454 replies to this topic

[maroubra_boy]

I've mentioned this a couple of times in a thread or two, but I think it is time to make a thread about this all of its own.

 

IT IS POSSIBLE TO HAVE A DEW-FREE corrector plate without needing to use heat.  Not one joule of heat.

 

We stuff around so much to get our scopes to acclimate and also insulate our scopes to slow the rate of cooling, and then we go and stick heating elements all around it.  Worse still the way heating elements are used they are most inefficient by wrapping the heat source around a metal tube...

 

I have a big dew problem where I live.  Home is where I do the majority of my lunar and planetary observing.  I live directly across from a heavily irrigated park whose lush green fields creates an especially humid microclimate all around it.  While the official dew point may be lower than the ambient, the moisture rich air coming from the park sees the surrounding streets soaking wet with dew.  It also doesn't help that my own backyard is turfed and with a lush vegie patch - dew is a constant battle for me.

 

I have been insulating my SCT's and Maks for several years now.  While this has helped me rip high magnification straight away upon setting up and has slowed the dewing process considerably, the corrector plate of these scopes will still dew up if the conditions are right (which is most nights).  I had used a hair drier for a number of years to deal with this, but it is only a temporary measure and it gets wearisome to be constantly getting up to dry off the dew...  Plus I have not been a fan of using heat on my optics so its been a love-hate situation.

 

There had to be an alternative.

 

This alternative has been staring me in the face for YEARS!!!  FANS!

 

First, professional observatories do not use heat to keep their equipment dry if dew is a likely problem on any given night.  Instead they use fans to push air all around their equipment making use of the evaporative power of moving air to dry things off.  By also only using fans they do not introduce unwanted heat and infrared interference into their equipment, not just for their cameras but also for the thermo-mechanical stability of the entire structure - different materials have different coefficients of expansion, and in the complex large structure of a professional scope, thermal stability means all the materials shrink/expand at predictable rates and amounts.  Start introducing heat and this predictability goes out the window.  How fans are used in professional observatories is critical too as these can cause thermal issues in materials too if incorrectly implemented.

 

Second, I have been utilising this principle of using fans on my open dobs and solid tube Newts for a number of years too.  I have had my scopes Newt optics stay bone dry while those of others have called it a night because of dew.  So how could I use this with my Cats???

 

The solution was so simple for me.  As I insulate my Maks with Coreflute, this material could be easily cut to accommodate a small 40mm 12V fan approx 50mm to 75mm in front of the corrector.  The fan is then wedge into the side of the dew-sheild end of the insulating wrap in such a way that it blows a vortex of air inside the dewshield compartment.  This vortex provides not just movement of air, but the corrector is also not subjected to a localized thermal strain from having a cold stream of air blow directly onto one spot on it.

 

12V was a no-brainer as it is already what I use to power the mount.  As an added bonus, the amount of power a 40mm fan uses is next to nothing compared to a heating strap even at lower power.  And also cheaper and easier to purchase, install and work with than a dew strap and controller be it commercially made or DIY.

 

I have been using this method for close to a year now with my Maks and refractors and have not had to stop a viewing session early because of dew.

 

Vibrations?  Nope.  None at all.  Not even with the Mak pushing 560X.

 

The day after my proof of concept first use of this fan system I thought "why couldn't this work with a refractor too?".  Well, it can.  I replaced the metal dewshield that came with the frac and replaced it with one made from Coreflute and installed a 40mm 12V fan to it, and PRESTO!  It works a treat too, and NO HEAT!

 

I have also attached a PWM unit so I can regulate the RPM's.

 

I do not need to use the fan every night of course as dew formation varies from night to night.  I also don't turn it on until I feel I need to.  If the corrector does start to dew up I just switch it on and the dew is dried off in a matter of moments.

 

You do not need to use Coreflute to make use of this method of dew control.  As long as you have a way of securing the fan inside of the dewshield you have made then it will work.

 

Alex.

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Edited by maroubra_boy, 01 February 2021 - 07:11 PM.

[CharLakeAstro]

I appreciate your research into this topic. However... it does have geographic limitations. I am in Canada, dew in the summer, frost in the winter. 

Once we get into air temperature -10C and below , fans moving the air will slow the frost formation, but will not eliminate it, without a supplemental heat source.

