Why calibration may be needed

Popular devices use a filter that approaches human eye sensitivity more closely, with an IR and R cut similar to daylight cameras. The CloudWatcher and PocketCW2 instead use a modern dichroic UV/IR filter with a bandpass similar to the ones commonly used in astroimaging—so it's fair to expect their results to differ slightly from other devices. Lunatico performs factory calibration on each unit, but several factors can still cause readings to drift:

  • Dust or condensation on the optical window
  • Sensor wear over time
  • Environmental factors specific to your site
  • Filter type differences (e.g. Johnson-V vs broadband)

Regular inspection of the optical surface is recommended, especially in dusty environments.

What you need

Calibration procedure

1. Set up ASTAP

Install ASTAP and download the v50 star database from SourceForge using ASTAP's built-in database manager. The v50 database provides the star flux data needed for photometric calibration.

2. Photograph your target area

Photograph the zenith (or multiple altitudes for a more complete picture) and record the simultaneous CloudWatcher or PocketCW2 SQM reading at the time of each exposure.

Image requirements for a valid calibration:

  • Astrometrically solved—ASTAP needs to identify which stars are in the frame
  • Background value must exceed pedestal/dark level—if not, expose longer
  • Single, unprocessed raw images (not stacked)
  • Providing dark frame(s) in the darks tab (Ctrl+A) or entering a pedestal value increases accuracy; a flat frame helps too
  • DSLR/OSC raw images require 2×2 binning—done automatically for DSLR images
  • Minimal interference from bright nebulae in the field
  • Accurate altitude recorded for atmospheric correction
  • UV/IR block filter only—no narrowband or colour filters

3. Calibrate and generate SQM report

In ASTAP, calibrate the image with Ctrl+U, then generate the SQM report with Ctrl+Q. ASTAP compares measured star fluxes against the v50 catalogue values to derive an accurate sky brightness figure.

ASTAP SQM report—calibration result screenshot

4. Compare and adjust

Compare the ASTAP SQM reading against the reading your CloudWatcher or PocketCW2 recorded at the same time. If there is a consistent offset, adjust the Darkness reference value in the device software accordingly.

Tip: Take several images on the same night and average the offsets before applying an adjustment. A single image can be thrown off by a passing thin cloud or aircraft trail.