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    Default Cooling of Newtonian Optics - all fans are not created equal



    Cooling of astronomical
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    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, and the same principles also apply for other
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    designs. This article also takes a big lead from how professional observatories have their
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    set up, as it is NOT the professional astronomers who design and build these instruments and housings, but a team of engineers, opticians and technicians. Professional astronomers just take the keys once the thing's been built!

    There is also an added bonus from implementing a better way of using fans - dew control

    This edition of this article also includes suggestions and corrections offered by many good people. Their suggestions made me aware of the visual bias I had unwittingly put into my original 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 instuments, 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:

    1, the draft of the fan is focused on one spot. This creates a constant cold spot on the primary creating a strain within the glass due to a forced temperature differential with the warmer extremities of the mirror. Plate glass in particular is more susceptible to deformation as a result, but even for borosilicate glass this is not ideal when a better solution is possible.
    2, the air within the tube is not being exchanged as the draft from the fan is bounced back off the mirror straight out the open cell structure – this is the path of least resistance for the draft and it will not go around the edge of the mirror and up the tube.

    Best practice
    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.
    Note: forcing a mirror to cool does induce a temperature differential, but this is a temporary one if the following system is employed.

    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
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    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.

    Photo application
    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.

    Dew control
    The better way of cooling optics brings with it an added bonus - dew control!

    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
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    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
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    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.

    Note that the hair drier solution only has intermittent effect as dew will form again requiring the use of the drier once again.

    Keep it cool from the start
    There is one very "easy" way to reduce the problems in needing to cool down optics - don't let the optics or scope get warm in the first place!

    If the heat is not in the instrument from the start, the entire cooling process is not only made easier and quicker, but can be a non-event from the start. Here, professional observatories are the example, and we ignore these to our peril. These massive instruments are housed in what are effectively giant coolrooms during the day. These instruments CANNOT be allowed to get warm, even for one day, as it will take several days to restore the night time temperatures within them. If many big amateur scopes just do not achieve thermal equilibrium just in the course of one night, a 200in lump of glass has no chance EVER to cool unless it is perpetually kept cool during the day.

    Storing your instruments in a cool environment is the first, be it at home or in a purpose built obs structure. Too many scopes are allowed to get hot during the day, and their owners confront significant complications from this.

    It is not the purpose of this article to give ways on how to keep your instruments cool. This is something each individual needs to do for their own circumstances and means. The purpose of this article here is to make you aware of this particular aspect.

    ~ x . X . x ~

    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 as I also build scopes, how glass behaves thermally has become more significant in my mind in order to best design the instruments I build.


    Alexander Massey.

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    Default Re: Cooling of Newtonian Optics - all fans are not created equal

    Wow that is quit a thesis on the subject. I learned a few things.

    Thanks.
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    Default Re: Cooling of Newtonian Optics - all fans are not created equal

    Great article, Alexander. Many thanks, I found it to very useful as I think about slapping a (reversed) fan on my 10"
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    dob.
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    Ian

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    Default Re: Cooling of Newtonian Optics - all fans are not created equal

    The timing of this is interesting. Saturday night as I was unscrewing the plate behind my Newt's primary to get at the
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    screws, I considered whether I should leave it off for the night to allow air flow. As usual I put it back on when I finished collimating. My main consideration was to avoid light link into the tube.

    That night was particularly humid, and the dew started early, to the point where dew formed on the primary. I will read through your article and see what I can apply from it.

    Bob
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    Default Re: Cooling of Newtonian Optics - all fans are not created equal

    If one converts to sucking the cooling air along the tube to exhaust out the back, then does it make any sense to add a dew heater to the tube?

    Where along the length of the tube should this pre-heater be located?
    Near the top where it can radiate a bit of heat into the secondary, or half way down to maximise the surface area over which the air can heat, or down near the primary mirror?

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    Default Re: Cooling of Newtonian Optics - all fans are not created equal


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    Originally Posted by bobharmony
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    My main consideration was to avoid light link into the tube.

    Light LINK into the tube???? Maybe light LEAK into the tube was what I was trying to say
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    Default Re: Cooling of Newtonian Optics - all fans are not created equal

    Watson, why are you looking at heating the tube? The whole purpose of this is to avoid heating of the primary and the structure.

    Ian, remember to close off the mirror cell all around the reversed fan. Only this way will you draw air all over the primary. An open cell is brilliant if natural cooling is all you need.

