Working with L1688 - Revision history

Rebull: /* Working with the data tables */

2010-10-20T20:11:57Z

Working with the data tables

Rebull: /* Bandmerging the photometry */

2010-06-17T18:22:22Z

Bandmerging the photometry

Rebull at 03:55, 15 May 2010

2010-05-15T03:55:39Z

Rebull: /* Doing photometry */

2010-05-05T20:14:52Z

Doing photometry

Rebull: /* Making the mosaics */

2010-05-05T20:14:03Z

Making the mosaics

Rebull at 18:35, 5 May 2010

2010-05-05T18:35:19Z

Rebull at 18:32, 5 May 2010

2010-05-05T18:32:52Z

Rebull: /* Downloading the data */

2010-05-05T18:21:19Z

Downloading the data

Rebull at 18:20, 5 May 2010

2010-05-05T18:20:46Z

Rebull: /* Doing photometry */

2010-03-29T23:53:28Z

Doing photometry

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 =Working with the data tables=
-I've provided the data tables in something called "IPAC table format" in the tarball above.  There is a long version with many columns, and a short version with many fewer columns.  Spot, Leopard, and IPAC's [http://www.ipac.caltech.edu/Skyview/ Skyview] all understand IPAC table format.  You can also import the file into Excel to play with it there.  NOTE THAT there are a LOT of sources in this relatively small mosaic.  You will want the computer to do as much work for you as possible - you don't want to be counting sources manually.
+I've provided the data tables in something called "IPAC table format" in the tarball above.  This amounts to plain text with a special header. There is a long version with many columns, and a short version with many fewer columns.  Spot, Leopard, and IPAC's [http://www.ipac.caltech.edu/Skyview/ Skyview] all understand IPAC table format.  You can also import the file into Excel to play with it there.  NOTE THAT there are a LOT of sources in this relatively small mosaic.  You will want the computer to do as much work for you as possible - you don't want to be counting sources manually.
 You can use Spot, Leopard, or IPAC's Skyview to overplot the sources onto an image of your choice.  NOTE AGAIN THAT there are a LOT of sources in this relatively small mosaic. Skyview (and maybe Spot/Leopard) don't understand the full table, with all those columns; that's why I provided the "catshort" version with just positions.

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 There is no GUI tool to bandmerge the photometry.
-The code I use is written in IDL (expensive to have a site license, even for academics), and for each band, starting with 2MASS JHK, looks for matches within 1 arcsecond in IRAC-1, then IRAC-2, then IRAC-3, then IRAC-4, and then within 2 arcseconds in MIPS-1, and then within 10 arcseconds in MIPS-2.  (Bonus questions: Why should I start with the 2MASS data and work up in wavelength? Why should this matching distance change?)
+The code I use is written in IDL (expensive to have a site license, even for academics), and for each band, starting with 2MASS JHK, looks for matches within 1 arcsecond in IRAC-1, then IRAC-2, then IRAC-3, then IRAC-4, and then within 2 arcseconds in MIPS-1, and then within 10 arcseconds in MIPS-2.  (Bonus questions: Why should I start with the 2MASS data and work up in wavelength? Why should this matching distance change?  hint: [[Resolution]])
 RESULTS: Bandmerged photometry (including 2MASS data!) is included in the tarball above if you wish to short-cut this process, and I promise I won't think any less of you.

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 This page uses as an example the Lynds 1688 cloud.  L1688 is a star-forming region, and our goal is to find old and new YSOs.  So, this same chain of analysis will work for any other Galactic star-forming region.  To analyze extragalactic data or for using Spitzer data for purposes other than looking for YSOs, while several of these steps are universal (downloading data, literature searching, creating mosaics), the rest are customized to this goal of finding old and new YSOs.
-NOTE THAT this page is not meant to be a cookbook to follow blindly (without thinking) for Lynds 1688 or any other cloud!  It is instead intended to provide you the tools to use to find the answers you seek for any step of the analysis for any cloud.
+NOTE THAT this page is not meant to be a cookbook to follow without thinking for Lynds 1688 or any other cloud!  It is instead intended to provide you the tools to use to find the answers you seek for any step of the analysis for any cloud.
 [http://web.ipac.caltech.edu/staff/rebull/outr/l1688.tar.gz Download the tarball with all my reduced images and data tables for Lynds 1688.]

