Gina

This is the ASC as it will fit on the pipe/mast.  It's big - about a foot high overall and up to 8" diameter.  It includes remote focussing and lots of space for the electronics.  I haven't sorted out the dome cover yet.  May be able to do that tomorrow.  I'm thinking the mast cover motor may go in the other side opposite the focus motor.

Attachment for light extension spring,

Wind rig working fine now - not seen any more funnies.  Leaving the weather station for the moment to concentrate on my ASC.

May change the focus lever slightly to make it easier to print.

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

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.

The top part of the ASC.

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...

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.

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.

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.

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

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.

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.

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.

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!!

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.

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.

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.

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).

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.

Stage 1 - Camera, lens and dome.  The top of the camera is going to have to be slightly above the centre of curvature to allow 180° FoV.  Hopefully, this won't be enough to cause distortion and elongated stars.  The smaller top part of the lens rotates to focus.

I've ordered the 4.5" dome and corresponding O ring.  Decided there was no need to give myself a harder job for the sake of a few quid!!

For even manual focussing I shall need a re-design.  Whether I can do it with the current size dome etc. remains to be seen.

One possibility is to buy a bigger dome and O ring, such as a 4.5 Inch Clear Acrylic Dome for use in all sky cameras.  This would provide more room to get a focussing mechanism in and also with the centre of curvature being near the bottom (as opposed to being 18mm above in the case of the present 3.5" dome) the mechanism would not need to be totally inside the dome. It would mean spending another £25 on parts but I guess this is a small price compared with the overall cost of the ASC and particularly with astronomy in general.  I'll work on the design and see.