Reflector Collimation
This example of reflector collimation uses a 200 mm f/6 parabolic Newtonian telescope.
Spot Diagram
In this example one collimation screw on the primary mirror is turned one-eighth of a revolution. The collimation screw is assumed to be a representative M4-0.7 screw, meaning a 4 mm diameter and a 0.7 mm thread pitch. Thus, one-eighth turn is 88 micrometers (microns) at one edge of the mirror. Below is the on-axis spot diagram on a CCD with 6 micrometers (microns) pixels.
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| Spot Diagram: 200 mm f/6 parabolic Newtonian, primary mirror tilted by one-eighth turn of collimation screw or 88 µm, CCD pixel size 6 µm |
The above image shows what should have been a pin-point sharp spot diagram spot has expanded to over a half pixel across due to a 88 micrometer (micron) misalignment of the primary mirror. Given that the FWHM of the Airy disk of this imaging setup is about 0.6 pixels, the aberration introduced by the misalignment is almost certainly noticeable in the final image even after astronomical seeing blurs the spot.
GoldFocus Plus Collimation Measurement
GoldFocus Plus uses a diffraction mask and analysis software to measure very specific locations in a star's diffracted spot diagram. Below is the idealized diffraction pattern of the above spot diagram for the 200 mm f/6 parabolic Newtonian with the primary mirror misaligned by 88 micrometers (microns). GoldFocus Plus measures the 200 mm f/6 parabolic Newtonian collimation as being out by 0.3 pixels.
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| GoldFocus Plus Collimation Measurement: 200 mm f/6 parabolic Newtonian, primary mirror tilted by one-eighth turn of collimation screw or 88 µm, CCD pixel size 6 µm |
