The LBC focal plane consists of four edge-butted CCDs with gaps of ~18 arcseconds between the arrays. You can find more details about LBC on its instrument page: LBCHomePage. To fill these gaps in order to make a continuous deep image, dithered observations must take into account the size and spacing of these gaps.

Built-in gap filling patterns.

The latest (03 July 2007) version of the LBC OB generating GUI has a number of built-in dither patterns which fill the gaps. These should replace the user-defined patterns that are downloadable from this page and described below. The new built-in gap-filling patterns have been broken up (10-point -> 2 5-point dithers; 9-point -> 3 3-point dithers) since each OB should be limited to less than 30 minutes execution time and it is not possible to stop and OB, focus and then resume where the OB left off. The patterns are listed in the following table - click on the link to display a plot of the sequence. Different colors are used to indicate the breaks in the 10pt and 9pt sequences.

First the sets that have both left and right handed senses. Click on the link to see a plot of the dithering offsets for a scaling factor of 20 arcseconds. The scaling factor is adjustable, but should be greater than 18 arcseconds to fill the gaps between individual CCDs. The remaining dither sequences are listed in the next table.

left handed		right handed
5-split L	5-splitLeft	5-split R	5-splitRight
9-splitL	9-splitLeft	9-splitR	9-splitRight
9-splitL 3#_1/3	9-splitLeft 3#_1/3	9-splitR 3#_1/3	9-splitRight 3#_1/3
9-splitL 3#_2/3	9-splitLeft 3#_2/3	9-splitR 3#_2/3	9-splitRight 3#_2/3
9-splitL 3#_3/3	9-splitLeft 3#_3/3	9-splitR 3#_3/3	9-splitRight 3#_3/3
10-splitL	10-splitLeft	10-splitR	10-splitRight
10-splitL 5#_1/2	10-splitLeft 5#_1/2	10-splitR 5#_1/2	10-splitRight 5#_1/2
10-splitL 5#_2/2	10-splitLeft 5#_2/2	10-splitR 5#_2/2	10-splitRight 5#_2/2

Other patterns.

Other patterns included in the GUI are listed below. User Provided allows for uploading a custom pattern. See below for some examples.

Pattern Name	Description	Dithering angle	Scaling
Random	random pattern	dithering angle adjustable	scaling adjustable
4-split default	moves a star from center of chip 2 to centers of chips 3, 1 and 4	dithering angle fixed to position angle	scaling fixed
10-split default	original gap-filling default - superseded by 10-splitL or 10-splitR	dithering angle fixed to position angle	scaling adjustable
Circular	circular pattern	dithering angle adjustable independently of position angle	scaling adjustable
Rectangular	rectangular pattern	dithering angle adjustable independently of position angle	scaling adjustable
User Provided	upload a pattern of RA;DEC offsets (separated by semicolons), one per line
None	use if no dithering wanted

User defined gap-filling patterns

The attached files are offset lists for some gap-filling dither patterns that can be loaded into the OB generating GUI as a user-defined dither pattern. In all cases, the given pattern is set up with 18 arcsecond offsets in the global pattern, so the gap padding in any pattern is basically zero. You can scale this up in the OB generating GUI as needed for your program, and we recommend that you normally use scale=1.1 to give at least a few pixels overlap on-sky from adjacent dithers. The sequencing of the exposures is designed to maximize offsets between temporally-adjacent images.

• 5 Point Patterns

The 5-point dither patterns are the standard 'dice' layout (four corners and the center of a square). The pattern is rotated with respect to the gaps. The two versions are rotated in opposite directions. Repeating pattern 1 and 2 at the same spot on the sky is not an optimal gap-filled 10-point pattern, as the gaps will repeat in 2 of the 10 exposures. Higher numbers of dithers in one OB should use the 9- or 10- point patterns. The 5-point patterns at scale=1 cover \xB136 arcseconds from the initial pointing.

5pt1.off: 5-point dither pattern, version 1

5pt2.off: 5-point dither pattern, version 2

• 9 Point Patterns

The 9-point dither patterns are in the standard 3x3 layout, also rotated with respect to the gaps in the array. The two versions are rotated in opposite directions. As with the 5-point pattern, repeating the 2 patterns at the same point on the sky will not produce an optimally-sampled 18-point pattern. The 9-point patterns at scale=1 cover \xB172 arcseconds from the initial pointing.

9pt1.off: 9-point dither pattern, version 1

9pt2.off: 9-point dither pattern, version 2

• 10 Point Patterns

The 10-point patterns are somewhat different that the 5- and 9-point patterns in that there is no exposure at the initial pointing center. The initial exposure in each pattern is offset by ~9 arcsec in RA and DEC in order to preserve the complete gap-filling property of this pattern. The complete pattern results in dithers covering \xB181 arcsec from the pointing center.

10pt1.off: 10-point dither pattern, version 1

10pt2.off: 10-point dither pattern, version 2

• Combining Dither Patterns

Even deeper imaging can be obtained by combining the above dither patterns, but some care must be exercised when doing so. The 5-point and 9-point patterns are not designed to be combined together. Doing so will result in gaps repeating on sky. However, both of them can be combined with the 10-point pattern to make 15-point and 19-point patterns with half-gap spacings, giving a more uniform sky coverage. Even better would be to scale the pairs of patterns by a factor >= 2.0 so that none of the gaps repeat on sky in the combined 15-point or 19-point patterns. This will, however, result in much larger dithers (\xB1162 arcsec from the initial pointing). Combining multiple 19-point patterns is possible (start with one of each version), but consider using different scaling factors for the repeated patterns.

Also keep in mind the need to re-collimate the telescope on ~30-minute timescales. Deeper imaging in the U-band filter should consider reproducing these relative on-chip positions spread across multiple OBs.

• On-Sky Layout of these dither patterns

The plot linked below shows the on-sky layout of three of the dither patterns (versions 1 of the 5-point (plus), 9-point (circle), and 10-point (box) patterns) in their scale=1 locations. The axes show offsets in arcseconds from the nominal pointing center. Note that only the center position of the 5- and 9-point patterns repeat. Version 2 patterns would result in a mirror-image (flip in X) of the patterns in this plot.

• DitherPatterns1.pdf: Plot showing on-sky layout of 5, 9, and 10 point patterns.

• Dithering to Fill Saturation Bleed Trails

It is possible to recover most of the sky covered by saturation bleed trails from bright stars by dithering the field rotation. These bleed trails can be a significant problem in long exposures obtained with the redder filters in LBC Blue. This cannot currently be done within one OB, however, so multiple OBs each with a set field orientation must be combined.

If you run into any problems using these patterns, please let me know!

-- DavidThompson - 02 Jan 2007

My experience with long exposures of deep fields is that square patterns like these do not make good flats and skies. A random dither that doesn't repeat is better. People doing
very long exposures may consider generating a random pattern.

-Jill Bechtold, Jan. 9, 2007