Galaxy Zoo Talk

How to use TOPCAT to match catalogs like GZ1 and GZ2 and sort?

  • JeanTate by JeanTate

    In a GZ forum thread, zkKyle wrote: "ps - if anyone is interested in looking at the actual data, I promise it's not as intimidating as it might occasionally seem. Publicly available tools like TOPCAT are really good at matching catalogs like GZ1 and GZ2 and sorting data"

    If anyone reading this has experience with TOPCAT (or similar) and the time to explain how to use it to match catalogs and sort, would you mind writing a tutorial for us ordinary zooites please?

    Posted

  • KWillett by KWillett scientist, admin, translator

    Sorry I didn't see this earlier, Jean. Have you tried downloading or using TOPCAT at all yet?

    If you give one or two goals for what you'd like to use it for, I can try to offer advice.

    Posted

  • JeanTate by JeanTate in response to KWillett's comment.

    Hi Kyle.

    Yes, I myself have used TOPCAT before, on (with?) the MPA-JHU "emission line analysis for the SDSS Data Release 7 (DR7)".

    As I explained to Meg, over in PH Talk, I'm not so much interested for myself - I think I can work it out on my own (when I return home, and do not need to rely upon just my laptop and uncertain internet access here in China 😉 ) - but to get an easily accessible, easy-to-follow, directly relevant to GZ, step-by-step intro so that other ordinary zooites who are undoubtedly interested (and who may not be as afraid of looking completely stupid in public as I am1) can follow and learn. Yeah, it's a kinda hobby-horse of mine; many zooites not only want to click/classify, but yearn to learn, and we oldbies should try harder to help.

    That said, perhaps a concrete example might help: how - in detail - would you go about finding the DR7 ObjIds of the galaxies classified as 'ACW' (with P_ACW >= 0.90 as the threshold) in GZ1 AND which were classified in GZ2 from Stripe 82 images (either version, no matter what the classifications were) AND which have DR7 spectroscopic redshifts > 0.02 but < 0.30?

    1 It's much more difficult to see in any Talk, but in the GZ forum, the number of "Views" of threads in which I ask these sorts of questions never ceases to amaze me; who ARE all these ordinary zooites who are reading my posts/these threads, but who never ask any questions (notable exceptions, of course)?!? In Quench Talk, three other ordinary zooites were brave enough to respond at one point (so I have proof that these interested-but-fearful-of-responding ordinary zooites do exist! 😃 ), and I continue to receive PMs/Messages from various interested ordinary zooites, who don't - it seems - want to post publicly.

    Posted

  • vrooje by vrooje admin, scientist

    Also, on the TOPCAT website there is a link to some documentation and an FAQ, both of which may help.

    To me, the key points are: the folder icon opens a new file, so you can use that to import data. If you're importing a FITS file or other binary format, you can use the "auto" selection when importing a file, but if it's a text file that you can read yourself you'll probably have to specify what kind of text file it is (csv, or VO Table, or others).

    Once you've read in a table use the icons immediately to the right of the little radio tower to see what's in the table after you've clicked on it. You can change and add columns, manipulate individual table entries, etc., using these. Sorting is really easy, as if you click on the table name the details will appear in the "Current Table Properties" part of the window, and you can choose what to sort by and whether to show only a subset of the table there.

    Subsets are defined in a window you can bring up using the icon that looks like a funny-colored egg.

    If you want to match two tables together, use the icon with two matchsticks further to the right in the icon bar. You can match by many different criteria, including by a single column in each ("exact value", such as object ID) or by position on the sky (e.g. RA and Dec) to within some tolerance that you specify. You can also simultaneously cross-match more than 2 tables by using the different options in the Joins menu.

    If you make new tables or make changes to old tables, you can save them using the floppy disk icon near the left side of the icon bar. You can also save the whole session of TOPCAT, a feature I use frequently to save my work in progress. If you save a session and your computer crashes, you can load the session and continue essentially right where you left off. (It doesn't save everything but it is pretty close.) Sessions are saved in fits-plus format and I usually end the filenames with topcatsession.fits to remind myself that it's not a regular fits table.

    Note: all the options I mentioned above are also in the TOPCAT menus... I just use the icons more frequently.

    Hopefully that plus the online documentation ought to get you started? But also, as Kyle said, if you have something specific that you're having trouble with, feel free to ask...

    Cheers,
    -Brooke

    PS: If you're loading really big tables and you run out of memory, check the documentation as there are instructions on how to increase the program's memory allocation.

    Posted

  • vrooje by vrooje admin, scientist

    Hi Jean,

    Sorry, cross-post!

    I think a tutorial on a specific case with TOPCAT would be really useful and there's probably nobody like a seasoned data expert such as yourself who is also using the program for the first time to lead other people through it after some exploration!

    Cheers,

    -Brooke

    PS: On most forums the "views" is a strict number of page views, including non-human views like the Google web crawler. So you can take some comfort in the fact that a higher fraction of people are willing to speak up than it seems -- though I have no doubt a lot of people "lurk" on all forums. 😃

    Posted