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swarp ncombine
12-05-2020, 08:54
Post: #1
swarp ncombine
Hi,

can I simply get as the result of image combination, in addition to the coo-add an image with the number of images combined in each pixel? This is acctually the NCOMBINE and is very usefull to estimate the total exposure time used in each pixel.

Thanks, Ana
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12-05-2020, 10:26 (This post was last modified: 12-05-2020 10:28 by oleg_astro.)
Post: #2
RE: swarp ncombine
Hi Ana,
do you need this information from coadd.fits header?
...
...
COMMENT
SOFTNAME= 'SWarp ' / The software that processed those data
SOFTVERS= '2.19.1 ' / Version of the software

SOFTDATE= '2010-05-01' / Release date of the software
SOFTAUTH= 'Emmanuel BERTIN <bertin@iap.fr>' / Maintainer of the software
SOFTINST= 'IAP http://www.iap.fr' / Institute
...
...
COMMENT
COMMENT DUMMY keywords for later use
TEXPTIME= 3920.00 / total Exposure Time
EXPTIME = 1.00 / effective Exposure Time (s)
GAIN = 3920.00 / effective GAIN for SExtractor
INST_TEL= 'OMEGACAM@VST' / Instrument and Telescope
FILTER = 'NB_659' / Filter Name
ASTRCAT = 'GAIA-DR1' / Astrometric reference catalogue
FWHM = 0.97 / Image seeing (FWHM, arcsec)
FWHM_ERR= 0.01 / Image seeing error (FWHM, arcsec)
FWHM_IMG= 4.54 / Image seeing (FWHM, pixel)
HFD = 1.22 / Image seeing (half flux diameter, arcsec)
HFD_ERR = 0.03 / Image seeing error (half flux diameter, arcsec)
HFD_IMG = 5.70 / Image seeing (half flux diameter, pixel)
NCOMBINE= 40 / number of individual frames in this stack
GABODSST= 6617 / first GABODSID of images in this stack
GABODSEN= 6641 / last GABODSID of images in this stack
DATE-ST = '2017-02-11' / first date of stack data (local time at tel.)
DATE-END= '2017-03-07' / last date of stack data (local time at tel.)
PROJECT = 'test' / The PROJECT name used in THELI GUI
...
...

My OS is Astronomy Linux 19.04 (64-bit) https://astronomylinux.ap-i.net
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12-07-2020, 11:37
Post: #3
RE: swarp ncombine
Thanks, but no, i don't need the total number (I know that in advance), but the number of images combined in each pixel of the final co-add. You see, I use dithering techinque and create large fields; but I need to estimate total exposure time used on different objects in my final co-add. To estimate that, I need to know how many images are "involved" in different parts of my co-add.
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12-14-2020, 19:59 (This post was last modified: 12-14-2020 20:13 by oleg_astro.)
Post: #4
RE: swarp ncombine
(12-07-2020 11:37)ana.lalovic Wrote:  ... but I need to estimate total exposure time used on different objects in my final co-add....
Ok.
Now I understand you.
Wait for more information...


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My OS is Astronomy Linux 19.04 (64-bit) https://astronomylinux.ap-i.net
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12-21-2020, 19:57
Post: #5
RE: swarp ncombine
(12-14-2020 19:59)oleg_astro Wrote:  
(12-07-2020 11:37)ana.lalovic Wrote:  ... but I need to estimate total exposure time used on different objects in my final co-add....
Ok.
Now I understand you.
Wait for more information...

Many thanks to Oleg for introducing me with new software. But I want to report for others who may face the similar problem that I've finally solved it. It is quite simple but needs twice as many files to be created. First, I made a single image the size of my individual frames and set all pixel values to 1. Then I copied and renamed this image so that I have one for each of my individual frames. Then I copied WCS header from individual frames to newly created single-value images so they share the same astrometric solution as their "parent" frames. Then I used Swarp to make a SUM. In this way, the final co-add that is the sum of single-valued images keeping original astrometry reflects the number of images combined in each pixel.

Hope the easier way shows up.
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