Tonight's forecast is approx -18C, last night was -21C

[maroubra_boy]

You have a tough situation during winter for sure.  And yes, of course the use of a straight fan under such harsh -10deg and colder is not necessarily an option.  As you say, a supplemental heat source for you could well be an option.  A hair dryer is very harsh too, and may compromise the coatings of the optics or even compromise the glass itself.  So a mild source of heat could be a good addition for you just to help take the edge of the cold without introducing a significant heat shock or introducing significant thermal uneven heating through out the glass as happens with only perimeter heating that a dew strap provides.

Edited by maroubra_boy, 01 February 2021 - 07:47 PM.

[Migwan]

Have you done any high power star tests before and after turning on the fan?  

 

Your fan idea is so easy, I think I'll give it a try come spring.  Not sure I want to try it in the dead of winter.  Who knows, it might work then too.  

 

A quick look puts the watts at around 1.8 to 2.4W.  That's efficient.

 

My dew control system (with an 11", a 5" and two eyepiece straps) requires between 6 and 9.6W on low and high respectively.  So probably 3 to 4.8W on just the 11".     Regarding expense, I have $27 total in the straps and controller, which includes enough nichrome to make lots of straps.  

 

Thanks for sharing the idea. 

 

jd

[maroubra_boy]

Have you done any high power star tests before and after turning on the fan?  

 

 

Yes, at 560X.  As I mentioned in my first post, ZERO vibrations.

 

As for cost, less than $3 for a 40mm 12V fan.

Edited by maroubra_boy, 01 February 2021 - 08:34 PM.

[Alrakis]

Just a thought (and maybe a crazy idea), but what about wrapping the fan with a dew heater? Maybe the dew heater on the fan could be turned on so that when frost is an issue the dew heater can provide a curtain of air that is just warm enough to keep the optics frost and dew free. If it is constant I would think it might not cause turbulence issues.

 

Chris 

[maroubra_boy]

Try to forget heat.  Unless you are trying to observe in sub zero temperatures, forget heat.  And even then heat would be best left for your gloves or jacket.

 

The whole premise of using a fan is exactly to get away from this ingrained notion we have that the only way to deal with dew is with heat.

 

YOU DON'T NEED HEAT.

 

Professional observatories don't use heat.  Look at how they do things.  There is nothing mystical or prohibitively exotic or expensive about how they do things.

 

I have also used fans successfully at below zero temperatures and had no dew or frost issues.

 

Benschop is talking about -18°C to -21°C, which makes me wonder why you are outside in the first place with a scope under such conditions????  

Edited by maroubra_boy, 01 February 2021 - 09:11 PM.

[MarMax]

I like the innovative thinking and I've not really liked the idea of applying heat to an SCT that already has a heat problem. Luckily I'm in an area where 90% of the time a dew shield without any heat works fine. If I find that I need to kick up my dew control a notch I'll be going with a fan.

[Migwan]

Yes, at 560X.  As I mentioned in my first post, ZERO vibrations.

 

So how was that first diffraction ring?   Wasn't really worried about vibrations.

 

jd

[CharLakeAstro]

I am not outside, but the telescope and camera are. I remotely control both the telescope, camera and focuser. Our longest nights are winter, the clearest skies are also the coldest nights. 

 

 

Try to forget heat.  Unless you are trying to observe in sub zero temperatures, forget heat.  And even then heat would be best left for your gloves or jacket.

 

The whole premise of using a fan is exactly to get away from this ingrained notion we have that the only way to deal with dew is with heat.

 

YOU DON'T NEED HEAT.

 

Professional observatories don't use heat.  Look at how they do things.  There is nothing mystical or prohibitively exotic or expensive about how they do things.

 

I have also used fans successfully at below zero temperatures and had no dew or frost issues.

 

Benschop is talking about -18°C to -21°C, which makes me wonder why you are outside in the first place with a scope under such conditions????  

[maroubra_boy]

So how was that first diffraction ring?   Wasn't really worried about vibrations.

 

jd

To start I see up to 8 diffraction rings through my Santel Mak, and none of these are disrupted when the fan is going if this is what you mean.  There is no thermal differential so there will be nothing to see with the diffraction rings.

[charlesgeiger]

I have seen past posts where folks using the Tempest fans with their newer Celestron scopes with the vents say that in high humidity where dew is prevalent that these fans can induce water inside their optical tubes.  I think you have a good concept going but was wondering if using in saturated air (high humidity) if the fan would simply blow condensing water directly onto the corrector?  At some point I would think the air saturation would take over.  

