Heat the lens

RJN · Post by **RJN** » Thu Aug 19, 2004 2:18 pm

Sometimes CONCAM images show dew or water on them. Here is a recent image from MK: http://nightskylive.net/mk/mk040819/mk0 ... 35549p.jpg

Several of us (including Noah Brosch at WO, Andre Phillips at SD, John Oliver at RH, and Dave Cook here at MTU) have been thinking of ways to minimize this. As dew images make up less than 10 percent of images, it is not absolutely critical, but it would be good.

Andre has championed putting a dome over CONCAMs and has implemented this at SD. See: http://www.phys.unsw.edu.au/~map/weathe ... .html#dome

CI has also implemented a dome. But the domes can act like cold fingers themselves, and may themselves dew up.

WO is the only observatory to actually heat their lens. Now they have a CONCAM2 and so far as I can tell, that has been completely effective. I therefore think that is the way to go for future CONCAM3s, etc. in the future: heating the lens. A lens warmer than the surrounding air will cause water anywhere on or in the lens to evaporate relatively quickly, usually, I would guess, in much less than a day.

Perhaps the WO crew can respond with details of how they heated their CONCAM2 lens, and we can then work out how to retrofit existing CONCAM3s for this and future CONCAM3s, CONCAM4s ,etc. We expect to use a new higher throughput lens for the next CONCAM3 and I will advocate working a small resistive heater into the implementation.

Thoughts?

- RJN

nbrosch · Post by **nbrosch** » Fri Aug 20, 2004 4:02 am

We experience quite often high humidity in the second halfs of nights, which can reach 100% and dewing on suraces. The result is a limitation in the observing time for our telescopes and dewing of the CONCAM lens.

This kind of dewing is well-known by amateur astronomers. They use a dew remover, which is a hot air blower (hair drier) to evaporate dew from the front lens of catadioptric telescopes, and a dew preventer. The latter is a long sleeve mounted before the telescope that creates a micro-climate in front of the lens where the dewing conditions are slightly different from those external to it. Sometimes, this sleeve is warmed electrically; tis forther prevents dewing because it raises the air temperature next to the lens above the dew point.

This is the approach we employed to prevent dewing on th elens of our CONCAM. We fabricated an aluminum ring that just fits around the lens. The ring has a notch in its central point where a series chain of resistors has been fitted. The resistor chain has been potted with RTV to prevent humidity from getting inside. The ends are connected to a low-voltage power supply in the CONCAM box and when this is ON the aluminum ring heats up to a temperature that is perceptibly hot but not burning your hand! This apparently is sufficient to keep dew from forming on the lens.

I am writing this from home and do not have with me details about the resistor value and the number of resistors in the chain. I'll try to post these next week.

Noah Brosch

RJN · Post by **RJN** » Sat Sep 04, 2004 1:52 pm

There are several "anti-dewing" systems that have a "flexible heating element (usually a tape)" that might be the simplist off-the-shelf method for heating the lens to avoid dew. One is made by BC&F Engineering and their web page is here: http://www.hfo.org.uk/Anti%20Dew/antidewingsystem.html

It seems that this system costs under $200., it can be adjusted in temperature, and there is a version that can plug right into the power strip inside the CONCAM box. The heating tape can just be wrapped around the outside of the lens assembly. If it can be also attached to the day-night switch, then it will not generate unwanted heat during the day, either. Possibly we will try to build one into the next CONCAM3.1 that we are making here at MTU.

- RJN

Mwaterson · Post by **Mwaterson** » Tue Nov 16, 2004 5:31 pm

I have used several flexible "heater tape" devices from Minco to stabilize various flat things such as lens mounts. They can also be made to any shape/size/resistance (for a price), and I can imagine an easy layout for a custom "comcam heater" that could be laminated to the aluminum lens mounting plate with PSA tape. Controling it can be as simple as an LM335 sensor, opamp, and FET, or you can buy a fancy PID controller for a few hundred$$. The critical thing is probably to insulate the top of the plate so that it does not radiate to the night sky, and I am planning to try doing only that this week (with a temperature logger inside to see how the computer/camea environment fares!) Based on other experience at the observatory, if the plate is insulated it should only take 10-20W to keep it warm under the conditions we see (sites which have colder conditions will need more power). I'll see if I can post some temperature plots in a week or so.

-mfw

RJN · Post by **RJN** » Tue Nov 16, 2004 9:15 pm

That sounds good. Here is another solution. We have a computer and camera that are too hot, and a lens that is too cold. Put them in thermal contact! Here's how:

Take some inexpensive accordian-like plastic tubing and connect one end to the exhaust fans of the computer and camera. Channel the other end of the tubing to blow the warm air onto the lens (the part inside the box). This is the same method my car uses to get dew off the windshield -- it effectively takes excess heat from my car engine and blows it onto the windshield glass. The warm glass then causes any frost to melt and the dew to evaporate.

I think the tubing, once found, will cost less than $10. Thoughts?

