TAOS Zipper Mode Operation (with Animation)

1. Trail Mode: Point your telescope to a specific direction with the tracking off. Target stars will drift across the field of view of the CCD camera and leave their trails as long as the shutter is opened.

   This is easy to implement but maybe difficult for analysis. You'll be monitoring target stars which keep drifting in and out of the field of view. The seeing condition before and after an occultation event may contaminate the real occultation signal. You alos need a more reliable shutter.

   • Hers is a simulated trail mode image.

2. Zipper Mode: This is a new technique proposed by one of us. You just point your telescope to a specific direction with the tracking on. You'll be monitoring the same patch of the sky. But the electrons accumulated in the CCD camera will be shifted electronically every, say, 0.2 seconds for a few (64 ?) rows instead. The shutter can be left open.

   This is consistent with the CCD operation (shifting and reading out), but, extra programming is needed. We can have better idea about which target star one is looking at. Each 0.2 second exposure is well seperated by, say, 64 rows. But, a bright star may still leave some electrons during shifting (a streak between two exposures). However, the sky background is accumulated many times more than it is in an ordinary exposure. All field stars will be compressed into one row block, say, an area about 64 by 2048 pixels. The star images in each row block might be exposures at different time.

   • Here is a simulated zipper image, which was originally produced by Tim Axelrod. The idea of zipper mode evolved into its current form (see the illustration below) since then.
   • Here is part of a real zipper image (616 x 443 pixels). The size of one row block is 2048 x 64 pixels. Several row blocks are included.

Zipper Mode: A Java Script

Zipper Mode: GIF Animations

• Here is a GIF animation (~1.8 MB) of the TAOS shutterless zipper mode operation (produced by Federica Bianco).
• Here is another GIF animation (~10 MB) on how the photoelectrons are accumulated in one row block and how that row block is read out (produced by Andrew Wang).

The TAOS Shutterless Zipper Mode: A Simple Illustration

• (a), (b), (c) and (d) illustrate the process of CCD readout under shutterless zipper mode.
• A square CCD chip covers a field where eight different stars happen to align across that field as shown in (a). One rowblock, which is shown right below each "CCD snapshot", is read out in each cycle. A cycle label is associated with the exposure of each star in a cycle. The readout direction is downward.
• It reaches a steady state at (d) after four cycles here while all eight different stars in this field are "squeezed" into one rowblock. Moreover, the star images in each rowblock may come from exposures in different cycles (as shown by their numerical labels). This can be better illustrated from the gif animation (~20 MB, produced by Andrew Wang) based on the real data taken during the 2006 Hengchun earthquake.
• In reality, target stars spread over a field randomly as illustrated in (e). It is impossible to reconstruct a field based on its zipper image.
• The TAOS raw data is collected as a stack of tens of rowblocks usually. Figure (f) shows what it may look like if the target field is the same one as shown in (e). But, keep in mind that as many as 32 cycles might be involved in a TAOS rowblock, which is too complicated to be shown here clearly. However, you can take a look at a real TAOS zipper image (~1.6 MB, in JPEG). Are you able to identify an asteroid occultation event there?
• To move a whole rowblock downward takes time as well. Therefore, a bright star usually leaves a clear streak behind which is really exposure during the movement. This is not shown in the plot, though, it is obvious from the real image above.