Posts with the Tag PyVO:

  • DASCH is now in the VO

    Black dots on a white-ish background.  In the middle, some diffuse greyish stuff around a relatively large black dot.

    This frame would show comet 2P/Encke during its proximity to Earth in 1941 – if it went deep enough. But never mind practicalities: If you want to learn about matching ephemeris against the DASCH plate collection (or, really, any sort of obscore-like table), read on.

    For about a century – that is, into the 1980s –, being an observational astronomer meant taking photographic plates and doing tricks with them (unless you were a radio astronomer or one of the very few astronomers peeking beyond radio and optical in those days, of course). This actually is somewhat fortunate for archivists, because unlike many of the early CCD observations that by now are lost with our ability to read the tapes they were stored on, the plates are still there.

    Why Bother?

    However, to make them usable, the plates need to be digitised. In the GAVO data centre, we keep the results of several scan campaigns large and small, such as HDAP, the various data collections joined in the historical photographic plate image archive HPPA, or the delightfully quirky Münster Flare Plates.

    I personally care a lot about these data collections. This is partly because they are indispensible for understanding the history of astronomy. But more importantly, they are the next best thing we have to a time machine; if we have a way of knowing how the sky looked like seventy years ago, it is these plate collections. Having such a time machine is important for all kinds of scientific efforts, including figuring out whether there are aliens (i.e., 2016ApJ...822L..34S) on Tabby's Star.

    Somewhat to my chagrin, the cited paper 2016ApJ...822L..34S did not use the VO to obtain the plate images but went straight to DASCH's web interface. DASCH, in case you have not heard of it before, is probably the most ambitious project concerned with plate digitisation at the moment – or perhaps: “was”, because they just finished scanning the core part of Harvard's plate collections, which was their primary goal.

    I can understand why Bradley Schaefer, the paper's author, did not bother with a VO search In 2016. For starters, working with halfway homogeneous data from instruments you are somewhat familiar saves a substantial amount of work and thought, in particular if you are, in addition, up against the usual lack of machine-readable metadata. Also, at that time DASCH probably had about as many digitised plates as all the VO's contemporary plate collections taken together.

    DASCH: The Harvard Plates

    Given such stats, I have always wanted to have at least the metadata from DASCH's plates in the VO. Thanks to a recent update to DASCH's publication system, this is now a reality. Since 2024-04-29, I am publishing the metadata of the DASCH plates via Obscore and and SIAP2.

    Followup (2024-05-03)

    This is now DASCH news, and one of my two main contacts on the DASCH side, Peter Williams, has written an insightful post on this, too. Let me use this opportunity to thank him for the delightful cooperation, and extend these thanks to Ben Sabath, who is primarily responsible for the update to the DASCH publication system I mentioned above.

    Matching plates are returned as datalink documents, pointing to a preview, photos of the plate and its jacket, and links to the science data, once downsampled by a factor of 16, once in the original size (example). For now, #this points to the downsampled version, as Amazon charges DASCH about three cents per full-scale plate at the moment, and that can quickly add up by accident (there's nothing wrong with consciously downloading full-scale FITS-es if you need them, of course).

    This is a bit fishy in that the size of the image in the obscore/SIAP2 fields s_xel1 and s_xel2 refers to the unscaled image, and thus I should be returning the full-scale image as datalink #this. I hope I will not cause much confusion with this design.

    In case you look at the links in the datalink documents, let me include a disclaimer: Although they point into the GAVO data centre, the data is served courtesy of the DASCH project. The links only go to us because we need to sign links for you. I mention this because you can save the datalink documents and the links within them; the URLs you are redirected to from there, however, will expire fast. Just do not look at them.

    Plates in Global Discovery

    So – what can you do with DASCH in the VO that you could not do before?

    Most importantly, you will discover DASCH in registry interfaces and its datasets in global queries (in particular the global dataset queries I have discussed a few weeks ago). For instance, DASCH is now in Aladin's discovery tree:

    A screen shot with many selected points, highlighted in green, on the right side.  On the left side, an tree display with many branches folded in.  On a folded-out branch, there is “DASCH SIAP2“ highlighted.  On the right side, there is a large rectangle overplotted in red.