@@ Line 70: / Line 70: @@
 '''IN ANY CASE''': Never just trust that the computer has done it right. It probably did what you asked it to do correctly, but you asked it to do the wrong thing. '''Always''' make some plots to test and see if the photometry seems correct.
-RESULTS: Reduced (and bandmerged) photometry is included in the tarball above if you wish to short-cut this process, and I promise I won't think any less of you.
+RESULTS: Reduced (and bandmerged) photometry is included in the tarball above if you wish to short-cut this process, and I promise I won't think any less of you.  NOTE THAT this represents several days/weeks of a professional astronomer's time to do this photometry. You should not expect to be able to reproduce this effort in a few hours, and it is NOT CHEATING to just take my photometry, or that delivered by a Legacy team to IRSA.
 For the data I gave you, I actually cropped the image and the catalog out of a MUCH MUCH MUCH larger mosaic and catalog.  The cropping of the image had to be done in pixel space, so north is not necessarily up, and the cropping is not necessarily strictly in RA/Dec.  The cropping in the catalog is strictly RA/Dec.  This means that there will be wedges of images without sources.  Sorry! If you really want those data, let me know, or go get the data yourself from the c2d delivery on the Legacy pages.

← Older revision		Revision as of 20:14, 5 May 2010
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	IF YOU DECIDE YOU NEED TO MAKE A MOSAIC, [[Make a simple mosaic \| go here]] and follow the tutorial to make the mosaics from the post-BCD pieces you have downloaded.		IF YOU DECIDE YOU NEED TO MAKE A MOSAIC, [[Make a simple mosaic \| go here]] and follow the tutorial to make the mosaics from the post-BCD pieces you have downloaded.

−	RESULTS: Mosaics from all 4 IRAC channels and 2 of the 3 MIPS channels (for L1688) are included in the tarball above if you wish to short-cut this process.	+	RESULTS: Mosaics from all 4 IRAC channels and 2 of the 3 MIPS channels (for L1688) are included in the tarball above if you wish to short-cut this process. NOTE THAT this represents several days/weeks of a professional astronomer's time to create these mosaics. You should not expect to be able to reproduce this effort in a few hours, and it is NOT CHEATING to download my mosaics, or start from the pipeline mosaics.

@@ Line 68: / Line 68: @@
 '''OPTION 3''': Using your own preferred software to do the photometry.  Be careful; read the stuff on [[Units]] first.
-'''IN ANY CASE''': Never just trust that the computer has done it right. It probably did what you asked it to do correctly, but you asked it to do the wrong thing. Always make some plots to test and see if the photometry seems correct.
+'''IN ANY CASE''': Never just trust that the computer has done it right. It probably did what you asked it to do correctly, but you asked it to do the wrong thing. '''Always''' make some plots to test and see if the photometry seems correct.
 RESULTS: Reduced (and bandmerged) photometry is included in the tarball above if you wish to short-cut this process, and I promise I won't think any less of you.
@@ Line 74: / Line 74: @@
 For the data I gave you, I actually cropped the image and the catalog out of a MUCH MUCH MUCH larger mosaic and catalog.  The cropping of the image had to be done in pixel space, so north is not necessarily up, and the cropping is not necessarily strictly in RA/Dec.  The cropping in the catalog is strictly RA/Dec.  This means that there will be wedges of images without sources.  Sorry! If you really want those data, let me know, or go get the data yourself from the c2d delivery on the Legacy pages.
-Questions, assuming you are using MOPEX:
+Questions:
-#Compare the source extractions from any one band with its corresponding mosaic. Is it getting fooled by detector artifacts?
+#If you are doing automatic source detection, compare the source extractions from any one band with its corresponding mosaic. Is it getting fooled by detector artifacts?
-#Overlay the source extractions from, say, IRAC band 3 with the image from IRAC band 1, or the source extractions from MIPS-24 with image from IRAC band 1. Are there a lot of stars (or other objects) in common? How does the nebulosity compare?
+#If you are doing automatic source detection, overlay the source extractions from, say, IRAC band 3 with the image from IRAC band 1, or the source extractions from MIPS-24 with image from IRAC band 1. Are there a lot of stars (or other objects) in common? How does the nebulosity compare?
 =Bandmerging the photometry=

