[ausastronomer (John Bambury)]

Hi Rod,

You got it "almost" right.

In regard to this quote, "I undertand now that in most circumstances one does not use ECES and ECEC at the same time, and that IE, CA and NPAE vary between sessions and should not be saved."

What i said was this:-

Irrespective of your telescope type or mount type, never save IE (Index error in elevation). It is not repeatable from one session to another. It is like a "super" fix alt ref and renders the altitude reference angle term redundant, if you use a TPAS model.

In terms of the other errors, I said whether you saved them or not was entirely dependant on your mount type and what was causing those errors. I said that with a "truss" dob, with a "sling" type mirror cell it was not a good idea to save NPAE (non perpendicular axes) or CA (collimation error) because each time the scope was set up the truss poles can bed slightly differently from session to session. In addition the mirror can settle differently in the sling making these errors "non repeatable" from session to session. I mentioned that ECES and ECEC "could" be saved from session to session, again dependant on what was causing those errors. For instance if ECEC (Elevation Centering Error Cosine) was your most significant error term and caused by something like your altitude encoder being improperly centered on the altitude bearing axis of rotation, it would be a repeatable error and the term could be saved. I also mentioned that I didn't save any terms with my truss dob.

With a scope like a CNC machine made tubed dobsonian, or an equatorially mounted scope, things are somewhat different as there is a lot more repeatability in the setup, in regard to "some" of the terms.

Cheers,
John B

Originally Posted by Rodstar

Thanks to John (Ausastronomer) for giving me the low down re TPAS on the phone this morning.

Sounds like I should not be using all of the terms, too. For example, I undertand now that in most circumstances one does not use ECES and ECEC at the same time, and that IE, CA and NPAE vary between sessions and should not be saved.

Hope I got that right?!