SCT & Mak total dew control without heat - a solution!
Hi all,
I've been working on this for a very long time - trying to keep my SCT's and Maks dew free over the course of a night. And one thing I HATE is needing to use heat - we fret so much trying to remove or control heat from our scopes, and then we go and stick heat sources to dry it up. Not exactly logical...
The starting point
As many of you may be aware I've been actively espousing the benefit of insulating our SCT's and Maks instead of waiting for them to cool. By insulating the OTA there is no heat differential created between the cold tube and the warm primary and baffle tube, so no heat plume can be generated as the air inside the OTA stays warm. With no heat plume in the OTA, you can rip high magnification from the scope from the very start, and not need to wait for the scope to reach ambient temperature. And with big SCTs and Maks, this can be several hours.
I've been wrapping my SCT's and Maks for several years now, and I have not waited for any cooling period in all this time. Instead, I set up my scope when I find I have the time (limited as it is), and get to work with sketching straight away. No heat plume, and 300X, 400X as I find best suits seeing conditions.
,
More info on insulating SCT's and Maks can be found in this thread:
This wrap-come-dewshield however won't stop dew from forming on the corrector. It will extend the dew-free period, but dew will still form if conditions are right. While the ideal solution would be to have the scope set up in a dew-free environment (like my dark sky site), most times an ideal location is not possible, and we need to work with dew and try to mitigate it (like my backyard - blooming Dew Central).
The heat-free solution - a FAN
I've been working with fans with my truss dobs also for several years. I've also worked out a way to keep my dobs dew free using fans too. But I've struggled to work out how to be able to implement fans with my SCT and Mak.
The solution came thanks to the material I use for the wraps I make for my scopes now, Coreflute.
Coreflute allowed me to cut a suitable flap in the dewshield section to accommodate a 40mm 12V fan. The flap is important here as it allows the fan to be placed at an angle so that it generates a vortex inside the dewshield. This whirlwind of air helps keep the corrector dry, or if dew has started to form, then to dry off the corrector.
I also connected the fan to a small PWM unit so to vary the rpm's. If dew is light, then I can drop the revs. If dew is heavy, I can increase the revs.
Proof of concept was done over a few nights - one of the benefits of the persistent cloud cover of late is it gave me the time in the field. I had the scope wrapped in two configurations, one as an all of scope wrap, and the other as a partial, more conventional corrector only dewshield (though thermally not an insulating design as the OTA is left exposed). I left the scope pointing up and waited for dew to form - with this 9" Mak, that took between two and four hours on the nights dew did form. With dew beginning for form, I turned on the fan, and PRESTO! The dew dried up and stayed away!
The position of the fan in front of the corrector is important. You don't want to place it too close to the corrector lest a cold spot is generated across the corrector which will put strain on the glass and distort it enough to stuff the image. I place my fan no less than 100mm in front of the corrector.
The choice of 12V was obviously to do with what is normally used with astro gear. I also added a DC power socket to the fan to make the connection to the PWM unit a cinch.
This wrap-dewshield-fan arrangement is an easy enough DIY project. Care needs to be taken when cutting the Coreflute to make the length-long slits and the flap for the fan - it's easy to accidentally make the flap too large for the fan, or make a dog's breakfast in making it too small and needing to widen it. By making this flap for the fan, the flap helps hold the fan in place, and if you don't need the fan the flap closes up the dewshield again.
No heat, no optical distortions, no dew and less power. Blooming brilliant!
Thanks for reading,
Alex.
Last edited by mental4astro; 11-04-2020 at 08:28 AM.
Reason: typo
Really interesting Alex,
I am just wondering if you have tried any other insulating material.
I am thinking in particular of something like the thin aluminium coated foam blanket that is used in house insulation. I think it would have a Higher R factor than the Coreflute though probably not as rigid for holding a fan.
Gary, if you want to use a fan you will need to see how you can accommodate a fan.
One material that has produced poor insulation properties is the aluminized car windshield sun shades. Myself and a couple of my friends have tried it, and Coreflute is superior.
As for which material is "best", it all depends on what you have readily available, what working properties you want from it (if you need to take it off and on, or if the scope is fixed in an obs).
Just don't have the outside of the wrap coloured black. Black cools to a lower temp than ambient at night, working against your insulation properties, and it attracts dew!
Last edited by mental4astro; 11-04-2020 at 12:53 PM.
Reason: Typo
What that article I posted the link to needs is a picture.
Here's what I've done with my solid tube 8" f/4 Newt. The primary cell is custom made. It totally closes off the rear except for a hole in the centre, where a fan draws air OUT of the tube, NOT blow onto the primary.