    You can now find DASCH in Aladin and do the usual “in view“ searches. However, currently this yields many matches that are, in practical terms, spurious, as they come from extremely wide-angle instruments. The red rectangle is the footprint of one of these images; note that the view here is a full two pi sky. We will probably do something about this “noise“.

    The addition of DASCH to the VO has a strong effect in some use cases. For instance, at the end of the GAVO plates tutorial, we do an all-VO obscore query that, at the time of the last update of the tutorial in 2019, yielded 4067 datasets (of course, including modern and/or non-optical observations) potentially showing some strongly lensed quasar. With DASCH – and, admittedly, a few more collections that came into the VO since 2019 –, that number is now 10'489; the range of observation dates grew from MJD 12550…52000 to MJD 9800…58600, with the mean decreasing from 51'909 to 30'603. That the mean observation date moves that much back in time is a certain sign that a major part of the expansion is due to DASCH (well, and certainly to APPLAUSE, too).

    Followup (2024-05-03)

    As discussed in my DASCH update, I have taken out the large-coverage plates from my obscore table, which changes the stats (but not the conclusions) quite a bit. They is now 10'098 plates and mean observation date 36'396

    TAP, Uploads, and pyVO on DASCH

    But this is not just about bringing astronomical heritage to the VO. It is also about exposing DASCH through the powerful ADQL/TAP interface. As an example of how this may be useful, consider the comet P2/Encke, which, according to JPL's Small-Body Database was relatively close to Earth (about half an AU) in May 1941. It would have had about 14.5 mag at that point and hence was safely within reach of several of the instruments archived in DASCH. Perhaps we can find serendipitous or even targeted observations of the comet in the collection?

    The plan to find that out is: compute an ephemeris (we are lazy and use an external service, Miriade ephemcc) and then for each day see whether there are DASCH observations in the vicinity of the sky location obtained in this way.

    As usual, it's never that easy because the call to the ephemeris webservice (paste the link into TOPCAT to have a look) returns cursed sexagesimal coordinates. We need to fix them before doing anything serious with the table, and while we are at it, we also repair the date, which is simpler to consume if it is MJD to begin with. Getting the ephemeris thus takes quite a few lines:

    from astropy import table
    from astropy import units as u
    from astropy.coordinates import SkyCoord
    from astropy.time import Time
    ephem =
    parsed = SkyCoord(ephem["ra"], ephem["dec"], unit=(u.hourangle, u.deg))
    ephem["ra"] =
    ephem["dec"] =
    parsed = Time(ephem["epoch"])
    ephem["epoch"] = parsed.mjd

    Compared to that, the actual matching against DASCH is almost trivial if you are somewhat familiar with crossmatching in ADQL and the Obscore schema:

    svc = pyvo.dal.TAPService("")
    res = svc.run_sync("""
        SELECT *
            JOIN tap_upload.orbit
            ON (1=CONTAINS(POINT(ra, dec), s_region))
            AND t_max>epoch""",
        uploads={"orbit": ephem})

    Followup (2024-05-03)

    You would probably query the dasch.narrow_plates table in actual operations; querying dasch.plates is probably more for people interested in the history of astronomy or DASCH itself.

    Inspect the query for a moment: This is a normal upload join, except we are constructing an ADQL POINT on the fly to be able to see whether we are in the spatial region covered by a DASCH dataset (given in obscore's s_region column). We could have put the temporal condition into the join's ON; but I think the intention is somewhat clearer with the WHERE constraint, and the database engine will probably go through identical motions for both queries – the beauty of having a query planner in the loop is that you do not need to think about such details most of the time.

    Actually, in this case there is one last complication: As said above, we have put a datalink service between you and the downloads to discourage accidental large downloads. We hence use pyVO's (suboptimally documented) datalink interface (iter_datalinks):

    with pyvo.samp.connection() as conn:
        for dl in res.iter_datalinks():
            link = next(dl.bysemantics("#preview-image"))

    Among the artefacts available we pick the scaled jpegs in this fragment (#preview-image), since these are almost free even on the Amazon cloud. Change that #preview-image to #this in the to get scaled calibrated FITS-es, which are still fairly small. This would, for instance, let you overplot the ephemeris in Aladin, which you cannot do with the jpegs as they lack astrometric calibration (for now). But even with #preview-image, we can use Aladin as a glorified image viewer by SAMP-sending the images there, which is why we do the minor magic with functions from pyvo.samp.

