Starship Asterisk*

The software for hl-concam should be now fully functional. Opacity maps are expected to be generated in just few days from now, after the software aquires some data.

I have noticed a persistant distorted area in the center of the Haleakala images, which to the untrained eye looks like some water internal to the lens. It has remained nearly the same for some time (week+?) so I don't think it is external dew. I will have a look inside on Wednesday when I plan to be at the site assuming the weather improves, but any thoughts/suggestions on where/how/what to look for would be appreciated...

mfw

It's not just Haleakala which has the distorted area in the middle of the lens. South Africa for example is suffering the same. Have you guys found black holes?

Yes, water has likely gotten into the fisheye lens and settled a minimum somewhere. I think it is likely caused or fueled by condensation in humid weather. Many times these spots evaporate in a day or three but the current HL spot seems to be pretty resilient. I still think that, left alone, it will evaporate within a few days.

The solution, in my opinion, it not a dome but to slightly heat the lens. A lens a few degrees warmer than the air will avoid acting like a cold-finger and cause any condensation that does seep into the lens to evaporate faster. The only part of a warm lens that needs to be waterproof is the very top.

- RJN

I examined the camera today & find a spot of water the size of a dime hanging beneath the upper/outer element. It seems like there is an airspace between the first 2 elements that has become saturated, as when I tilted the cover open, the drop slid only slightly but the rest of the area became misted/fogged. It seems like a major drying-out operation is indicated here - any advice?

This is only going to get worse as we go into winter, so I will try to figure out where the moisture is coming from (whether inside from humidity drawn in by the ventilation fan or outside from leakage)...

mfw

It seems like a major drying-out operation is indicated here - any advice?

mfw,

That is the most detailed description of where the water in a CONCAM3 lens I've ever heard! Interesting. There has been some debate as to where lens water comes from in general and I believe the concensus is that it is condensation, not water leaking in during rain. If so, putting a dome over it should do little to stop these spots from occuring.

On the bright side, so far, every spot like this has dried out eventually. Also, the spot takes up relatively little sky and does allow observers to figure out how cloudy it is. The CI people once took their CONCAM down and put it inside where it was dry to get rid of a similarly stubborn spot.

On the negative side, the spot may persist in damp weather for quite a while. The spot compromises opacity maps and variability measures in its vicinity. Also, it looks annoying and unprofessional.

What to do? Ideas: turn off the big fan that blows in damp air when humidity is high. Watch out, though, that a running box does not get too hot during the day.

Heat the lens. Either short term at high T to quickly evaporate the spot and then remove the heater, or long term at small (T_lens - T_ambient) and keep the heater on all of the time.

Blow internal air at the lens. You might try just putting a small new fan in that points directly at the lens and blows the relatively warm air inside the box at the outside of the lens.

Either way, please keep us posted as to what you do and how it worked out!

- RJN

The weather being grim, we took down the camera at ~noon to try to dry it out. I'll leave it in a warm, toasty officce for a couple of days and see if that helps. There was no water/dew inside the enclosure when I openned it up, so the moisture in the lens must be condensation or else its leaking from the top. Will keep you posted...

mfw

I put the camera back up at noon 12/14, with a "blanket" of 3/8" foam insulation to try to prevent the outer aluminum plate from supercooling to the sky. I have a temperature logger inside to monitor the lens, the plate, and the ambient temp & Rh inside the box to get a better idea of how much the lens is cooling and how much humidity is ending up inside.
Unfortunately I made the hole for the lens a bit too small, and last night's images are slightly obstructed; I'll fix this today by removing the part that is above the horizon.

mfw

Concam was inside to dry out over the weekend, sorry someone forgot to email. It is back up and appears to be working as of tuesday 1/25.
Hopefully we won't have too many more storms this winter!

Haleakala continues to have problems with the "blurry center" phenomenon.
This is caused by moisture trapped in the lens, most likely from condensation.
The following series of images show the formation of this distortion.

It has been another amazingly clear night in Hawaii, unfortunately moisture still distorts the pretty pictures from HL.

