All Sky Camera Revisited

[Gina]

Taking a few images tonight but I'm suspicious of the image - it would seem to show dew or condensation.  Now this could be on the outside of the dome and of little concern but it could be in the "dry" zone, maybe in the lens.  The silica gel was still orange this evening so maybe I'm concerned unnecessarily.  Dew heater is on.

Exposure 90s, gain 0 and camera temperature -1.6°C.

Edited April 11, 2019 by Gina

[JamesF]

There was a lot of dew this evening when I went out to shut up the chickens for the night, so it wouldn't surprise me if there was some heavy dew on the dome.

James

[Gina]

The dew appears towards the SE where the breeze was coming from so that makes sense.  There was some dew around the bottom of the dome earlier but that cleared after the dew heater had been on for a while.  There is no sign of the central dew dispersing.

[Gina]

Well I didn't see any before but now the oval is just a tiny bit smaller.

Edited April 11, 2019 by Gina

[Gina]

Gamma reduced to 30.  FITS image enhanced in PixInsight and saved as PNG.

[JamesF]

The misty patch is a touch smaller, I'd agree.  Might need a dew heater with a bit more welly though?

James

[Gina]

Yes, it looks like it.  I'm currently running the dew heater at 12v - I can run it at 13.8v or I can add more heating.

[Gina]

I think I may arrange to switch the dew heater on from the outside RH rather than manually via the INDI Control Panel.  I'm planning to measure RH and temperature outside the observatory anyway.  I can show whether the dew heater is on with a light on the control panel maybe with a manual override.  Also, I think it would be interesting to measure the temperature at the bottom of the dome inside (higher up would obscure the FOV).

Edited April 11, 2019 by Gina

[Gina]

I ran the image capture overnight until dawn.  The dew eventually went from the dome but the clarity wasn't as good as the previous night and cloud came in just before dawn so no Milky Way visible in this set of images.  More dew heating is definitely needed and it wants turning on well before sunset to stop dew forming.

I shall leave the ASC as it is for now as there are other projects I want to work on.  The inside is staying dry so far and while that remains the case, I probably wont touch it.  Next clear night that's moon free I'll try tweaking the imaging settings to see if I can improve the results.

[Gina]

Been thinking about the question of the humidity level in the ASC.  What matters most is that the camera doesn't get wet and fail (like the last one did) but I think I've done everything possible to prevent this.  The camera itself is in a sealed casing with silica gel beads to absorb any damp, this casing is also well above the bottom of the outer casing so there would have to be lots of water inside before it even reached the camera casing.  Damp in the lens would be the next problem though I have had condensation in the lens before and it dried out fine so not a critical problem.  In fact I think this would be a good indication of failure of the outer casing sealing and well before water could cause a problem with the camera I reckon.

[Gina]

Cloudy earlier but now the clouds have gone and there's just the moon high up in the sky.  Dew heater has been on since well before dusk but there's a ring round the moon which is not there when viewed with no.1 eyeball!  Not sure if this is dew or just image sensor overload.

[Gina]

Not dew.

[Gina]

Left the image capture running overnight.  This is an image from just before dawn with a bit of curves processing.

Edited April 13, 2019 by Gina

[Gina]

No night sky images to post due to the weather but I can report that the outer casing is still sealed and dry inside.  It does need a more powerful dew heater as I mentioned earlier.  Also, I want to be able to dry off water droplets from earlier rain.

Edited April 16, 2019 by Gina

[Gina]

The raindrops dried of naturally during the daytime - we even had some sunshine this afternoon before the clouds came back.  Now the clouds have moved away again but been replaced by the nearly full moon.  Just the very brightest stars showing.  With the dew heater on the dew is gradually going.  This is 60s exposure with gain 0.

[Gina]

The silica gel has turned green so the inside of the main casing is no longer dry though there is no sign of misting of the lens - maybe some on the dome.  I shall power down and bring the ASC indoors to investigate.

[Gina]

Have ASC and mast indoors, taken the ASC off the mast and opened it up and, yes, it's wet inside.  Only a couple of drops of free water but moisture all over the inside parts.  I guess I now need to think carefully as to whether trying to keep the inside totally dry is a practical proposition.

[Dr_Ju_ju]

Sounds like you need the opposite of fully sealed, but open'ish with a forced air flow....

[Gina]

Yes, that is just what I was thinking as an alternative.  I shall have to think how I can manage it.  Ideally, dry(ish) air from inside the observatory.  OTOH is the air in the observatory dry?  Alternatively, I'm wondering if I could make a tiny dehumidifier.

Edited April 18, 2019 by Gina

[Gina]

The simplest dehumidifier would be to blow the air through packets of silica gel which would need replacing every so often with fresh or dried packets.  However, experience has shown that the silica gel gets "full" pretty quickly even with the air indoors. 

The other way is to cool the air below dew-point so that the water condenses out.  I could do this with a Peltier TEC.  Of course the dried air would then need heating otherwise the dome will be cold and dew will condense on the outside.  I guess the cooled and dried air could be passed through a heat exchanger on the hot side of the Peltier TEC and warmed up for feeding up into the dome.  This would become a combined dehumidifier and dew heater.  Hmmm...  I wonder...

[Gina]

Both the cooler and heater heat exchangers can be simple CPU heat sinks.  3D printed ducting could carry the air round the system.  The air coming out will be warmer than the air going in due to the inefficiency of the Peltier TEC but warmed air is what we want for feeding up into the dome to stop dew both outside and in (though the dehumidified air should not contain much moisture).  Seems to me this doesn't need a particularly efficient Peltier TEC and the ultra cheap Chinese ones should do.  It only needs to cool the air below dew-point.  I wonder if I can make this small enough to put inside the ASC casing.  A small tube would be all that is required to take the condensate away.

Edited April 19, 2019 by Gina

[Gina]

Ordered a pair of TEC1-12706 Peltier TECs plus a pair of 40mm square heatsinks from Amazon (Prime) arriving tomorrow or Saturday.  OK cheaper on ebay but wouldn't get them until Wednesday or Thursday next week.  I have a variety of fans I can choose from.  The heatsinks would go on either side of the Peltier TEC.

Edited April 18, 2019 by Gina

[Dr_Ju_ju]

Does it have to be super dry ??  I usually find that just the fact that there's circulating air. means things stay dry. It's when air is stagnant that allows for any moisture to condense out...  

[Gina]

The lens is directly on top of the cooled camera body and is likely to be several degrees below freezing.  I have had moisture inside the lens when I've used it outdoors without the dome and outer ASC casing.  OTOH warm air circulating around the lens might warm it up enough to prevent condensation.  I'll try it.  Then there's the question of how to heat the air.  I might try combining cooling the camera with heating the air.  Although far less efficient than water cooling, it might be sufficient, particularly if I used two stage Peltier TEC cooling.  Using water cooling is not necessarily carved in stone.

Edited April 18, 2019 by Gina

[Gina]

One possibility to reduce condensation in the lens would be to provide thermal insulation of the lens from the cold camera body by using a 3D printed plastic for the CS to T2 adapter instead of the supplied aluminium one.  That would then just leave the optical window in the camera itself.  Any dampness in the camera would probably condense on the cooled camera body in preference to the electronics or window but I'm rather concerned where it might end up.  I'm concerned about cold, damp nights with lots of moisture in the air so I'm still thinking that keeping moisture out of the ASC would be best.  I'm unlikely to want to image when it's actually raining.