Fast scan

Take a fast scan image

This is an experimental feature.

A fast scan image can be used to observe occultation's of stars by the moon or minor planets.
The technique is like a drift scan image, but the telescope drive is on, the CCD is read out with a high speed, so you get a trail of the object, showing differences in the brightness of the object.
Make extended tests before observing occultation's, using artificial stars and natural stars and check the time stamp file.
The accuracy of your results will depend on the precision of the timing which depends on the Operation System, the computer, installed software, the Settings of WinScan, the camera ...
I could not check all that combinations, so broad testing is required.
The time stamp file should be always used to get the exact time of the occultation.

I got a message about problems identifying the start of the trails when using an USB camera. It should be used the "Strobe readout time" feature to avoid that.
It was also reported to me, that there may be a brighter pixel all 256 pixels followed by a dimmer pixel (only USB). Make some tests.
This may also depend on the version of the driver.

The Ethernet interface should not be used for fast scan images.

The tracking of the telescope is on.
Use the tracking CCD, place the object you want to expose, near the right side of the image .
There should not be any other bright objects at the same columns.
Chose Camera - Settings menu.
Declination, Focal Length, Sidereal Rate - no entries required for this kind of imaging.
CCD - imaging CCD or tracking CCD, use the tracking CCD, the read out speed will be higher.
Binning - vertical binning if your are using the imaging CCD or 1 x 1 binning for the tracking CCD.
Planetary mode - Use the planetary mode to get a higher read out speed
Start - the first column to read out
Length - the number of columns to read (about 20)
Focus (check Box) - not checked
Light or dark frame - light frame
Auto Grab number of images - 1 (or more)
Brake during taking an image - "only Esc key" recommend, enabled or disabled,are also possible, "only Esc key" or "disabled" the precision of the timing will increase
"only Esc key" or "disabled" are only available when using Windows 95,98 or Me.
Using NT, 2000 or XP you may change the priority of WinScan, see: Change Priority
Check the time stamp file, to see, if the timing is exact enough. Windows XP may interrupt the fast scan image from time to time.
Setting the priority may reduce that problem, but there may be some delays left. They may be caused by some of the services in XP. Try to set some services to manual or to disable them. You may check: services (extern link)
Or use Windows 98 or ME and set "Settings - Brake(ESC) while taking an image - disable "
Or use the time stamp file to correct your data.
At the bottom of the form at the left, there is a Page Control
Chose the FastScan Tab Sheet
Time per line - using the tracking CCD about 0.0015 sec per line for very bright stars, make some tests, check the timestamp file
Number of lines to read - about 10000 or more
Store first to memory - should be checked
After checking all entries, leave this form.
( If you want to use the "Take drift scan image" function to take a fast scan image (for example, if you want to read the last lines), check the "FastScan" CheckBox on the "DriftScan" Tab Sheet to avoid the subtraction of a time offset for the dark reference lines. )
Then chose: Camera - Take Fast Scan image.
It is asked for the file name and the exposure is started.

To evaluate the fast scan image:
Load it to View Scan.
Save the columns with the trail of the object as txt file
Load that file to Excel
Delete the first 4 lines ( Shiftime, Number of lines, KindOfImage, IsFastScan).

To calculate the time of the start of the occultation:
Find the grab in the trail.
Find the start of the trail.
Example:
start of the trail: x13 y164
start of the grab:x13 y1300
1300 - 164 = 1136
Look for SHIFTIME in the file header (example: 0.006 sec)
1136 * 0.006 = 6.816 sec
Add 6.816 sec to the start exposure time (DATE-OBS) in the file header to get the time of the start of the occultation
This will only work, if "Start delay" or "Strobe readout" are not used.
Better check the offset in line 1136 in the time stamp file (more exact).
(same file name as the image, extension "tsf")
You may also open the "tsf" file also using Excel.
To find the start of the trail, it may be useful, to compare the column with the trail with a column of the background. (for example by creating a diagram in Excel).
You may use the "Start delay" and "Strobe readout time" features to add marks to the trail.

Start delay:
Set to it to 0, if you want no delay at the start of the image.
If you set it to a value > 0, the program will wait at the start of the image.
It waits: Start_delay * shiftime.
You may set Start_delay to values about 1-6 or more.
Example: If you set Start_delay to 5 and shifttime is 0.03 sec, the program will wait 0.15 sec at the start of the program, and line 0 is read out at 1*0.03 + 5*0.03 = 0.18 sec, line 1 is read out at 0.21 sec ... . If Start_delay is set to 0, line 0 is read out at 0.03 sec, line 1 at 0.06 sec ... .
May be useful when observing occultation's to mark the start of the trail.
The first pixel in the trail will be brighter. (and the surrounding pixels may be also)
To evaluate the occultation, you may use the time stamp file, or you have calculate the time like: t0 + pixels * shiftime + shiftime * start_delay .
Just count the pixels in y pos from the start of the trail to the gap caused by the occultation.

Strobe readout time:
May be checked if you observe occultation's and there is no extra autoguider at the scope or if you got problems identifying the start of the trail.
You may check at: Settings- Fast Focus - Strobe readout time
and fill in at "At LineNr" a value of about 1000.
At strobe delay fill in a value about 1-3 or more
The program will wait "if (line mod LineNr == 0)" strobe_delay*shiftime
(if you assume first line =0 it will wait at line 1000 if you assume first line = 1 it will wait at line 1001)
There will be a brighter points in the trail at that places.
A time stamp file must be saved (Settings Computer)
At "last line to strobe" you may fill in the number of the last line, at which the program will strobe the readout. May be useful to avoid strobing the readout during the occultation. If the program should strobe the readout in the whole image, just fill in a huge value like 1000000. . Example:
Assume: Star is at pixel x13 y164. The trail starts at that coordinates.
(y coordinates related to the whole CCD, but comparing it with a standard image, as there are dark reference lines near the readout register, you have to add "Dump extra" (check CCD Info) to the y value of the standard image to get the coordinates related to the whole CCD ) LineNr is 1000. At the pixels x13 y164 x13 y1164 and x13 y2164 and x13 y3164 ...
there should be brighter pixels.
(the program waits when reading out a line 0, 1000, 2000... for "Strobe_delay * shiftime" ) Assume there is a gap from x13 y1300 to x13 y1500 caused by an occultation,
then you can calculate the time of the start of the occultation
using the time stamp file and the brighter pixels.
1300 - 1164 = 136.
1000+ 136=1136.
Check the offset in sec in the line 1136 in the time stamp file.
Assume the offset is 6.798 sec.
Add 6.798 sec to the Start Exposure Time in the File Header.
(There is a difference to the first example because the program waits at line 0 and line 1000 0.012 sec instead of 0.006 sec.)
If the star moves form its position, due guiding errors, the bright pixels may be different positions like x 15 y 1170 for example.
then you get:
1300-1170 = 130
1000+ 130 = 1130
At line 1130 in the time stamp file the offset may be: 6.762sec.
If you set "last line to strobe" to 1005, the program will strobe the readout at line 0 and line 1000, brighter pixels will be at: x13 y164 and x13 y1164

I would suggest to call a SNTP time server by "Get time" before the image is started.
Or use a GPS receiver to get the exact time GPS

"SNTP - WinScan - Write Offset File " is not implemented for Fast Scan images.