0 votes

I have been downloading DR_7 and DR_6 images for the g, r, and z bands using the DataLab image query for a list of known objects using RA/DEC and I've been having a few issues.

1) For about 1/2 of the objects this is working great, returning the 5'x5' cutout, but for the others I am getting only small parts of the image. I thought this could be an edge issue at first but that shouldn't be the case for so many of them.

2) The images I downloaded are "N up, E right", instead of "N up, E left", is this how they are actually calibrated or is there an error in how I am downloading them?

3) For DR_6 I have been able to get images for the g and z bands but using the same coordinates and code nothing is returned for the r band.

Here is the code I am using:

DEF_ACCESS_URL = "http://datalab.noao.edu/sia/ls_dr7"

svc = sia.SIAService(DEF_ACCESS_URL)

fov = (5./60)

for ra,dec,ID in zip(R_A,D_EC,I_D):
        imgTable = svc.search((ra,dec),(fov/np.cos(dec*np.pi/180),fov),verbosity=2).to_table()
        sel0 = startswith(imgTable['obs_bandpass'].astype(str),'g')
        sel = sel0 & ((imgTable['proctype'].astype(str)=='Stack') & (imgTable['prodtype'].astype(str)=='image')) # basic selection
        Table = imgTable[sel] # select
        row = Table[0]
        url = row['access_url'].decode() # get the download URL
        urllib.urlretrieve(url, ID + "_g.fits")

Please let me know how I can resolve these issues.

Thank you!
asked Jul 17, 2019 by anonymous | 93 views

1 Answer

0 votes
Hi there,

1) For your first question, can you give a list of example ra, dec where this is happening, as well as some where the result is as expected?

2) This is how the Legacy Survey stacks have their WCS defined.  You can verify that the WCS is correct by over plotting a catalog.

3) So for LS DR6, the filter is recorded as "bokr", and so does not start with "r".  If you modify your code to look for "r" or "bokr", it should return r-band images.


answered Jul 17, 2019 by kolsen (2,700 points)
Thank you for the help!

Here are a few objects where the images are cut off:
RA: 256.4  DEC: +23.36
RA: 202.09 DEC: +4.32
RA: 184.6   DEC: +32.35

Here are some that returned the 5'x5' cutout:
RA: 179.9  DEC: +24.27
RA: 27.15  DEC: +5.51
RA: 151.35  DEC: -24.42

190 questions

187 answers


1,326 users

Welcome to Data Lab Help Desk, where you can ask questions and receive answers from other members of the community.