Dew Solutions for Newtonians

Hi Alan
​​​​​​In my case the manufacturer Orion Optics installed their own dew heating system for primary mirror sandwiched in with the carbon fiber tube on my Newtonian. The secondary mirror has a Kendrick dew heater installed. Kendrick is a Canadian company supplying dew heating solutions and I think they offer primary mirror dew heaters. Check it out and good luck
https://www.kendrickastro.com/

My primary also fogs up sometimes in the early hours of the morning (after the session finishes) when it just starts to warm up and any dew on the OTA starts evaporating. Not something I worry about as it soon clears.
More broadly, I prevent dew with a dew strap at the secondary end of the OTA with a home-made dew shield wrapped around it (just a cut of thin foam camping mat with velcro for attaching). I also have a dew strap on the OTA near the primary end but I only turn it on if the dew is forecast to be bad. I find minimizing dew on the OTA goes a long way to preventing dew forming on the mirrors.

I made myself a dew shield out of thick construction foil (it's used for windows on construction sites). The material is stiff enough and I put some velour in it.
On the back site on my 10" I made a simple "cap" out of cardboard that works as dew cap and kind of a protection for the screws and stuff if I need to stand the scope up on the ground.

Works well well for me here in the misty Spessart-Forest

All the best
Mike

Alan Brunelle:
Hi All,

I am new to imaging with a Newtonian and upon my first attempt I noticed dew forming on the primary and secondary during the morning.  This was slight and my session had ended at the time.  My other scope, RASA 11 is basically sealed and I do not have a dew heater for the primary.  The dew "catcher" on that scope is the corrector plate and I have a heater mounted at the front.  I have never had dew as an issue with such a setup.  However, the corrector is in contact with the metal OTA and corrector mount, so I expect that the heat communication is reasonably efficient between the metal and glass.

No such situation exists with my Newtonian (TS, 12" UNC, Carbon Tube).  The primary is well isolated from the OTA and has minimal contact points with the mirror cell.  This includes the retention clips and the floating mirror supports.  In such a situation, how do Newton users here successfully deal with the issue?  I feel that simply wrapping the mirror cell OTA rim with a standard dew strap will generally just heat the periphery of the cell and generate tube currents, given the open nature of the OTA.  Are there heater strips or pads that can be stuck onto the back of the mirror and are gentle enough to not warp the mirror during use?  Also solutions for the secondary?

In my current situation, the dew and fog appear during the morning and after first light starts, so I am thinking that I may just wrap the tube near near the base and not heat during imaging, but only after the scope is parked horizontally.  I could then turn the heater on fairly agressively and hope that this keeps the OTA and air within the OTA fairly above the dew point, and not worry about imaging efffects at that point.

Thanks,
Alan

Hello Alan,
I use pretty much all newts for my imaging with the exception of a wider field setup here in Michigan. There are several methods to removing or prevention of dew with these scopes. I if you ad a shorter extension/dew shield to the front of the OTA that will SIGNIFICANTLY help with the secondary mirrors dew issue! However you don’t need to use like a full length one that you buy for SCT’s. I made mine from Kydex material and I only made it so there was only a 10-12” extension. Made a huge difference! See the pix below.

For the secondary mirror, I’ve used several different dew heaters but I’ve found the one that works best for me is at this link: https://www.kendrickastro.com/newtonian.html
all the way at the bottom of the page model: 2028-NXT-2 / I have used the ones that are mounted on the back of the secondary mirror (the flat split elliptical units) but if you don’t have the correct size mirror they can be a real pain to deal with. Then there is the unit that mounts on the back of the mirror under the secondary holder. I’ve found with those units the work well for visual but not so well for imaging because a visual scope generally will have a smaller secondary mirror. These heaters are smaller so they fit between the mirror and secondary holder. They are the Astro Systems units: https://www.astrosystems.biz/dewgrd.htm
I’ve also used a traditional eyepiece dew heater and that does work well but I didn’t like the extra thick cord that you have to electrical tape across the edge of the spider. In my case I opted to use the cooper tape for the conductivity and soldered wire to it and used an RCA jack to connect power to the unit. This eliminates adding the added wire thickness to the 1 spider vane.

