https://bugs.kde.org/show_bug.cgi?id=424637
Bug ID: 424637
Summary: Suspicion that KStars does not precess position angles
correctly
Product: kstars
Version: git
Platform: Compiled Sources
OS: Linux
Status: REPORTED
Severity: normal
Priority: NOR
Component: general
Assignee: mutla...@ikarustech.com
Reporter: akarsh.si...@kdemail.net
Target Milestone: ---
I suspect that KStars does not precess position angles correctly.
A position angle listed in a catalog with a given epoch (say, J2000.0) would
likely be the angle with respect to the north direction that the object makes
with the NCP (North Celestial Pole) at that epoch (i.e. the J2000 NCP). Since
the current NCP is different from the catalog NCP, and since deep-sky objects
remain relatively fixed as seen from space, the catalog position angle should
remain fixed at the present day with respect to the *catalog NCP* and not the
JNow NCP.
However, reading the code in `Projector::findNorthPA` and `Projector::findPA`,
it appears that we render deep-sky objects assuming that their catalog position
angles are referred to the JNow NCP rather than the J2000.0 / catalog NCP.
Two potential solutions to this are:
1. In the existing Projector::findNorthPA code, change the "small displacement"
vector to point towards the J2000.0 NCP
2. Use the formula from Page 116 of Jean Meeus' book to compute the position
angle of an object relative to the precessed location of the J2000.0 NCP in
JNow, i.e.:
* First, we precess the coordinates (undefined, 90°) from J2000.0 to JNow to
get the current epoch coordinates of the J2000.0 NCP
* Next, we use Meeus' relative position angle formula to compute the relative
position angle of the J2000.0 NCP in JNow coordinates with the JNow coordinates
of the object of interest
* Then we add appropriately the PA of the object to this angle and render it
This method is hard to get correctly with all the signs, so we can test it
against the first method which is "hackish", but reliable.
STEPS TO REPRODUCE
1.
2.
3.
OBSERVED RESULT
EXPECTED RESULT
SOFTWARE/OS VERSIONS
Windows:
macOS:
Linux/KDE Plasma:
(available in About System)
KDE Plasma Version:
KDE Frameworks Version:
Qt Version:
ADDITIONAL INFORMATION
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