    If you want to try this yourself or mangle the program to do something else that requires querying against a reasonable number positions in time and space, just get and hack away. Make sure to start Aladin before running the program so it has something to send the images to.


    This is a contrived example, and it is likely that this particular use case is astronomically wrong in several ways. Let me enumerate a few things that would need looking into before this approaches proper science:

    • We compute the ephemeris for the center of the Earth. At half an AU distance, the resulting parallax will not shift the position enough to hide a plate we should know about, but at least for anything closer, you should try to do a bit better; admittedly, for a resource like DASCH – that contains plates from observatories all over the place – you will have to compromise.
    • The ephemeris is probably wrong; comet's orbits change over time, and I have no idea if the ephemeris service actually uses 2P/Encke's 1941 orbit to compute the positions.
    • The coordinate metadata may be wrong. Ephemcc's documentation says something that sounds a lot as if they were sometimes returning RA and Dec for the equator of the time rather than for J2000 (i.e., ICRS for all intents and purposes), but of course our obscore coverages are for the ICRS. Regrettably, the VOTable returned by the service does not contain a COOSYS element yet, and so there is no easy way to tell.
    • If you look at the table with DASCH matches, you will see they all were observed with an extremely wide-angle instrument sporting an aperture of a mere three inches. Even at the whopping exposure times (two hours), there is probably no way you would see a diffuse object of 14th mag on a plate with a 1940s-era photographic emulsion with that kind of optics (well: feel free to prove me wrong).
    • It would of course be a huge waste of bandwidth to pull the entire plates if we already had a good idea of where we would expect the comet (i.e., had a reliable ephemeris). Hence, a cutout service that would let you retrieve more or less exactly the pixels you would like to use for your research and not the cruft around it would be a nifty supplement. It's in the works, and I'd say you can almost hold your breath. The cutout will simply appear as a SODA service in the datalink documents. See 2020ASPC..522..295D for how you would operate such a service.
  • Global Dataset Discovery in PyVO

    A Tkinter user interface with inputs for Space, Spectrum, and Time, a checkbox marked "inclusive", and buttons Run, Stop, Broadcast, Save, and Quit.

    Admittedly somewhat old-style: As part of teaching global dataset discovery to pyVO, I have also come up with a Tkinter GUI for it. See A UI for more on this.

    One of the more exciting promises of the Virtual Observatory was global dataset discovery: You say “Give me all spectra of object X that there are“, and the computer relates that request to all the services that might have applicable data. Once the results come in, they are merged into some uniformly browsable form.

    In the early VO, there were a few applications that let you do this; I fondly remember VODesktop. As the VO grew and diversified, however, this became harder and harder, partly because there were more and more services, partly because there were more protocols through which to publish data. Thus, for all I can see, there is, at this point, no software that can actually query all services plausibly serving, say, images or spectra in the VO.

    I have to say that writing such a thing is not for the faint-hearted, either. I probably wouldn't have tackled it myself unless the pyVO maintainers had made it an effective precondition for cleaning up the pyVO Servicetype constraint.

    But they did, and hence as a model I finally wrote some code to do all-VO image searches using all of SIA1, SIA2, and obscore, i.e., the two major versions of the Simple Image Access Protocol plus Obscore tables published through TAP services. I actually have already reported in Tucson on some preparatory work I did last summer and named a few problems:

    • There are too many services to query on a regular basis, but filtering them would require them to declare their coverage; far too many still don't.
    • With the current way of registering obscore tables, there is no way to know their coverage.
    • One dataset may be availble through up to three protocols on a single host.
    • SIA1 does not even let you constrain time and spectrum.

    Some of these problems I can work around, others I can try to fix. Read on to find out how I fared so far.

    The pyVO API

    Currently, the development happens in pyVO PR #470. While it is still a PR, let me point you to temporary pyVO docs on the proposed module – of course, all of this is for review and probably not in the shape it will remain in[1].