Another thing that I do is insulate the OTA with reflectix. (See pix) While I’m not a 100% sure that it really does much for dew, I think in its own way it at least slows it down. Its mainly for slowing down the cooling/heating of the metal tube in my case so focus doesn’t change so fast. Any way its just another thing I do is all.

All this is just been my experience an when I had DBA Astronomy Products I used these secondary dew heaters on my Certified line of scopes.

Hope this helps!

Dale

Alan Brunelle:
Thanks All for your help!

I will likely go with the flow and see what my local environment demands of my setup.  I know that I do not suffer from Florida-like dew issues, so I hope not to have to solve such a problem.  From what I gather, the secondary appears to be the primary issue with regard to dew.  Followed closely by the primary.  I may wrap the scope like Dale does (and others that I have seen on this site).  But for the secondary, I am leaning toward one of the Kendrick solutions.  I will have to measure my secondaries to see what solution is best.  Yes, I have several secondaries.  One is very large and will only be employed if I should try to image with a full frame camera.  In such case, I would need to modify the tube to replace the 2 inch focuser, so that is unlikely to be an issue anytime soon.  So that leaves me with 2 secondaries.  I will see if the split heaters will fit, if not, I will go with a strip heaters.  I like the copper film leads to power them, and since the secondary heaters are essentially permanent, I will fashion detachable leads to the copper strips so that when I swap the secondaries I can plug them into the power.  Not sure I will drill my tube for the pass-through power socket to my Pegasus Power controller. 

Switching to issues that may arise with the primary, I will see if there are other solutions, other than heating that will solve or avoid the need.  If I find I need too, I will figure something out.  I am certain that I will not employ the Kendrick, full back heater pad for the primary!  Those are huge and seem to eat a great deal of power.  If I have room on the primary between the cell elements, I may stick some pads onto the back of the primary and daisey chain them together.  In that case, I would not intend to heat during imaging for fear of creating "zones" on the mirror, but I might pulse prior to imaging and let the heat spread out.  But most likely, since my dewing seems to occur after my imaging session ends (and the sun comes up and heats the air), I would likely heat only at that time.

Thanks all!
Alan

I’ve been luck in the sense that I have never had dew on my primary for imaging anyway. Personally if you get a dew heater and a tube extension/shield I think that will help you significantly. Those are the easiest to work with I’d think. 

Best of luck!

Dale

John Tucker:
The best solution I've found is a fan, at least if you have access to AC power.

One of my first nights out with my newt was at the Texas Astronomy Society dark site in southern OK.  It had rained for several days and the ground was noticeably damp.  I went into the observatory and noticed they had some big industrial grade fans in there, and set one up about 10 feet from my telescope to reproduce the effect of a 5 mph wind.  Lots of people gave up that night but I had no problems at all. 

Dew is a huge problem here in FL.  My first night shooting in Florida, the dew began to settle on things just 30 minutes after sunset.  An hour later the clothes I had left on a picnic table were too wet to change into.  After about 4 hours there was condensation on the primary and the secondary, not a thin mist but to where you could tip the tube and have some drops of water come out. 

I use the same trick here.  I went to Home Depot and bought an outdoor-rated fan for about $60.  Not one of the mega huge ones, but maybe 20 inches in diameter.  As before I set it about 10 feet from the scope and set it to create a roughly 5 mph wind.  Since the air speed is constant, it doesn't cause problems - my guiding is typically 0.5 arcsec.

I tried using a fan last night and it really worked well!!!  Autumn in upstate NY can be very damp and the previous night I checked my gear around 2am and the everything was soaking wet including my secondary mirror. Last night I set the fan up and had no issues at all. On nights where the temperature is going to approach the dew point I will definitely be setting up the fan from now on. 
This is a very simple solution to an issue all of us Newt users have had at one time or another. 
Thanks for the great tip John!!!

First of all, thanks for all the experiences and solutions offered on this forum question.  I wanted to follow up with which solutions that I have adopted.

1. Until such time as I get the rig into an observatory (which I desperately need), I will forego an extended dew shield.  Such an addition offers a number of good reasons, even beyond the issue of dewing.

