Gina's Mini Dome Observatory for Widefield Imaging Rig

[carastro]

HI Gina, I haven't read every single post on here, but as an owner of a Skypod, I would not think it would be possible to motorise the dome.  The moulded panel heights can be uneven and the whole thing can take a real effort to push round at times.  Having said that, you get such a huge expanse of sky with the POD that I rarely find it necessary to rotate the POD in any case.  

The roof has a rubber gasket to stop rain, though it can leak at the pivots either side, but not in the centre.  I put a small pot each side to catch the rain.   It also prevents imaging at or near the Zenith and i had to build a Table extension to unhook the dome and slide it off (another task requiring some muscle as it's not easy).  

Whilst I am really glad I have it, if I ever started again, I would not go for a Skypod for imaging because of the need for a Pod Zenith table.  

Carole 

Edited May 4, 2017 by carastro

[Gina]

Thank you Carole - useful to hear your experiences   I went off the SkyShed Pod idea for that reason (zenith view).  My latest clamshell design avoids this by opening right up except for northern view down to the horizon but I have rising ground and trees to the north so no loss.  My design is for a much smaller and lighter roof than the Pod - it's only 2ft diameter and will be 3D printed from plastic.  The 3D printing process uses a sort of honeycomb lattice structure with lots of air space inside the print making it light but strong. 

Here's a repeat of the cross-section of the rig with roof open, north to the left.

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[wimvb]

Just two questions, Gina. How good is the durability (especially uv resistance) of the plastic used? Some plastics become brittle when exposed to uv light for extended periods.

The other question is about the middle section of the cover. From your image, it looks like this section can become stuck behind the fixed section? But you've probably already seen to this.

[Gina]

2 hours ago, wimvb said:

Just two questions, Gina. How good is the durability (especially uv resistance) of the plastic used? Some plastics become brittle when exposed to uv light for extended periods.

I'm not sure but it can be painted.  I've had an all sky camera made with ABS in use for quite some time and so far it's alright.

2 hours ago, wimvb said:

The other question is about the middle section of the cover. From your image, it looks like this section can become stuck behind the fixed section? But you've probably already seen to this.

Yes, the model is not quite right - the lip on the fixed section is bigger than it should be.  In fact I might not have a lip at all on the fixed section and increase the overlap.

[Gina]

Actually, this spherical clamshell design is beginning to look far too complicated and I'm having second thoughts.  A cylindrical rather than spherical design would be far simpler and I already have one for a bigger setup that could be downsized for the widefield imaging rig on fork mount.  Being smaller and therefore lighter it should be capable of being remote controlled relatively easily.  Before proceeding further on the enclosure/obsy I might finish the fork mount and the fork mounting on the imaging rig itself so that I can see how big the roof needs to be in practice.

I would have preferred a dome shaped micro observatory but sometimes practicalities have to take precedence over aesthetics.

[Gina]

Here is a cross-section diagram of a possible cylindrical clamshell roof and the imaging rig plus the earlier spherical design for comparison.  The new one is the smaller.  This assumes the rig can be parked before closing the roof otherwise the roof will need to be bigger.  It may be worth having a larger roof to avoid the need for parking which would also mean a quicker cover-up if rain comes.

[Gina]

It's looking like the widefield mini observatory I designed for using the NEQ6 mount would be suitable if shortened from around 700mm-1000mm down to around 350mm.  If I used that it would save me quite a lot of work - it would only need the aluminium sheets cut down with tinsnips though the 12mm marine ply might be a touch of over-engineering.  This would alloy the roof to be closed with the rig pointing in any direction.  It could probably be a couple of inches smaller diameter but I doubt this would be worth the extra work.

Someone suggested cutting several large holes in the plywood quadrants to reduce weight and cover with a lightweight waterproof cloth or something.  I might go for that as with the much shorter aluminium sheets, the plywood may be the biggest weight.  I could probably work it out.

[Gina]

I've just been outdoors looking at where I shall put this little unit and I think I have a problem.  The basic problem is my main observatory - it's in the way of lower views to the east if I put the new one where I was thinking of.  There is a small space around the SE corner which might be a possibility.  The only other thing I can think of is to have it quite high up which has other problems.  Of course, when I built my main observatory I had no plans for adding another, albeit a tiny one.

It would be easier if I didn't need to see Polaris - it could go further away from the main obsy to the west.

Edited May 5, 2017 by Gina

[wimvb]

Do you need to see Polaris? Polar alignment can be done without. For (initial) drift alignment, you don't need it in view either.

[Gina]

I did wonder that.  I'm not sure how far away from the main obsy I could go and still see Polaris anyway.  Too far west reduces western view because of trees otherwise I could go past the house to get a northern view. 

I think I'll stick with original plan of mounting it on my Skywatcher Pillar Mount on the southern boundary in front of the house.  Behind the main observatory would be too awkward - the corner of the plot is less than a right-angle and although there is a space, there are only small gaps for access.  I can get all round for painting but wangling a mini observatory in would be difficult - probably easiest to go through the warm room and scope room and lower it down somehow.  No - don't like it!