    To quote from there, the basic usage would be something like:

    from pyvo import discover
    from astropy import units as u
    from astropy import time
    datasets, log = discover.images_globally(
      space=(274.6880, -13.7920, 0.1),
      time=(time.Time('1995-01-01'), time.Time('1995-12-31')))

    At this point, only a cone is supported as a space constraint, and only a single point in spectrum. It would certainly be desirable to be more flexible with the space constraint, but given the capabilities of the various protocols, that is hard to do. Actually, even with the plain cone Obscore (i.e., ironically, the most powerful of the discovery protocols covered here) currently results in an implementation that makes me unhappy: ugly, slow, and wrong. This is requires a longer discussion; see Appendix: Optionality Considered Harmful.

    datasets at this point is a list of, conceputally, Obscore records. Technically, the list contains instances of a custom class ImageFound, which have attributes named after the Obscore columns. In case you have doubts about the Semantics of any column, the Obscore specification is there to help. And yes, you can argue we should create a single astropy table from that list. You are probably right.

    PyVO adds an extra column over the mandatory obscore set, origin_service. This contains the IVOA identifier (IVOID) of the service at which the dataset was found. You have probably seen IVOIDs before: they are URIs with a scheme of ivo:. What you may not know: these things actually resolve, specifically to registry resource records. You can do this resolution in a web browser: Just prepend to an IVOID and paste the result into the address bar. For instance, my Obscore table has the IVOID ivo://org.gavo.dc/__system__/obscore/obscore; the link below the IVOID leads you to an information page, which happens to be the resource's Registry record formatted with a bit of XSLT. A somewhat more readable but less informative rendering is available when you prepend (“landing page”).

    The second value returned from discover.images_globally is a list of strings with information on how the global discovery progressed. For now, this is not intended to be machine-readable. Humans can figure out which resources were skipped because other services already cover their data, which services yielded how many records, and which services failed, for instance:

    Skipping ivo://org.gavo.dc/lswscans/res/positions/siap because it is served by ivo://org.gavo.dc/__system__/obscore/obscore
    Skipping ivo://org.gavo.dc/rosat/q/im because it is served by ivo://org.gavo.dc/__system__/obscore/obscore
    Obscore GAVO Data Center Obscore Table: 2 records
    SIA2 The VO @ ASTRON SIAP Version 2 Service: 0 records
    SIA2 ivo://au.csiro/casda/sia2 skipped: ReadTimeout: HTTPSConnectionPool(host=&apos;;, port=443): Read timed out. (read timeout=20)
    SIA2 CADC Image Search (SIA): 0 records
    SIA2 European HST Archive SIAP service: 0 records

    (On the skipping, see Relationships below). I consider this crucial provenance, as that lets you assess later what you may have missed. When you save the results, be sure to save these, too.

    A feature that will presumably (see Inclusivity for the reasons for this expectation) be important at least for a few years is that you can pass the result of a Registry query, and pyVO will try to find services suitable for image discovery on that set of resources.

    A relatively straightforward use case for that is global obscore discovery. This would look like this:

    from pyvo import discover
    from pyvo import registry
    from astropy import units as u
    from astropy import time
    def say(s):
    datasets, log = discover.images_globally(
      space=(274.6880, -13.7920, 1),
      time=(time.Time('1995-01-01'), time.Time('1995-12-31')),"obscore")),

    (the watcher thing lets you, well, watch the progress of the discovery).

    A UI

    To get an idea whether this API might one day work for the average astronomer, I have written a Tkinter-based GUI to global image discovery as it is now: tkdiscover (only available from github at this point). This is what a session with it might look like:

    Lots of TOPCAT windows with various graphs and tables, an x-ray image of the sky with overplotted points, and a play gray window offering the specification of space, spectrum, and time constraints.

    The actual UI is in the top right: A plain window in which you can configure a global discovery query by straightfoward serialisations of discover.images_globally's arguments:

    • Space (currently, a cone in RA, Dec, and search radius, separated by whitespace of commas)
    • Spectrum (currently, a single point as a wavelength in metres)
    • Time (currently, either a single point in time – which probably is rarely useful – or an interval, to be entered as civil DALI dates
    • Inclusivity.