@@ Line 40: / Line 40: @@
 Additional questions:
-#Look for image artifacts in the mosaics - they are certainly still there.  What are real features of the sky and what is a feature of the instrument and/or reduction?  [http://ssc.spitzer.caltech.edu/irac/features.html IRAC artifact examples] and [http://ssc.spitzer.caltech.edu/mips/features.html MIPS artifact examples] are online.
+#Look for image artifacts in the mosaics - they are certainly still there.  What are real features of the sky and what is a feature of the instrument and/or reduction?  [http://ssc.spitzer.caltech.edu/irac/features/ IRAC artifact examples] and [http://ssc.spitzer.caltech.edu/mips/mipsinstrumenthandbook/70/#_Toc254160618 MIPS artifact examples] are online, both as part of the corresponding instrument handbook.
 #Which objects are saturated, in which bands?
 #How big are any of the features in the nebulosity? (What do I mean by big?) in pixels, arcseconds, parsecs, and/or light years? (Hint: you need to know how far away this thing is, and it's about 125 pc away. If it helps, there are 3.26 light years in a parsec.  If you're working with another object, you should look in the literature for distance estimates to it.)
-#In this case, asteroids matter. The ecliptic latitude of L1688  is just -2.7 deg. (You can use Spot/Leopard to convert coordinates for any other target you may be working with.)  The IRAC and MIPS maps were taken in two pieces.  I gave you just one IRAC map, the combination of all frames, but I gave you 2 MIPS-24 frames, one from each epoch (separated by 3-5 hrs).  Can you find the asteroids?  (personally, I'd use the blink feature in ds9, but you can use whatever you want.)
+#In this case, asteroids matter. The ecliptic latitude of L1688 is just -2.7 deg. (You can use Spot/Leopard or the SHA to convert coordinates for any other target you may be working with.)  The IRAC and MIPS maps were taken in two pieces.  I gave you just one IRAC map, the combination of all frames, but I gave you 2 MIPS-24 frames, one from each epoch (separated by 3-5 hrs).  Can you find the asteroids?  (personally, I'd use the blink feature in ds9, but you can use whatever you want.)
 #For the 70 um data, the SSC pipeline produces two sets of BCDs: filtered and unfiltered.  The filtered data attempts to automatically remove some of the instrumental artifacts, but does not conserve flux for extended objects or for very bright point sources.  I gave you a copy of both the filtered and unfiltered mosaics.  Can you find the sidelobes around bright objects in the filtered image that are an artifact of the filtering process?
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 [[Photometry | read about photometry in general]], and finally read the [[I'm ready to go on to a more advanced discussion of photometry| advanced intro to photometry]].
-The decision you now face is the following.  You have a mosaic with 100s to 1000s of sources, over at least 4 bands (and recall with HDR IRAC data, there are actually 2 frames per band, a long and a short one), so that's 8 IRAC frames, and probably you want to at least do MIPS-24 as well, for a total of 9 frames.  You need to decide if you are interested in photometry of just a few objects, in which case you might consider doing all of the photometry by hand, one object at a time, one band at a time.  On the other hand, you might be interested in having the computer automatically detect and measure the photometry of all the objects in the frame.  If this is the case, you need to tune the parameters of your photometry extraction software to the specific application you have here -- the specific band, source density, background, etc., etc.  No matter what you do, in order to check the accuracy of your photometry, you will probably end up doing this more than once on the same data set.
+The decision you now face is the following.  You have a mosaic with 100s to 1000s of sources, over at least 4 bands (and recall if you have HDR IRAC data, there are actually [at least] 2 frames per band, a long and a short one), so that's 8 IRAC frames, and probably you want to at least do MIPS-24 as well, for a total of 9 frames.  You need to decide if you are interested in photometry of just a few objects, in which case you might consider doing all of the photometry by hand, one object at a time, one band at a time.  On the other hand, you might be interested in having the computer automatically detect and measure the photometry of all the objects in the frame.  If this is the case, you need to tune the parameters of your photometry extraction software to the specific application you have here -- the specific band, source density, background, etc., etc.  No matter what you do, in order to check the accuracy of your photometry, you will probably end up doing this more than once on the same data set.
