SV-VTTcheetsheet

Using StarView and VTT

StarView and the Visual Target Tuner can now interact to allow you to search for data starting from an image in the VTT or by displaying the DSS image of the area around an observation (or observations) and display an outline of the observed aperture(s) overlaid on the image to show where data were taken. You can start from either the VTT or SV.

You will need to have APT version 3.0 or later installed in order to get the VTT and SV to work together. You can install this from apt.stsci.edu.

Starting from StarView

Displaying an observed aperture with the VTT

For this example we assume you have worked with StarView and know how to do a basic search. For more information about StarView and searching, please refer to the StarView help system at http://starview.stsci.edu.

To make StarView work with the VTT, you use the version of StarView bundled with the APT. This version has the VTT as its default DSS and Overlay display tool. If you wish to revert to the old JIPA interface, go to the Edit menu and select Preferences. Then click the Viewers tab. You will see the DSS Viewers: selection menu. Pull down on the menu and select JIPA.

If the VTT is not being used and you did not change this, check the Environment panel. Note if VTT does not appear in this menu, StarView was unable to find the VTT program. Please reinstall the two as described above.

Once you have changed this, you can now display things in VTT. With a form loaded and search results returned, select the record(s) you wish to display in the VTT. If the VTT is not running, you can simply click the Overlay button to display a DSS image AND the apertures.

The VTT will start up and after it loads both the DSS image and the apertures to be displayed it will show you the following.

Note if the VTT is already running there is currently no mechanism for determining what is displayed in the VTT. The Overlay button will only send the apertures to the VTT. If you have not displayed a DSS image in the VTT that includes the coordinates for the overlay you are sending, the overlay will not be displayed over a DSS image. To send a new DSS image to the VTT, click on the DSS button in StarView, or go to the DSS and load an image there.

With an observed (e.g. StarView generated) aperture displayed in VTT you can do several things. By clicking the SV icon at the left you can put VTT into StarView mode. By clicking in the image when the cursor is a cross hair (e.g. a +) you can push coordinates back to StarView's active search form. If no form is active, StarView will load the Quick Search form and put the coordinates in it. You can then use these coordinates and any other search parameters that you enter in the StarView window to search for objects near where you clicked. In StarView mode, when the cursor gets close to corners of an aperture and the cursor becomes a pointing hand, you can double-click on an aperture to select it and send the aperture back to StarView. If that dataset is in the StarView search, it will be highlighted. Note this also works for multiple apertures that appear on top of one and other in the image. All of the apertures will be highlighted in the StarView form.

Starting up from VTT

You can also start the duo working from the VTT side. With the VTT running you can load DSS images (as described in the VTT help system at http://apt.stsci.edu). Once an image is loaded, you can go into StarView mode by clicking the SV logo on the left of the VTT tool. When the SV logo button is depressed you are in StarView mode. Selecting any of the other buttons above the SV button will put you in one of those modes (i.e. the magnifying glass puts you into zoom mode so clicking on the image zooms in or out).

To search for data around a particular object in the DSS frame, click on the object in StarView mode. StarView will start up, create a quick search form, and put the coordinates of where you clicked into the RA and Dec search qualification table for this form. You can then enter other information, such as instrument names or start time/dates and click the Search button to search.

With the search returned, you can then send observed apertures back to the VTT by selecting them in StarView and clicking the Overlay button. VTT will load these apertures and display them

Notes about coordinates and orientation angles in the archive:

The coordinates in the archive acquired from the data headers are not the best estimates available for the true coordinates of the observation. The orientation angles from the headers are also only estimates of the orientation angle of the telescope plus the best estimate of the conversion of that angle to the relevant position angle of the aperture, as known at the time of the first processing.

COORDINATES

Most of the coordinates used for archive searches come directly from the header keywords of the observations themselves. These header keywords are populated by the commanded positions, not calculated positions. The quality of the coordinates thus depends critically on the nature of the acquisition of guide stars, the guiding modes, and the accuracy of the the guide star positions. Differences in the positioning of imaging observations can be up to a couple of arcseconds, although if the field was acquired on gyros only, the accuracy of the position can be much worse. Spectroscopic observations are often "peaked up", but as spectroscopists can tell you, placing a spectroscopic aperture with accuracy and 100% confidence is not always possible.

To definitively confirm the presence of an object in the FOV of the imaging observation, particularly for objects near the edge of the FOV, you will need to consult the preview and perhaps even the data itself. The coordinates in the preview itself are only based on the header parameters, but the preview may show the object in question. To get a better assessment of absolute astrometry, one needs to use the OMS data, the jitter files, and to get the best assessment of absolute astrometry, you need to retrieve the image data, geometrically flatten the image and identify astrometric standards (if they are available) on the image itself.

For more than most archive users need to know, but it's all here:

Pointing Calculations and Sources of Error

ORIENTATION ANGLES

The orientation angle of the telescope is set to within ~0.003 degrees of the commanded orientation (computed from the Phase II specs), based on the measurements of the guide star positions. This accuracy is achieved barring any difficulties in acquisition, pointing, or header creation.

The relevant position angle for the aperture itself is also not a perfectly known quantity, and the conversion from the position angle of the telescope to the position angle of the aperture is a correction which has been updated and improved. (For example the STIS data taken prior to 15 Sept 1997 can have problems in the orientation keyword in the header leading to errors of up to 0.5 degrees.) For more details see: How do I best determine an observation's actual orientation?

Always remember: STARS MOVE!

Also remember that not only do planets and other solar system objects move, but so do nearby stars. The outline of the aperture of an observation of a nearby star will not necessarily center up on the image of the target star in the DSS because of proper motion.

Authored by Niall Gaffney
Last Modified: September 7, 2005