    When you run this, this basically calls discover.images_globally and lets you know how it is progressing. You can click Broadcast (which sends the current result to all VOTable clients on the SAMP bus) or Save at any time and inspect how discovery is progressing. I predict you will want to do that, because querying dozens of services will take time.

    There is also a Stop button that aborts the dataset search (you will still have the records already found). Note that the Stop button will not interrupt running network operations, because the network library underneath pyVO, requests, is not designed for being interrupted. Hence, be patient when you hit stop; this may take as long as the configured timeout (currently is 20 seconds) if the service hangs or has to do a lot of work. You can see that tkdiscover has noticed your stop request because the service counter will show a leading zero.

    Service counter? Oh, that's what is at the bottom right of the window. Once service discovery is done, that contains three numbers: The number of services to query, the number of services queried already, and the number of services that failed.

    The table contains the obscore records described above, and the log lines are in the discovery_log INFO. I will give you that this is extremely unreadable in particular in TOPCAT, which normalises the line separators to plain whitespace. Perhaps some other representation of these log lines would be preferable: A PARAM with a char[][] (but VOTable still is terrible with arrays of variable-length strings)? Or a separate table with char[*] entries?


    I have promised above I'd explain the “Inclusive” part in both the pyVO API and the Tk UI. Well, this is a bit of a sad story.

    All-VO-queries take time. Thus, in pyVO we try to only query services that we expect serve data of interest. How do we arrive at expectations like that? Well, quite a few records in the Registry by now declare their coverage in space and time (cf. my 2018 post for details).

    The trouble is: Most still don't. The checkmark at inclusive decides whether or not to query these “undecidable” services. Which makes a huge difference in runtime and effort. With the pre-configured constraints in the current prototype (X-Ray images a degree around 274.6880, -13.7920 from the year 1995), we currently discover three services (of which only one actually needs to be queried) when inclusive is off. When it is on, pyVO will query a whopping 323 services (today).

    The inclusivity crisis is particularly bad with Obscore tables because of their broken registration pattern; I can say that so bluntly because I am the author of the standard at fault, TAPRegExt. I am preparing a note with a longer explanation and proposals for fixing matters – <cough> follow me on github –, but in all brevity: Obscore data is discovered using something like a flag on TAP services. That is bad because the TAP services usually have entriely different metadata from their Obscore table; think, in particular, of the physical coverage that is relevant here.

    It will be quite a bit of effort to get the data providers to do the Registry work required to improve this situation. Until that is done, you will miss Obscore tables when you don't check inclusive (or override automatic resource selection as above) – and if you do check inclusive, your discovery runs will take something like a quarter of an hour.


    In general, the sheer number of services to query is the Achilles' heel in the whole plan. There is nothing wrong with having a machine query 20 services, but querying 200 is starting to become an effort.

    With multi-data collection services like Obscore (or collective SIA2 services), getting down to a few dozen services globally for a well-constrained search is actually not unrealistic; once all resources properly declare their coverage, it is not very likely that more than 20 institutions worldwide will have data in a credibly small region of space, time, and spectrum. If all these run collective services and properly declare the datasets to be served by them, that's our 20-services global query right there.

    However, pyVO has to know when data contained in a resource is actually queriable by a collective service. Fortunately, this problem has already been addressed in the 2019 endorsed note on Discovering Data Collections Within Services: Basically, the individual resource declares an IsServedBy relationship to the collective service. PyVO global discovery already looks at these. That is how it could figure out these two things in the sample log given above:

    Skipping ivo://org.gavo.dc/lswscans/res/positions/siap because it is served by ivo://org.gavo.dc/__system__/obscore/obscore
    Skipping ivo://org.gavo.dc/rosat/q/im because it is served by ivo://org.gavo.dc/__system__/obscore/obscore

    But of course the individual services have to declare these relationships. Surprisingly many already do, as you can observe yourself when you run:

    select ivoid, related_id from
    natural join rr.capability
    standard_id like 'ivo://'
    and relationship_type='isservedby'

    on your favourite RegTAP endpoint (if you have no preferences, use mine: If you have collective services and run individual SIA services, too, please run that query, see if you are in there, and if not, please declare the necessary relationships. In case you are unsure as to what to do, feel free to contact me.