I would think that your solution (along with supplied heat) would work similarly to the defrost system in a car.  No matter the conditions on the outside, the windshield will stay clear as long as there is some supplied fan air and heat.  What do you think?

Charlie 

[luxo II]

Charles agreed, I’d like to hear  how Linwood fares in Florida with his C11.
 

Alex and I are in Sydney where we have two scenarios.

 

Firstly, two sites very near the ocean in temps typically 10C on a winter night, maybe 20C on a summer night, grassy suburban locations with humidity around 90%. Dew is a real problem at these. If the winds drops a typical SCT will dew over in 15 minutes without any preventive measures, though the maks do fare better.

 

Second, a dark sky site 100km inland at 1000m altitude, 10 degrees cooler and thanks to the terrain, a dry microclimate where nighttime humidity is approx 50% and frost and dew really aren’t a problem. There I might use a dewcap and often nothing at all.

 

The northern US or Canadian winter scenario of -20C is altogether different and without an observatory for shelter I’m a bit surprised anyone would try to observe in that. I’ve lived in a colder climate before and recall observing in -5...-10 C - I froze my cornea to the eyepiece a couple of times - and it’s not something I’d willingly do often.

Edited by luxo II, 02 February 2021 - 06:19 AM.

[maroubra_boy]

Charles, we first must get past the thinking that we somehow must use heat.  There are consequences with heat.

 

Where did you read that the Tempest fans induce condensation inside SCT's?  As I mentioned in my first post how fans are implemented is critical for their proper function and to not introduce unwanted effects.  There may well be other factors at play as to why condensation may be happening inside these Tempest cooled scopes that are not being noticed and only being blamed as the sole perpetrators of this condensation.  I am not advocating these instruments or these particular fans as I have no experience with them.  But what I do know is if one does not take the right precautions (design & utilization), then adverse consequences often follow.  Just flicking on a switch without considering other factors, such as environmental microclimate you are using the scope in, you may be introducing other problems.

 

My use of fans around the corrector is in an open environment.  The front aperture of the dewshield is totally open so the air being drawn into the dewshield is expelled out the path of least resistance - out the dirty big aperture opening.  The fan is also not blowing directly onto the corrector.  NEVER should a fan ever blow directly onto an optical element as this will create a cold spot on the glass which in turn will induce a thermal strain in the glass due to the uneven heat distribution inside it, and the fine figure of the optic will be distorted.  This is exactly what also happens when you heat a corrector plate from its outside circumference - a thermal strain is induced into the glass because the outside is warm but the inner section around the secondary mirror remains cool, PLUS the secondary mirror cell is a heat sink that acts to keep the inner part of the corrector cool - it is all working against you.  Also, a cold spot on the optics becomes a focus for condensation when the scope is packed up for the night.

 

What you will find if you introduce a heat source to the fan is not so much a heating of the corrector, but of the dewshield itself.  This I found when I dried off the corrector with a hair dryer.  The small heat mass of the dewshield and its black inside surface means it will absorb a lot of heat quickly, and when you look through your scope immediately after using the hair dryer is the thermal effects caused by the warm dewshield and you then need to wait for these to dissipate.  If a small fan is not pushing enough air, then try using a larger fan.

 

One thing I do not claim is that the use of fans is a panacea.  When I mention professional observatories, the location of these facilities is NOT just their remoteness from city lights, but many other environmental factors are also taken into consideration in the studies undertaken in the site selection process.  If you are in an environment that routinely has high humidity, you will be doing the proverbial up the creek...  To this end, professional observatories are not designed or located according to an astronomer seeing a mountain peak and saying "yup, that'll do".  Instead the entire design, build and site selection process is done by teams of engineers, technicians opticians, geographers, meteorologists, all of whom bring their own niche specialities.  While the microclimate around professional observatories is especially sought and we all too often do not have access to such environmental conditions and must work with what we have, the principles they employ is what we can take away.

 

If you have your scope in an observatory, then a fan in the dewshield is not necessarily how you should employ a fan.  Rather employ the trick professional observatories do of moving air all around the inside of the structure.  If air is moving, condensation/dewing is far less likely to happen on those surfaces.

