Ted

There have been a couple recent threads on building APPM models during bright moon times.  Here is one.  https://ap-gto.groups.io/g/main/topic/81388396

I do not think there is a max limit on focal length, as long as plate solving still functions.  I build models on a 2500 mm OTA with a full frame sensor / OAG and plate-solving is not a problem. 

Image scale may be more important.  The APPM section states
"If your image scale when binned in less than 3 arc-seconds/binned pixel then plate solve accuracy
will not be affected."



Bryan