    Future Directions

    At this point, this is a rather rough prototype that needs a lot of fleshing out. I am posting this in part to invite the more adventurous to try (and break) global discovery and develop further ideas.

    Some extensions I am already envisaging include:

    • Write a similar module for spectra based on SSAP and Obscore. That would then probably also work for time series and similar 1D data.

    • Do all the Registry work I was just talking about.

    • Allow interval-valued spectral constraints. That's pretty straightforward; if you are looking for some place to contribute code, this is what I'd point you to.

    • Track overflow conditions. That should also be simple, probably just a matter of perusing the pyVO docs or source code and then conditionally produce a log entry.

    • Make an obscore s_region out of the SIA1 WCS information. This should also be easy – perhaps someone already has code for that that's tested around the poles and across the stitching line? Contributions are welcome.

    • Allow more complex geometries to define the spatial region of interest. To keep SIA1 viable in that scenario it would be conceivable to compute a bounding box for SIA1 POS/SIZE and do “exact” matching locally on the coarser SIA1 result.

    • Enable multi-position or multi-interval constraints. This pretty certainly would exclude SIA1, and, realistically, I'd probably only enable Obscore services with TAP uploads with this. With those constraints, it would be rather straightforward.

    • Add SODA support: It would be cool if my ImageFound had a way to say “retrieve data for my RoI only”. This would use SODA and datalink to do server-side cutouts where available and do the cut-out locally otherwise. If this sounds like rocket science: No, the standards for that are actually in place, and pyVO also has the necessary support code. But still the plumbing is somewhat tricky, partly also because pyVO's datalink API still is a bit clunky.

    • Going async? Right now, we civilly query one service after the other, waiting for each result before proceeding to the next service. This is rather in line with how pyVO is written so far.

      However, on the network side for many years asynchronous programming has been a very successful paradigm – for instance, our DaCHS package has been based on an async framework from the start, and Python itself has growing in-language support for async, too.

      Async allows you to you fire off a network request and forget about it until the results come back (yes, it's the principle of async TAP, too). That would let people run many queries in parallel, which in turn would result in dramatically reduced waiting times, while we can rather easily ensure that a single client will not overflow any server. Still, it would be handing a fairly powerful tool into possibly unexperienced hands… Well: for now there is no need to decide on this, as pyVO would need rather substantial upgrades to support async.

    Appendix: Optionality Considered Harmful

    The trouble with obscore and cones is a good illustration of the traps of attempting to fix problems by adding optional features. I currently translate the cone constraint on Obscore using:

    "(distance(s_ra, s_dec, {}, {}) < {}".format([0],[1], self.radius)
    +" or 1=intersects(circle({}, {}, {}), s_region))".format([0],[1], self.radius))

    which is all of ugly, presumably slow, and wrong.

    To appreciate what is going on, you need to know that Obscore has two ways to define the spatial coverage of an observation. You can give its “center” (s_ra, s_dec) and something like a rough radius (s_fov), or you can give some sort of geometry (e.g., a polygon: s_region). When the standard was written, the authors wanted to enable Obscore services even on databases that do not know about spherical geometry, and hence s_region is considered rather optional. In consequence, it is missing in many services. And even the s_ra, s_dec, s_fov combo is not mandatory non-null, so you are perfectly entitled to only give s_region.

    That is why there are the two conditions or-ed together (ugly) in the code fragment above. 1=intersects(circle(.), s_region) is the correct part; this is basically how the cone is interpreted in SIA1, too. But because s_region may be NULL even when s_ra and s_dec are given, we also need to do a test based on the center position and the field of view. That rather likely makes things slower, possibly quite a bit.

    Even worse, the distance-based condition actually is wrong. What I really ought to take into account is s_fov and then do something like distance(.) < {self.radius}+s_fov, that is, the dataset position need only be closer than the cone radius plus the dataset's FoV (“intersects”). But that would again produce a lot of false negatives because s_fov may be NULL, too, and often is, after which the whole condition would be false.

