Converting Galactic Coordinates to Equitorial J2000 coordina

[foravalon]

Hey guys,

In my quest to create accurately positioned fictional stars I find that my source material is based on the Galactic Coordinate System using figures like L2 and B2. Celestia, of course, uses an Equitorial (J2000) system using RA, Dec, and Distance. The need arises to convert my figures!

Selden was helpful enough to point me to the Chandra X-Ray "Precess: Coordinate Conversion and Precession Tool"...

http://cxc.harvard.edu/toolkit/precess.jsp

... which is awesome and does exactly what I'm trying to do! BUT, I find my self needing to convert a lot of figures with no desire to plug each of them manually into this lovely website. So I've begun a hunt for the equations BEHIND the Chandra site so I can use them in a program of my own or even plug them into excel or something.

I'm no math wiz but I found this in a text and I'm wondering if it's what I'm looking for...

Galactic Coordinates


Sometimes, coordinates are given in a galactic reference frame, rather than a geocentric one. Given galactic coordinates (b, l), the equatorial coordinates ?? (declination) and ?± (right ascension) can be computed from the formulas
?° ?± ?? b l

cosbcos(l - 33?°) = cos??cos(?± - 282.25?°)


cosbsin(l - 33?°) = sin??sin62.6?° + cos??sin(?± - 282.25?°)cos62.6?°


sinb = sin??cos62.6?° - cos??sin(?± - 282.25?°)sin62.6?°


The reverse transformations are:

cos??cos(?± - 282.25?°) = cosbcos(l - 33?°)


cos??sin(?± - 282.25?°) = cosbsin(l - 33?°) cos62.6?° - sinbsin62.6?°

sin?? = cosbsin(l - 33?°) sin62.6?° + sinbcos62.6?°


The first system was defined in 1932 using optical observations of the Milky Way Galaxy. The new system was defined in 1958 in terms of 21 cm observations of HI (Sullivan 1984, p. 140).

It looks great on paper but I have no idea what it means, I'm also concerned about their use of "b" and "l" as opposed to b2 and l2 because it makes me wonder if this indicates a different type of galactic coordinate system that I'm unaware of, but maybe it's just the math-phobe in me jumping at shadows.

Do these equations make sense to anyone out there and do you think they're a step in the right direction?

[Guckytos]

That remembers me of my search for a conversion tool. Had to do a lot of SciFi coordinates given in Galactic coordinates.

The equations look about that, what I remember from my search.

But be aware, which Year you take as base! Y1950 galactic Koordinates and Y2000 differ!

Found once a nice usefull programm, that could convert nearly everything.
It was called "Coordinate Conversions in Astronomy" or "CooC"
That programm is also able to batch convert, by giving it a preformatted CSV file for input. Try searching for it, I think there was also a WWW version of it.

If someone knows of a newer program that can do the same I would like to her about it, too.

Regards,

Guckytos

[foravalon]

That remembers me of my search for a conversion tool. Had to do a lot of SciFi coordinates given in Galactic coordinates.

The equations look about that, what I remember from my search.

But be aware, which Year you take as base! Y1950 galactic Koordinates and Y2000 differ!

Found once a nice usefull programm, that could convert nearly everything.
It was called "Coordinate Conversions in Astronomy" or "CooC"
That programm is also able to batch convert, by giving it a preformatted CSV file for input. Try searching for it, I think there was also a WWW version of it.

If someone knows of a newer program that can do the same I would like to her about it, too.

Regards,

Guckytos

Danke Shun Guckytos, I haven't had any luck so far, but the quest continues!

[Guckytos]

What do you mean by saying taht you didn't have any luck so far?

That you know of no newer program or that you can't find CooC? Or that CooC isn't working for you?

Here is a link to a download location of the offline version of CooC: http://www.astro.utu.fi/EGal/Tools.html

Regards,

Guckytos

[foravalon]

What do you mean by saying taht you didn't have any luck so far?

That you know of no newer program or that you can't find CooC? Or that CooC isn't working for you?

Here is a link to a download location of the offline version of CooC: http://www.astro.utu.fi/EGal/Tools.html

Regards,

Guckytos

I wasn't able to find it under "Coordinate Conversions in Astronomy" so thank you so much for the link! I'll have to play around with the program more when I have bit of time but it looks very promising! Do you know if there's any way to get the program to displaythe results in decimal degrees the way celestia likes them? If not it will probably still be okay, it might just mean I have to add another translating equation into my spreadsheet.
Thank you for this!

[foravalon]

I've finally had the chance to play around with Cooc and unfortunately, although I've tried many different permutations of the settings on the program, I haven't been able to get a match for the figures that the Chandra site has given me. this is a real shame.

Have you had any different luck?

Here's that page again:
http://cxc.harvard.edu/toolkit/precess.jsp

I have confidence that this site is giving me good figures, but perhaps I am just using Cooc incorrectly.

Are you able to get matching results between Cooc and the Chandra page?

[Guckytos]

Perhaps you should give some example coordinates here, of what you want to have converted, so that the same things are used.
And what program delivers what results.

This way it would be a lot easier to find out what went where wrong, I think.

Regards,

Guckytos

[Adirondack]

http://www.astro.utu.fi/EGal/CooC/CooCaccuracy.html

Adirondack

[foravalon]

Perhaps you should give some example coordinates here, of what you want to have converted, so that the same things are used.
And what program delivers what results.

This way it would be a lot easier to find out what went where wrong, I think.

Regards,

Guckytos

Okay, here's a sample of Galactica coordinates for two different stars:

################L2##########B2
Star One ###228.8546634###2.694648087
Star Two ###209.6547228###4.533520394


Here are the results from 3 different converters, the first is the Chandra X-Ray site, the second are from Cooc, the third converter I found at the NASA/IPAC Extragalactic Database (NED) website http://nedwww.ipac.caltech.edu/forms/calculator.html


Results:

Star 1###########RA###########Dec
--------------------------------------------------------------------
Chandra#########07 30 50.49#### -12 51 11.34

Cooc galactic######07 28 30.765####-12 44 49.09
Cooc galactic (oldsys) 06 33 13.309 ### +16 27 41.74

Ned B1950.0######07 30 50.47155###-12 51 11.2940
Ned J2000.0##### #07 30 50.49278###-12 51 11.1161



Star 2###########RA###########Dec
--------------------------------------------------------------------
Chandra#########07 01 38.80#### +04 58 38.16

Cooc galactic######06 58 59.380####+05 02 58.91
Cooc galactic (oldsys) 06 00 29.167 ### +34 15 14.71

Ned B1950.0######07 01 38.77553###+04 58 38.1808
Ned J2000.0##### #07 01 38.78424###+04 58 38.2420

------------------------------------------------------

It's obvious that the Cooc Galactic (old system) is the wrong system to use, so that's way out.

The J2000 Epoch of NED is the best match for the Chandra Site.

But unfortunatlely the converter I'd like to use, the program that can convert batch files, the Cooc Galactic conversion is not very close to either Chandra or NED.

[foravalon]

HOORAY for NASA!

Good news to anyone who might be on a simliar quest as me!
Check it out!
http://heasarc.gsfc.nasa.gov/cgi-bin/Tools/convcoord/convcoord.pl

The converter is accurate, it gives you output options, it's easy to understand, and it converts in batch files!

hooray! thanks NASA!