【HST, IRAC】make good photometry for IRAC image with HST high resolution image

This note keeps some tips of my work, so it may be hard for you to understand the details. You can give me emails to if you have difficult to make iraf photometry for faint objects.

How to make photometry?

   .... add soon

Be careful!

*) In galfit parameter file,
    Using a convolution box size larger than the image to make sure all the flux of bright objects in included.
    Resampling the psf to make sure the PSF fine-sampling factor is an integer value.
    The ellipticity is defined different in Sextractor which is "#ELLIPTICITY = 1-B_IMAGE/A_IMAGE".

How to improve the photometry?

Some steps for check

*) clip a large region for HST and IRAC images to check whether there is extended bright objects nearby.

*) for some objects, you can fake an object close to the target, and galfit them in the mean time. If  there is some bias about the magnitude, you should check your route.

Some improvement for galfit

*) Include all objects with HST magnitude brighter than 1 sigma IRAC detection limiting. 

*) Delete objects that are not detected in IRAC, and combine objects that are too close to be separated in IRAC image. Delete objects which are too close to the 

*) make sersic model for object larger than 1 IRAC pixel and estimate the position angle (the position angle are not define as the same in Sextractor and galfit, image1).

*) fixed all the positions, but release the position for quite bright or sersic model. fixed the ELLIPTICITY in sersic model.

*) release the postion of all object in the last run.

*) after complete the fit for ch1, then apply the same parameters for ch2. 

still not resoloved!!

*) The shift between IRAC and HST images, also between ch1 and ch2 IRAC images.

*) if the number of free parameters is larger than ??

*) which psf should be used?  one lose the information for the larger radius, the other is not so accurate.

*) galfit magnitude vs aperture photometry magnitude?

*) how to define whether one object is detected?

    the chi^2 improvement ?

Image1. Position angle

How to describe the results?

  Undetected

       too faint

       bright contaminated

  detected

       no other contamination

       bright galaxy nearby. give a flag to such candidates.
       faint galaxy in that cannot separated with the target.