    On top of that, it is virtually impossible that such an expression would be evaluated using an index, and hence with this code in place, we would likely be seqscanning the entire obscore table almost every time – which really hurts when you have about 85 Million records in your Obscore table (as I do).

    The standard could immediately have sanitised all this by saying: when you have s_ra and s_dec, you must also give a non-empty s_fov and s_region. This is a classic case for where a MUST would have been necessary to produce something that is usable without jumping through hoops. See my post on Requirements and Validators on this blog for a longer exposition on this whole matter.

    I'm not sure if there is a better solution than the current “if the operators didn't bother with s_region, the dataset's FoV will be ignored“. If you have good ideas, by all means let me know.


    If you want to try this (in particular without clobbering your “normal” pyVO), do something like this:

    virtualenv --system-site-packages global-datasets
    . global-datasets/bin/activate
    cd global-datasets
    git clone
    cd pyvo
    git checkout global-datasets
    pip install .
  • Towards Data Discovery in pyVO

    When I struggled with ways to properly integrate TAP services – which may have hundreds or thousands of different resources in one service – into the VO Registry without breaking what we already had, I realised that there are really two fundamentally different modes of using the VO Registry. In Discovering Data Collections's abstract I wrote:

    the Registry must support both VO-wide discovery of services by type ("service enumeration") and discovery by data collection ("data discovery").

    To illustrate the difference in a non-TAP case, suppose I have archived images of lensed quasars from Telescopes A, B, and C. All these image collections are resources in their own right and should be separately findable when people look for “resources with data from Telescope A“ or perhaps “images obtained between 2011-01-01 and 2011-12-31”.

    However, when a machine wants to find all images at a certain position, publishing the three resources through three different services would mean that that machine has to do three requests where one would work just as well. That is very relevant when you think about how the VO will evolve: At this point there are 342 SIAP services in the VO, and when you read this, that number may have grown further. Adding one service per collection will simply not scale when we want to keep the possibility of all-VO searches. Since I claim that is a very desirably thing, we need to enable collective services covering multiple subordinate resources.

    So, while in the first (“data discovery”) case one wants to query (or at least discover) the three resources separately, in the second case they should be ignored, and only a collective “images of lensed quasars” service should be queried.

    The technical solution to this requirement was creating “auxliary capabilities” as discribed in the endorsed note on discoving data collections cited above. But these of course need client support; VO clients up to now by and large do service enumeration, as that has been what we started with in the VO Registry. Client support would, roughly, mean that clients would present their users with data collections, and then offer the various ways to to access them.

    There are quite a number of technicalities involved in why that's not terribly straightforward for the “big” clients like TOPCAT and Aladin (though Aladin's discovery tree already comes rather close).

    Now that quite a number of people use pyVO interactively in jupyter notebooks, extending pyVO's registry interface to do data discovery in addition to the conventional service enumeration becomes an attractive target to have data discovery in practice.

    I have hence created pyVO PR #289. I think some the rough edges will need to be smoothed out before it can be merged, but meanwhile I'd be grateful if you could try it out already. To facilitate that, I have prepared a jupyter notebook that shows the basic ideas.

    Followup (2023-12-15)

    I have just prepared a slightly updated version of the notebook.

    To run it while the PR is not merged, you need to install the forked pyVO. In order to not clobber your main installation, you can install astropy using your package manager and then do the following (assuming your shell is bash or something suitably similar):

    virtualenv --system-site-packages try-discoverdata
    . try-discoverdata/bin/activate
    cd try-discoverdata
    git clone
    cd pyvo
    git checkout add-discoverdata
    python3 develop
    ipython3 notebook

    That should open a browser window in which you can open the notebook (you probably want to download it into the pyvo checkout in order to make the notebook selector see it). Enjoy!

  • LAMOST5 meets Datalink

    One of the busiest spectral survey instruments operated right now is the Large Sky Area Multi-Object Fiber Spectrograph Telescope (LAMOST). And its data in the VO, more or less: DR2 and DR3 have been brought into the VO by our Czech colleagues, but since they currently lack resources to update their services to the latest releases, they have kindly given me their DaCHS resource descriptor, and so I had a head start for publishing DR5 in Heidelberg.

