Posts with the Tag Plates:

  • Computing Residuals of an Astrometric Calibration

    Two plots, left a fairly good correlation, right a cloudy wave

    The kind of plot you can make following the recipe given here: Left, a comparison of the photometry, right, a positional residuals, not taking into account the SIP plate solution, when comparing the HDAP plate B3261a against Gaia DR3. Note that the cut-off a 4 arcsec is because of the match radius when obtaining the calibrator stars.

    I recently had to assess the quality of the astrometric calibration of a photographic plate. What I am going to show you in this post will of course work just as well for CCD frames, and if these have a sufficiently large field of view, this may be an issue for them as well. However, the sort of data that needs this assessment most typically are scans of plates, as these tend to have a “wobble”, systematic offsets in the scan direction resulting from imperfections in the mechanics.

    Prerequisites: An astronomical frame with a calibration in ICRS (or some frame not very far from it), called my-image.fits in the following, SExtractor (in Debian and derivatives: apt install source-extractor – long live Debian Astro; since it's called source-extractor in Debian, that's what I'll use here, too), and of course TOPCAT.

    Step 1: Extract Sources. Source extraction is of course a high science, and if you know better than me, by all means do it the way you think is appropriate. Meanwhile, the following might very well work for you sufficiently well.

    Create a working directory and enter it. Then, to create a file telling source-extractor what columns you would like to see, write the following to a file default.param:


    Next, give a few parameters to source-extractor; depending on the sort of image you have, you may want to play around with DETECT_MINAREA (how many pixels need to show a signal to register as a source) and DETECT_THRESH (how many sigmas a pixel has to be above the background to register as a candidate for belonging to a source). Meanwhile, write the following into a file default.control:

    CATALOG_NAME     img.axy
    PARAMETERS_NAME  default.param
    FILTER           N
    SEEING_FWHM      1.2

    – but if the following call gives you a few hundred sources, that ought to work for the present purpose.

    Then run:

    source-extractor -c default.control my-image.fits

    This will give you a catalogue of extracted objects in the file img.axy.

    Step 2: Fix source-extractor's output. Load that img.axy into TOPCAT. Regrettably, source-extractor does not add any useful metadata to the columns of its output table. To add the absolute bare minimum, in TOPCAT go to ViewsColumn Info. In that window, check UCD in the Display menu, and then put pos.eq.ra and pos.eq.dec into the UCD fields of the ALPHA_SKY and DELTA_SKY columns, respectively; double click to change fields in TOPCAT.

    To see if you have done the annotation right, in TOPCAT's main window, click GraphicsSky Plot. If the objects show up, you have just provided enough annotation to let TOPCAT figure out the position for each row.

    Step 3: Get calibrators. We will now try to add counterparts for Gaia DR3 to the extracted sources. To do that, click VOTable Access Protocol, and in the window popping up double click the entry for the GAVO DC TAP.

    In the Find box, type dr3lite to look for this site's version of the Gaia DR3 source catalogue. Click on gaia.dr3lite to select that table, and then select the Columns pane. This should show some of the Gaia DR3 columns.

    Now ExamplesUpload Join will generate a query that will cross-match your extracted sources with the Gaia sources. You should edit it a bit, only selecting the columns you will actually need, removing the TOP 1000 (at least on large images with more than 1000 sources), and reducing the match radius a bit when the calibration is not actually completely off and your epoch is sufficiently close to J2000.

    Hint: you can control-click in the Columns pane and then use the Cols button to insert all the column names in one go[1]. For me, the resulting query would be:

       source_id, ra, dec, phot_bp_mean_mag,
       FROM gaia.dr3lite AS db
       JOIN TAP_UPLOAD.t1 AS tc
       ON 1=CONTAINS(POINT('ICRS', db.ra, db.dec),
                     CIRCLE('ICRS', tc.ALPHA_SKY, tc.DELTA_SKY, 4./3600.))