 

High humidity environments present many challenges for telescopes.  I for one chose to avoid those nights of extreme dew because of the paper I use with my sketching (for which I have created a new sketch pad gizmo but have not had the opportunity to do proof of concept - bloody frustrating too), and that I just won't subject my gear to such conditions and the problems that come with trying to dry out soaked gear, namely watermarked optics and mould.  However in my development of this system of dew control I did seek out these heavy dew nights, and it works.  A 40mm fan may not be enough under some really extreme dew conditions, to which two options are available to me: 1, use a larger fan; 2, pack up.  And only you can decide upon pushing on under poor conditions or not.

 

Alex.

[Steve C.]

This is interesting. I never thought about this, but it makes sense. I only see dew on still nights, never on breezy ones. Something to try out.

[Migwan]

To start I see up to 8 diffraction rings through my Santel Mak, and none of these are disrupted when the fan is going if this is what you mean.  There is no thermal differential so there will be nothing to see with the diffraction rings.

  That's what I wanted to hear.

[kathyastro]

Newtonian owners have used this method for years.  It works.

 

The objective in dew prevention is to keep the optics from falling below the ambient air temperature to the dewpoint.  By moving ambient air over the glass, you are using the air itself as a source of heat to keep the glass at or close to ambient temperature.  If the temperature-dewpoint spread is very small (i.e. the air is almost saturated), it will be difficult to keep the glass above the dewpoint temperature and dew will likely result.  But if the spread is more than a couple of degrees, the glass can extract enough heat from the air to keep up with heat loss by radiation, especially if the dew shield is long and well-insulated.

[Alrakis]

I am guessing this would remove one of the major impediments to large maks; the cool down of the corrector. Wonder what you would need for a substantially larger (10 to 12+ inch) mak cass.

 

Chris 

[maroubra_boy]

Chris, this is not a technique to cool the corrector of a Mak.  There is no need to cool the corrector either.

 

The use of the fan is optimal when you insulate the SCT or Mak.  It is a two tier dew control system as a spin-off from the insulation of SCT's and Maks.

 

The insulation is to slow the cooling of the OTA which eliminates the internal heat plume - if the tube is prevented from cooling too fast then there is no heat differential between the primary mirror & baffle and the tube, and hence no heat plume.  I am able to rip 400X from my 9" and 10" Maks immediately after setting up with no cooling period and no heat plume.  Luxo ii also insulates his 10" Mak and needs no cooling period wait.

 

The corrector is part of this insulation system which in turn helps to extend the dew-free period of the corrector.  Depending on the individual night, this may be enough to keep the corrector dew-free all night, but when dew does begin to form then this is when you turn on the fan.  We are not looking to force the cooling of anything.  The fan works because the corrector is reached ambient temperature and there will be no heat differential impacting on the corrector from the fan.

 

If you notice on the photos of my 9" Mak in the first post, the dewshield wraps around the whole of the OTA.  I've been insulating my SCT's and Maks for several years.  The fan is now an added bonus following on from using fans in a similar way with my Newts 

 

Alex.

[smasraum]

Chris, this is not a technique to cool the corrector of a Mak.  There is no need to cool the corrector either.

 

The use of the fan is optimal when you insulate the SCT or Mak.  It is a two tier dew control system as a spin-off from the insulation of SCT's and Maks.

 

The insulation is to slow the cooling of the OTA which eliminates the internal heat plume - if the tube is prevented from cooling too fast then there is no heat differential between the primary mirror & baffle and the tube, and hence no heat plume.  I am able to rip 400X from my 9" and 10" Maks immediately after setting up with no cooling period and no heat plume.  Luxo ii also insulates his 10" Mak and needs no cooling period wait.

 

The corrector is part of this insulation system which in turn helps to extend the dew-free period of the corrector.  Depending on the individual night, this may be enough to keep the corrector dew-free all night, but when dew does begin to form then this is when you turn on the fan.  We are not looking to force the cooling of anything.  The fan works because the corrector is reached ambient temperature and there will be no heat differential impacting on the corrector from the fan.

 

If you notice on the photos of my 9" Mak in the first post, the dewshield wraps around the whole of the OTA.  I've been insulating my SCT's and Maks for several years.  The fan is now an added bonus following on from using fans in a similar way with my Newts 

 

Alex.

I'm very interested in this method. 

