Cooling of Newtonian Optics
Submitted: Monday, 27th April 2015 by Alexander Massey (mental4astro)
Cooling of Newtonian Optics - an Insight
Cooling of astronomical telescope optics is a field that is misunderstood in amateur astronomy circles. Many of the ideas being used at first sound feasible, but they ignore the very thermal properties of the materials that are involved. This can result in dissatisfaction with a new instrument we have bought, or incorrectly blaming a manufacturer for problems that are of our own making. Here I will mainly discuss the cooling of Newtonian optics in relation to the different ways they are presented (solid tube or open ‘truss’), and the different requirements of visual and photographic, but only token mention of other scope designs. This revised edition of this article also includes suggestions and corrections offered by fellow IIS members. Their suggestions made me aware of the visual bias I had unwittingly put into the first article. For this I am grateful!
Glass and how we use it
First thing to mention is glass. Whatever material the substrate is, plate, borosilicate, etc, all are poor conductors of heat. This means that they will take time to cool down. However, as oils ain’t oils, glass ain’t glass. Borosilicate glass types are the preferred material for astronomical instruments as they have a much smaller coefficient of expansion compared to plate glass. This means that they expand less for the same increase in temperature. The benefit for astronomical instruments, particularly for photography, the mirror is most dimensionally stable during the course of the night. Yet as all glass types are poor conductors of heat, how they respond to cooling, natural or forced, needs to be understood and work with it, not against it.
In the original version of this article, I mentioned that the mirror doesn’t need fan cooling. This is not incorrect, but it was misunderstood by some people, though poorly phrased by myself. If the instrument is used for photographic it is advantageous to quickly have the primary mirror reach equilibrium with the ambient temperature to maximize productivity. Fans ARE effective for this, but their application is what is most important here. Incorrect application can have deleterious results.
Solid tube instrument and all photo
We know the fundamental that hot air rises. In a long tube system, heat only has one way to go – straight past the secondary mirror and on occasions the focuser too. The metal tube will cool very fast due to its large surface area, but the primary will be dissipating heat for a longer period. When using high magnification, these captive convection currents can become visible, and very disruptive for photography.
Two things conspire against us here - the length of the tube and the bulk and consequential residual heat of the primary. The only way to change this is to have an efficient way to create a heat exchange. The easiest is to have the rear of the scope as freely open as possible to have cool air be drawn in to replace the warm. However, this is a slow process.
Now, the quintessential rear fan…
The other way to create the heat exchange is to force air. This is done by the use of fans. But the way they are employed is the key. Most mass production instruments make use of a very open, almost skeletal, mirror cell assembly. This is fine for a long slow cool, but a poor option for the use of fans. Most of these instruments that have a fan attached to the cell have the fan blowing directly onto the back of the mirror. Two problems here:
The best way to force a mirror to cool, with the least way of causing a constant temperature differential within it, is to draw the air over the largest possible surface area.
In situations of a solid tube, and for all photo applications including with an solid tube or open, the mirror cell needs to be very open, but the rear opening of the scope immediately behind the cell needs to be closed, and the fans that are attached to the openings of this closed end need to blow OUT of the scope, not into the tube and hence the back of the primary mirror. This is contrary to the initial thought of exchanging heat out of a scope. However, several things happen in our favour: with air being drawn out the back of the instrument, it makes it impossible for standing convection currents to form in the tube and the ‘boundary layer’ that forms in front of a warm mirror is disrupted; the largest possible surface area of the mirror is being subjected to the draft that is created instead of a single spot; and the draft of air helps in the prevention of dew formation. This last point is very significant for instruments that have an open tube structure.
Open tube or truss scopes
Many of us today make use of open tube or truss scopes. These instruments can give us access to large and portable instruments. But the cooling requirements of these, particularly for visual use, are a whole less burdensome. The first thing in favour of open structures is there is no closed tube to allow a standing convection current to develop – warm air vents straight out through the open structure of the scope. The typically very open structure of the rear cell allows for air to very easily and quickly move around the primary mirror to evenly cool the mirror. And lastly, a slowly cooling mirror is less problematic for visual use as we constantly switch between eyepieces, and so alter focus, any change in the position of the focal point is a whole less an inconvinence. The one instance when this can be a problem is if high magnification is used from the very start of a session and the eyepiece is not swapped – then, maybe a tiny and most likely imperceptible change in star size might be noticed by the most acute of eyes as the mirror cools over a couple of hours.
Fans on the cell of such open instruments are a futile exercise. Cold spots will be created which on very large mirrors can create noticeable distortions in out of focus stars if these fans are employed constantly during the course of the night. These fans will also not blow air up the tube structure for the same reason as above – the path of least resistance is straight back out the open cell. Any chance of dew control from this arrangement is also lost.