    This should result in about as many matches as your extraction had – a few more is ok, because you will have some spurious matches, a few less is ok, too, as there are always some outliers and artefacts, but you should clearly not pull a magnitude more or less objects here than you put in; fiddle with the match radius as necessary.

    See if there is a rough correlation between the Gaia calibrators and your extracted sources by plotting phot_bp_mean_mag against MAG_ISO. Absent more information, MAG_ISO, source-extractor's guess for the magnitude of the extracted object, will be just some crazy number, but it should have some discernable correlation with the actual magnitude. Do not expect too much here, in particular with old plates, for which good photometry is a science of their own.

    In my example, this looked like this:

    Plot: a rough correlation in red with a green tail

    The green points certainly are spurious matches; this observation did not reach beyond 14th magnitude or so, and there are many weak stars on the sky, so a few of them will show up in just about any cross match. See the opening picture for an example with a better correlation.

    Step 4: Do the correlation plot. Do GraphicsPlane Plot and then plot ra-alpha_sky or dec-delta_sky against X_IMAGE or Y_IMAGE. You could get something like this:

    Plot: A single wavy thing

    This rather certainly reflects some optical distortion; source-extractor regrettably does not take into account SIP corrections yet, so it is likely that a large part of this would be taken care of by the polynomials of the plate solution (the github issue I am linking to tells you how to be sure).

    But it can also look like this:

    Plot: Multiple wobbles

    This certainly is not the result of a lens or anything optical at all. It's the scanner's gears that you are looking at here. With an amplitude of perhaps three arcseconds this is rather excessive here; but something like this you will rather likely see even on good scanners – though it may essentially be invisible, as of the Heidelberg scanner we used for HDAP:

    Plot: A vertical cloud with no discernible structure.
    [1]I'm using the BP magnitude in the query below as most historical plates tend to be “blue sensitive“ (in some sense). Hence, BP magnitudes should be a bit closer to what source-extractor has extracted.
  • APPLAUSE via Obscore

    A composite of two rather noisy photo plates

    Aladin showing some Bamberg Sky Patrol plates (see towards the end of the post for what this is and how I made it).

    At the Astroplate conference I blogged about recently, the people behind APPLAUSE gave a couple of talks about their Data Release 3. APPLAUSE is a fairly massive endeavour to make available data from some of the larger plate archives in Germany, and its DR3 even hit the non-Astronomy press last February.

    Already for previous APPLAUSE releases, I've wanted to bring this data (or rather, its metadata) to the VO, but it never quite happened, basically because there was always another little thing that turned out to be too tedious to work out via mail. However, working out things interactively is exactly what conferences are great for. So, the kind APPLAUSE folks (thanks, Taavi and Harry) and I used the Astroplate to map their database schema (“schema” is jargon for what boils down to the set of tables and columns with which they describe their data) to the much simpler (and, admittedly, less powerful) IVOA Obscore one.

    Sure, Obscore doesn't deal with multiple exposures (like when the target field and the north pole were exposed on one plate to help precision photometry), object-guided images, and all the other interesting techniques that astronomers applied in the pre-digital age; it also doesn't usefully cope with multiple scans of the same plate (for instance, to correct for imprecisions in the mechanics of flatbed scanners). APPLAUSE, of course, has to cope with them, since there are many reasons to preserve data of this kind.

    Obscore, on the other hand, is geared towards uniform discovery, where too funky datasets in all likelihood cause more harm than good. So, when we mapped APPLAUSE to Obscore, of the 101138 scans of 70276 plates that the full APPLAUSE holds in DR3, only 44000 plate scans made it into the Obscore table. The advantage: whatever can be sensibly mapped to Obscore can now be queried together with all the other data in the world that others have published through Obscore.

