JEL's star-streak calculator 2011.
Important:
Opens with the default Microsoft Works Spreadsheet (version 9.0).
Info:
Use it to quickly estimate how much smear (how long the star-streaks will be) will result from various camera- and lens- settings when taking pictures of stars with a non-tracking camera (a camera on a static tripod)
Usage:
ONLY red color is inputs!
Enter your values on the C column for every red-colored row (that's from C1 to C11)
Do NOT enter anything on any other colored row (black, blue and green are outputs NOT inputs)
More detailed info:
Remember to take into account the travel-direction of the stars relative to your image-composition (IE the rotation of earth relative to your shooting direction and camera-tilt etc). X is horisontally on your image (left/right), Y is vertically on your image (up/down).
C13 and C14:
These are your max shutter-speeds for a non-smeared image (IE an image where a star doesn't fill more than 1 pixel) in X (horisontal width) and Y (vertical height) image-pixel coordinates. A value of 0.2, just as an example, is equal to 1/5th of-a-second shutter-speed. The lower the value, the faster your shutter must be.
C16 and C17:
How many pixels a star will cover in 1 second.
A value of 4, for example, means a star will draw a 4-pixel long streak at your current settings (the star will be elongated by 4 pixels in the travel-direction)
C19 and C20:
How many pixels a star will cover in the time you have entered in C11 (exposure time)
If you enter 30 seconds in C11, for example, to take a picture with the camera-shutter open for 30 seconds straight, then if you get a resulting value in C19 and C20 of 128, for example, then it means that the star will draw a 128-pixel long streak at your current settings (the star will be elongated by 128 pixels in the travel-direction)
C26 and C27:
Shows how much angular field-of-view a single pixel covers at your current settings.
The rest should be self-evident.
Don't forget to share your pictures with us
Have fun
JEL