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MPO Connections - Drift Scan Imaging (TDI)

An exciting technique is drift scan imaging. This involves keeping the telescope stationary and letting the stars drift across the chip. The trick is that the camera is rotated such that the stars drift from the "bottom" of the chip to the "top". In other words, say you have the camera setup such that north is at the top of the image and east is to the left. If you turn off the scope’s drive and take an image, the stars would drift from the left side to the right side of the image. Now, rotate the camera 90 clockwise (when looking at it from behind) so that west is at the top of the image and north was to the left. This causes stars to drift from the bottom to the top.

The trick in drift scanning is that it takes into account that as the top line of the CCD image is read, all the other lines move up one line. If the program reads a line at the same rate that a star drifts from one line to the next, the star stays on the same original line of pixels as it moves up the chip. When the given line reaches the top, it’s read into a buffer. Every so often, a group of lines is written to a file as a FITS image.

With this technique, you can let the sky drift into view of your camera for hours on end. The net result is a series of images that form a strip 15 long for every hour you keep the exposure going (if shooting at the equator; as you move away from there, the stars move cos(declination) x 15/hour. How wide that strip is depends on the width of the chip and the focal length of your telescope.

As you might have guessed, the further from the equator you go, the more curved the trails of the stars. There is a limit as to how far above or below the celestial equator you can go before the trails are curved enough to show the effect. The field of view of the camera, which is determined by the chip size and focal length of your scope determine the limit. Only experimentation will tell you the real limit.

Automatic Image Parsing

From the above, you might guess that one could get a very large image, i.e., 765 pixels wide by thousands of pixels high. It’s pretty hard to work with an image under such circumstances. Connections removes this difficulty by automatically writing the lines that have been read to the camera to a standard FITS file at given intervals. That interval is equal to the height of the chip, in pixels, at the binning setting used for the exposure.

For example, say you’re using an ST-8 at 2x2 binning. The height of the chip is 510 pixels. All images saved by Connections will be 765 x 510 (765 being the width of the chip at that binning). This is accomplished by forcing the exposure time to be an integral multiple of the time it takes for a star to drift the height of the chip (with one additional frame for "Dead Time"). See below for additional information on how the exposure time is adjusted.

This process of writing to a file while still reading from the camera is accomplished by copying the read buffer to a write buffer before the next image is started. The write buffer is handled by a low priority "thread" (process within a process) that has a minimum impact on the critical timing required for reading lines from the chip. Because the next image is not started until the read buffer is copied over to the write buffer, it’s possible that one line will be missed in each exposure. This depends on the speed of your CPU, the amount of RAM (the more RAM, the less likely Windows is using "virtual memory - a hard drive substituting for RAM), and if you have other programs running that are demanding CPU time. The latter reason is the most likely to cause problems so you should not be running other program while drift scanning, especially a screen saver!

Dead Time

Each time you take a drift scan image, an initial set of lines is discarded. The reason is that not all lines in the image would have the same amount of total integration time. For example, say the chip is 100 lines high and it takes 100s for a star to drift across the chip. If the lines were saved immediately after the exposure started, line 1 at the moment the shutter was opened, would have 1s of integration, line 2 would have 2s, and so on. Only the last line, line 100, would have the fully expected integration of 100s.

So, Connections automatically adds the time it takes for a star to drift the height of the chip to the overall exposure, in this case, 100 seconds, and throws out all the lines on the chip when the exposure started. This assures that all lines had the full integration time.

When the exposure first starts, the status line on the exposure progress form shows "Dead Time", indicating that the program is reading lines from the camera at the proper rate but not saving them.

Self-adjusting Exposure

DriftScanSetup.GIF (9324 bytes)

As you change the settings for the focal length during drift scan setup, Connections automatically calculates the scale of the image (arcseconds/pixel) which, in turn, determines the time it takes a star to drift the height of the chip. Note that changing the binning doesn’t affect the time because the time is based solely on the physical size of the chip, i.e. the Field of View.

When you enter a value in the Min. Exposure field, you’re telling Connections you want the exposure to be no less than the indicated time. Going back to the example that the drift time is 100 seconds for a given setup, the minimum exposure is going to be 100s (not counting the Dead Time mentioned above – we’ll get to that in a second). If you enter anything less than 100, the program forces the exposure to be 100s. If you enter 110, that’s 1.1 exposures. The program always goes up to the next highest integral number, so the minimum is two exposures or 200s.

 

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This page was last updated on 01/19/11 16:11 -0700.
All contents copyright (c) 2005-2011, Brian D. Warner
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