Quick check for optical aberrations at the WHT
Out-of-focus images of a star
usually exhibit approximate circular symmetry.
Departures from symmetry may 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 (at e.g. Cassegrain or prime focus).
- Point the telescope at a bright star, V ~ 9.
- Defocus the telescope, by 4 mm if working at WHT Cass, and by 20 mm if
at WHT prime focus.
- Expose for 60 sec (i.e. long enough for seeing effects to average
out).
- Check image quality. The diameter will be ~ 7 mm (a few 100 pixels on a
typical WHT CCD) 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 may be 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.
- A similar recipe can be followed for other telescopes, but the
optimum defocus, star mag and exposure time will be different.
Example out-of-focus image showing evidence of coma (actually from the INT, October 2024):
Recipe for analysis:
- COMA - If the central hole is not concentric with the outer circumference,
(as in the above image) and the shift is in the same
direction on the images on both sides of focus, 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.
If the measured coma exceeds 0.5 arcsec (it's difficult to measure
it more accurately than this) there's 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.
|