Matching Images

If you import images or download them through Virtual Observatory, those that have been plate solved, .i.e., contain all information to relate image pixels to sky coordinates, will be shown with a special badge in the browser:

Matched Image Browser Badge
Matched Image Browser Badge

Observatory refers to these images as having been matched. The reason for using this slightly different term is that Observatory performs extra steps for these images:

Other benefits of matched images in Observatory are:

If one or more of your images are not plate solved, you can ask Observatory to do this for you. Select one or more images, and choose Image ▸ Match… (⌃⌘M).

Astrometric Match
Astrometric Match

If Observatory can find coordinates in the image metadata, it will shown them in the Dialog that appears. Images often contain valid coordinates, but not all image orientation and scale information, and this may suffice for Observatory to fully plate solve the image. Do this by clicking the Match All button.

If you started with multiple images, multiple coordinates may be returned. If they all contained coordinates, you once again click Match All, but if only a few contained coordinates, or you want the plate solver to use only one of those retrieved coordinates, select it and click the Match button.

Object Lookup

Your image may not contain any coordinates of course. The master FITS or XISF file may not contain this information, or you are using a different file format (e.g. SBIG or RAW). In such case, or when the coordinates are incorrect, you need to provide a hint to Observatory.

Astrometric Match: Lookup Object
Astrometric Match: Lookup Object

Just type the name of the object in the image, its catalog identifier or its coordinates in the text field towards the top of the Dialog. Observatory will attempt to resolve your entry, and display the coordinates and description. Observatory is able to resolve many common names used for objects, as well as Messier, NGC/IC, PGC2003 and Tycho–2 objects. If it is unable resolve it, click the SIMBAD and NED buttons to have these services resolve it. For these two services an internet connection is required.

What about moving objects?

Currently, only “fixed” objects can be resolved, and their coordinates used as a starting point for the plate solver. If you for example enter “Neptune”, the table in the center of the Dialog will remain empty.

Observatory has one extra trick up its sleeve though. If the image version contains the name of a planet or Pluto in its name, and its exposure date/time is correct, choosing Image ▸ Match… will have Observatory compute the position of that object at that date/time, and display these coordinates in the Dialog.

This can be handy if you have for example an image of one of the fainter planets and their moons, like Uranus and Neptune, or even Pluto, with background stars, but you don’t know the coordinates of the image.

Currently this only works for the eight planets and Pluto of the solar system.

Detection and Matching Settings

If this was all there is to plate solving, then that would be great, but typically you will need to adjust the default settings to make it succeed.

Click on the triangle at the bottom left of the Dialog so you can make adjustments to the settings.

Astrometric Match: Detection and Matching Settings
Astrometric Match: Detection and Matching Settings

The following settings are available:

You can reset the settings to their default values by clicking the button towards the bottom right of Dialog. You can collapse the settings by clicking the triangle once again.

Please be aware that the star detector also uses the current centroid settings. You can change those by clicking the “target” button in the Editor Bar. Additionally, for RGB images, please note that currently the green channel is always used for matching, regardless the selection in the Channels panel.