The article explains the principle and how the airflow works to cool the primary more efficiently and evenly, eliminate tube currents, and keep it dry.
Yeah that's pretty much how my stock gso680 was set up.....
Ah, but yours was blowing onto the back of primary mirror.
That is the problem with the way mass produced OTAs have the fans placed on them. Do you expect the air to go into the OTA that way with the skeletal cell they have? It won't and it can't.
And the fan blowing onto the back of the primary mirror creates a cold spot on the mirror, thereby creating a thermal strain in the glass, and distorting the figure.
What my photo shows, AND how the fan is being used is not what mass produced OTAs have.
Read that article and understand what the photo is showing. It is not all that it seems at first sight.
Alex.
Last edited by mental4astro; 14-04-2020 at 03:38 PM.
Reason: Typo
Ok, if it did suck instead of blow, the problem still remains of the fan drawing air from around the open cell. No air is being drawn from around the front of the primary. This is where the fan is being most efficient at cooling the primary evenly, disrupting tube currents and keeping the mirror dry
Brilliant idea Alexander. I am thinking of doing the same for Skywatcher 180 Maksutov. Did you find one layer of cornflute is enough? Or you put something in between scope and cornflute (when you cut every second line it is only one layer of plastic protecting scope of cooling, right?) Maybe heater strip around primary would help? (back side would be still open).
I guess velcro strip to tighten insulation around scope would be on one side.
Thank you for sharing!
PS: what did you put inside dew shield? It looks like some black foam (yoga mat?)
Cheers
I have two dewshield wraps for the 9" Mak. The shorter one has foam rubber, yes, but on the longer one I have used black felt contact. The foam rubber I find too thick and not black enough.
Don't use any heat! This is the entire purpose of this method of dew control - it is thermally passive. The insulation slows down to cooling process, enough for a heat plume not to develop inside the OTA. Introduce a heat source to the rear of the OTA and you will re-establish a heat plume inside the OTA as the inside of the scope will have cooled down.
One layer of the Coreflute is plenty. The Velcro does keep the Coreflute tightly wrapped around the OTA. Coreflute is just one material that can be used. You can use whatever material that is at hand. The only thing that you need to do without exception is not have the OTA section of the wrap black in colour. Black will cool below ambient temperature, and will work against the very things you are trying to achieve. The pic below shows how I have laid out the black felt inside the wrap, but free from section that goes over the OTA.
Have a read of this following link which talks about the insulation of Maks and SCT's:
Adrian, you cannot freehand cut Coreflute - can't control knife and not safe. A metre long metal ruler, or metre long strip of aluminium. And work out a way that will allow you to line up the ruler to the cell that will be cut, and ONLY that cell. And then move onto the next...
Quote:
Originally Posted by AG Hybrid
Also, just pointing the irony. You created a solution with power because you hated a solution that used power....
Yeah, I can see that I really never hated using power. I just did not like how it has been used by amateurs, with heat. I too started using heat, but one post many years ago by an old IIS friend warned me against it with Newt mirrors. This got me thinking about the whole thing more closely and looking at how the Big Boys do it in the professional observatories, and then work out ways I can apply the same methods. Using fans seems simple at first, but use them incorrectly and you will induce the same thermal strain issues into glass but with cold.
Adrian, you cannot freehand cut Coreflute - can't control knife and not safe. A metre long metal ruler, or metre long strip of aluminium. And work out a way that will allow you to line up the ruler to the cell that will be cut, and ONLY that cell. And then move onto the next...
Yeah, I can see that I really never hated using power. I just did not like how it has been used by amateurs, with heat. I too started using heat, but one post many years ago by an old IIS friend warned me against it with Newt mirrors. This got me thinking about the whole thing more closely and looking at how the Big Boys do it in the professional observatories, and then work out ways I can apply the same methods. Using fans seems simple at first, but use them incorrectly and you will induce the same thermal strain issues into glass but with cold.
I've used a 140mm fan on the back of my 12" for years as a tool for dew prevention as well cooling. I leave it running all night. 4 nights ago I left my 12" out to cooldown with out a fan and the primary had dew on it in only 2 hours. I pulled it out of a relatively warm garage too.
I really never hated using power. I just did not like how it has been used by amateurs, with heat. I too started using heat, but one post many years ago by an old IIS friend warned me against it with Newt mirrors. This got me thinking about the whole thing more closely and looking at how the Big Boys do it in the professional observatories, and then work out ways I can apply the same methods. Using fans seems simple at first, but use them incorrectly and you will induce the same thermal strain issues into glass but with cold.
