Setup - Alignment, Focussing and Collimation
The telescope needs to be properly set up if you want to get good imaging
For visual work with the telescope, a reasonably good North/South alignment is
adequate. It does not matter if your target moves a little across the field of
view over a period of minutes.
For long exposures, alignment is critical. If exposures are going to be above
(say) 15 seconds then I use the following procedure:
- The telescope is initially set up such that the North/South axis
is aligned visually with a feature of the horizon known to be due south.
This enables me to do the initial setup before dark and be ready to capture
twilight "flat frames". Alternatively it can be aligned using the pole star in
the normal way.
- With the CCD camera operating, align on a reasonably bright star near the meridian (due South) and near
the celestial equator. Display this star on the computer screen and observe
its drift. (I also use this star to effect focussing and to check collimation
as described below)
- If the star is drifting South the axis of the
telescope mount is pointing too far East, make a small correction to the
mount (for my LX200 this means rotate the equatorial head anti-clockwise)
and realign the telescope to display the star again.
- If the star is drifting North the axis of the
telescope mount is pointing too far West, make a small correction to the
mount (for my LX200 this means rotate the equatorial head clockwise) and
realign the telescope to display the star again.
- Repeat until there is an acceptable amount of N/S drift, (i.e.
negligible for the length of the longest exposure).
- Now align the telescope to a reasonably bright star near the Eastern
horizon and near the celestial equator. Observe its drift:
- If drifting South then the polar axis of the
equatorial head is too low, adjust it upward (screw in on my LX200). If
drifting North then the polar axis is too high, adjust it down, (screw out
on the LX200).
- Repeat until drift is acceptable.
How much time you spend on this will depend on how long exposures you want to
take. I find that two or three cycles of adjustment will allow me to take good
exposures up to 2 minutes.
Focussing is one of the most difficult aspects of CCD Imaging. The level of
magnification in effect when an image is projected onto a CCD chip is usually
quite high and consequently the adjustment of focus is sensitive. Some advice:
- Select a moderately bright star and bring it to a good focus before
attempting to image deep sky objects, planets or the Moon. It is impossible to
tell when large or fuzzy objects are in focus, especially when the atmosphere
is moving them about on the screen.
- Be prepared to spend a fair amount of time getting a good focus. Adjust in
very small amounts and let the image settle before deciding whether more
adjustment is needed. Atmospheric turbulence will give instances of apparent
good/bad focus so don't jump to conclusions too quickly. Take and assess
multiple images if using a still camera.
- If the first star you use is too bright then its 'disc' in the image will
be large. Move to a fainter star and continue focussing.
- If possible/affordable fit a remote focussing motor so that you do not
have to run backwards and forwards between the telescope and computer screen
when adjusting focus.
- Once you have a good focus try to record it for future use. Replace the
camera with an eyepiece and move the eyepiece in/out until there is good
visual focus. Mark the eyepiece stem or place a collar round it so that you
can return it to exactly the same position next time the telescope is set up.
Good focussing will pay high dividends later when processing the images,
allowing sharpening filters and 'wavelets' to bring out details or give nice
round bright stars.
It is sometimes possible to compensate for a degree of poor focussing using
'deconvolution' filters, but it is best not to rely on them.
required for most reflecting telescopes to ensure the primary and secondary
mirrors are properly aligned. When using the telescope visually it may be
acceptable to put up with a certain amount of mis-alignment of mirrors. The
sharpness of focus may be a little reduced, but depending on the subject being
viewed this may not be too important.
However, when taking CCD Images collimation needs to be good otherwise
unwanted artefacts can appear in the images. Stars will develop 'whiskers' and
features like lunar craters can develop what appear to 'moustaches'. Collimation
problems may not be visible when taking the initial images but will appear after
applying sharpening filters or contrast stretch. So its best to check
collimation before any imaging session.
It is convenient to check collimation during telescope set-up when initially
focussing with the camera. Align on a reasonably bright star and display it on
the computer screen. The diagram on the right shows different degrees of focus
with good (left) and bad (right) collimation.
- When well out of focus, the star will appear as a
symmetrical 'doughnut' with good collimation but the central 'hole' will be
off-centre with poor collimation.
- When nearly focussed, good collimation will show a
bright ring but poor collimation will show a fuzzy star with a thin or
- When properly focussed there may not be much
difference between the good or bad collimation when viewing on the screen,
but the difference in the quality of processed images will be significant.
Details of how to adjust collimation vary from telescope to telescope, so
refer to the user manual.