Suck or blow

Forums Telescopes Suck or blow

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    Grant Privett

    I have found that the mirror in my 12″ f/4 Newtonian has a worrying tendency to get water droplets forming on it during the day when the air temperature rises quickly. Water is, also, often dripping off the NEQ6 itself to to the floor…

    To try to avoid this accumulating condensation, I removed the tube caps and left the whole thing horizontal under a waterproof blanket, but this didn’t seem to help.

    When the scope is being used in anger I use a fan pre-mounted in the mirror cell. This seems to encourage the water to evaporate. Consequently, I am planning to use a fan all the time.

    So, the question is, should you suck air down into the tube, past the secondary down to the mirror and out round the gaps round the mirror or, should I attach a motor at the secondary end and draw air into the tube at the mirror end and out via the open tube end.

    Which would be most likely to minimise the condensation?

    Chris Dole

    Hello Grant.
    I tend to suffer exactly the same problem with my Dall Kirkham. Rarely any moisture when observing but if I check the scope during the day there is often condensation on the primary at this time of year.
    My scope has three small cooling fans behind the primary. They pull air down the tube from the open/secondary end past the primary. They are very effective at clearing the moisture even though that isn’t their primary purpose. To combat any dust concerns I put a doubled up old piece of net curtain over the open end to act as a filter but to ensure good airflow.
    I’ve recently started using packets of silica gel inside the tube when the scope isn’t in use. This seems to help, as long as they are regularly recharged in a warm oven and the tube is fairly well sealed.
    It’s a really annoying problem that I’m still working my way through.


    Peter Carson

    Hi Grant,
    My Planewave 315mm CDK has forced tube ventilation that sucks air down the tube past the secondary, primary, and out the back of the primary mirror cell.
    I’ve noticed that all ASA reflectors have a fan that blows air into the side of the tube just above the surface of the primary. I understand this is to reduce or eliminate condensation as they don’t have any form of mirror heating.


    Daryl Dobbs

    I fitted to my Newtonian a fan which blows air up the tube, the rationale behind this is to break up the boundary layer in front of the mirror, a while ago I wrote an article which is found in tutorials on cooling Newtonians which might help.

    Gary Seronik has written a couple of very useful articles on cooling and condensation which might be helpful.

    David Strange

    I’ve wondered about the three fans fitted in the back of my RC10. I have never bothered to use them, do they serve a purpose when imaging?
    I’ve never noticed a difference. Should they be switched on before a session to equalise temperature inside the tube?
    Any RC owners welcome to comment!


    Martin Lewis

    I suck and blow!

    My 18″ mirror is in a square mirror box and I have a soft surround to the mirror with 4 small fans one in each corner to pull air down the tube during cool down prior to imaging. Meanwhile I blast the middle of the mirror rear with a 100mm fan which has a large baffle around it so the air get funnelled raidally across the mirror. This cools the primary at the maximum rate and faster than if I were sucking air past the back. I can control the small fans separately from the main primary fan and both sets are speed controlled. The idea was that I could switch off the main fan at the start of an imaging session but still have the small fans sucking down the tube. Trials showed little benefit in doing this and generally I give everything an hour’s blast and switch them all off during imaging. Blowing during hi-res planetray imaging is definitely is a bad idea from my trials.

    If the scope or mirror is below the dew point, condensation will form on it, so you either have to warm the scope and mirror or decrease the dew point of the surrounding air. Don’t think there are any other options. You could bag the whole scope and put sachets of dessicant in the bag to drop the moisture in the surrounding air? You’ll get through a lot of dessicant, though it is rechargeable through heating. Or put an electric dehumidifier unit in the vicinity.

    Good luck

    Dr Paul Leyland

    If the scope or mirror is below the dew point, condensation will form on it, so you either have to warm the scope and mirror or decrease the dew point of the surrounding air. Don’t think there are any other options.

    I can think of another, one employed by myself, Kevin Hills and numerous others.

    Move your scope to a site where dew never happens. Operate it over the interweb thingy if you don’t fancy moving yourself as well.

    Admittedly, not very appealing to some folk.


    Grant Privett

    Thanks for all the input. I’ve been pondering this a bit.

    From what has been said, its clear that cold air and a warm mirror will degrade the images, but is not a dew hazard – cooling your mirror for an hour or so at the start of the night and possibly using a gentle dew heater later, seems to sort that issue.

    The question then, is what to do when the air is warm and the mirror cold?

    A frequently seen example would be when someone who has been outside on a cold day wearing glasses goes into a warm house. The glasses quickly dew up. At that point many people wipe the water from the glass surface but, if you wait, the dew clears unaided as the temperature of the surface of the glass approaches that of the air and evaporation, caused by air movement, takes it away. This suggests that, for my telescope, the question is how to minimise the condensation and speed up the evaporation?

    Warming the surface of the mirror by attaching heaters isn’t possible as they block the light and I am not aware of cheap tube mountable infra-red LEDs that could do the job.

