"Star chart accuracy." by Raymond Bruton: (email:pam99999XXX @ XXX,  remove XXX)

Dated: 2000-3-21


In consideration of star charts, especially those produced by computer programs, several things must be examined critically. These questions are related to the star catalogues used by the program.

A. Completeness: How complete is the plot of stars to a given limiting magnitude?

B. Accuracy: How accurate are the positions and the photometry (specifically the magnitudes, especially the visual values?)

C. Duplicity: Is the same star plotted in more than one place?

These questions should shape the evaluation of charts for the purpose of identification of deep sky objects which are also plotted on them.The deep sky objects that we seek can be at the very limit of human vision so the true representation of galactic stars in the foreground is very important. The representation of objects within the solar system will not be considered here.

The question of completeness is very important. Missing stars confuse the star patterns for identification of a faint planetary nebula or galaxy which itself may be plotted slightly out of place. Deep sky catalogues often have crude coordinates. The observer needs good representations of the star field to successfully identify the "faint fuzzy" he or she is searching for. Incompleteness also mars the appearance of major open clusters such as the "double cluster" (NGC 869 and NGC 884.) This last example is particularly important as the astrometric catalogues used to plot the charts are dreadfully incomplete in "crowded regions" such as clusters and the Magallenic Clouds. This is a by product of the very process by which they were produced. Although fainter components of close double stars are likewise excluded this is not too important as the primary is almost always included. No catalog is perfect for plotting the sky.

The SAO Catalogue has many "missing" stars. The star Zeta Sculptoris is not included and hundreds of seventh and eight magnitude stars were "missing." In spite of these shortcomings the SAO is still used by many programs to plot charts. The PPM Catalogue has been hailed as the successor to the SAO. The PPM adds many more stars and has greatly improved coordinates compared to the SAO. The SAO and PPM include many stars to magnitude 10 (visual,) yet neither is truely complete to magnitude 9.0. These two catalogues which are widely used to plot charts are notable because they include the Henry Draper spectral classes for most of the stars. The SAO or PPM in conjunction with the Hubble Guide Star Catalogue (GSC) was all that was available for many years to plot charts. The GSC was produced by digital scanning of Schmidt Plates for the use of the Hubble Space Telescope. It was never intended for use as a general catalog complete to 13 mag. in rich milky way fields or to 15 mag. away from the plane of the galaxy. Yet for many years it was the only thing available to extend plots of stars beyond magnitude 9 or so. In the magnitude range 9-13 it is much more incomplete than the SAO or PPM used to plot stars to magnitude 9. Whole large areas of the sky are poorly represented due to improper scans in conjunction with plates which were often overexposed. Many areas of improper scanning give the appearance of intervening dark nebulae in areas of the sky where none in fact exist. The GSC has proved useful to plot charts in spite of many shortcomings because of it denser star coverage.

Then the ESA released the Hipparcos and Tycho Catalogues. The Tycho catalogue in particular is nearly complete to magnitude 10.5 and plots many to 11.0. Its use in programs to plot star charts was thus assured. It was more complete than the PPM and plotted stars to a greater density. Some stars especially those in open clusters were excluded. This catalog in conjunction with the Hipparcos catalogue did attempt to at least "sample" the brighter stars in the Magellanic Clouds. Yet the GSC catalogue reached greater limiting magnitudes with a resulting greater star density. Since even a fairly small telescope can reach greatly beyond magnitude 11, greater star densities are needed to properly identify the embedded "faint fuzzies." Although the Tycho Input Catalog (TIC) was used by some programs to try to extend Tycho to mag. 12.5 this had only limited success. A few improved versions of Tycho came out such as the ACT and Tycho reference catalogues. These ;however, had fewer stars than the original Tycho and were in many ways less suitable for charting despite improved coordinates.

The recent Tycho-2 catalog, with its supplements, looked very promising as a single star catalogue to plot stars to magnitude 12.5. This would produce a good star density for many uses while avoiding the many shortfalls inherent in the GSC. Although many software developers took note of its value only one man rushed to put it to the test and give the world a look at it. Han Kleijn, the creator of the excellent program "Hallo Northern Skies," posted a tool to compile this catalogue on his website even before the Tycho-2 catalogue was even available from the CDS or NASA.

Mr. Kleijn then posted compiled versions of this catalogue on his webb site. The heavy demand caused him problems in serving it which are not yet fully solved. The catalogue was an instant winner and other software developers are getting ready to incorporate it. The freeware program "Sky Charts" and the commercial program "Guide" will make it available later this year. Although currently the best, Tycho-2 still has some problems with "incompleteness." An effort is well underway to supplement Tycho-2 with stars from the various other flavors of the original Tycho catalog. This work is being undertaken by Arizona amateur Bill Anderson (USA) in close cooperation with Han Kleijn who hopes to incorporate it into Hallo Northern Skies. Although some cluster stars will probably remain unplotted, my hat is off to both Mr. Anderson and Mr. Kleijn for their efforts. Next the question of accuracy is considered.


Astrometric catalogues used by Star Map plotting programs usually have good accuracy as far as the coordinates are concerned. All except the GSC take proper motion into account for most stars. The stars are plotted in the correct places by all catalogues from the SAO to Tycho-2 in most instances although Tycho-2 is far superior to the SAO. The more modern PPM does even better. With the stars in the correct places the question of photometric accuracy will be considered. The SAO used old visual values often from the older "Durchmusterung" Catalogues. These actually were not to bad to say magnitude 9.0. The suggestion of a magnitude of "9.5" for stars north of the celestial equator is to be met with much caution for it is usually taken from the Bonner Durchmusterung and may well be two magnitudes FAINTER! These values are at least visual. The PPM uses crude photographic values for the north and pretty good photovisual values for the south. This means that red stars in the northern hemisphere are plotted as much as one and a half magnitudes two dim. This can cause confusion in the identification of a narrow star field at the telescope. Yet this catalogue (the PPM) is very widely used to plot charts as it has very good positional accuracy and is fairly complete to magnitude 9. How many amateurs have read the introduction to the PPM (separate for north and south) and are aware of the photometric issues? Very few, this writer suspects are aware of this problem. The Tycho catalog magnitudes (Vt) are close but not exactly visual magnitudes. The accuracy of even the fainter stars of 11th magnitude is still within one tenth of a magnitude or at most two tenths, a vast improvement over any of its predecessors. For this reason charts plotted with Tycho look correct to 11th magnitude. The photometry for fainter stars listed by Tycho-2 are progressively poorer but still useful for charting purposes. The photometry of the GSC is so poor and hetrogenious in nature that it will not even be discussed here. It has been discussed by Bryan Skiff of the Lowell Observatory (Arizona, USA) for those who might be interested. Bluntly, the GSC magnitudes are terrible! I will now move on to the third consideration of good star charts, the question of duplicity.

Programs like "Hallo Northern Skies." which use only one catalogue at a time to plot the desired chart, has virtually no star plotted separately at two slightly different locations. Other programs suffer, sometimes greatly from this problem. To be fair the SAO may well have a dozen or so duplicates, but these were largely removed from the PPM. At present Hallo Northern Skies is the best program to use to avoid confusing duplication in the chart. Less duplication means a cleaner, more realistic chart upon which to plot deep sky objects for identification at the telescope.

Less duplication, high positional and photometric accuracy and denser star coverage make Tycho-2 and Hallo Northern Skies a winner to help an amateur astronomer find the faint fuzzies.

Raymond Bruton (USA)