-OPTION 1: Using MOPEX to do many sources at once: [[I'm ready to advance to a highly technical and in-depth discussion on downloading Spitzer data. | Go here]] and get the (admittedly still cryptic) doc file that is linked in partway down. Find the part on using Apex-1Frame.  Follow that for each channel (twice for HDR data!) to get the photometry.
+'''OPTION 1''': Using MOPEX to do many sources at once (not particularly recommended!): [[I'm ready to advance to a highly technical and in-depth discussion on downloading Spitzer data. | Go here]] and get the (admittedly still cryptic) doc file that is linked in partway down. Find the part on using Apex-1Frame.  Follow that for each channel (twice for HDR data!) to get the photometry.
-OPTION 2: Using [http://spider.ipac.caltech.edu/staff/laher/apt APT] to do one source at a time: [[Aperture photometry using APT| Go here]] and work through the (admittedly skeletal) page.  If you take this option, I recommend you start with a short list of famous objects from the above step.  If you have suggestions for specific improvements for this tutorial, please let me know, or just implement them!  NOTE THAT APT now can handle lists of targets, so you're no longer limited to doing one object at a time using APT, but it is STRONGLY recommended to try several by hand until you understand what is going on and optimize your parameters.
+'''OPTION 2A''': Using [http://spider.ipac.caltech.edu/staff/laher/apt APT] to do one source at a time: [[Aperture photometry using APT| Go here]] and work through the (somewhat skeletal) page.  If you take this option, I recommend you start with a short list of famous objects from the above step.  If you have suggestions for specific improvements for this tutorial, please let me know, or just implement them!
-OPTION 3: Using your own preferred software to do the photometry.  Be careful; read the stuff on [[Units]] first.
+'''OPTION 2B''': Using APT to do a list of sources at once: Only AFTER you have thoroughly explored the parameter options in APT and thoughtfully considered what parameters to use, you can now send APT a list of targets, so you're no longer limited to doing one object at a time using APT.  I STRONGLY recommend that you try several by hand until you understand what is going on, and optimize your parameters correspondingly for the mosaic and channel and background levels you have in your specific mosaic.
 RESULTS: Reduced (and bandmerged) photometry is included in the tarball above if you wish to short-cut this process, and I promise I won't think any less of you.

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 =Downloading the data=
-[[How do I download data from the Spitzer Telescope? | Go here]] (well, really specifically [[Using_the_SHA,_a_concrete_example | here]]) and follow the tutorial to investigate what observations exist and download the data. Search on L1688, or another object of your choice.  Find the AORs, look at the coverage, decide if you need to download the IRAC and MIPS mosaics from more than one AOR or just one AOR.  The data I'm using for this example are all from pid 177, but you should investigate the coverage of the other programs too.
+[[How do I download data from the Spitzer Telescope? | Go here]] (well, really specifically [[Using_the_SHA,_a_concrete_example | here]]) and follow the tutorial to investigate what observations exist and download the data. Search on L1688, or another object of your choice.  Find the AORs, look at the coverage, decide if you need to download the IRAC and MIPS mosaics from more than one AOR or just one AOR.  The data I'm using for this example on L1688 are all from pid 177, but you should investigate the coverage of the other programs too.
 It turns out, though, that L1688 is part of the c2d Legacy program (pid 177 is part of that), so you may also wish to consult the [[How can I find already-reduced Spitzer data? | page on already-reduced Spitzer data]].