    With some minor updates, here it is now: Over nine million medium-resolution spectra covering large parts of the northen sky – the spatial coverage is like this:

    Coverage Healpix map

    There's lots of fun to be had with this; of course, there's an SSA service, so when you point Aladin or Splat at some part of the covered sky and look for spectra, chances are you'll see LAMOST spectra, and when working on some of our tutorials (this one, for example), it happened that LAMOST actually had what I was looking for when writing them.

    But I'd like to use the opportunity to mention two other modes of accessing the data.

    Stacked spectra

    Tablesample and TOPCAT's Plot Table activation action

    Say you'd like to look at spectra of M stars and would like to have some sample from across the sky, fire up TOPCAT, point its TAP client the GAVO DC TAP service ( and run something like:

      ssa_pubDID, accref, raj2000, dej2000, ssa_targsubclass
    from tablesample(1)
      ssa_targsubclass like 'M%'

    This is using the TABLESAMPLE modifier in the from clause, which isn't standard ADQL yet. As mentioned in the DaCHS 1.4 announcement, DaCHS has a prototype implementation of what's been discussed on the IVOA's DAL mailing list: pick a part of a table rather than the full one. It takes a percentage as an argument, and tells the server to choose about this percentage of the table's records using a reasonable and fast heuristic. Note that this won't give you perfect statistical sampling, but if it's not “good enough” for some purpose, I'd like to learn about that purpose.

    Drawing a proper statistical sample, on the other hand, would take minutes on the GAVO database server – with tablesample, I had the roughly 6000 spectra the above query returns essentially instantaneously, and from eyeballing a sky plot of them, I'd say their distribution is close enough to that of the full DR5. So: tablesample is your friend.

    For a quick look at the spectra themselves, in TOPCAT click Views/Activation Actions, check “Plot Table” and make sure TOPCAT proposes the accref column as “Table Location” (if you don't see these items, update your TOPCAT – it's worth it). Now click on a row or perhaps a dot on a plot and behold an M spectrum.

  • Small Telescopes, Large Surveys

    Image: Blink comparator and survey camera

    Plate technology at Bamberg observatory: a blink comparator with one plate mounted, and a survey camera that was once used at Boyden Station, an astronomer outpost in 60ies South Africa.

    I'm currently at the workshop “Large surveys with small telescopes: past, present, and future” (or Astroplate III for short) in Bamberg, where people are discussing using and re-using the rich heritage of historical observations (hence the “plate” part) as well growing that heritage in the age of large CCDs, fast computers and large disks.

    Using and re-using is of course what the Virtual Observatory is about, and we've been keeping fairly large plate collections in our data center for quite a while (among them the Archives of Landessternwarte Königstuhl or the Palomar-Leiden Trojan surveys, and there is the WFPDB TAP-accessibly). Therefore, people from GAVO Heidelberg have been to all past astroplate conferences.

    For this one, I brought a brand-new tutorial on plate scans in the VO, which, I hope, also works as a general introduction to image discovery in the VO using SIAP, Datalink, and Obscore. If you're doing image stuff now and then, please have a quick look at the thing – I am particularly grateful for hints on what to improve or perhaps particularly obvious use cases for the material discussed.

    Such VO proselytising aside, the conference is discussing the wide variety of creative, low-cost data collectors out there as well as computer-aided re-analysis extracting new knowledge from decades-old data. If I had to choose a single come-to-think-of-it moment, it would be Norbert Zacharias' observation that if you have a well-behaved object and you'd like to know where it was in 1900, it's now more accurate to extrapolate Gaia astrometry to the epoch of observation than to measure it on the plate itself. Which is saying a lot about the amazing feat of engineering that Gaia is.

    This is not, however, an argument for dumping the old data. Usually, it is exactly what is not so well-behaved (like those) that's interesting – both in terms of astrometry and in terms of photometry (for which there's a lot more unruly behaviour in the first place). To figure out how objects don't behave well, and, for objects disguising as well-behaved only on time scales of the (say) Gaia mission, which these are, the key is “old” data. The freshness of which we're discussing this week.

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