 

When you say you insulate the entire OTA (I see the pics) you mention coreflute which is, I think just corrugated plastic.  Is there some other insulator between the OTA and the coreflute or is the coreflute adequate?  Did you have to score the outside to let the coreflute bend in a nice curve?  Did you paint or using flocking at the end to reduce light at the dew shield end?

I'm trying to get a solid idea of what exactly you're doing (besides the fan part).

 

Thanks

[maroubra_boy]

The insulation of SCT's and Maks is quite well discussed here on CN.  There are any number of materials that are being used, and there are only two principles that need to be followed:

 

1)  It cannot have black or other dark colour on its exterior - black cools to below ambient temperature and will work against the insulation properties of the wrap.  Black is the very worst colour that any telescope can be painted because of its thermal properties.  You will find black painted OTAs will dew over long before white tubes, and even form frost while white OTAs remain wet if the ambient temperature is close to 0°C.

 

2)  It needs to wrap around at least 2/3 of the OTA, preferably the whole length, and extend to become the dewshield.

 

The back end of the OTA does not need to be wrapped, but nothing to stop you from doing it.

 

Many people use "reflectix".  I have used Coreflute and old yoga mats

 

The inside of the dewshield is just stick-on flocking.  Coreflute is difficult stuff to have paint adhere to it, and with the flexing of the wrap on and off the OTA the paint will begin to flake off.  After I apply the flocking I always take to it with a vacuum cleaner with a brush attachment to clean up any loose particles from it.  The old yoga mat I just painted the black onto it.  No matter what material you have you will also find that the insulation and its light colour will also form less dew on it than the naked OTA, and protect the OTA from dew.

 

The first pic below shows the yoga mat wrap around an orange tube C8.  The second pic shows how each second cell of the Coreflute has be cut so that it easily wraps around the OTA.  The third pic shows the inside surface of one of the Coreflute wraps and how slots and holes have been cut into it to accommodate the various appendages that come off the OTA.  The last pic is of my mate's 180 Skywatcher Mak and how he's used a Reflectix type of material to wrap the OTA + the rear cell.

 

There are any number of threads here on CN on insulating STC's and Maks.  Others may chip in here with their own pics of their insulated SCTs and Maks.

 

NOTE:  many people just have a dewshield that hangs off the front of their OTA.  This is nowhere near the same thing as it leaves the body of the OTA totally exposed and subjected to the ravages of dew seeping into its nooks and crannies.  These provide no thermal insulation and the dew-free period of the corrector is token compared to that of an insulated OTA.

 

Alex.

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Edited by maroubra_boy, 03 February 2021 - 04:48 PM.

[maroubra_boy]

Newtonian owners have used this method for years.  It works.

 

The objective in dew prevention is to keep the optics from falling below the ambient air temperature to the dewpoint.  By moving ambient air over the glass, you are using the air itself as a source of heat to keep the glass at or close to ambient temperature.

 

Thanks Kathy

 

However, the movement of air is not a heat source.  It is actually the evaporative capability of moving air that keeps things dry.

 

One may have noticed during an astro session that the windward side your car is dry yet the lee side is wet (depending on the wind access around your car).

This is the very effect that is keeping not only your Newts & open cassegrains AND professional observatories dry.

 

Don't think of it as heat.  There is no heat involved here.  It is entirely the evaporative capability of moving air.

 

This technique of using moving air is not my invention.  Only thing I've done is work out how it can be utilized with SCTs, Maks and refractors.

 

Alex.

[Tyson M]

It's curious how you don't induce bad seeing in front of your scope via this method. If it works as well as you say it does, it could be a another boon to cat designs....much of the same way reflectix has been.

[maroubra_boy]

Tyson, read post No 11 to answer your question. 

[luxo II]

Tyson I can confirm it is effective here, and a nice part is that the power to run the fan is negligible - far far less than a heater strap. What I don’t know is how effective it would be in the extremes of a northern US or Canadian winter at say -10...-20C.

I had reservations about the fan causing vibration viable at high power but this is not an issue esp with maglev fans. Also a key aspect is that a vortex rotating inside the dewcap is most effective - it’s not just a matter of an air current blowing on the corrector.

The same principle is used in agriculture to protect high-value crops from frost. In cold-climate vineyards in New Zealand and Australia it’s not uncommon to see huge electric fans used to move cold air at night to prevent frost ruining the grapes - in some cases a helicopter is used.

Edited by luxo II, 04 February 2021 - 11:47 PM.