Gina's Mini Widefield Imaging Observatory

[Davey-T]

I was just thinking the same thing yesterday about the need for counter balance not only to take the load off the drive but also if it fails the counterbalance would automatically close it.

A length of builders bar and one of those steel door stops would do then when you get the right length by trial and error you could cut the spare off.

Dave

[Gina]

Yes, I was thinking in terms of a bar and weight (x4 ie. 2 sections x 2 ends).  It certainly needs something.

I have the NEQ6 mounted on the 8" extension pipe and that on a baulk of timber to raise the rig to just below the roof when closed as seen in this photo.

Now, with the roof open I have been moving the rig about to check clearance.  BIG PROBLEM - the imaging rig doesn't clear the box when approaching the meridian as seen in this second photo   The main RA axis of the NEQ6 is too long - it would have been relatively easy to have a shorter neck!

SO... what to do about it?  I think the easiest would be a longer version.   Back to the drawing board

[Davey-T]

You could finish it and flog it to someone with an HEQ5

Dave

[Gina]

OK...  I have learnt a few things from this project...  

1. This observatory is NOT big enough for what I want to put in it , particularly usung the OTT NEQ6 mount  
2. The south side does not need to be as low as I made it.
3. Thin sheet aluminium is not very stiff even when curved to a quadrant.  So will need both ends driven.
4. Counterbalancing is needed.

I now need to have a good old thinking session.  I knew there was a good chance that this unusual design might have some problems but these are not insurmountable.

Can this version be saved?  Yes, I think so.  An extra 100mm in length should solve the problem.

[Gina]

You could finish it and flog it to someone with an HEQ5

Dave

Posssibly   Or buy/make another mount   But I think I shall try to modify this version to fit the NEQ6.

[Gina]

To accommodate the NEQ6 mount I could either increase length or increase diameter.  The former will mean buying some more aluminium sheet or using something else - the latter a new piece of marine ply and a lot of work.

[toxic]

you could always use a similar counter balance like the eq use .

and as for the width just make it 12 inch's wider and use 6mm marine ply and screw it to the edge it curves quite well and when you are satisfied with how it works just glue it and screw it then it will be extremely rigid hope this is of some help after all your hard work good luck Gena.   

[Gina]

This photo gives an idea of the extra length required to allow all round movement of the rig.  Also shown is a possible way of driving both ends at the same time though I don't know if the axle would clear the rig in all required positions.  I shall still be adding counter balancing though.  I can get 1000mm x 500mm x 0.7mm aluminium sheet for £18 a sheet - a bit thicker but not too much.  I'll need 2 but may be able to make use of what I've got for the fixed one.

[Gina]

you could always use a similar counter balance like the eq use .

counter ballance.jpg

and as for the width just make it 12 inch's wider and use 6mm marine ply and screw it to the edge it curves quite well and when you are satisfied with how it works just glue it and screw it then it will be extremely rigid hope this is of some help after all your hard work good luck Gena.   

I don't think 6mm marine ply bends that easily   2mm or 3mm thickness might.  I could use a polygon rather than a circle for the profile but I'm not keen on that.  No, I think thin aluminium sheet wins.

[saac]

Gina could you stiffen the aluminium sheet using bonded stringers - rather like an aircraft monocoque.   You can see the same technique used on a car bonnets - the bonnet itself is no more than thin sheet steel but its stiffness comes from stringers welded onto it.  I was wondering if a similar solution would be available in the form of say 3 d printed stringers bonded (epoxy resin) to the aluminium sheet.  

Jim

[toxic]

Gina i see that you have a router just cut groves in the play every inch from end to end and at half the depth with the thinnest cutter you have. it will bend no problem, but to make it easier you could do all that then fix it to at the bottom corners then stick a kettle under it, there will be enough steam from it to make it warm and bendy to fix the rest in place then when it has cooled it will have formed its shape.

[Horwig]

Agree with Toxic about bending ply. But to make it extra strong, use two thicknesses, glued together. 

[Gina]

Thanks all

I had wondered about cutting grooves in one side of the ply to make bending easier and I have used steaming to make wood bend in the past when I was making a sailing dinghy many decades ago   I might have a go at this - I haven't ordered any more aluminium as yet.

Later...  I've been looking at the cost of 6mm marine ply and it works out at about the same as the 0.7mm thick aluminium sheets for a single layer.  So it seems like the aluminium wins.  But I'll sleep on it.

[saac]

I'm sure you are already onto this Gina but with care, and a few good router bits, it is possible to use your router to cut and shape thin sheets of aluminium.  Several passes with shallows cuts but it can work well. The ally sheet needs to be screwed securely down in case the bit bites in and cutting fluid, while messy, will help.

