All Sky Camera Revisited

[Gina]

Minimal camera cover added.  This provides thermal insulation so the thicker the better.  This includes a bit of clearance to allow for 3D printing artefacts.

[DaveL59]

wow, so much more complex than my thrown together mod'd cctv rig 🙂 But then I'm not super-cooling mine so a lot less to worry about.

[Gina]

One possibility for the camera casing also used as part of the main ASC casing (shown in clear blue).  The dome clamping ring (clear red) and the O Ring (orange).

[Gina]

This shows the original dew heater in the new dome and casing.  It seems too small so I plan to replace it with a ring of resistors round the inside of the dome.  The dew heater was barely adequate for the smaller dome so more dew heater power will help.  Also with resistors round the edge the focus mechanism should be easier to design.

Edited September 9, 2020 by Gina

[Gina]

One possible focus mechanism.  A lever on the focus sleeve and a cam on a shaft.  A spring would press the lever against the cam.

[DaveL59]

would that risk focus shift tho depending on how you drive the cam? Might be ok if using a worm drive to adjust the cam rotation.

Rigidity of the cam may be an issue too under hot conditions? or become brittle under extreme cold?

Figured best to ask as I don't know how the 3d printed material stand up to heat/cold in a stressed component so I'll bow to your knowledge on that front  🙂 

[Gina]

For remote focussing I would drive the cam using a small stepper motor with 64:1 gearbox so equivalent to a worm drive.

I'm thinking of arranging a remote controlled dome cover to keep hot sun off the most susceptible parts of the ASC.  In winter cold the cam will be in the warmer part of the ASC, with dew heater and warm rising from the electronics which will how be inside the ASC casing to avoid lots of wires from ASC into the obsy.

[Gina]

I shall not be using silicone sealant in the new version of the ASC.  The dome will be using an O ring and clamping ring and I have decided to use the same approach to sealing and holding the lower parts of the casing together.  I want the ASC to be fairly easy to take apart and put back together - there always seems to be problems of some sort or other!!

[Gina]

I definitely think it would be a good idea to arrange a cover for the dome when the ASC is not in use.  I think the main benefit would be in strong sunshine to stop things getting too hot and to keep UV off the dome and prolong it's life.  I think a 28BYJ-48 stepper motor with gearbox should be strong enough, same as I use for remote focussing.  I'll probably use Hall sensors for open and closed limit switches.

Seems to make sense to have the RPi 3, with the cooling and dew heater controls and motor drivers in the ASC casing.  Makes it bigger but saves having over a dozen wires going into the observatory and only the power wires go into the ASC.

[Gina]

I think the water cooling reservoir, circulating pump and radiator with fan will still have to go in the observatory so the water pipes will need sealing.  However, previous builds have shown that feeding them through holes in the casing slightly smaller that the OD of the pipes produces a good seal.

[Gina]

So far none of my ASC builds has proved completely sealed and allowed use of silica gel sachets to keep the air inside dry, preventing condensation or frost on the aluminium camera casing (or dew on the dome).  One possible solution is to provide another pipe into the ASC with air pressure applied to raise the ASC internal air pressure above atmospheric and prevent moisture getting in but this is more to do and be nice to avoid.  A simpler alternative is just a pipe and desiccant in a jar to stop damp air getting in but without using pressure.

[DaveL59]

shame you can't draw a vacuum to help reduce/eliminate moisture in the air, but then you'd need a strong pump to boil moisture off and risk the casing collapsing. Might be worth trying with a large syringe to at least get a low vacuum tho...? Or perhaps go get a helium filled balloon and use the gas in it to purge the air 😉 

[Gina]

I was going to try pressure in the last build but never got round to actually applying pressure.  I'm going to think about this. 

I could use a small radial fan like I use in my 3D printers to increase the pressure slightly above ambient.  I plan to move the ASC mast to the NW corner of the "shed" rather than the middle of the north wall, to get it out of view of the imaging rig on the pier, so the water cooling reservoir, pump, etc. will be in the warm room where the humidity will be lower.  The air in here should not be all that damp but could be easily dried with a couple of silica gel sachets. 

