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Written by Matheus J. Castro matheusj_castro@usp.br
Under MIT License.
Copyright (c) 2020

Combine Catalogs Steps

This tutorial shows how to use raw data from a set of reduced images, create and combine catalogs using this images.
The process described here needs to be done for each set o filter independently.
Here we are using a set from Observatorio Astrofísco de Javalambre (OAJ) and the filter used was rSDSS. More infromation at https://oajweb.cefca.es/

Legal Disclaimer: All data published here were previously authorized by PI Astronomer responsible for these images. The author is not responsible for any unauthorized use. The purpose of this tutorial is only educational.

Getting the Data

Download in TACDATA website (OAJ) inside the TACDATA_1500052 observations the images with the filters you want.

https://tacdata.cefca.es/

SAOImage DS9 - Extracting

DS9 Website

Open DS9 and transform the .fz files in the .fits just saving again.

Astromatic SWarp - Combining Images

SWarp Website

Combine each of the images you get for each exposion time using SWarp. To do that follow this steps:

  • Create the configuration file:

    1. swarp -d > default.swarp
    2. Change the IMAGEOUT_NAME and the WEIGHTOUT_NAME to what you want
    3. Change the WEIGHT_TYPE from NONE to BACKGROUND
    4. The COMBINE_TYPE to MIN
    5. SUBTRACT_BACK to N

  • Create a list with the .fits you want to combine:

ls *.fits > list_name.lis
  • Run SWarp:
swarp @list_name.lis -c default.swarp

Astromatic SExtractor - Creating Catalogs

SExtractor Website

Now run SExtracor to generate the catalog file following this steps:

  • Create the extended configuration file and the parameters file:
sex -dd > default.sex
sex -dp > default.param

  • Inside the config file (.sex), modify:

    1. CATALOG_NAME to what you want
    2. CATALOG_TYPE to FITS_LDAC
    3. DETECT_THRESH to 5
    4. FILTER to N
    5. WEIGHT_TYPE to MAP_WEIGHT
    6. WEIGHT_IMAGE to the name of SWarp generated weight

  • Now, on the parameters file leave only this params:

NUMBER
X_IMAGE
Y_IMAGE
MAG_AUTO
MAGERR_AUTO
FLAGS
FLAGS_WEIGHT
FWHM_IMAGE
BACKGROUND
ALPHA_J2000
DELTA_J2000
  • Finally, run SExtractor:
sex name_of_the_combined_image.fits

With that, now we have a nice catalog for this exposure time. Do the same for the other exposures.

TOPCAT - Zero-Point fix

TOPCAT Website

For the last modification before combining the catalogs, we need to correct the zero-point based on one of them. To do that we need TOPCAT:

  • Match the base catalog with one another (max error 3").

  • Then open the table, click with right mouse anywhere and select "New Synthetic Column", this will open a window, fill the boxes with the information bellow:

    • Name: MAG_DIF
    • Expression: MAG_AUTO_1-MAG_AUTO_2

  • Close the table and open the box with sigma symbol, will open a table with statistcs of each column of your table. Go to the column that we just created, MAG_DIF (Note if nothing appear, click on the reload box at the top of the window). We are looking for the Mean value, save this to later use.

  • Now, open the catalog table that we used to match with the base catalog and add a synthetic column whit this fields:

    • Name: MAG_AUTO_CORRECTED
    • Expression: MAG_AUTO+(the Mean value that we saved in the previous step, note that if the value is negative it will subtract)
      Allways verify if the result values agree with the base catalog.

  • Repeat the process with the other catalogs, allways using the same base catalog that before.

  • Finally, verify the results ploting an histogram.

Runing Python3

Now you problably have 3 catalogs for each filter, it's time to run the combine_catalogs.py script to combine them. Here is how to use it:

  • On terminal type:
python3 combine_catalog.py [options] arguments

  • Arguments need to be file names of FITS catalogs or a list of files.

  • Each list must contain only names of files, one per line, and need to have the extension ".lis".

  • The first input catalog will be the base for others, and only their columns will be present in the output catalog.

  • The [options] are:

 -h,  -help		|	Show this help;
--h, --help		|	Show this help;
	-s  [option]	|	Save or not the output catalog
			|		0 to not save the output (default);
			|		1 to save a ".cat" file;
			|		2 to save ".csv" file;
			|		3 to save both;
	-t  [option]	|	Integer to set the cross-match radius in arc-second;
			|		(by default is 3)
	-e  [options]	|	A list with the extension needs to use for each file.
			|	It need to be in this format n,n,n... changing "n"
			|	by the extension you want to use. The order is the
			|	order of your files input. 1 is the default value,
			|	if one argument is missing, the default is considered. Zero
			|	values (like ...n,0,n...) makes the file correspondent
			|	be set to the default value;
	-ep [option]	|	Change the default value for the extensions;
	-ra [option]	|	The name of the column for Right Ascension;
			|		(the default is ALPHA_J2000)
	-dc [option]	|	Same as -ra but for Declination;
			|		(the default is DELTA_J2000)
	-m  [options]	|	Same as -e but for the column names of the magnitudes,
			|	The values need to be in format n,n,n... and the
			|	default value is MAG_AUTO;
	-mp [option]	|	Change the default value for the column name of the magnitudes;
	-o  [option]	|	The name of the output catalog WITHOUT extension and spaces. 
			|	Also option -s must be given. The default it "Results_Combined".

Example:
python3 combine_catalog.py -s 1 -e 2,1,3,0 -mp MAG_AUTO combine.lis catalog1.cat catalog2.cat