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GoldFocus - Calculator


Enter your GoldFocus mask, telescope, camera, and imaging data in the boxes below.  Calculated data cannot be entered.
 GoldFocus High-Resolution mask
 GoldFocus Plus Collimation mask
Aperture mm
Focal length 651  mm (calculated)
CCD pixel size micrometers
Wavelength nm
Diffraction limit 1.1  arc-seconds (calculated)
Seeing arc-seconds
Total seeing 3.2  arc-seconds (calculated)
Imaging seeing 3.7  arc-seconds (calculated)
Enter any ONE of the following four parameters and click the corresponding "Calculate" button to calculate the other three parameters.
Focus tolerance percent
NCFZ micrometers
Focuser travel micrometers
GoldFocus error pixels


Following are descriptions of the parameters and calculated values:
GENERAL Parameters in edit boxes may be input, all others are calculated
Aperture Aperture of your telescope in millimeters (mm), typically 70-500 mm (INPUT)
f/ratio f/ratio of your telescope, net of flatteners, focal reducers, Barlows and focal extenders, eyepiece projection, and others, typically 2-50 (INPUT)
Focal length Effective focal length of your telescope in millimeters (mm) (CALCULATED)
CCD pixel size CCD pixel size in micrometers (microns), typically 5-24 micrometers (INPUT)
Wavelength Wavelength of light in nanometers (nm), typically 550 nm for the average of visible light, in the range 400-700 nm for visible light, or in the range 300-800 nm for UV-IR (INPUT). Some typical wavelengths:
    Visible light - 550 nm (average)
    Red filter - 640 nm (average)
    Green filter - 530 nm (average)
    Blue filter - 440 nm (average)
    H-alpha - 656 nm
    H-beta - 486 nm
    O-III - 501 nm
    S-II - 672 nm
    N-II - 658 nm
Diffraction limit Diffraction limit of your telescope in arc-seconds, at the given wavelength (CALCULATED)
Seeing Astronomical seeing in arc-seconds, typically 0.4-6.0 arc-seconds (INPUT)
Total seeing Total seeing in arc-seconds, combined effects of diffraction limit and astronomical seeing (CALCULATED)
Imaging seeing Seeing experienced during imaging in arc-seconds, combined effects of total seeing and defocus (CALCULATED)
Focus tolerance Percentage tolerance from best case of total seeing, typically 1-20 percent
NCFZ New Critical Focus Zone in micrometers (microns), typically 10-1000 micrometers
Focuser travel Focuser travel to reach perfect focus in micrometers (microns), typically 5-500 micrometers
GoldFocus error Focus error as measured by GoldFocus analysis software in pixels, typically 0.1-10.0 pixels


  • After entering the parameters for your telescope, camera, and astronomical seeing conditions, a good starting point is to calculate using a focus tolerance of 15%.  A focus tolerance of 15% can generally be discerned by naked eye without the aid of a mask or software, so it is a good reference for potential focus improvements.
  • If you are focusing with the GoldFocus analysis software and have a focus error, then enter that error in the GoldFocus error box and click the Calculate button next to it.  This will calculate the focus tolerance you have achieved.  Depending upon your telescope (smaller aperture and faster f/ratio telescopes are more difficult to focus) and the quality of your focuser, you should expect to achieve a focus tolerance under 10 percent, perhaps as little as 1 percent - smaller is better.
  • If you have an electric stepper motor focuser, then enter its minimum step into the NCFZ box and click the Calculate button next to it.  This will calculate the focus tolerance and GoldFocus error you can expect to achieve with your focuser.  Note that this will be a calculation for the worst case scenario where perfect focus might fall exactly midway between two consecutive focuser steps.  Better focus may be achieved, but it will be coincidental and may not always be reproducible.

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