### Author Topic: Aperture Photometry question  (Read 1345 times)

#### CraigNZ

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• Posts: 94
##### Aperture Photometry question
« on: 2016 June 27 18:19:20 »
In the output Image Table there are 4 columns: CATRA, CATDEC, IMGRA, IMGDEC.  Quoting from the documentation:

CATRA, CATDEC

Equatorial J2000 coordinates of the star as stored in the catalog.

IMGRA, IMGDEC

Equatorial J2000 coordinates of the star as measured in the image. This value is calculated unprojecting the measured pixel coordinates of the star.

When converting the IMGX and IMGY pixel values to IMGRA and IMGDEC, is the proper motion of the stars used in the calculation?

#### Andres.Pozo

• PTeam Member
• Posts: 918
##### Re: Aperture Photometry question
« Reply #1 on: 2016 June 28 01:11:28 »
When converting the IMGX and IMGY pixel values to IMGRA and IMGDEC, is the proper motion of the stars used in the calculation?
It is not necessary to calculate the proper motion since the image contains the stars as they where at the time when it was taken. When doing the astrometry of these stars the coordinates are valid for the epoch of the image.

The values of CATRA and CATDEC are the result of applying the proper motion correction for the given epoch to the coordinates extracted from the catalog. Of course this is only valid if the catalog contains proper motion data.

#### CraigNZ

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##### Re: Aperture Photometry question
« Reply #2 on: 2016 June 28 01:23:31 »
Where I was concerned was in the documentation it stated that IMGRA and IMGDEC are the equatorial J2000 coordinates of the star, and were calculated by 'unprojecting' from IMGX and IMGY.  I wasn't sure if 'unprojecting' meant applying only the distortion correction of the image or if also applying proper motion in reverse to show the equatorial coordinates had it been measured in J2000.

Would it be correct then to say that IMGRA and IMGDEC are the equatorial coordinates of the star at the epoch of the image?

#### Andres.Pozo

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##### Re: Aperture Photometry question
« Reply #3 on: 2016 June 28 10:26:42 »
Where I was concerned was in the documentation it stated that IMGRA and IMGDEC are the equatorial J2000 coordinates of the star, and were calculated by 'unprojecting' from IMGX and IMGY.  I wasn't sure if 'unprojecting' meant applying only the distortion correction of the image or if also applying proper motion in reverse to show the equatorial coordinates had it been measured in J2000.

Would it be correct then to say that IMGRA and IMGDEC are the equatorial coordinates of the star at the epoch of the image?

The references to J2000 in my scripts are to the "J2000 equinox" not "J2000 epoch". IMGRA, IMGDEC are coordinates in the J2000 equinox for the epoch of the image.

When the catalog has proper motion data the values of CATRA and CATDEC are in the J2000 equinox for the given epoch (it should be the epoch of the image).

When the catalog doesn't have proper motion data the values of CATRA and CATDEC are in the J2000 equinox and epoch (as served by the catalog server).

#### CraigNZ

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##### Re: Aperture Photometry question
« Reply #4 on: 2016 June 28 14:06:35 »
Sorry, I missed that .. reading equinox but thinking epoch.

Is it possible then to indicate in the output file if CATRA and CATDEC are the epoch of the image, ie, corrected using proper motion values.  This would make it easier when doing differential astrometry to know if the reference stars are using positions corrected for proper motion.  Ideally if the proper motion is used, a flag is set and reported in the output table.

#### CraigNZ

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• Posts: 94
##### Re: Aperture Photometry question
« Reply #5 on: 2016 June 28 14:29:45 »
Andres, cancel that request, I came up with a different solution.  So there is no need to flag the proper motion correction in the output table.

Thanks for helping on this and clarifying my misunderstanding.