I would be interested to know how good the sealing of the ASC casing actually is but have yet to think of a practical way of finding out

Edited September 10, 2020 by Gina

[DaveL59]

hmm Helium cannisters aren't cheap tho would be able to do a lot of refills. If you don't mind CO2 then air pistol cannisters are a lot cheaper.

You'd have to figure a way to safely purge the ASC and then seal, bike schrader valves perhaps or a suitable stop valve that you can turn on/off as required. Also the pressure from a canister may (will) be a bit much so you'd need to regulate that too. Either way you'd purge the moist air out and it'd be replaced with whichever gas and then it'll just be ingress of outside air with moisture, which silica gel should handle till you re-purge.

If you can get an inflated helium balloon of course then it'd be low pressure and you can just squeeze to flush via tube/valves and if you're sniffing the exhaust you'll know it's done when your voice goes all squeaky 😄 

[pete_l]

9 minutes ago, DaveL59 said:

hmm Helium cannisters aren't cheap tho would be able to do a lot of refills. If you don't mind CO2 then air pistol cannisters are a lot cheaper.

And CO2 molecules are much larger that Helium ones, so the (inevitable) rate of leakage will be much lower.

The gas in toy Helium balloons is a mixture of He and air. There will be no guarantee that it will be dry. I.e, no water vapour in it. Does that apply to those CO2 cannisters as well?
Acetone is quite hygroscopic. A few drops of that somewhere strategic might get the water out of any gas you use.

[DaveL59]

no idea re dry-ness of either, but there would be a much smaller quantity of ambient air if you can purge the ASC body was my thinking.

As an alternative, how about...
https://www.amazon.co.uk/Vacu-Vin-Wine-Saver-Concerto/dp/B00005U2FA/ref=sr_1_7?dchild=1&keywords=vacuum+pump&qid=1599749527&sr=8-7

So long as you can make the unit airtight then this may get you a vacuum that you can easily top up and help keep moisture low. Downside tho, any slight leak in the casing would potentially suck water in if its raining. I'd prefer the purge approach tho, at least there's little/no residual pressure or if anything a slightly higher pressure than the outside of the casing.

Edited September 10, 2020 by DaveL59

[pete_l]

If you create a vacuum, would you have problems with heat dissipation? No air means no heat loss through convection.

[Gina]

Thank you for your suggestions but I will not be going with purging with a gas other than air nor will I be reducing pressure in the ASC as that would just encourage damp to get in if the sealing wasn't perfect.  If sealing was perfect, simply placing a few silica gel sachets inside is all that's needed. If sealing is not perfect (as seems likely) then a slightly raised internal pressure provided with dry air seems the simplest solution.

Assembly of the ASC in my living room will mean the air inside is relatively dry.  Cooling the camera casing will further dry the air as damp condenses onto the surface and then turns to frost if I cool below freezing.  Actually, I don't find it necessary to cool very much generally though the camera casing my well get below zero even if the sensor temperature doesn't.

Edited September 10, 2020 by Gina

[Gina]

This screenshot shows a model of the ASC with the lever and cam focussing.  One thought is to connect a shaft to the cam and to the output shaft of the stepper motor below the camera.  Another is to extend the lever and take it down below the dome seal and put the cam directly on the motor shaft with the motor in a casing extension.   There will also have to be a casing extension for operating the dome cover though I haven't worked out the details yet.

[Gina]

This sort of arrangement.  Of course, the casing needs the extension to house stepper motor but virtually anything is possible with 3D printing.  I've removed the dome clamping ring for clariy.

Edited September 10, 2020 by Gina

[Gina]

Edited September 10, 2020 by Gina

[Gina]

The new dome has arrived and looks huge compared with the previous one.  If I want remote focussing as it seems I do then I need the space and room for the focussing mechanism.  In fact focus has been a problem with the ASC all along so this does seem the way to go.

I'll post new design screenshots shortly.  It's going pretty well...

[Gina]

The top part of the ASC.

[Gina]

Added focus motor mounting and trough for dew heater resistors with ventilation holes.  Might change the holes for slots.

This shows the main casing in transparent blue to show the motor cam and focus lever mechanism.

[Gina]

Here is the main casing with slots instead of holes.  Looks better to me.  There is still plenty of thermal lagging for the camera.