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# andrea1984

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Vacuum

• Location
Rome
1. ## DSLR plus lens - rule of thumb to avoid star trails?

I want to give a contribution to the calculation, this is the calculation that I use to know how many seconds of exposure can be set on the camera before the trail is noticeable, based on the calculations made by the site (http://www.sceneplanner.com/tool3.php): Tha calculation does not take care of: - seeing - resolution of the optics (optical resolution = 116 / focal lenght) Example when the orientation is portrait: Input parameters: HorSize = Horizontal Sensor size (mm) HorPx = Horizontal resolution of the sensor (pixel) FL = Focal length of the optics (mm) Dec = Declination (degree) TrailPx = Maximum number of pixel we accept before before assuming the photo has the trail Calculated parameters: FOVHor = Field of view orizontal (rad) Res = Resolution (rad / pixel) AngSpeed = Angular speed (rad / s) Seconds = number of seconds that a pixel takes to move of TrailPx pixels FOVHor = 2 * atan( HorSize / (2* FL)) Res = FOVHor / HorPx AngSpeed = (2 * Pi) / 24 * abs(cos(Dec * Pi / 180)) -> Seconds = Res * TrailPx / AngSpeed * 3600 To calculate the maximum exposure for the top or bottom frame add or subtract FOVHor / 2 when calculating the AngSpeed: AngSpeed = 2 * Pi / 24 * abs(cos((Dec + FOVHor / 2)* Pi / 180)) For landscape orientation use the vertical resolution and size. Setting the TrailPx to 12.5 pixel this calculation gives the same results as the site previously published
2. ## Adding motor to focuser

I'm not even able to remove the wheel from the rod, even after removing the screw. Attaching the photo. Thanks
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