    You can immediately see the effect when you run the little python program doing the global discovery we gave in our plates tutorial. Here's what it prints now (values from pre-APPLAUSE-in-Obscore are in square brackets):

    Column t_exptime: 3460 values
      Min   12, Max 15300, Mean 890.24  [previous mean: 370.722]
    Column em_mean: 3801 values
      Min 1.8081e-09, Max 9.3e-07, Mean 6.40804e-07 [No change: Sigh!]
    Column t_mean: 4731 values
      Min 12564.5, Max 58126.3, Mean 49897.9 [previous mean: 51909.1]
    Column instrument_name: 4747 values
      Matches from , Petzval, [Max Wolf's residence in
      Heidelberg, Maerzgasse, Wolf's Doppelastrograph,
      Heidelberg Koenigstuhl (24), Wolf's
      Doppelastrograph,] AG-Astrograph, [Zeiss Triplet
      15 cm Potsdam-Telegrafenberg], Zeiss Triplet,
      Astrograph (four 10-cm Tessar f/6 cameras),
      [3.5m APO, ROSAT PSPCC, Heidelberg Koenigstuhl
      (24), Bruce Astrograph, Calar Alto (493),
      Schmidt], Grosser Refraktor, [ROSAT HRI,
      DK-1.54], Hamburger Schmidt-Spiegel,
      [DFOSC_FASU], ESO 1-metre Schmidt telescope,
      Great Schmidt Camera, Lippert-Astrograph, Ross-B
      3", [AZT 22], Astrograph (six 10-cm Tessar f/6
      cameras), 1m-Spiegelteleskop, [ROSAT PSPCB],
      Astrograph (ten 10-cm Tessar f/6 cameras), Zeiss
    Column access_url: 4747 values [4067]

    So – for the fields selected in the tutorial, there are 15% more images in the global Obscore image pool now than there were before APPLAUSE, and their mean observation date went a bit farther into the past. I've not made any statistics, but I suspect for many other fields the gain is going to be much higher. For a strong effect, try some random region covered by the Bamberg Sky Patrol on the southern sky.

    But you have probably noticed the deep sigh in the annotations to the statistics above: Yes, we don't have the spectral band for the APPLAUSE data, which is why the stats on em_min doesn't change. As a matter of fact, from the Obscore data you can't even guess whether a plate is “more red” or “rather blue”, as Obscore doesn't have an (agreed-upon) field for “qualititive bandpass indicator”.

    For some other data collections, we did map known emulsion/filter combinations to rough bandpasses (e.g., the Palomar-Leiden Trojan Survey, which only had a few of them). For APPLAUSE, there are 435 combinations of filter and emulsion (that's a VOTable link that you can paste into TOPCAT's load button in order to have a look at the table). Granted, quite a few of these pairs are (more or less) spurious because of inconsistent spelling. But we still gave up on researching the bandpasses even before we started.

    If you're a photographic plate buff: You could help us and posteriority a lot if you could go through this list and at least for some combinations tell us what, roughly, the lower and upper limits of the corresponding bandpasses might have been (what DaCHS already knows, plate-relevant data near the bottom of the file). As usual, send mail to if you have anything to contribute.

    Finally, here's the brief explanation of the image for this article: Well, I wanted to find some Bamberg Sky Patrol images for a single field to play with. I knew they were primarily located in the South, and were made using Tessar cameras. So, I ran:

    SELECT t_min, access_url, s_region
    FROM ivoa.obscore
    WHERE instrument_name like '%Tessar%'
    AND 1=CONTAINS(POINT(345, -38), s_region)

    on GAVO's TAP service. Since Aladin 10, you can do that from within the program (although some versions will reject this query because they mistakenly believe the ADQL is bad. Query through TOPCAT and send the result over to Aladin if that bites you). Incidentally, when there are s_region values in Obscore tables, it's a good idea to use them as I do here, as it's quite a bit more likely that this query will use indices than some condition on s_ra and s_dec. But then not all services fill s_region properly, so for all-VO queries you will probably want to make do with s_ra and s_dec.

    From that result I first made the inset bar graph in the article image to show the temporal distribution of the Patrol plates. And then I grabbed two (rather randomly selected) plates and had Aladin produce a red-blue composite of them. Whatever is really red or really blue in that image may correspond to a transient event. Or, as certainly the case with that little hair (or whatever) that shines out in blue, it may not.

  • 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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