Quick check for optical aberrations at the WHT
Out-of-focus images of a star
usually exhibit circular symmetry.
Departures from symmetry indicate that an optical aberration is present.
A pair of out-of-focus images, symmetrically either side of best focus,
should be taken
after instrument changes, or after work on
the main telescope mirrors, and whenever the in-focus image quality is
suspect. Recipes for observation and analysis are given below.
Recipe for observation:
- Focus the imaging camera (e.g. aux-port, Cass TV or prime focus).
- Point the telescope at a bright star, V ~ 9.
- Defocus the telescope, by 4 mm if working at Cass, and by 20 mm if
at prime focus.
- Expose for 60 sec (i.e. long enough for seeing effects to average
- Check image quality. The diameter will be ~ 7 mm (~300 pixels on a
TEK CCD, ~ 500 on an EEV), with ~ 10000 counts per illuminated
pixel. The diameter and count rate are not critical (but
saturation should be avoided). The top-end support vanes are
usually visible (cross-shaped shadow).
Concentric rings are from the original mirror-polishing.
Small blobs are usually due to water stains on the primary mirror.
- Defocus the telescope by the same amount the other side of best focus,
and repeat the exposure.
Recipe for analysis:
- COMA - If the central hole is not concentric, and the shift is in the same
direction on both images, coma is present.
Measure the shift d between the centre of the illuminated part and
the centre of the hole. Convert d to arcsec, then multiply by 1.2
to get coma extent.
E.g. if d is 10 TEK pixels at aux port, corresponding to 1.1 arcsec
in the focal plane, the coma has extent 1.3 arcsec.
If the measured coma exceeds 0.5 arcsec (it's difficult to measure
it more accurately than this) there's clearly a problem.
- ASTIGMATISM - If the
image is clearly elongated in both images, but in directions at
90 deg to one another, astigmatism is present,
refer images to telescope manager.
- SPHERICAL ABERRATION
- If both images have circular symmetry, but the central hole
is clearly larger in one image than the other (in practice this
is difficult to see), spherical aberration is
present, refer images to telescope manager.
- If coma or astigmatism are present, determine the direction
on the CCD relative to elevation.
The most frequent problem at the WHT is coma (tailed images),
caused by misalignment of the optics e.g. after incorrect
replacement of the primary mirror (1996, 1997),
or movement of the secondary mirror (Sep 2001).
Astigmatism and spherical aberration are rarely a problem.
The coma formula is from an ESO memorandum 13/6/80 by Ray Wilson.
[Example images to be added when available.]