Orientation of the image:
The orientation of the image is:
The begin is at the west. (First line)
WinScan saves images in this orientation. (It is saved line after line)
ViewScan can only evaluate images in this orientation.
Horizontal stripes:(North-South) example (The stripes are not exact horizontal, but only the orientation of the camera is not correct.) The orientation of the camera is not correct. Take a stripe image with CCDOPS (with shut of telescope drive) and rotate the camera, so that the stripes will be exact vertical. Trails, (39KB) The orientation of an ST-7 must be more exact than 0.11 degree = tan -1(1/lines) (An ST-7 has 510 lines)
Vertical stripes:(West-East) example The read out speed is not correct. A wrong declination(6) or a wrong focal length(10) may be set in the Settings menu. Or SCAN(<=version 1.5) is running under windows in a DOS window. (Use DOS mode or use version >=1.6 and the clock cycle counter option(rdtsc))(see readme.txt) If the camera can't be found under DOS, change the BIOS (carefully) (see readme.txt). The time reading out a line can be also put in direct (7). Example: vertical stripe, 51 pixels, 0.164 sec to read out a line, 510 lines, put in 0.149 or 0.1804. The read out speed shout be more exact than 1/510.
Slanted stripes in the hole image: example The read out speed and the orientation of the camera are not correct.
Vertical curved stripes in the hole image: At a high declination the declination is not correct.
Vertical (curved) stripes at both sides of the image It is taken a image at a high declination. See: differential trailing In most cases the stars at the sides of the image look only a little oval an the image is looking blurred.
Depending from the size of the CCD, the focal length and the declination (the field of view and the declination) differential trailing will appear. At the equatorial of the sky, the stars move by the rotation of the earth with about 15 arc seconds per second. Near the pole the speed is much lower. In an image north of the equatorial, the stars in the northern part of the image move slower than the stars in the middle, the stars in the south move faster. WinScan can calculate or simulate the differential trailing. Calculate differential trailing Near the middle of the image, the stars are pictured still point like.
If it is calculated exact, the trailing at the side of the CCD near the pole is a little higher than the trailing at the side near the celestial equator.
The size of the trailing (in pixels) depends on:
Focal length: twice the focal length=half the size of trailing
Binning: 2x2 binning=half the size of trailing
Number of pixels(same size of a pixel) of the CCD: twice the number= twice the size of trailing
Size of the pixels(same number) of the CCD:0.018mm*0.018 mm instead of 0.009mm*0.009 mm = twice the size of trailing
Using short focal lengths, it should be used the partial tdi mode. There is les differential trailing. Example: 50 mm focal length, ST-7, declination 30 degree, it will occur a 22 pixel differential trailing if all 510 lines are read in with opened shutter. If only 22 lines read in with open shutter, the differential trailing will be only (22/510)*22=0.94 pixels.
The differential trailing should be < 1 pixel (2-3 pixel may not decrease the quality of the image very much).
Focus: By changing of the temperature during the exposure, the focus can change. Then it should be focused again. Focusing during an drift scan exposure is difficult, better end the exposure and focus using CCDOPS.
Orientation of the camera: If the polar alignment is not exact, the field will rotate by turning the telescope. The orientation of camera is to be adjusted new.
Alt Az mount I have not tested a Alt Az mount. You have to calculate the rotation of the field and adjust the camera new for every new Alt Az position ( tolerance about +- 0.11 degree (ST-7),+- 0.056 degree (ST-8)). Adjusting the orientation may take some time. Using a Alt Az mount as a meridian telescope the orientation of the camera will not change. Focusing the camera will be a little more difficult.
Accuracy of the timer
WinScan uses the clock cycle counter for an intern timer.
You should adjust the system timer before you start an
image, for example by an internet time server.
After that is done, chose: "Change_Time_Date"
and press the "Set WinScan Timer to system timer <-- ".
You can also calibrate the intern Timer of WinScan.
I found an error of about 1 second after 12 h of exposure, but
you should make some tests on your computer system.
There are problems when using an "Intel Speed Step" processor (used in notebooks). Better use an other computer system. The speed step feature can be disabled, but there may be also other problems.
Do not run a second (active) program at the same time. (Check the time stamp file, if there are problems)
Evaluate the image: CCDOPS for Windows can show images of about 15 MB. ViewScan can be used to slice the images in smaller parts.
Others: If clouds pass during the exposure, the brightness of background will be different at different parts of the image.If the CCD gets saturated, the next image may be damaged, the pixel count may be 0. You should take a few dark frames, which you should discard, before taking the next image or the next dark frame.