Downloading from the Gemini Observatory Archive

For this tutorial we provide a pre-made package with all the necessary data. Here we show how one can search and download the data directly from the archive, like one would have to do for their own program.

If you are just interested in trying out the tutorial, we recommend that you download the pre-made package (Downloading the tutorial datasets) instead of getting everything manually.

Query and Download

This tutorial uses observations from a Science Verification program done during the commissioning and characterizing phase of the GMOS-N Hamamamatsu CCDs. The program ID is GN-2017A-SV-151.

The first step of any reduction is to retrieve the data from the Gemini Observatory Archive (GOA). For more details on using the Archive, check its Help Page.

Science Data

Access the Gemini Observatory Archive (GOA) and fill the search form as follows:

  • Program ID: GN-2017A-SV-151-382
  • Instrument: GMOS-N
  • Filter: i’

Press the Search button in the middle of the page.

The table will show you 10 files. Mark the checkbox for the first 5 files in the list. Normally, you would use all 10 files, but for the purpose of the tutorial, 5 files will do and will run faster.

You can also copy the URL below and paste it on browser to see the search results:


The calibration files could be obtained by simply clicking on the Load Associated Calibrations tab. You will see that the Gemini Archive will load much more files than we need (129 files, totalling 0.53 Gb). Obviously we don’t need all that.

For this data, we need a few biases and a few twilight flats, all taken around the time of the observations. How many to download depends on your personal philosophy to some extend. For the biases, using 10 to 20 raw biases works well. For the twilight flats, make sure that they are set to “Pass”, do not use the “Usable” if you can avoid it. In this case, because it was commissioning data, the quality status was not set and all calibrations are set to “Undefined”. It will be fine for our purpose.

For this tutorial, we will pick the 10 biases taken on the day previous to our observations since none were taken on the day. The twilight flats from 2017 July 2, GN-CAL20170702-3, are the closest in time to our observations, we will use those.

For the biases, let’s pick the first ten (10) on the list, skipping the very top one which comes from an engineering program (the GN-ENG- in the program ID gives it up). The selected biases are from observation ID GN-CAL20170613-3 and GN-CAL20170615-14. Select the checkboxes on the left.

For the twilight flats, scroll down the table until you see them, about half way down. Be mindful of the last column, we normally must select the flats with a “Pass” status. Here all the flats are set to “Undefined” because this was commissioning data so we will have to make due with them. Let’s pick the flats from the night of 2017 July 2 with observation ID GN-CAL20170702-3. Let’s pick the first 5 flats. Select them checkboxes on the left.

Now scroll all the way down and press the “Download Marked Files” button.

Unpacking the data

Now, copy all the .tar files to the same place in your computer. Then use tar and bunzip2 commands to decompress them. For example:

$ cd ${path_to_my_data}/
$ tar -xf gemini_data.tar
$ bunzip2 *.fits.bz2

(The tar files names may differ slightly depending on how you selected and downloaded the data from the Gemini Archive.)


If you are using the manually selected data to run the tutorial, please remember to put all the data in a directory called playdata, and create a parallel directory for running the tutorial called playground. The tutorial makes assumption as to where everything is located.