2.  Secondary:  Given the diy solutions found on CN and/or the commercial solutions, I have instead opted to use the heaters that I found on-line commercially, and at a very reduced price point.  Starting with the secondary, I could not be sure that I could fit the Kendrick split ring item to my secondary.  And there is no way I was going to pry off the secondary from its mount to put a full disk heater between the two.  The strip heaters I could find on-line for a few bucks seemed not that different than the Kendrick model, so I went for the non-astro model.  Here are some photos of the heater and the wiring via the spider using copper tape as suggested and shown above by @Dale Penkala.
Note the strip heater runs down along the aluminum secondary mount and maybe just 1 cm onto the actual secondary.  Most of the heat flows into the aluminum.  The electrical connetions are via spade connectors.  I need the ability to disconnect these so that I can change out the secondaries that I have for different imaging demands.  The heater comes with an adhesive so the fabric tape is for light and insulation.  I'll probably add a bit of tape to the other side of the secondary mount rib to balance heat flow and temperature across the full back of the secondary and mount.  A bit of black "Shoe Goo" anchors the wiring near the connective end.  Shoe Goo is extremely tough stuff.  


You can see the copper strips used to connect to power.  They are painted black.  Very low profile (see next photo).  A lot of copper tape is the 1 mil variety.  Here, I found 3 mil tape, which I thought would be a better conductor and easier to solder.  Tape is used to anchor, along with black Shoe Goo.  Also, I added a barrel connector through the wall of the OTA.  Yes I drilled my carbon tube!  But I wanted a solid connection and in a location that did not interfere with the dust cap.  Also, if I add a dew shield, I wanted the connection to be solid and in a consistent location.  And no, none of the visible connections fall within the light path of this OTA, which itself offers a generous amount of space on the margins.


A not quite face on view, showing the low profile nature of the copper tape setup.  My only concern is that the sellers of the copper tape claim that the adhesive is highly conductive.  (I actually doubt that after testing with my meter.)  So I was concerned that I might get a short between the two strips of tape and the metal in the spider.  The black paint on the spider is sufficiently insulating.  Lets hope is stays that way...

My concerns about the strip heater that I used is that its wattage is 10 watts at 12 V.  This is a 3X more than the commercial strip heater I was looking at.  However, upon use and setting the aggressiveness during use to a low setting, the heater works fine.  Getting 4 heaters for $15 was also very very painless.  Time will tell if these are durable.  They are claimed to be sealed against the environment and indeed appear to be so.  The resistive conductors appear to be thin foil, rather than NiChrome wire.  It is quite flexible and can be closely pressed onto the changing surfaces of the secondary and mount.  I actually used the other 3 strip heaters as temporary solutions on my primary (see below).

3.  Primary heaters.  Unlike the experiences mostly seen on this forum, my primary has been dewing up over the last month or so.  So I have no choice but to heat my primary.  Since my rig is open air, a fan seems illogical and I will not extend my scope until such time as it comes "indoors" with an observatory. 

Because the heaters I planned to use for my primary were on a slow boat from China, I went ahead and temporarily mounted the remaining three strip headers that I got for the secondary to allow me to image for a few of the last clear nights here in Oregon.

For this, I did not want to remove the adhesive protective backing and just taped them directly on to the primary in symetrical locations.  I also learned that using such heaters demands that they be wired in parallel.  Doing so in series would effectively raise the resistance seen at the load and force the wattage output to extremely low max values.  The heaters that I planned to use are now delivered and I will pull these strip heaters off and save them in case I need to use them for other projects.  BTW, they worked spectacularly well at very low power draws from my PowerBox.  Under very dewy conditions (every other surface ended up dripping wet), the primary stayed dry.  

