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coverdisC supplement december 91
How to
clean a refractor
Prolong your refracting telescope’s useful life, and boost image quality
Tools and
materials
AIR BLOWER
A high-power bulb air
blower with the brush
attachment removed
– used to shift loose
dust particles without
touching the lens
surface.
The MiniSolarCam can
be piggybacked on a
scope, as it is here, or
mounted on a tripod
alcohol
A small bottle of
isopropyl alcohol to
dissolve stuck-on debris
and greasy marks.
LAMP
A bright reading light to
allow close examination
of the lens surface.
lens cleaning
cloth
A special micro-fibre
cloth to remove any
smearing left by the
tissue and alcohol, and
to give the lens surface
a final polish.
lens cleaning
tissues
Disposable lens cleaning
tissues to apply the
alcohol without leaving
any lint behind, while
carefully cleaning the
lens surface with no
chance of scratching it.
T
aking good care of your astronomy gear
and treating it with respect is common
sense. Precision equipment responds
well to a bit of TLC, and ensuring that
components like your focuser operate smoothly, or
that nothing gets knocked to put your optics out of
alignment, should be normal practice. Keeping dust
covers on the equipment when not in use, storing it
in a dry but well-ventilated area and waiting for it
to dry out before storing it are all measures that you
should take to keep your pride and joy in tip-top
condition. But what if, despite your best efforts, your
optics do end up with dust, stains or fingerprints on
them? Should you clean them, and if so, how?
A little bit of dust is not a major issue, because as
soon as you remove your dust covers, tiny particles
will start to settle on exposed optical surfaces.
For the most part, these can be left alone: even for
imaging purposes, these particles will be too far from
the focal plane to cause problems. However, if they
do concern you, they can be easily removed with the
use of a high-power blast from an air blower bulb.
You should remove the brush and dispose of it, as
the brush hairs can deposit grease and debris from
previous cleaning sessions, causing more problems
than they cure!
Grime-busting
Moisture from the typical air conditions in which
you use your refractor can dissolve some particles,
such as pollen, so that when they dry, they can be
well and truly stuck to your delicate lens surface. If
this happens, you will have to use more than just the
blower bulb to deal with them. Fingerprints and
other greasy substances can also accumulate on
the lens, and when this happens, it really is time for
a clean.
You will need unencumbered access to your lens’s
surface, so start by retracting your dew shield, or
removing it altogether if it’s not retractable. Having
used the high-power blast of air technique to
remove as much dust as possible, grease and other
coverdisC supplement december 91
contaminants can be removed using a
proprietary lens cleaning kit, available from
any good photographic shop. The main
components of these kits are isopropyl
alcohol, lens cleaning tissues and a microfibre cloth. Use a reading light or good
natural light to examine the lens surface
carefully and identify the problem areas.
STEP-BY-STEP GUIDE
A clean start
To use the cleaning kit, remove a lens
cleaning tissue from the pack and add two
drops of the isopropyl alcohol to it. Gently
wipe the lens surface, starting from the
centre and moving carefully outwards,
being especially careful not to touch the
lens with your fingers. Don’t let any of the
fluid accumulate on the edges of the lens
surface, and never apply the fluid directly
to the lens. Excess alcohol could end up
inside the lens cell through capillary action,
and dissolve any lens cement present in the
construction of the lens cell. Throw this
tissue away and use a fresh tissue to soak up
any residue. Re-examine the lens surface
to check for any greasy smears and if there
are any, use a fresh lens cleaning tissue
impregnated with another two drops of the
alcohol as before. Throw this tissue away as
well. Finally, very carefully polish the lens
surface with the micro-fibre cloth, using a
pristine area of the cloth.
The design of a refractor is essentially
a closed tube, so it is unlikely that dust
or other material will be deposited on
the rear surface of your telescope’s primary
lens. If this happens, the lens cell can
be removed carefully, provided you can
mark its original position in some way.
However, we would advise against this
unless you know exactly what you’re doing.
If you get it wrong, you could seriously
upset the alignment of the optical elements
and end up with a telescope that’s damaged
as well as dirty.
STEP 1
It is important that you have easy access to the
front of your primary lens, so if you have a
retractable dew shield, fully retract it before
you start. Many refractors have a fixed dew
shield, but these can usually be carefully
removed with a gentle turning action.
STEP 2
Hold the telescope with the lens pointing
downwards. Hold the high-power blower bulb
near to the front of the lens (with the brush end
removed) and direct several sharp blasts of air
at the surface. This should dislodge the
majority of accumulated dust particles.
STEP 3
Some soluble particles, grease and fingerprints
will need to be dissolved away. Apply two
drops of the isopropyl alcohol to a fresh lens
cleaning tissue and carefully wipe the lens from
the centre outwards, then use a second, fresh
tissue to remove any liquid residue.
STEP 4
Use the special micro-fibre lens cleaning cloth
to very gently polish the lens surface. This will
remove any smears left by the previous cleaning
process. Be sure not to press too hard while
you’re doing this, and only use a fresh area of
the cloth, turning it as you work.
STEP 5
Carefully examine the lens under good daylight
or a bright light to check for any stubborn
particles or greasy marks, and repeat from step
three onwards, using fresh lens cleaning tissues
each time until you are happy that you have got
the surface as clean as possible.
STEP 6
Re-install the dew shield if you had to remove
it. Finally, clean the inside of your dust caps
carefully, using a lint-free cloth, to ensure that
they don’t impart any unwanted dust onto your
freshly cleaned lens when you put them back on
for protection.
find out MORE
Watch the author’s own video showing the
lens cleaning process in action:
www.youtube.com/watch?v=1RVeuK0daF4
Celestron Knowledgebase on lens cleaning:
http://tinyurl.com/37yhcda