With open tube instruments, the primary mirror of these can be very exposed, on occasions the mirror completely exposed. In these situations if the primary mirror is to be force cooled, the ‘best practice’ solution described above is still the best arrangement. This will require some type of dew shield or cuff to be placed around the primary mirror, and the back of the mirror cell closed off in order to create the most favourable movement of air around the primary.
Cooling of mirrors is important for imaging to increase productivity. But it needs to be a controlled cool. If you have a closed tube scope, reflector, refractor, SCT, Mak, etc, letting your scope cool to the ambient temperature is advantageous as I mentioned above if you are viewing under high magnification. If you seek to employ fans, there are best practice methods that will be most effective. If your instrument is totally closed, like a Mak or SCT, unless the instrument comes with factory installed fans/cooling system, there is little that can be done to accelerate cooling. If your scope is an open tube/truss one, a cooling period is not necessary for visual use.
If you would like your scope to cool prior to using it, this will take anything from 1/2 hour to a couple of hours, all depending on the temperature differential between the ambient and that of the mirror, and the size of the mirror - small mirrors will lose heat faster than large ones.
There are three ways to prevent dew formation, heat, air flow and shelter.
Solid tube Newt's have their primary mirror sheltered down the end of a long tube. Those long 'light shrouds' that are often seen wrapped around open tube Newt's do the same thing, but they introduce other complications as they get wet, and can also trap convection currents while the primary mirror is still warm. On my scopes I use what I call a 'cuff', or dewshield, that comes up only about half way up the open tube from the mirror box. Since using this on my 17.5" I have never had dew problems plague it. My 12" the same, and save for impossible nights when even fog forms it has also not had dew problems, but when this occurs it also coincides with the time to pack things up for the night as seeing conditions have gone pear shaped too.
Air flow is the way professional observatories stop dew formation on their mirrors. Vibration from the fans is the biggest hurdle here. Once the primary is at thermal equilibrium with the surrounds, this 'boundary layer' does not exist that can form from a warm primary. The fans then help prevent dew formation. This method is trickiest to apply to most Newtonians. Many big dobs made by specialist dobs builders incorporate a battery of fans that blow across the face of the primary mirror as their mechanism of dew control. A small fan can also be used to control dew formation over the secondary. For both primary and secondary mirrors, vibrations from the fan/fans needs to be dampened.
There are some instruments that employ fans that blow across the face of the primary, and draw air out the rear of the primary mirror cell. These are typically high end photographic instruments.
Heat can be used on scopes, but it needs to be carefully employed. Secondary mirrors benefit from gentle heating as it eliminates potential problems from fan vibration. But heating a secondary is more complicated than just sticking a heater onto it. The secondary mirror holder plays a vital part in this. The wrong holder and it will render any heating efforts futile. Heating of the secondary is very effective as it is small and the heating system can be designed to quite uniformly and gently heat it. Heating of the primary mirror I would suggest is not the best option, for the optical problems un-uniform heating creates is the same as for fan cooling.
But there is one method of dew formation that incorporates both air flow and heat – the good old hair drier! Heating of moving air increases the evaporative capacity, but it also needs to be applied carefully. If the hair drier is focused too much on one spot, it will affect the optical properties of the optics. The hair drier needs to be constantly waved over the primary mirror and surrounding structures, and from a distance, and for no longer than is required to remove dew. This will soften the intensity of the heat, and applies a gentle and minor heating to the face of the mirror’s face that won’t compromise optical performance. And to state the bleeding obvious, if the ambient temperature is very frigid, extreme care needs to be taken if a hair drier is to be used lest thermal shock is induced to the corrector plate of an SCT or Mak which in extreme cases could cause the plate to fracture. In ALL instances, the use of a hair drier needs to be a gentle one. 12V hair driers are excellent as they have limited heating capacity. The box of my own 12V unit is in a very sore and sorry state I have been using it for so many years. I should look at replacing this long serving little cardboard box.
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If you look at professional observatories, they do use fans. But looking closely at how they are used, not one single fan is blowing onto the primary mirrors directly. The fans are used to blow across the face of the mirror to blow away any dew that can form on it. Also, these mirrors are not allowed to get warm during the day. These observatories are giant cool rooms, air conditioned to the temperature the evening is expected to drop to, so these huge mirrors do not get warm, ever. Such massive mirrors may take all night to reach equilibrium, if not several days. Allowing these to get warm is something professional astronomers cannot afford to do. In an ideal world we might all like to be able to keep our instruments under temperature controlled conditions. But this is not possible for most of us. So if we do consider the cooling aspect of our scopes, we need to do so respecting the thermal properties of the materials being used, and the way that air does and does not move.
It took me a while to rationalize the misuse of fans on scopes. Particularly now that I also build scopes, how glass behaves thermally has become more significant in my mind in order to best design the instruments I build.
As this article is not intended as a how-to for any specific instrument, if you would like specific information for your instrument, please discuss on IceInSpace. I am sure that you will find both visual and photo minded people who would be able to best assist.
Clear skies, sharp pencils and cool cameras,