Why calibration may be needed

The CloudWatcher and PocketCW2 use a dichroic UV/IR filter for sky quality measurement, and Lunatico performs factory calibration on each unit. However, several factors can 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

  • ASTAP — free FITS viewer with photometry features (download from hnsky.org)
  • v50 star database — downloadable from SourceForge via ASTAP's database manager
  • Raw (unprocessed) sky images of the zenith
  • Dark frames and flat frames when possible

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
  • Single, unprocessed raw images (not stacked)
  • Dark and flat frames provided where possible
  • DSLR images: 2×2 binning recommended
  • 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.