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:
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:
It automatically generates tags so you can more easily find the images.
For example, if you import a plate solved image of M 1, Observatory will automatically tag it with
M 1, the equivalent
NGC 1952, the
SNR tag to indicate that this is a supernova remnant, as well as additional tags for other objects in the image.
You can search for images containing these tags in the Browser by typing them in its search field, as well as by creating Smart Albums in the Sidebar or by using one of the predefined ones. In this case, if you select the predefined Smart Album Objects ▸ Nebulae ▸ Supernova Remnants in the Sidebar, your M 1 image will appear in the Browser.
It computes additional information, for example the constellation the center of the image falls in.
Other benefits of matched images in Observatory are:
The editor displays overlays: image scale, orientation, RA/Dec grid, but also various built-in and external catalogs.
The Astrometry Inspector displays right ascension and declination coordinates, angular separation of stars and their position angle.
The Photometry Inspector will automatically attempt to provide the magnitude of the selected comparison star from the Tycho–2, UCAC4 or USNO-A2.0 catalogs.
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… (
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.
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.
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.
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.
The following settings are available:
This slider determines the sensitivity of the star detector. If you move it all the way to the right, it detects stars that are only slightly above the background noise. If your image is noisy, you may want to increase it a notch, but if you make it too sensitive, it may detect false positives. The default usually is good for a well exposed image, but if you have a large image with many well exposed stars you may want to decrease the sensitivity, since it makes the star detector faster.
Use brightest N stars
Limits the maximum number of extracted stars. The default is usually good, but you may want to increase it if you are uncertain of the pixel scale.
Determines the area of the image the star detector is using. When set to Automatic, the detector uses the whole image except around the corners, to ensure detected stars always completely fall within the image. Manual adjustments range from the center 25% of the image, to the center 50%, then 75% and the complete image. The automatic setting is recommended, but if you have a large image with optical distortions you may want to reduce the detection area.
The estimated pixel scale in arc seconds per pixel. It only needs to be an estimate, but if it is not accurate within a factor of two, this significantly reduces the chances on success. The reason is that the estimate is used in combination with the frame size above and the search radius below to retrieve the stars from the catalogs. This may lead to the situation that effectively for only a small area of the image stars are obtained from the catalogs, or for a way too large an area. If you don’t know the pixel size, you may want to try with a few different values, e.g. 1, 2, 3 or 0.5.
If you check Use pixel scale from image then Observatory attempts to obtain the value from the image itself. If it cannot, then it will set it to 1x1, or 2x2 for 2x2 binned images, etc.
Observatory comes with the Tycho–2 catalog built-in. It is recommended to at least set it up with the UCAC4 catalog as well. Observatory will use the selected catalog for matching. The choices are Tycho–2, UCAC4 and USNO-A2.0. If set to Automatic, it will first try Tycho–2, then UCAC4 (if installed) and lastly USNO-A2.0 (if installed).
By default the frame size and pixel scale determine the area of the sky for which stars are retrieved from the catalogs for matching the brightest stars in the image. If your estimate for the pixel scale is inaccurate, or if your image is distorted and you only use the center of the image for the star detector, this may lead to the situation that too small an area of the catalogs is sampled for stars.
Drag the search radius slider to the right to increase the area of the sky for which stars are retrieved from the catalogs.
An alternative to adjusting the Search radius is to keep it to its default value, and let Observatory do a spiral search for matching. If you move the slider all the way to the left, no spiral search is performed. By default Observatory does a spiral search in an area that is approximately four times the size of the image. The area increases fast when you move the slider to the right, so use this with caution.
By default the maximum allowed RMS in arc seconds of the solution is the HWFM of the stars in the image. It is a reasonable default, but you can override this default here.
Minimum number of matches
The minimum number of matches required for the solution usually depends on the catalog that was used for the match. When using the Tycho–2 catalog, Observatory requires less matching stars than for the UCAC4 catalog, which in turn requires less matching stars than the USNO-A2.0 catalog. You can override it here. The minimum is 3, but this may lead to false positives.
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.