Here is one of the 4 heaters that I received this week.  It cannot be seen in this photo, but the resistive conductor in this heater is clearly a wire, not a foil strip.  I will assume it is NiChrome.  This heater also is "sealed" in a much thicker silicone/fabric sandwich than the strip heaters.  This heater is lower power than the strip heaters (7.5W spec, but likely under 7W as measured directly with the meter) and I plan on using 4 of these.  3 symetrically a few cm inside the perimeter of the primary and one directly in the center.  I will attach these with a thin layer of silicone adhesive to ensure full coverage of the heater to primary.  I will likely add a bit of silicone to the wire entries as I am not sure the there is a good seal into the wiring solder joints.  This will provide 30 watts/hour of power, which happens to be roughly the power that the commercial heater for this size mirror offers.  I would never expect to use more than a few watts/hour as I have learned from using the strip heaters.  These heaters are relatively thick and the silicone adhesive will add a bit more thickness overall.  I expect that this will slow and diffuse the heat transfer to the primary.  That is fine.  I hope not to have to cover these heaters with tape or other insulation for them to be effective.  Either way, it has to be better than a peripheral heater wrapped around the edge of the primary.  I do worry that if I need agressive heating, that uneven heating might cause a mirror surface artifact that might resemble pinched optics.  But the expectations are that the heaters will not come on until very late in these sessions.  I'll post a final photo of this setup.

Finally, I would like to stress that the total cost of materials, heaters, spade connectors, copper tape, and fabric tape amount to much less than the cost of a single small commercial astrophoto strip heater.  Some of this, the copper tape, fabric tape and spade connectors, I would have bought regardless of which heater I went with.  I do acknowledge that I am accepting a significant level of risk by using these cheap heaters.  Cost was not the only factor.  In fact, the commercial astro heaters did not offer a good or at least desirable solution for heating my 12 inch primary.  And I could have gone full-on diy with NiChrome wire, but I think the cost of buying NiChrome wire, getting it gauged properly and sized properly, assembled safely, etc. would probably not beat the costs that I incurred.

Alan Brunelle:
A final update with the promised photo of the finished assembly for the heating of the primary mirror.  The 4 heaters appear to draw 0.6 Amps at 12.1V, which works out to ~7.25 W.  This was confirmed after assembly and the 4 heaters and under full power setting with the Pegasus Power Box.  So a bit under spec.  A total of 29 W at 12.1 V and a power draw of under 2.5 Amps.  This at full power.  I can feel the heat under these conditions and likely will never run these at more than 20% agressiveness in reality.  As such, the draw of all the heaters, including my guide scope is well within the power I supply to the Power Box, though heating is one of the biggest draws overall.


To repeat, I used 4 heaters.  Three placed symetrically on the perimeter, a couple of cm from the edge of the primary.  The 4th heater is centered under the fan.  There appears to be a lot of wire spaghetti, but I decided to not shorten the cables.  I wanted all the cables to be the same length so that each heater would output the same power.  And it is nice to have the extra cable in case I need to make some repairs in the future.  The heaters are fixed in place with a thin layer of silicone adhesive.  These were pressed down with blocks of wood to ensure an even and thin spread so that heat transfer is consistent from heater to heater.  Silicone adhesive is tough stuff, but I know that if any heater were to fail, it would not be that difficult to remove if need be.  The spade connectors pretty much offer me no real advantage other than the connections are crimped rather than soldered.  If I ever have to replace a heater, I will have to cut these connections and redo it.

Hopefully, this description will be of use to someone.  Also, If anyone has used the type of heaters described in this thread and found them to be wanting, or have failed, please post that here for anyone on the future.

Hello Alan,
Great explanation on your process! I didn’t see the 3mil copper tape when I was looking for it so that’s a big upgrade to my setup. I’m sure it would have been much easier to solder to without melting or damaging the copper strip! Also was not aware of this “sho goo” that you used either! Thanks for that info as well!
I know what you mean about the split elliptical heaters. The wrap around was the one that I settled on for that reason. I machined my own secondary mirror holder and then with the over sized secondary mirror it gave me more then enough room to cover and keep everything behind the mirror wo pertrueding into the light path. Great alternative method you did!
Also I agree with you drilling thru the CF tube! Its a much better strength wise not to mention cleaner look! Just an fyi, I used the right angle phono connection/jack to help keep the profile as low as possible.
I’ll be interested in your primary heater method as I have not run into that happening with my setup, so far.  It will give me an idea to consider in the future if I encounter this. I think the reason I haven’t had this issue is I’m out in the country with corn fields all around me not to mention I’m in my observatory. I know I have had secondary dew issues thus my fix for that, but out here there is always a bit of wind or breeze coming across of the fields so I think this helps with blowing or redirecting that moisture that normally would collect on the primary. Whatever the reason I just haven’t had that problem.