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    Default Re: Cooling of Newtonian Optics - all fans are not created equal

    Anyone know which way the fans on the popular ritchey christen astrographs run?


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    Default Re: Cooling of Newtonian Optics - all fans are not created equal


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    Originally Posted by mental4astro
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    Watson, why are you looking at heating the tube? The whole purpose of this is to avoid heating of the primary and the structure.

    Ian, remember to close off the mirror cell all around the reversed fan. Only this way will you draw air all over the primary. An open cell is brilliant if natural cooling is all you need.
    On my
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    I use what I call a 'cuff', or dewshield, that comes up only about half way up the open tube from the mirror box.
    I am new at this, until recently I did not know that anti dew heaters were needed or even existed.
    Here we have the Thread opener stating he uses a "cuff" or dewshield and he also states
    There are three ways to prevent dew formation, heat, air flow and shelter.
    If we draw new air into the newtonian tube to waft over the primary mirror and finally exhaust out the back then that air will contain some water vapour.
    If the air is cooled then there can be condensation.
    Pre-heating the air will help prevent that condensation.
    A cold primary mirror that is in a humid airflow will be at risk of condensation forming on the cold surface.

    Now I get to ask a question:
    Why do we see users adding dew heaters to their scopes?

    I suggest they are doing that to preheat the air and thus reduce the risk of condensation.

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    Default Re: Cooling of Newtonian Optics - all fans are not created equal

    Good questions, Watson.

    There are some aspects here that I can see as seeming to be counter intuitive. Remember a few things:

    Preheating the air is not necessary to prevent dew formation. Movement of air also will not create the formation of dew.

    Recall going to your car of a morning and find it totally soaked with dew? Now, you start driving, and in a matter of minutes the dew has totally dried off. It is air movement that is causing evaporation, not dew formation on the "cooled" car surface.

    As long as there is movement of air, dew will not form.

    What is important is that there not be localised "focus" of the draft of air on any one spot on the optics. The air just needs to be a gentle flow uniformly moving over the glass. This will not cause a differential in the internal temperature of the glass, and therefore preserve the figure of the optics. Remember, the primary mirror is parabolic. A temperature differential will induce stress and strain within the glass, and therefore will alter the shape of the paraboloid. Gentle movement of air at the ambient temperature will not only prevent dew formation, but also creates the least temperature differential on the optics. This is why professional observatories use this method.

    Now, I've just said that optics shouldn't be heated, but in my article I say that the secondary mirror is ok to heat. What is important to understand here is that the secondary mirror of a Newtonian is flat. This is the key to being able to introduce a gentle heat. It is applied from the rear surface, and needs to be gentle. Ideally it shouldn't be heated, and best practice is to gently blow air onto it. Biggest problem for us here is to not introduce vibrations from a fan.

    Much of what is done using heat with
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    is from poor understanding of thermodynamics. All this means is the way materials react to heating and cooling. What does not help is that many people also set up their gear in places that are nothing more than "dew central", and rig up their equipment with incredibly power hungry gizmos to control dew and frost, not being aware of the effects that this heating has on their optics. This then brings up the other lesson to be had from professional observatories - site selection.

    Where you set up is the single biggest factor to control, even prevent dew from forming or even being an issue.

    Professional observatories are not set up where they are just because they are dark sky locations. More so, these sites also provide a set of environmental conditions that see dew pretty much non existent, and with consistently superior
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    .

    * Remoteness
    * Elevation
    * Local topography
    * Surrounding vegetation and land use

    All of these contribute to eliminating dew as a problem.

    It would then be a good moment to now introduce the link to the other article that I've written on site selection for astronomy purposes:


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    The dark sky site I regularly visit I've come to find after exploring locations that satisfy the above conditions. As a result, dew has never been something that has been a problem for myself or any of my observing buddies who share this site with me. Yet, there are even astro clubs and societies that have set up their dark sky sites in locations without any consideration to an appropriate place for astro, and their locations are nothing more than dew ponds, and visitors to these locations need to take extraordinary measures to control dew.

    You want the best way to prevent dew from forming on your optics, start by selecting a site that provides the best conditions for dew NOT TO FORM in the first place.

    Ventilation methods and site selection are the two key ways to preventing dew from forming.

    Alex.

 

 
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