it points well.
Do you have any perspective on how to address non-orthogonality on the AP900?