The moisture on the lens persists, perhaps it is time to attempt another modification of the lens?

For those who are curious, suggested methods to correct this problem can be found at

http://asterisk.apod.com/viewtopic.php?t=62

new ideas are always welcome!

Mwaterson wrote:... I have a temperature logger inside to monitor the lens, the plate, and the ambient temp & Rh inside the box to get a better idea of how much the lens is cooling and how much humidity is ending up inside...


mfw

Is this data being recorded? Could we have a look at what the sensors have measured?

Since people are expressing interest, I attach a link to a couple of plots showing the internal temperature of the system over 2 week-long periods, in Nov 04 and Jan 05. (Note that the nov data doesn't have Relative Humidity data; sorry, no way to make this real-time...)
The probes were placed on the computer, in the "ambient" air near the middle of the box, and wedged up into the joint between teh lens and the top cover. This last was to hopefully give some indication of how much colder the lens was getting, but it is still not thermally contacting the front glass surface. The report is at http://koa.ifa.hawaii.edu/~waterson/ConCam/ConcamTemps/
My interpretation of the data is that the dewing problem is in fact due to IR radiative super-cooling of the front glass element of the lens, as RJN has suggested; I don't see any indication that the internal temperatures of the box are falling below the dewpoint, so there should not normally be a problem with condensation in the rest of the system. Having said that, I don't have data for a real storm, but still the computer seems to keep the inside of the box warmer than outside ambient so I don't think that is the problem.
-- mfw

It would seem to me that the actual collection of moisture is not a very frequent event but rather the problem is that once it is there it is very difficult for it to escape under normal circumstances. Why it does not evaporate would probably require data taken from inside the lens, I suspect that because the lens has a relatively tight seal there is just nowhere for it to go, I would guess that it is trapped there and even though it may evaporate to vapor it recondenses to liquid before it can escape the lens. This "blurry center" phenomenon is a fairly common problem and I believe it should be one of the primary issues addressed in the development of the next generation of concams. My idea would be to put a vacuum behind the lens but that may prove impractical for multiple reasons both physical and optical.

Thanks, Mark and Vic. Yes, it seems reasonable that dew is accumulating on the outermost lens element of the fisheye. Dew might accumulate on both sides of the lens, but the outside dries out during the sunshine of day. The question is how to stop dew from forming on the inside of the outer fisheye lens element that cools radiatively.

As always, my solution is (drum roll ...) heat the lens! One idea is to possibly drill small holes in the plastic side that holds the fisheye glass elements. These small holes might allow moist air to get out, while allowing warm air inside the box to get to the inside of the outermost lens element. This may effectively heat the lens! Perhaps some sort of resistive wire can be thread around the outside of the box that carries a smal current and can heat the outermost lens element. Lastly, a wire on the inside of the box heating the inside lens element significantly might conduct heat to the outermost lens element. That is what the Wise Obs. people do, I think.

BTW, can someone remind me of why the horizon of the HL frames is partially blocked? Can they be easily unblocked?

- RJN

BTW, can someone remind me of why the horizon of the HL frames is partially blocked? Can they be easily unblocked?

Someone forgot to remove the tape/thermal blanket when he was up there last week
should be ok tonight...

MFW

Over the weekend a power failure took out a router in the path from Haleakala to the Real World, so the network link to the mountain is down, though the concam is ok as far as I know (the failure was at the downhill end of the link).
We have not yet heard any estimate of how long it will be out, but will post if/when we do.

mfw

The HL Concam is back in action tonight and the lens even seems to have finally dried up

The visible streaks occur often in the HL movie for Mar 8th.
http://nightskylive.net/hl/hl050308/movie-hl050308.gif

They seem to originate from a common origin and follow a pattern. My guess would be that they are lasers used for adaptive optics. Anyone who knows what they really are let me know.

HL Concam is online and returning real nice images (aside from what appears to be a drop of water in the center) and lots of photometry. Opacity not available.

The waxing moon on the east is fading most of the stars in the west. A drop of water is seen in the north-east portion of the lens.

The HL contact is Garry Nitta
nitta@ifa.hawaii.edu