 

[Gina]

I've just been outdoors and looked at the geometry.  I think it's alright.  The residence is a bungalow and the roof would seem to be pitched at 45° and a line extending from the roof slope appears to reach the ground near the boundary fence.  Since the latitude here is 51°, Polaris should be in view with the imaging rig a metre or so above ground and less than that from the fence.  Tomorrow, if it's fine I'll see if I can set up a sighting device and measure some angles.

[Gina]

OK... Initial measurements.  I've set up the pillar mount in roughly the position I would like and adjusted it to level the top.  Then I took a 2ft angle measuring device with spirit level and opened the arms, placing one on the top of the pillar and aligned the other with the object.  Measured the gap at the open end with steel rule - the angle device doesn't have a protractor included (which would increase it's usefulness considerably!).

Results :- Angle from top of pillar mount to roof apex is 9.5" at 22.5", to obsy roof 3.5" and to neighbours oak tree 6".

Angles work out from half opposite side divided by the hypotenuse gives the sine of half the angle.

House roof sin (a/2) = opposite/hypotenuse = 4.75/22 = 0.2375.  a/2 ≈ 13.5° and a ≈ 27°.  So no problem with Polaris

Obsy roof sin (a/2) = 1.75/22.5 = 0.077r.  a/2 ≈ 4.5°.  a ≈ 9°.

Tree sin (a/2) = 3/22.5 = 0.133r.  a/2 ≈ 7.5°.  a ≈ 15°.

Edited May 6, 2017 by Gina

[Gina]

So no problem with the view - I don't expect to do much to the north or north-west.  One thing I am wondering though is whether the pillar mount is up to holding the mini observatory in a storm.  Maybe I should use guy ropes!!  Alternatively, I could use what I had for the much earlier ROR version - fence posts in spikes driven into the ground to support the observatory and use the pillar mount just to hold the fork mount and imaging rig.  Be better with light breezes when imaging too.  Yes, I think the observatory needs substantial ground attachment and to be separate from the mount.

Edited May 6, 2017 by Gina

[Gina]

Just had a look outside in the jungle and under the tarpaulin I spread out over the ground I want to clear for the gravel path to the observatory and garden, I found the four fence post ground spikes and two of the three concrete blocks I had the pillar mount on - it's about 5ft nearer the observatory than the position I've just measured the angles from so maybe slightly higher angle to the obsy roof but we'll see.  I might as well use the fence post spikes as they are really solidly in the ground - no point in giving myself extra work for the sake of a degree more view to the east.

[Gina]

Only thing is that the post spikes are not in the best positions but I may be able to sort things out - tomorrow probably.  Maybe orientate the roof at 90° to the design above if that's feasible.  ie. have the roof axles angled N-S.

Edited May 6, 2017 by Gina

[Gina]

I think this project may proceed faster than I expected - it seems to be going rather well

[Gina]

This photo from 2014 shows the post spikes in place together with the frame for the roof and the pillar mount with NEQ6 on it.

[Gina]

Been looking at the drive for the roof and it only needs the inner moving section driven as the outer one will be held by the lips to the inner one.  The outer is always trying to open due to gravity.  During opening the outer section will rotate with the inner until it reaches the stop at the end of its travel.  Then the lips separate and the inner continues until the roof is fully open.  During closing the inner will rotate until the lips engage and then it will pull the outer round with it until closed.

[Gina]

Here are new cross-section diagrams.

[Gina]

Here's a photo of the ground with post spikes and concrete blocks.  That's a 1 metre carpenter's rule included in the photo.  The left hand block used to be flat on the ground so may want turning and bedding in.  The right hand block and front slab look well installed and will probably suffice.  Top of photo points about 15° west of south

[Gina]

The post spikes are certainly not in the best positions - E-W frame dimension will be about 400mm and N-S about 500mm whereas the outside dimensions of the spikes are about 740mm E-W and 480mm N-S - but they are 750mm long and I can't see me getting them out of the ground.  I'll just have to adjust the woodwork appropriately.  I had considered rotating the roof from axles about E-W to about N-S but with the base section being not far off square, there seems little point in this.

[Gina]

Been out and turned the east block flat down but not yet bedded it in.  It was a much as I could do to turn it over - is a hard concrete block 18" x 9" x 6" and very heavy!  Later on I'll move it out of the way and take some topsoil off.  Then it can be re-bedded.  I've moved the pillar mount onto the blocks and slab and levelled it with it's levelling screws (a very useful feature).  Measured the angle to the observatory roof from the top of the pillar but the rig will be some 450mm higher up so should be less than the 9° I calculated before.

[Gina]

Looking at where the pillar mount is, the eastern posts should be reasonably placed to take the eastern side of the roof framework but the western side will need bringing in eastwards.  Two thoughts on how I might do this are shown in the attached diagram.

[Gina]

Some photos.

[Gina]

That 500mm x 250mm x 10mm aluminium plate is quite a lot bigger than needed and causing problems with the roof box.  I'll sort out something smaller.