Stellarium

Bug #1527856
Comment #4

Comment 4 for bug 1527856

Revision history for this message

Andrew James (andjames) wrote on 2015-12-24:

Thanks for the replies here, and apologies for the delay in response. Correcting code that someone else has written is hard work, and I've spent much time looking how tings work.

To gzotti, I ask this obvious question "Are the proper motion calculated independently of the precession,..." just to confirm that there is no other routine affecting this.

(I thought these proper motions seem to curve against the background sky, suggesting another motion added into this proper motion. Any motion should be almost linear. If this is not true, then looking at the calculated magnitudes, there should be a problem too. I note that some other User comments on apparent star disappearing and reappearing when accelerating overtime, may also be the cause of these issues too.)

Thanks for the response.

To Alex, I couldn't find the routine in StarWrapper.hpp or Star.hpp, but a routine does exist in ParseHip.C and ParseHip.v2.c. I.e.

==============
PM VECTOR ROUTINE
==============
     void ChangeEpoch(double delta_years,
                    double &ra,double &dec, // degrees
                    double pm_ra,double pm_dec // mas/yr
                   ) {
     ra *= (M_PI/180);
     dec *= (M_PI/180);

     const double cdec = cos(dec);
     Vector x = {cos(ra)*cdec,sin(ra)*cdec,sin(dec)};
     const Vector north = {0.0,0.0,1.0};

     Vector axis0 = north ^ x;
     axis0.normalize();
      const Vector axis1 = x ^ axis0;

const double f = delta_years*(0.001/3600)*(M_PI/180);

x += pm_ra*f*axis1 + pm_dec*f*axis0;

     ra = atan2(x.x[1],x.x[0]);
     if (ra < 0.0) ra += 2*M_PI;
     dec = atan2(x.x[2],sqrt(x.x[0]*x.x[0]+x.x[1]*x.x[1]));

ra *= (180/M_PI);
dec *= (180/M_PI);

==========

The tan ambiguity (atan2) seems absolutely fine here.

I suspect that two possible problems are here;

1) That for some reason the HIP2 data is in error, either switched around or rounded in correctly.
2) Or somethings wrong with the pm_ra or pm_dec equation measured in mas per year.

As for the two stars flying apart there is a routine to correct for this ParseHip.C (lines 171-240) under mergeSameComponents (line 213), suggest the flag in the .dat file has not been set. [I'd suggest writing an additional routine for special cases of double stars that have not been flagged.[

=======================
Past and Future Magnitudes
=======================

A further problem, that might be related to this is that 29,240+/-1370 AD, Alpha Centauri makes its closest approach to the Sun at 2.970+/-0.012 light years when the parallax is 1.098 mas (See Mattews (2004)). The apparent total visual magnitude -1.05.

In Stellarium 14 here, on that date, finds the result is still 4.39 light years and the magnitude of both components is +0.10 and +1.20. (Total about -0.2V magnitude.) Also Sirius remains at -1.45, even though it significantly changes too. In ParsHip.C (lines 154-253) there seems no formal correction for the changes in the parallax affecting these apparent magnitudes.

If this is not correct, then I'd suspect the proper motions changing values (specifically accelerating or deaccelerating) over time are not being account for either. I.e. Line 139 of ParsHip.C calculated value "x +m", being the calculated vector of both the pm_RA and pm_Dec . This is somehow 'lost' when lines 347-354 are executed.

Note: I do recall this actually working correctly in Stellarium version 6 or 7, which looks like around 2008 when Johannes Gajdosik's (2007) routines were introduced.

Hope this helps,

Regards,
Andrew James

Unrelated Magnitude Calculation Comment.

Routine in Lines 304-346 makes corrections for B_T and V_T HIP magnitudes for spectral class then star colours, however, they do not convert these two magnitudes into B and V Johnston equivalents. You should be using photometry conversions in something like the recent paper by Turnbull, M.C., "ExoCat: The Nearby Stellar Systems Catalogue for Exoplanet Imagiing Mission" ArVix, 1510.01731 pg.6-8 (See www.http://arxiv.org/abs/1510.01731 ), which will immediately fix this particular issue.

Thanks for the replies here, and apologies for the delay in response. Correcting code that someone else has written is hard work, and I've spent much time looking how tings work.

To gzotti, I ask this obvious question "Are the proper motion calculated independently of the precession,..." just to confirm that there is no other routine affecting this.

(I thought  these proper motions seem to curve against the background sky, suggesting another motion added into this proper motion. Any motion should be almost linear. If this is not true, then looking at the calculated magnitudes, there should be a problem too. I note that some other User comments on apparent  star disappearing and reappearing when accelerating overtime, may also be the cause of these issues too.)

Thanks for the response.

To Alex, I couldn't find the routine in StarWrapper.hpp or Star.hpp, but a routine does exist in ParseHip.C and ParseHip.v2.c. I.e.

const double cdec = cos(dec);
     Vector x = {cos(ra)*cdec,sin(ra)*cdec,sin(dec)};
     const Vector north = {0.0,0.0,1.0};

Vector axis0 = north ^ x;
     axis0.normalize();
      const Vector axis1 = x ^ axis0;

const double f = delta_years*(0.001/3600)*(M_PI/180);

x += pm_ra*f*axis1 + pm_dec*f*axis0;

ra = atan2(x.x[1],x.x[0]);
     if (ra < 0.0) ra += 2*M_PI;
     dec = atan2(x.x[2],sqrt(x.x[0]*x.x[0]+x.x[1]*x.x[1]));

ra *= (180/M_PI);
     dec *= (180/M_PI);

==========

The tan ambiguity (atan2) seems absolutely fine here.

I suspect that two possible problems are here;

1) That for some reason the HIP2 data is in error, either switched around or rounded in correctly. 
2) Or somethings wrong with the pm_ra or pm_dec equation measured in mas per year.

As for the two stars flying apart there is a routine to correct for this ParseHip.C (lines 171-240) under mergeSameComponents (line 213), suggest the flag in the .dat file has not been set. [I'd suggest writing  an additional routine for special cases of double stars that have not been flagged.[

=======================
Past and Future Magnitudes
=======================

Note: I do recall this actually working correctly in Stellarium version 6 or 7, which looks like around 2008 when Johannes Gajdosik's (2007) routines were introduced.

Hope this helps,

Regards,
Andrew James

Unrelated Magnitude Calculation Comment.

Routine in Lines 304-346  makes corrections for B_T and V_T HIP magnitudes  for spectral class then star colours, however, they do not convert these two magnitudes into B and V Johnston equivalents. You should be using photometry conversions in something like the recent paper by Turnbull, M.C., "ExoCat: The Nearby Stellar Systems Catalogue for Exoplanet Imagiing Mission" ArVix, 1510.01731 pg.6-8 (See www.http://arxiv.org/abs/1510.01731 ), which will immediately fix this particular issue.