- RJN

RJN · Post by **RJN** » Wed Nov 17, 2004 11:39 pm

After some more thinking, speaking with Lior, and a brief experiment, here are some more thoughts. It appears that the SBIG cameras, at least an older ST-7 model that was nearby, has a fan that primarily blows air INTO the camera, not warm air out of the camera. My first thought on learning this was that it undermined any ability to move warm air from the camera to the lens. But since there are no other major air vents on the camera, Lior points out that air would likely move in both directions in a connected tube, and is therefor sure to be warmer than ambient air in the box or outside.

It is not clear which direction the PC fans go. The CONCAM boxes themselves have very large fans that blow air into the box.

But here is a similar idea. Just put in a small PC-type fan that blows air already inside the box onto the lens. All of the electronics ensures that this air will be warmer than the outside air and hence help make the lens warmer as well. Also the blowing air should increase evaporation from the lens, which is what we want anyway. The small fan can be glued somewhere in the box to point at the lens, possible someplace that is away from the big box fan and appears warm to the touch. There should be room on the power strip to plug this fan in.

- RJN

Mwaterson · Post by **Mwaterson** » Thu Nov 18, 2004 1:53 am

Some investigation here revealed that the dew is forming inside the lens, beneath the first element - ie it isn't as waterproof as might have been thought. The inside of the box is warm enough that any trapped moisture is sure to migrate to the lens, so some effort to circulate air & warm it inside sounds like a good thing.
The fans on the PC also appear to blow in, allowing air to escape through a distributed set of slots, so not so useful either. I agree that a fan near the lens would at least help.
I put temperature logger inside today, with the sensor wedged up by the lens/cover joint as high as I could get it, to see what the thermal profile is for a few nights. Hopefully we'll get an idea how much colder the lens is getting.

There appears to be a significant amount of moisture inside the lens (both condensation & droplets, the central layer is ~ the size of a dime, so I think I'm going to need some direction as to how to dry it out....

mfw

RJN · Post by **RJN** » Thu Nov 18, 2004 3:06 am

I think I'm going to need some direction as to how to dry it out....

Unfortunately, past what is being discussed in the HL Camera Status thread, I don't know of any quick ways to dry to lens. There is no easy way to remove the lens for drying. - RJN

solarblast · Post by **solarblast** » Tue Mar 08, 2005 4:16 pm

I'm only mildly acquainted with your setup, but for our Sentinel Fireball Network, we use the equipment I show on my web site at <http://home.earthlink.net/~mtnv95959a/m ... nings.html>. A resistor and thermostat is inside the cylinder below the dome area. We never have dew problems on the inside, at least at my site, but I do have dew problems on the outside of the dome.

I dropped in on your site to see if someone might have a solution to the dewing of a dome such as used at my site.

nbrosch · Post by **nbrosch** » Tue Mar 08, 2005 4:49 pm

solarblast wrote:I'm only mildly acquainted with your setup, but for our Sentinel Fireball Network, we use the equipment I show on my web site at <http://home.earthlink.net/~mtnv95959a/m ... nings.html>. A resistor and thermostat is inside the cylinder below the dome area. We never have dew problems on the inside, at least at my site, but I do have dew problems on the outside of the dome.

I dropped in on your site to see if someone might have a solution to the dewing of a dome such as used at my site.

Looking at your web site, it seems to me that you are using the "display dome" of Edmund Scientific. These are made of transparent acrylic material. We considered those, but were told that they become opaque due to de-polimerizing of the acrylic material by the solar UV, and they get scratches and crazing from dust scouring and from the heating/cooling cycles. How long have you operated with these domes? Have you seen any deterioration?

Also, these domes are manufactured for shop displays, thus do not boast of a high optical quality. Has the person who did the triangulation had any computational difficulties becuase of unaccounted distortions?

Thanks you for your attention,
Noah Brosch

Guest · Post by **Guest** » Thu Apr 21, 2005 8:47 pm

nbrosch I agree with your words.

craterchains · Post by **craterchains** » Thu Aug 04, 2005 4:09 am

Might you try infrared heat directed at the dome, and or lenses? Or, would it cause interference with the CCD?

It heats objects, not the air.
Norval

nbrosch · Post by **nbrosch** » Thu Aug 04, 2005 5:42 am

[quote="craterchains"]Might you try infrared heat directed at the dome, and or lenses? Or, would it cause interference with the CCD?

It heats objects, not the air.
Norval[/quote]

The dome material is transparent to near IR thus it would not heat. A small fraction would be reflected by the dome internal surface. As this is hemi-spherical, there is a chance the reflection would be concentrated (not focused) close to the center. This, normally, is where the CCD normally is. The one thing CCDs hate is heat...

Starship Asterisk*

Heat the lens

Heat the lens

Lens heating for a CONCAM

No Dew Inside This Dome. Outside, Yes.

Re: No Dew Inside This Dome. Outside, Yes.

Heating