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Astronomy Calculations:
Max Exposure time on tripod

With this tool you can calculate how many pixels a star moves on your sensor along RA axis during one exposure when having the camera on a tripod. For a wide angle lens it will be very different movement along the DEC axis, it calculate movement at center, top and bottom of sensor along DEC axis. With this you find the max exposure time you can have without getting star trails.

  • But maybe you want star trails?

    More to read about star trails:
    Star trail, Wiki

Galaxy M33

Type in your data

It start to calculate as soon you change or write new figures in the dark red boxes. Do not exceed the maximum number of characters, delete characters if necessary.

You use the information on your own risk! There can always be a mistake in my equations behind the calculations, check that the result is correct. Let me know if you find something wrong and I try to correct it. Some calculations are very simple done and not correct in the small details.

Sensors data:

pixel pitch in my
(1.00 to 9.99 my)

Optical data:

focal length in mm
(1 to 1000 mm, distorted lenses as fisheye will not be correct)

= Calculated results arc sec / pixel (pixel scale)

Sensors data:

number of pixels along DEC axis
(1 to 9999 pixels, it depends if the camera is oriented in portrait or landscape mode relative DEC axis)

Coordinate data:

center of image point at DEC?
(-90.0 ... 0 ... +90.0 degrees)

= Calculated results degrees (sensor upper point at DEC)

= Calculated results degrees (sensor bottom point at DEC)

Exposure data:

exposure time seconds)
(0.01 to 999.99 seconds)

Output data:

Below you find how much a star will move on the senor during one exposure at different DECs.

= Calculated results arc sec move per exposure along RA axis.

= Calculated results number of pixels move per exposure upper part of sensor against north/south pole.

= Calculated results number of pixels move per exposure center part of sensor.

= Calculated results number of pixels move per exposure bottom part of sensor.

How much can we tolerate that a star move and get an elongated ugly look?

It's a bit personal, but with a sharp lens, in center 2 or 3 pixels, at the edges where the lens is softer maybe a bit more.

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