This is to respond to a question in the “polar alignment” feature request thread. I was asked:
“Why not use a external focuser like you would on a RC? Seems to be the tried and true method for it. I understand you want the focuser to work remotely… seems like you’d get a better product that way. The only limitation is cost.”
There are many answers to this - but a simple one is that my system works extremely well as is, and it would all work fine except for the focus routine insisting on moving from right to left through the focus curve. I should not need to add on an additional focuser - especially since one is already built into the sct.
In fact - high end rc’s do not have an external focuser. They have the imaging train attached to the back rigidly - and the secondary does the focusing. With an sct I can have the same thing - except the primary moves. I find the focusing in an sct to be extremely repeatable as long as backlash is removed. There is no downside to using the sct focuser - with the right technique and software - and there are many upsides.
Having a motor and motorized focusing has nothing at all to do with being “remote” - it has to do with achieving fine focus with an autofocus routine - and that is central to automated imaging - and even manual imaging of a single object - and touted by SGP.
Finally - when using OAG, there is limited backfocus with many telescopes, including sct’s, so the addition of a focuser is not only unnecessary, it may be impossible because it takes up too much backfocus. But more importantly - it serves no benefit at all except to add flexure to the otherwise fixed imaging train. In my case I have a rotator also involved - which takes up additional backfocus.
Even if you did use an external focuser - if you want to remove backlash, it would not work if the numbering happened to go the wrong way.
But more to the point of the request - I would claim that SGP has many features specifically to address the issue of backlash and correct behavior of in vs. out. Look at this from the flyover help text for backlash compensation in sgp:
“Compensation will be invoked when the move is opposite the compensation direction.”
So - backlash compensation is important enough to be supported in SGP, and it is critical to have it set the right way for a given setup, and options are provided to reverse the sense of it for a user’s setup - but when it comes to autofocusing it ignores everything and goes from high to low. If backlash compensation is important, and may be either way for a given setup - this is simply a bug, or at the least an overlooked problem that could and/or does impact many users.