Jim

[skybadger]

if it is the edge of the ally sheet you are worried about, I suggest putting a lip on it, either by folding the edge back on itself or bending it a round a thin steel rod. This will perform the same as the stringers for stiffness and keep the edge straight. You don't need to keep the rod in there after...

Mike

[Gina]

I think I have decided to go for aluminium again and yes, bending the edge down certainly strengthens it.  To get the length it will have to be a bit thicker anyway - 0.7mm rather than 0.5mm.  The roof will be a metre overall rather than 750mm which should be plenty.  I will be buying two pieces 1000mm x 500mm for the moving roof parts.  The fixed part will use either two of my original ali panels or timber.  I can pop rivet the ali panels together to form a longer piece.

The small motor gearbox I bought is very unlikely to provide enough power for the roof so I've bought a recycled 12v windscreen wiper motor which I think will be quite adequate.  It's the same physical size as the motor gearbox I shall be using for my main observatory ROR automation.  It's out of a Volvo car so should be good quality.

[Horwig]

Wiper motors are good for astro use, I'm using a pair of ford focus motors on my dome. Very high torque for not many pounds is a good equation. Best of luck with it.

H

[Gina]

Thank you Hugh

[skybadger]

I tend to find that the spigot for mounting things on the end of the wiper motors is a pain -- its meant for bolting flat or conical recesses in wiper driver bars  onto. That works best for wheels if you have a flat wheel profile you can do the same with. I had to come up with an extension for mine that I could then bolt the driver gear onto. The extension was a length of thread with a tap in the end, slitted crosswise and expanded to fit the cone. A nut came down the thread to grip the cone tight. It was a bit of a faff but it worked.

Alternatives you might want to consider are the car window motors which work on a reel of wire  - maybe wrapped around a sector on the outside for gearing or even a small motor from motionco. I had to go small to fit the small gap between internal and external walls on the dome. I can't hold the wheel on that I can on a wiper motor.

HTH

Mike

[Horwig]

I have to agree with Mike (Skybadger) about mounting anything onto wiper motors.

My solution was to drill and tap a piece of hex bar, a cone section was cut into the end with the lathe, to match the cone section of the motor. A couple of flats were filed into the cone section of the motor spigot, and grub screws to match put through the hex section.

Just to be safe, the whole assembly was then epoxied. Seems to have worked, at least the dome still turns.

Huw

[Gina]

The cone section on mine has ridges - could be a sort of spline or just for extra friction.  It came without a nut but a standard M8 nuts fits fine so that's no problem.  I don't know yet how I shall drive the roof but I have various options for making parts using 3D printing or lathe and various hand tools.  When I get the new ali sheets and can make the roof up and sort out counterbalancing I can see just how much power is required.  Main bearings are ball bearings so little friction.

Mine has a connector with 4 lugs so I shall do a spot of testing when I find where I've put my DMM   OTOH maybe someone knows what the connections are.  I suspect two for motor power and maybe they contain limit switches.  I don't know if they just go round and round with a crank when used in a vehicle or whether they actually reverse.

[Tinker1947]

I think they go round and round with a crank to sweep back and forth....

[Gina]

That's what I thought.  Wonder why there's 4 connections - could be redundancy I guess

[Tinker1947]

Different speeds...??

Found this

https://www.google.co.uk/search?q=car+windscreen+wiper+motor+wiring&espv=2&biw=1920&bih=1099&tbm=isch&imgil=tMUzNK--tgifRM%253A%253BvR9Rnssk91_R-M%253Bhttp%25253A%25252F%25252Fforum.woodenboat.com%25252Fshowthread.php%25253F64204-Wiring-a-self-parking-windscreen-wiper-DC-motor&source=iu&pf=m&fir=tMUzNK--tgifRM%253A%252CvR9Rnssk91_R-M%252C_&usg=__xFh11QWoOAqcnT5vC-Latw8-sGE%3D&ved=0CCoQyjc&ei=zxjIVOu5CcuuUbPsgfgP#imgdii=_&imgrc=H1R_GPsRRDPebM%253A%3BvcdTapjcaX8eDM%3Bhttp%253A%252F%252Fwww.s-v-c.co.uk%252Fresources%252Fimages%252Fwiperplug.gif%3Bhttp%253A%252F%252Fwww.s-v-c.co.uk%252Fwiper-systems%252F%3B400%3B281

[skybadger]

Different speeds is right . I have had two with different contacts pairs for different speeds and one with a built in low res

encoder...