When taking a sequence of exposures for stacking/coaddition, what dither patterns are most commonly desired? Why?

Question

When taking a sequence of exposures for stacking/coaddition, what dither patterns are most commonly desired? When visiting a telescope, what default dither patterns would a visiting astronomer like to see in the observing software (assuming custom patterns are also supported)?

Answer 1

Dithering is as much an art as a science and depends on many factors including, but not limited to:

• The type of object being observed (point source, small extended object, large extended object)
• Telescope parameters (The field of view of the telescope relative to the size of the object, optical quality, size and type of abberations, etc)
• The quality of the detector (flat fielding, vinetting, bad pixels, linearity, etc)
• The type of readout electronics (single or multiple channels and where they fall on the chip, gains and biases, etc)

At the very least, you should support an arbitrary two point dither pattern. Basically, you position your object on the detector in a "good" spot and than have the ability to specify an offset in pixels in both x and y where you want to dither to so that your object ends up on another "good" spot on the chip. Ideally, the camera desginer has identified these "good" spots on the detector and provided the necesary positions and offsets so that operators can choose where to dither to. This mode should support both ABAB and ABBA patterns. The first is jumping back and forth between the two positions. The latter takes an image, dithers to the second position, and then dithers back for a final image at the first location. This pattern is best for point sources, or small extended sources that completely fit within the two good regions on the chip.

For the two point pattern, one possible reduction method would be to subtract the adjacent AB images ( A-B and B-A ) to remove sky background and many instramental effects, then all of the A images can be coadded and all the B images can be coadded, finally, the two combined images can be shifted and coadded if needed.

If your targets are larger, or the chip has a lot of cosmetic variations (bad pixels, electronic artifacts, etc) you'll also want to support a four point dither pattern that moves the target around on the chip cyclicly (i.e. ABCDABCD patterns) Typically you would just start at an initial postion, take an image, offset in x, take an image, off set in y, take an image, reverse the x offset, and take the final image. If you were running through the pattern more than once, you'd then reverse the y offset (which takes you back to the original postion) and start over. If there are four "good" spots on the chip that are relatively distant from each other (say one in each quadrant of the chip) the camera designer might implement a specialized four point dither that takes sources to each of these postions in turn and may not just be a simple square offset.

In this case the three other postion images are averaged and then the averaged image is subtracted from the remaining image to remove skybackground. This is repeated for each postion and then the images from the four different postions are shifted and coadded. This pattern helps to guarantee that if there are a lot of bad pixels, you get actual flux measurements from your source at all postions. With only two dither positions you have a chance that there is a bad pixel in the same postion relative to the source in both dither postions (This obviously increases as the number of bad pixels on the detector increases). The chance that there will be a bad pixel in that location in all four dither positions is very, very small.

If the observing at the telescope is interactive, i.e. the observer is looking at the data as it is being taken and can influence the observation patterns, the dither offsets should be variables that can be specified. If it is queue observering where astronomers submit proposals and a telescope operator takes the data and sends the astronomer the data, it is even more important that there be copious documentation of the quality of the chip and it's physcial and cosmetic characteristics so the observer can make informed dithering decisions. In this latter case, it is also a good idea for the camera designer to identify the good parts of the chips and set up automatic two and four point dither patterns that utilize those postions to maximum effect.

Finally there should also be an option to move the target image completely out of the field of view. This is used to get sky background images for large targets that either nearly fill or are larger than the FOV of the camera. However, this is usually accomplished by nodding and chopping rather than dithering.