【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.

         

GALFIT_Sextractor_aperture photometry_BCG substraction

How to make the aperture photometry for this faint galaxies???
Try galfit
Try sextractor check image
Try image-image method
Try image-selfsmooth method
Try image-selfsmooth+correction method
Try smooth > galfit method (large smooth or small smooth?)
THEN Try photometry!
However, the best method is still my own method. Could be published soon.

To be continued.

Useful python script in astronomy.

Out of numpy, scipy, astropy that you have already used, there may be more detail small script you may want for daily work. I have made some script myself, like to read a catalog,  simple plot from a catalog. However, the use of them are quite limit. Here I introduce for you some good scripts from the website. They are more powerful and can be used in a wide area. Hope this will help you to have more time to focus on your job!

http://www-int.stsci.edu/~ferguson/software/pygoodsdist/
http://www.stsci.edu/~ferguson/software/pygoodsdist/doc/index.html

pygoods.tar			Full package of the utilities listed below.
angsep.py	html	txt	Angular separation between two celestial sources
coords.py	html	txt	Utilities for parsing and converting coordinates.
parseconfig.py	html	txt	Utilities for reading SExtractor-style parameter files.
numprint.py	html	txt	Utilities for printing columns of numpy one-dimensional arrays.
readcol.py	html	txt	Utilities for reading columns of numbers from ascii files.
sextutils.py	html	txt	Utilities for reading SExtractor catalogs.
match.py	html	txt	Utilities for coordinate matching.

[Oct28] The choice of slit size, one method in work

About the slit size:

You know tht the slit width will impact the spectroscopy. And do you know what is the proper width you should choose?

In an observation that you need both high resolution and perfect normalization, you can do like this. Observed with an narrow slit with enough exposure time to get the spectrum with enough resolution, then with a quite short time exposure with a wide enough slit which contain all of the flux of the source to get the real flux level. The width slit spectrum is only useful to determine the normalization of the flux. Just shift the the high resolution spectrum and you will get the perfect spectrum with both high resolution and real flux level.
You can have a simulation here: http://terpconnect.umd.edu/~toh/models/AbsSlitWidth.html

About the normal study:
    检查类比
Usually, you make a code or you make a simulation similar with others', but you do not know whether it is correct. Then try to compare the different results to make sure it is correct and also can help you to improve the results.

Orc02

Daily tips:
I have received the draft of the paper. Should start to deal with the problems.
To complete the selection of Lyman break galaxies in z~ 1-3 as in xinwen's email.


Today, Genevieve Graves (Princeton University) gave us a talk about two works.  The first one is quite interesting. "They have recently developed a new method for weak lensing using background source magnification instead of using gravitational shear. " In the traditional method, people measured the shear signal by comparing the lensed morphology of a group galaxies with the random sample.
However, "Traditional magnification methods have struggled to match the signal-to-noise (S/N) per background source achieved by shear because the intrinsic dispersion of galaxy luminosities and radii are much larger than the intrinsic dispersion of ellipticities." 

We have solved this problem using knowledge about the galaxy population to predict the intrinsic radii to within ∼ 40%. Our new magnification method thus yields a signal that is nearly comparable to shear. Moreover, the dominant sources of systematic error are different from those in shear-based measurements. This means that combining shear and magnification can alleviate the worst biases in each method and produce a substantially more robust, higher S/N measurement of the dark matter distribution for a given survey than can be achieved with shear alone. As a proof-of-concept, we have used this technique to make a galaxy-galaxy lensing measurement using SDSS imaging and spectroscopic data. This talk will describe the new method, and present our first lensing measurements.