Intro

Over the past weeks persistent cloud cover over southern Austria had effectively halted any deep-sky work — until a brief clearing yesterday finally allowed a few hours of imaging. While a colleague targeted the interacting galaxy pair NGC 935 / IC 1801 (Arp 276), I happened to be nearby with a C9.25 reduced to 1306 mm focal length and also captured Arp 276 about two hours just with luminance.

During inspection of the frames in PixInsight I noticed that the readout galactic coordinates of the reference star TYC 1218-420-1 differed dramatically from the values listed in SIMBAD. A forum discussion suggests PixInsight reports galactic coordinates tied to the ICRS realization rather than the classical FK4/FK5-based IAU system — effectively referencing quasars instead of historical fundamental stars. However, the magnitude of the discrepancy I obtained appeared far too large to be explained by the reference-frame change alone.

This observation triggered a deeper investigation into how “galactic coordinates” are actually defined today — and why two technically correct systems can still yield very different numbers.

Readout Data in PixInsight …

As PixInsight offers enormous possibilities, every project is the chance to dive deeper… This time the ReadOut-Cursors figures attracted my attention: if an image is plate solved (=has an astrometric solution), the ReadOut-Cursor shows of course Right Ascension (α) and Declination (δ) for every pixel as well as the Pixels Intensity „K“ with integer values (in my case the maximum brightness of 65.535 for the fully saturated star core ( I use the 16-Bit ZWO ASI 2600 MM Pro).

Right underneath the ICRS equatorial coordinates, given in α and δ – as it was my assumption – the Galactic Coordinates (l) and (b) are displayed. As I will describe in the following this assumption was somehow wrong …

PixInsight does not follow the IAU conventions for the Galactic Coordinates!

After running the SetiAstro-Script („Whats in my Image 2.7.1“) a very comfortable search is possible with object information directly made available from SIMBAD and other databases. Having identified the brightest star in my image as „TYC 1218-420-1“ (with a Magnitude of 9.93 mag) I found that the Galactic Coordinates did not match with the ReadOut in PixInsight. The SIMBAD coordinates were 152.1270890687861 -37.7420794951435 whereas the reading in PixInsight showed l = 285 40 21.97 and b = -40 53 06.63.

As I understood it (until now…) Galactic coordinates describe the viewing direction of an object on the celestial sphere as seen from the Sun relative to the plane of the Milky Way:

The Galactic longitude l is measured along the plane, with the zero meridian defined by the direction towards the Galactic Centre (Sgr A*), for which $l=0^{\circ },b=0^{\circ }$. The Galactic latitude b is the angular distance from the Galactic plane. The North Galactic Pole corresponds to $b=+{90}^{\circ }$.

However, PixInsight does not adopt the commonly used IAU definition of Galactic coordinates, which is tied to the FK4/FK5 reference frames and therefore inherits small systematic offsets when used together with modern astrometry. Instead, PixInsight implements a Galactic coordinate system that is rigorously consistent with the International Celestial Reference System (ICRS). The ICRS was introduced 1998 using extragalactic quasars as reference objects. This approach follows the formulation described in a paper by Liu, Zhu, and Zhang (2011), ensuring that coordinate transformations remain internally coherent with contemporary astrometric catalogs such as Gaia. As a result, the Galactic longitudes and latitudes reported by PixInsight will differ from traditional IAU values, but they are mathematically consistent with the underlying ICRS reference frame used by modern plate-solved images.

One would therefore expect the coordinates derived from either system to differ only slightly, perhaps by a small offset. What I find difficult to imagine however, is a discrepancy of the magnitude observed for the star TYC 1218-420-1: PixInsight reports a galactic longitude of 285°, whereas SIMBAD lists 152°. Such a large difference hardly seems attributable solely to the use of different reference systems — or could it?

The question therefore remains unresolved. Should anyone be able to contribute to clarifying this issue, or point out a fundamental aspect I may have overlooked, I would be most grateful.