Ritchey-Chrétien Collimation
This example of Ritchey-Chrétien collimation uses a 200 mm f/8 Ritchey-Chrétien telescope.
Spot Diagram
In this example one collimation screw on the secondary mirror is turned 10 degrees or one-36th 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, a 10 degree turn is 19 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/8 Ritchey-Chrétien, secondary mirror tilted by one-36th turn of collimation screw or 19 µm, CCD pixel size 6 µm |
The above image shows what should have been a pin-point sharp spot spot diagram has expanded to nearly a full pixel across due to a 19 micrometer (micron) misalignment of the secondary mirror. Given that the FWHM of the Airy disk of this imaging setup is about 0.7 pixels, the aberration introduced by the misalignment will certainly affect 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/8 Ritchey-Chrétien with the secondary mirror misaligned by 19 micrometers (microns). GoldFocus Plus measures the 200 mm f/8 Ritchey-Chrétien collimation as being out by 0.5 pixels.
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| GoldFocus Plus Collimation Measurement: 200 mm f/8 Ritchey-Chrétien, secondary mirror tilted by one-36th turn of collimation screw or 19 µm, CCD pixel size 6 µm |