    Warming the back of the mirror can be done and will have an effect, but glass is a poor thermal conductor and a strap dew heater (or two) on the outside of the tube wall near the mirror may be the best we can do.

    Air movement will encourage the evaporation of the dew, so a fan is also needed. If the fan is off, the air above the mirror will stagnate and the condensation will persist for hours. If the fan is on the mirror will reach equilibrium faster and, while the dew will form faster, it will also evaporate faster. So, the question becomes: what is better for the mirror short lived dew ups or long lived dew ups?

    To my mind, water droplets sitting on the mirror for hours is very bad news so, when the telescope is not in active use, I will try running my dew heater on a higher setting than I normally use, plus I will run my mirror cell fan.

    I hope this makes some sort of sense – please let me know if I am being exceptionally dumb as I may have missed something. I will report back here in a week or two. I wil inspect the tube every morning for a week or two.

    One aside: I have used a Pulsar, a SkyShed POD and a roll off wooden roof and never opened up the roll off roof and discovered the telescope significantly dewed up. I am not entirely clear why that would be. Bigger air volume – insulated walls/ceiling/floor?

    Grant Privett

    A cunning plan indeed. 200 clear nights a year and dark skies, whats not to like? … Apart from the hosting charges/costs. 🙂



    I have found that the best way to push air through a tube in a laminar fashion is to have fans at the top and bottom. As to direction in this case, I am not sure.

    Martin Lewis

    Definitely having dew sitting on the mirror for prolonged periods is bad news if there is any acidity present in the liquid pools but if the water is neutral I don’t think a long dewing is worse than a short dewing. I think the main issue with dewing is that is tends to collect up loose surface debris into rings and spots – these agglomerations of particles are then no longer loose and become stuck to the surface causing scattering and loss of contrast. The collecting action locally concentrates up any agressive debris on the surface increasing its likelihood of attack of the surface.

    Fans to blow on the surface of the mirror to speed up evaporation will only work if the air being blown over the mirror is lower than the temperature of the mirror and blowing ambient air over a mirror below the dew point will just make matters worse.

    Some observatory owners do have electric dehumidifiers in their observatories and these can be triggered on relative dew point or just switched on after a session to dry everything off.


    Grant Privett

    I think I prefer a short duration dewing as that gives less time for the water droplets to absorb gases from the atmosphere and become acidic. Though, with the reduction in heavy industry over the last 50 years and diminished use of coal powered fire stations, perhaps I am being over cautious. But, realuminising mirrors ain’t cheap.

    I was also disappointed that parking the telescope horizontally didn’t encourage the droplets to run off better – most of them seem to stay attached to the mirror. I assume its a surface tension effect.

    Yes, I take your point regarding air temperature, but think the combination of glass being a poor thermal conductor and a fan means the surface of the mirror (not the bulk) can follow the air temperature quite closely. Thats very handwavium, but I don’t have time to make a mathematical simulation. Anyone?

    I can remember many still nights with dew, but not that many windy ones – though it may be a selection effect as, before the dome, I didn’t observe on windy nights for fear of tube vibration.

    I also considered a dehumidifier, but need to check the energy consumption first – a dome isn’t exactly a sealed system. A small computer fan is very low power, hence my enthusiasm.

    I think the main change I need to apply now is a very fine mesh net over the secondary end of the tube to try to catch the bigger debris and discourage spiders.

    Just checked. Third morning in a row without any condensation, ice or droplets on the mirror and none present when I used the telescope on the 27th. Will keep checking every morning.

    Martin Lewis

    Hi Grant,
    The reason breezy nights are less dewey is that the warming effect of the ambient air prevents surfaces facing the much colder night sky from being so depressed relative to the air temperature. On still nights surfaces with high emmisivity like plastics and painted surfaces can be several degrees below ambient temperature and well below the prevailing dewpoint. If there is a breeze around it has a moderating effect.

    I’ve got measurements of a 2.4C depression for a plastic surface on a still night dropping to 1.5C on a breezy night. Radiative sky temperature in UK is typically 30C lower than ambient so there is a strong cooling effect for anything facing it – like opening the freezer door!

    Aluminium foil, with its very low emmisivity was 0.9C and 0.2C respectivley on those two different nights making shiny aluminium a great anti-dew blanket!


    Dr Paul Leyland

    Interesting. Radiant sky temperature in La Palma is also about 30C lower than ambient.

    I have never seen dew anywhere except inside a SBIG camera after the desiccant needed renewing. That was excusable because it was generally run at -20C.

    Perhaps having the scope inside a dome is advantageous from this point of view too.

    Grant Privett

    In the spirit of full disclosure…. 🙂

    Was -7.2C here last night (still -3.2C by 9:15) with a very heavy hoarfrost/fog in the early hours.

    Looked in dome at 11:00 – by which time the air was registering as +5C – and found some condensation on the mirror.

    Well , there goes my theory….

    So, henceforth will also use a dew heater band near the secondary plus the fan. Can’t help matters having frost on the inside of the tube some nights. Don’t want to use too much power though.

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