Do It Dry… INT Primary “Vapour Cleaning”
M. F. Blanken, A. K. Chopping, K. M. Dee (ING)
June 2003 ‘vapour cleaning’, a concept invented by ING staff, was used to
clean the Isaac Newton Telescope’s primary mirror (2.5m) for the very first
The primary mirrors of the telescopes at the Isaac Newton Group are regularly
cleaned to decrease the frequency of aluminising. Over the last few years
ING has moved from annual aluminising to condition based aluminising, only
doing it when the reflectivity and scatter measurements indicate it is needed.
The advantage is that an extra three nights are available to observers every
year that aluminising is not carried out, not to mention the real risk of
damage to the primary mirrors every time they are removed from the telescope
Regular cleaning is currently done by a method called “snow cleaning” or
cleaning”. This cleaning method uses liquid CO2
that forms snowflakes once it is in the open air. These snowflakes hit the
mirror surface and capture dust particles. The temperature shock between
the cold snowflake and the “warm” mirror will easily break the bond between
the dust particles and the mirror. The particles together with the snowflakes
fall down onto the telescope structure. There the dust can be wiped away
from the structure. This way of cleaning the mirrors is quick and easy restoring
the reflectivity by about 1–2% and decreasing the scattering.
Unfortunately stains like water and oil cannot be removed using this method.
A better way of cleaning the mirror is to use water, soap and natural sponges.
First we wet the mirror surface with water to flush away all the big dust
particles. By dabbing and with the use of soap on the sponges the water and
oil stains can be removed. The rest of the soap has to be washed away by using
water before drying. The best way of drying is to keep the surface wet until
the very last moment when the water is blown away with filtered clean air.
All the dust and most of the heavy stains can be removed using this method.
The reflectivity and scattering can be recovered to values close to those
retained after aluminising. Therefore this method is much better than the
A disadvantage of the “washing in situ” method is that it uses roughly 5–10
litres of water per square meter. This can be a problem when a copious amount
of water is running around mirror cells and associated equipment. Particularly
electronics have to be protected. So normally novel ideas have been developed
to seal the mirror or optical component to stop the water leaking around the
telescope which reduces the risk of water damage. By using the “water vapour
method” only 1–2 litres of water is used per square meter. The advantage is
optical results equal to “water washing” without the risk of water damage.
The small amount of water used is easily controlled with sponges or towels
placed at the bottom of the telescope structure.
The ING invested in 3 industrial vapour cleaners to be used for the “vapour
cleaning” process. Before the machines were used on a telescope mirror, extensive
tests were done on similar coated mirrors. It was found that it was very difficult
to cause any damage to the aluminium coating. Indeed only one test, which
involved holding the vapour stream only a couple of centimetres away from
the surface and in one position for 20 minutes caused a slight degradation
of the coating.
|Vapour cleaning (left) [ JPEG | TIFF ] and drying
(right) INT primary mirror [ JPEG | TIFF ].
The primary advantage of vapour cleaning starts with wetting the mirror.
For this part of the procedure a soapy vapour can be used by pre-mixing water
with soap. By wetting the mirror this way, the soapy vapour will start cleaning
whilst removing the large dust particles. The vapour is heated to a temperature
of about 35ºC. Therefore the temperature shock between the warm vapour
and this time the “cold” mirror helps to release the particles from the mirror
(reverse of the “CO2
cleaning” method). Before drying the mirror,
the soap can be cleaned away from the surface and the steam can keep the
surface wet. Even without touching the mirror surface the “vapour cleaning”
will give a better result than the “CO2
cleaning”. Finally to
achieve the same results as the “water washing” method, sponges and dabbing
still need to be applied to the mirror surface after the wetting.
The result of the washing of the INT 2.5m primary was so successful that
plans are made to repeat this procedure on the William Herschel Telescope
primary (4.2m). This is believed to be the very first time that such a process
has been used on a major telescope mirror anywhere.¤
Email contact: Maarten