Congrats on your mod here Alan and keep us posted on how it all works for you!

Dale

Alan Brunelle:

Dale Penkala:
Great explanation on your process! I didn’t see the 3mil copper tape when I was looking for it so that’s a big upgrade to my setup. I’m sure it would have been much easier to solder to without melting or damaging the copper strip! Also was not aware of this “sho goo” that you used either! Thanks for that info as well!
I know what you mean about the split elliptical heaters. The wrap around was the one that I settled on for that reason. I machined my own secondary mirror holder and then with the over sized secondary mirror it gave me more then enough room to cover and keep everything behind the mirror wo pertrueding into the light path. Great alternative method you did!
Also I agree with you drilling thru the CF tube! Its a much better strength wise not to mention cleaner look! Just an fyi, I used the right angle phono connection/jack to help keep the profile as low as possible.
I’ll be interested in your primary heater method as I have not run into that happening with my setup, so far.  It will give me an idea to consider in the future if I encounter this. I think the reason I haven’t had this issue is I’m out in the country with corn fields all around me not to mention I’m in my observatory. I know I have had secondary dew issues thus my fix for that, but out here there is always a bit of wind or breeze coming across of the fields so I think this helps with blowing or redirecting that moisture that normally would collect on the primary. Whatever the reason I just haven’t had that problem.

Thanks Dale!  Here is the first phrase of the Amazon description for the 3 mil tape: vootape 3.15 Mil Copper Foil Tape [1/4 inch x 66ft].  $5!  That should get you to the item.  I will assume that I cannot post the link here...  It says that it is double-sided adhesive.  Its not.  Only one side of the tape has adhesive.  If you lived close I could give you a bunch, since I will never use the 66ft!


You can get Shoe Goo on Amazon as well.  It resembles a number of the better rubber adhesives that you can buy at a hardware store.  But you can get it in black.  I actually first used it as a repair for a detached sole in boots that I liked and it worked very well.  While flexible, it is pretty hard.  Sticks well, and retains its flexibility forever.  But mostly I used it because I had it on hand, and unlike white or clear adhesives I had, it was black!

Yes the split heaters from Kendrick are nice, but they basically require a perfect fit.  Something I did not think I had and did not want to deal with waiting for delivery only to just have to send it back.

The strip heater I used actually resembled the Kendrick heater in dimensions and I applied it basically as suggested by Kendrick for their heater.  The resistive conductor in these small strips are foil-like, very thin and sealed in plastic film that is also very thin and adhesive-backed on one side.  I think the Kendrick strip heater is sized similar to the one I used and I applied it as best as I could to how Kendrick shows it in their diagram.


Kendrick supplies a nice neoprene "jacket" to hold the heat in.  I am using the fabric tape for the same reason.  My holder is deeply ribbed.  Not the nice cylinder shown here and typical of so many holders.  So I could not use the wrap method that would have been logical.  

I trust that most of the individuals who posted on this thread were truthful in that they do not suffer from primary fogging.  In fact, I had never seen it until this fall.  But this is the first time I have used an open tube design telescope in the fall around here.  My other scope is closed, and I had heaters set up for that from day one.  But I got fogging once we started getting the cool mornings and atmospheric fog, which started in September.  What bothered me was that we were in a drought, typical for this area of the Pacific Northwest.  But we have had a good deal of wildfire smoke in the area and my mirror had picked up a light coating of smoke during my sessions.  I believe that these particulates act as nucleation sites for fogging.  What scared me is that these particulates can release acids upon contact with water.  So it was a good time to give the primary a rinse with distilled water and then alcohol while I had the primary cell off of the OTA for the application of the heaters and wiring.

BTW, thanks for the suggestion for getting a right angle for that secondary power cord.  I am sure this will not be the last improvement!  Given the fact that the rainy weather has settled in here, I will have many months to work on getting this rig refined.  Kind of like the person who works on high end cars, never to have the rubber meet the pavement...

Alan

Nicely done Alan and I’m sure it will work great for you! I’m sure this thread will help others with the same problem.

Dale

Alan Brunelle:
Sorry to resurrect this old thread, but I thought it my duty to report on any issues that I have seen on my setup in case someone adopts what I have done.

