relationship between mapqk and ampqk

[danielsf]

Reading the documentation for mapqk and ampqk, I get the sense that they are supposed to invert each other. Mapqk transforms the mean position of a star into the apparent geocentric position. Ampqk does the opposite. However, Mapqk accepts parallax as one of its arguments. Ampqk does not. Indeed, in using the functions, I have noticed that they do invert each other to sub-milliarcsecond precision when parallax is zero. When parallax is not zero, running the roundtrip through mapqk and ampqk can result in a residual on the order of a few 0.1 arcseconds. Is there some reason these functions are not exact inverses of each other? I understand that subtracting out the effects of proper motion and radial velocity might not make much sense, but I would have assumed that parallax was something that we would want to be able to account for in converting from geocentric apparent to mean position.

Thanks.

[dsberry]

This is a question for Pat Wallace - author of slalib, upon which pal was based. I suggest you re-post your question on the starlink mailing list STARLINK@jiscmail.ac.uk, which Pat usually checks, or email him directly.

[danielsf]

Thanks. I emailed the mailing list.

[danielsf]

Apparently, I am not allowed to post to the STARLINK list unless I am registered there. Do you have Pat Wallace's direct email address handy? Thanks.

[timj]

I'll copy it to you offline on Slack.

[timj]

The PAL implementations are at https://github.com/Starlink/pal/blob/master/palAmpqk.c and https://github.com/Starlink/pal/blob/master/palMapqk.c

[danielsf]

Thanks. I was able to look at those via the git submodule that gets cloned with PALPY.

[danielsf]

I emailed Pat. The conclusion of that conversation was that ampqk was never meant to invert the ICRS-to-apparent-geocentric transformation for an actual celestial object (i.e. something with a parallax). It is only useful for going from (for example) alt, az to apparent geocentric to a theoretical position on the celestial sphere.