If the seeing is really terrible – i.e. there is a
strong turbulence in the atmosphere – the
out-of-focus star may change "wildly", and
change in a fraction of a second in size,
color, shape (image at right). If this is the
case, the seeing is simply too bad to
continue. You have to wait another night (or
use an artificial star). If this happens when
you have just open your "warm" dome in a
very cold night, just wait one hour to check if the problem is due to local turbulence (the telescope
is much warmer than the air). It is a good practice to open the dome (and start telescope's fans)
as soon as the sun sets. By astronomical twilight, the telescope will be very close to thermal
equilibrium with the environment, minimizing the thermal (local) turbulence.
5.2) How to
check
collimation
1. Be sure your telescope is in thermal equilibrium with the environment.
2. Start with the lowest power (longest focal length) eyepiece available. DO NOT use a 90° prism.
3. Choose a bright star at least 45° above horizon. Bring the star in the center of the field of view
and look at the defocused image. You MUST use a star, a planet cannot be used since it is not
a point-like source of light.
4. If the dark shadow is clearly NOT in the center of the white disk jump to section 5.3,
“Collimation procedure”.
5. If the dark shadow of the secondary mirror looks in the middle of the white circle, change a bit
the focus, using the telescope focuser, NOT the electric motor of the secondary mirror, if
present. The white disk will became smaller or bigger. Check if the shadow is always in the
middle (the human eye is a very good "device" to control if two circles are concentric). Always
keep the image in the center of the field of view using the telescope mount. Focus from a "very
big" to a "very small" image, and continue to check if the shadow is always in the middle of the
white spot. Find the size of the spot that is more comfortable for your eye to detect
concentricity. If you are using a camera you may have to change exposure time; you may also
measure the single images using specific software (or a simple ruler, working directly on the
computer screen). If you are using a camera set the exposure time so you can see different
shades of gray in the star image.
6. If the image is too bright or too dim to see clearly the shadow of the secondary mirror, choose
a different (dimmer or brighter) star, or change exposure time.
7. If the shadow always look concentric, change eyepiece with a more powerful one (shorter focal
length) or zoom the electronic image, and repeat steps 3-5. For this step a visual magnification
of around 200-300x is a good choice.
8. If the shadows always look concentric, the collimation of your scope is "good enough" for any
reasonable application. Jump to sections 6.5 and 6.6, for “fine tuning”.
The image at left gives you an idea about what
you will see during this procedure while you
change focus.
Remember, always keep the star in the center of
your field of view, using the telescope mount.
5.3) Collimation procedure
If the shadow
is not
in the middle of the white disk, start collimation from the primary mirror, for the
very simple reason that 90% of the times you can reach a good collimation working only on the
primary mirror, and you need no particular tool to do so.
16
