LBC Offset Iterator

The LBC Offset Iterator allows users to specify complex dithering patterns in the Absolute Det X/Y coordinate system. The iterator consists of: a Top Frame, called the Offset Table, where X/Y offsets appear in the order of execution next to an assigned index, an Iterator Action Item Menu, with buttons that control the addition, subtraction and order of entries in the table and allows for upload and download of dither patterns, Pattern Generators, which contain common dither patterns, random and grid offset generators, and a bottom Position Frame with text boxes for editing or inputting offset values by hand.

Iterator Action Item Menu

Most of the buttons in this menu are self explanatory, simply adding, removing, or moving items in the Offset Table to change the overall sequence. However, the Upload and Download buttons at the far right of the menu are of interest. The Download Table Position button is a useful way to save complex dither patterns. Once depressed, the button will bring up a dialog box requesting the user save an ASCII file to their local hard drive.

If you would like to use this downloaded dither pattern in another program or for an observation in the same program, you can use the Upload Table Position to ingest the file and convert it into values on the Offset Table. You may also upload homemade offset files; be sure they are ascii files that conform to the correct file format. An example is here.

 

Pattern Generator

These base pattern, in Absolute RA/Dec

• Chip Centers: designed to position the target on the center of each of the four chips. This is the pattern used for observations of standard stars.
• 3-point: A 3-point tilted triangle dithering pattern was added to the LBC dithering pattern list.
• 5-point: in a left (5-point L) or right (5-point R) leaning 5-dice pattern to fill gaps;
• 9-point: in a left (9-point L) or right (9-point R) leaning 3×3 grid to fill gaps. These have been separated into 3 sets of 3-point dithers.
• 10-point: a left (10-point L) or right (10-point R) leaning 10-point grid to fill gaps.

The user may input a scale factor (in arcseconds) that will apply to the pattern they have selected. If the user does not want to choose a scale size themselves, the tool will insert a default value that was selected to adequately fill the gaps. This default is recommended in most cases. If the user would like to set their own scale, a tool tip in the dialog box provides recommendations based on whether or not the observation is homogeneous or heterogeneous binocular.

Once the user has selected the pattern, input the scale, and click the “create” button, the OT will generate a dither pattern. The pattern will apply the scale and convert the RA/Dec patterns listed above and output a table of offsets in Absolute Det XY. For instance, if we select a 5-point L pattern with the default 20 arcsec scaling, the offset table will appear as follows:


 

In the Position Editor, this pattern would appear as yellow circles numbered from 0 to 5.

Position Frame

The Offset Table is not interactable; one cannot click on a value in in the table and change it in situ. If a user would like to enter or change an offset, he or she must click on a line in the table and then interact with the X and Y text boxes on the bottom frame of the Offset Iterator.

 

There are three pattern generator tools in the offset iterator, the random generator, the grid generator, and the Dithering Pattern Generator. Since the goal of dithering with LBC is to carefully fill the gaps between the chips,  we recommend using one of our pre-generated dither patterns which are available in the Dithering Pattern button.