Imaging - filters and adaptors

• Mounts/adaptors not available for all filters for wheels and focal-plane slide.
• Required filter not yet available.

Imaging - PSFs

• Images have larger FWHM than expected for seeing (at that wavelength) - telescope focus probably needs tweaking, maybe because the filter-specific focus offset used is not correct.
• Images are elongated - may be atmospheric dispersion.
• Images are otherwise distorted - may be due to PSF degradation by the filter itself (especially if one of the 50-mm square narrow-band filters is being used).

Imaging - background light

• Diffuse scattered light when observing near the moon.
• Arc-shaped light leaks, usually caused by light leaking around the edges of inadequately-masked narrow-band filters.
• Ghosts.
• Sky flats aren't flat.

Imaging - other

• Vignetting at left of imaging area.
• Central wavelength shifts with radius in the field, for filters in wheels 1 or 2 (not important for broad-band filters).
• Narrow-band filter of required central wavelength is not available - filters can be tilted to tweak the central wavelength.

Spectroscopy

• Arc/sky lines and/or spectrum are curved/rotated.
• Spectrum contaminated by second-order light in the blue - order-sorting filter needed
• Light leaks near top-left and top-right of spectrum
• Position of spectrum (e.g. arc lines) changes on CCD (flexure)
• Acquisition tool unavailable - acquire manually

General

• Target not visible - possible causes are bad telescope pointing (ask the operator to point the WHT to a star of known position) or something blocking the light path (see below).
• Not enough photons - possible causes are clouds / dust, dome not aligned with telescope, mirror petals not fully open, A&G flat mirror not correctly positioned, use of a filter with poor throughput (or ND filters left in light path of calibration lamp), incorrect assumed object magnitude (or calibration lamp off, or failed), CCD gain (detected photons per count) not taken into account. Try imaging a photometric standard, using SIGNAL to calculate how many counts are expected.
• CCD not responding correctly - try dasreset acam . You'll need to reset the readout window (and speed and binning) afterwards.