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Tool for converting FK4/FK5 [message #93165]

Thu, 05 May 2016 06:48

Miguel
Messages: 18
Registered: April 2015

Junior Member

Hi,

I am currently working with two fits files but with different coordinates system.
One is the FK4 (B1950) and the other is FK5 (J2000) and I would like to know if there is a procedure in IDL to convert from one to another.
I have found the (j/b)precess but it works for only for one position and hprecess converts only the equinox in the FK5 system.

I prefer IDL but if you know something in Python, it would be great.

Thanks for your help.

Miguel

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Re: Tool for converting FK4/FK5 [message #93167 is a reply to message #93165]

Thu, 05 May 2016 07:46

wlandsman
Messages: 743
Registered: June 2000

Senior Member

Have you tried vector input for jprecess.pro

http://idlastro.gsfc.nasa.gov/ftp/pro/astro/jprecess.pro

It seems to work for me.

a = [3.24,23.44,31.66]
dec = [-0.23,22.44,34.44]
mu_radec =[ [3.2,2.3],[0,0.1],[-2.3,1.2]]
jprecess,ra,dec,ra2,dec2,mu_radec=mu_radec

Also, you can always just loop over calls to any procedure that only processes scalars.

--Wayne

On Thursday, May 5, 2016 at 9:48:42 AM UTC-4, miguelfigue...@gmail.com wrote:
> Hi,
>
> I am currently working with two fits files but with different coordinates system.
> One is the FK4 (B1950) and the other is FK5 (J2000) and I would like to know if there is a procedure in IDL to convert from one to another.
> I have found the (j/b)precess but it works for only for one position and hprecess converts only the equinox in the FK5 system.
>
> I prefer IDL but if you know something in Python, it would be great.
>
> Thanks for your help.
>
> Miguel

Report message to a moderator

Re: Tool for converting FK4/FK5 [message #93168 is a reply to message #93165]

Thu, 05 May 2016 07:50

wlandsman
Messages: 743
Registered: June 2000

Senior Member

Oops, I just realize what you meant by "one position" -- you want to transform the FITS headers.

Although it is not documented, HPRECESS does calls jprecess if the input year is 1950 and the output year is 2000, and calls bprecess if the input year is 2000 and the output year is 1950. This is just what (I think) you want.

On Thursday, May 5, 2016 at 9:48:42 AM UTC-4, miguelfigue...@gmail.com wrote:
> Hi,
>
> I am currently working with two fits files but with different coordinates system.
> One is the FK4 (B1950) and the other is FK5 (J2000) and I would like to know if there is a procedure in IDL to convert from one to another.
> I have found the (j/b)precess but it works for only for one position and hprecess converts only the equinox in the FK5 system.
>
> I prefer IDL but if you know something in Python, it would be great.
>
> Thanks for your help.
>
> Miguel

Report message to a moderator

Re: Tool for converting FK4/FK5 [message #93173 is a reply to message #93168]

Thu, 05 May 2016 16:27

Miguel
Messages: 18
Registered: April 2015

Junior Member

So my 1950-fits is well converted to fk5-fits using hprecess ?
That's what I used but I noted that there is a slight difference between the converted file and a another fits representing the same region and already in the fk5 system.

Does a different projection (RA--SIN for one, RA--GLS for the other) could lead to this difference ?

Miguel

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