Partial TDI mode

Taking drift scan images using foto lenses with a short focal length


If you are using for example a ST-7 and a foto lens with 50mm fl you will get an effective exposure time of 1258 sec. (The time, the star needs to move across the CCD.) Because of that long exposure time the CCD will become saturated. Even if you stop down the lens, or if you use filters, there may be problems as the orientation of the camera must be very exact, the focal length must be well known, the CCD must point to 0 declination, the image must not have any distortions caused by the lens. You may get problems with trailing, the stars become trails or will look elongated.
You may use the partial TDI mode to avoid that.
Check stardial:
Stardial (extern link)
You will take many images, instead of one long stripe. Taking such an image the shutter opens, the exposure is started, and the CCD is read out line after line at sidereal rate, but after about 25 lines, the shutter is closed, and the CCD is read out complete. The effective exposure decreases to 1285*25/510= 61 sec. Settings in WinScan:
Focal len: 50 mm or the exact focal length of your foto lens. Declination: center of the image. Partial TDI tab:
Number of lines to read: 25 (about 10-100)
Store ramping: Check this, if the ramping (for example 25 lines at the top and the bottom of the image) should be stored.
If the ramping is stored, the image will be 537 lines high, if not, 487 lines.

Start the image:
"Camera - Take Partial TDI Image"
If the CCD still gets saturated, set Number of lines to a lower value, if the background in the image is less then about 10000 you may fill in a higher value.
To check the orientation of the camera, take two (or more) images, and check if a star moves form the bottom of the images to the top, at exact the same column. (You may use standard images, with shut of telescope drive) If every thing works, you may fill in at "Autograb, number of images" the number of images you want to take. In the images, you will see ramping at the start and at the end of the image. (25 lines)
I have not made much tests in this mode. The time the shutter is opened is stored in the file header. The effective exposure time is the time form opening the shutter until closing it, (or Shifttime * number of lines to read).
It is also stored in the file header.
(If you want to use the "Take drift scan image" function to take a partial TDI image, check the "FastScan" CheckBox on the "DriftScan" Tab Sheet to avoid the subtraction of a time offset for the dark reference lines. ) The mid exposure time is constant, except in the ramping.
WinScan should also support the ST237/ST255/ST5C, but I have not tested it using that cameras. If you got it working, please tell me about the results.