The resonant frequency of the resonant scanner can change over time due to environmental and mechanical factors (heat, zoom factor, etc.) Two-photon lasers can also heat the resonant mirror which contributes to these changes.
Variation in Resonant Scanner Frequency
In ScanImage, variation in the Resonant Scanner Frequency may impact acquisition in the following ways:
- Missing frame triggers
- Increases or decreases in the field of view.
- Changes in bidirectional alignment in bidi mode.
For example, on a ScanImage system equipped with an 8kHz Cambridge Resonant Scanner, the resonant scanner frequency just 30 seconds after start of acquisition measures 7.9145kHz (126.35 us). After 5 minutes, the resonant scanner frequency was measured again and found to be 7.9176kHz (126.30 us) which is a change of -50ns, or -25ns per line in bidi mode. As detailed in the Scan Phase documentation, the minimum scan phase adjustment is one tick. The length of this tick depends on the digitizer sampling rate, but is either 12.5ns or 8.33ns. Therefore, a correction must be made to the scan phase of 2 or 3 ticks for the 80mHz and 120mHz digitizers, respectively.
Automatic Scan Phase Correction in ScanImage
ScanImage continuously monitors the resonant frequency and adjusts the scan phase accordingly. Because the monitoring and adjustment are performed on the FPGA, ScanImage can react to very quick changes in the resonant frequency. This is especially relevant for high zoom levels, since the resonant frequency is less stable at a smaller resonant scanner amplitude.