Manual Guiding Tolerances Back | Up | Next

The desired resolution on the imaging sensor is first calculated with Formula 5 and then that figure is plugged into Formula 15 or 16 below to calculate the allowable star drift in relation to the size of the cross hair box.

From the equations above we would take the spot size in microns times our allowable drift factor (usually less than 2x) and convert to millimeters for the formula below, remembering that 1 millimeter equals 1000 microns and 1 micron equals 0.001 millimeter.

Lets use our 5 inch f/8 telescope that can form a star with an Airy Disk diameter to the first interspace of 10.7 microns. This is 0.0107 millimeters. We will use this figure for the most strict criteria of no allowable drift DR in the equation below.

Formula 15    Manual Guiding Tolerance

TOL = (DR/B) * (GFL/CFL)

Where:

  • TOL = tolerance in guide box diameters
  • B = guide box size in millimeters
  • DR = allowable star drift on the sensor in millimeters
  • GFL = guide scope focal length in millimeters
  • CFL = photo camera lens or scope focal length in millimeters

Example 1 - What is the guiding tolerance for a 5 inch f/8 imaging scope that is guided with a 90mm f/10 guidescope with 3x Barlow and a cross-hair guide-box size of 0.2mm?

TOL = (DR/B) * (GFL/CFL)
TOL = (0.0107/0.2) * (2700/1016)
TOL = (0.0535) * (2.657)
TOL = 0.14

The guiding tolerance calculated from this equation is 0.14 box diameters or about 1/8 of a box diameter. But again, this is for movement in both directions, so the working value is 1/16th of a box diameter in either direction.

Example 2 - What is the guiding tolerance for a 50mm f/2.8 camera lens that is guided with a 90mm f/10 guidescope with 3x Barlow and a cross-hair guide-box size of 0.2mm?

Guidescope focal length G = 2700mm (900mm x 3x Barlow)
Photo camera lens focal length F = 50mm

TOL = (DR/B) * (GFL/CFL)

TOL = (0.0107/0.2) * (2700/50)
TOL = (0.0535) * (54)
TOL = 2.9

The guiding tolerance calculated from this equation is almost 3 box diameters. But again, this is for movement in both directions, so the working value is about 1.5 box diameters in either direction.

Real World Tolerances

In the real world, these guiding tolerances can be relaxed easily by a factor of 2x because optical systems will never form a star at their theoretical limits during long time exposures because of factors such as seeing, tracking, guiding, and flexure.


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