Mostly I should state that I am please with how things are working.  But, I am still getting used to how to use it with the controller, which in my case is the Pegasus Astro power box.

I have had a stretch of clear weather this week, so finally got this telescope out for a few nights of imaging.  Most nights have failed due to high winds, but at least I tried.

In the case of the primary heaters, I note that when the heaters are turned on, quickly I find that the out of focus star images change into the shape of a trefoil.  That is three brighter areas equally spaced around what should be the out-of-focus donut that is normally seen.  Because it has been around freezing here, the heater power, when set to auto, has been pretty constant.  This not counting the cycling of the pulse generator.  I have also set the Power Box controller at the lowest agressive setting of one.  This certainly has kept the dew at bay.  I believe that the exposed nature of my setup means that heat migrating from the heater pads outward to the exposed areas means that this forms a heat gradient, and therefore stress gradient between the exposed areas and areas under the pads.  And this does not change during the night.  The in-focus star image appears to align the three spots of the trefoil and therefore seems to resolve into a minor issue, but I still am not happy with that occurring.  So I have gone to manually setting the power levels to lower levels and stopped the cycling.  I use settings under 10% for the primary and just a few percent for the secondary.  With that, I can eliminate the trefoils and do not see any dew.  So in this case, I think that pulsed heating is not ideal. 

I am not sure that the heater on the secondary causes any issues.  But I expect that the heat would in theory cause a minor expansion of the aluminum secondary holder which could cause slight changes in the focus.  This is dealt with if one uses autofocus and hfr assessment to trigger refocusing.  I am not sure that pulsed heating hurts here, but steady power to the heaters, at a lower setting should be better.  But the insulation over my heater that I have done with multiple layers of cloth tape (or the foam that is supplied with some kits or used by others), likely reduces any real temperature fluctuations with shortly spaced pulses.

I can only imagine that any heating setup for a Newtonian primary will have some heat-induced issues optically.  For that reason, solutions, such as the Kendrick heating pad that covers the full back of the primary may avoid such issues.  But I was not about the remove my primary from its cell, stick the rubber heater on the back and try to fit it back into its cell.  I am not even sure there is room for that and not sure how stable the primary mirror would be without the floating supports in contact with the glass of the primary.  Alternatively, heating just the periphery of the mirror offers challenges because of interference from mirror clips and the fact the heating, isolated to the periphery of the mirror, should also cause figure changes, albeit different that what I see.  In that case, it is also a long way for the heat to travel to get to the center of a larger mirror.  

I think I have the issue solved by using the lowered settings on my power controller.  However, not wanting to just let things be (which would be the smart thing to do!), I will likely buy a thin sheet dense foam that I can cut with scissors and cover the back of my primary in segments that cover the heaters and most of the exposed areas of the primary without interfering with the floating mirror cell suspension.  I am hoping that this allows the heat that enters the glass at the location of the heaters can more freely migrate horizontally out from the heaters and not so easily exit the exposed primary back around the heaters (see figure above).  Given the Pacific NW weather here, I will have a good number of months to work that mod out.  I am not ready to close off the whole back of the OTA for fear that this might cause other issues with imbalanced temperature within the optical tube itself.  Carbon fiber tubes may well be less reactive to temperature changes, but I can clearly see that the effects are not zero, and I do not want that to become a secondary problem.

Thanks,
Alan

Good update Alan,
I’ve always kept my secondary dew heater set at the most minimum setting as I can with my controller. I’ve always figured the less the heat the better.
You have dove into something I’ve never had to do or honestly want to do with primary mirror issues. While I do have times here in Michigan with very humid nights (especially in the summer months) I have not found my primary to ever really have dew. Its always been my secondary which as I stated I’ve controlled that with my setup now and works great.
Keep us updated on your primary dew controlling ideas as you work that situation out!

Dale

Does anyone have an actual example of a case in which they experienced dew problems in spite of using a large fan?  I live in Florida where the dew typically starts just after sunset and can mimic a few tenths of an inch of rain, in terms of the water deposited on the ground.  So far it always works for me.

Not trying to be dogmatic here, but its a simple solution that doesn't involve applying heat to your mirror.  Of course it doesn't get real cold here and isn't a great approach if you don't have access to AC power.