DIY mount for widefield - some thoughts..

[Gina]

I think a reliable and accurate friction drive might be quite tricky to engineer, especially using a material such as rubber across a working range of maybe 20 to 30C.

James

Hmm, yes, the accuracy might be a problem.  The rubber idler is going to displace, particularly on the smaller wheel/axle and this would cause inaccuracies.  Whereas a timing belt drive will be accurate - there won't be any slippage and negligible displacement on the large pulley.  What timing belt drive lacks is stiffness.

This is going to need some thought.  The Dec axis would probably be fine with friction as the only purpose is to correct polar misalignment.  The RA axis should move at exactly sidereal rate - that may be a problem.

[Gina]

Would this do you?

http://www.considine.net/aplanatic/fork/fork_mount001.htm

I do rather like using wood - I wonder...  That one looks like overkill though

[Gina]

And how about this as a stepper?

http://www.active-robots.com/motors-wheels/bipolar-stepper-motors/3319-42bygh40-m-160-4a-nema-17-bipolar-99-55-1-planetary-gearbox-stepper.html

They look rather nice - except for the price   Step angle is about a quarter of the little steppers I was thinking of using so I'd only need 40:1 reduction ratio.

[Gina]

Been thinking about the drive method.  Since the plan is to use guiding, I think stiffness may be more important than accuracy.

RA rate required is 15 degrees per hour (I think) - Dec rate is zero.

[JamesF]

Hmm, yes, the accuracy might be a problem.  The rubber idler is going to displace, particularly on the smaller wheel/axle and this would cause inaccuracies.  Whereas a timing belt drive will be accurate - there won't be any slippage and negligible displacement on the large pulley.  What timing belt drive lacks is stiffness.

This is going to need some thought.  The Dec axis would probably be fine with friction as the only purpose is to correct polar misalignment.  The RA axis should move at exactly sidereal rate - that may be a problem.

I'm really not sure you need that much stiffness for the application you're proposing.  Holding a couple of kilos of camera steady is not going to require the same sort of engineering as holding 25kg of sail-like large aperture newt steady.  I reckon a belt drive will be fine as long as you have some mechanism (probably by allowing the motor to move slightly) to maintain a small amount of tension on the drive.

I wouldn't disagree that a friction drive would be perhaps the best solution, but I'm not convinced about using one based on a rubber wheel even for the DEC axis.  I'd be concerned that the physical characteristics of the wheel might change over the likely temperature ranges it will be used at (say -10C to +20C) and that the wheel might be prone to slipping as it gets colder.  Some sort of spring mechanism to hold the driving wheel against the driven surface might help with that, but then the wheel might develop a flat spot if unused for a while.   I'm still inclined to think that for an amateur build a belt drive would be a more practical solution.

James

[Horwig]

That one looks like overkill though

Yes, agreed, but it's for a large Schmidt camera, but could be scaled down nicely for your mount.

A polar disk mount is very economical to produce, only one polar bearing, the disc itself being part bearing and part drive,

this would make a blank, if you have a tame engineering facilityhttp://www.ebay.co.uk/itm/6mm-316-Grade-Stainless-Steel-Disc-8-Diameter-203-2mm-/221131391279?pt=UK_BOI_Metalworking_Milling_Welding_Metalworking_Supplies_ET&hash=item337c75412f

I've also seen a 185 mm dia x 52mm ali offcut on ebay for £20!!! 

Cash job for sombody with a good lathe to true them up,

and In theory, any remaining errors would (could? might?) be corrected by guiding.

You then have a mount for pocket money, well at least the price of a few bottles of good wine.

As you see, I'm with you on this one, why buy something when you can build it...

Huw

[Gina]

I'm really not sure you need that much stiffness for the application you're proposing.  Holding a couple of kilos of camera steady is not going to require the same sort of engineering as holding 25kg of sail-like large aperture newt steady.  I reckon a belt drive will be fine as long as you have some mechanism (probably by allowing the motor to move slightly) to maintain a small amount of tension on the drive.

I wouldn't disagree that a friction drive would be perhaps the best solution, but I'm not convinced about using one based on a rubber wheel even for the DEC axis.  I'd be concerned that the physical characteristics of the wheel might change over the likely temperature ranges it will be used at (say -10C to +20C) and that the wheel might be prone to slipping as it gets colder.  Some sort of spring mechanism to hold the driving wheel against the driven surface might help with that, but then the wheel might develop a flat spot if unused for a while.   I'm still inclined to think that for an amateur build a belt drive would be a more practical solution.

James

I agree that a rubber tyred idler wheel has it's faults - difficult to retain sufficient friction at low temperatures and avoiding flats.  Yes I would have a spring system to hold it/them against wheel and axle.  This needs a double linkage and is not all that easy to implement, which is one reason I've rejected this sort of drive for other things and gone for timing belts and pulleys.

Having thought about the pros and cons of each type of drive I'm coming down in favour of timing belt and agree with you James

[Gina]

Yes, agreed, but it's for a large Schmidt camera, but could be scaled down nicely for your mount.

A polar disk mount is very economical to produce, only one polar bearing, the disc itself being part bearing and part drive,

this would make a blank, if you have a tame engineering facilityhttp://www.ebay.co.uk/itm/6mm-316-Grade-Stainless-Steel-Disc-8-Diameter-203-2mm-/221131391279?pt=UK_BOI_Metalworking_Milling_Welding_Metalworking_Supplies_ET&hash=item337c75412f

I've also seen a 185 mm dia x 52mm ali offcut on ebay for £20!!! 

Cash job for sombody with a good lathe to true them up,

and In theory, any remaining errors would (could? might?) be corrected by guiding.

You then have a mount for pocket money, well at least the price of a few bottles of good wine.

As you see, I'm with you on this one, why buy something when you can build it...

Huw

I didn't realise at first that the disc drive also acted as a bearing.  Interesting idea.

[Horwig]

As an obsy mounted widefield rig, there's lot going for the polar disc mount, or its close relative, the horseshoe mount:

 

with two rollers, one, or both driven, there should be an evening out of any irregularity in the disc, I'm not biased,I just like its simplicity. Also, with long term drift alignment, there's no need for a dec drive, apart from initial pointing

H

[laser_jock99]

Beacon Hill Telescopes still make worm wheel sets:

http://beaconhilltelescopes.org.uk/pricelist.html

I used one of these on my original 1980's home made portable mount!

[Horwig]

Beacon Hill Telescopes still make worm wheel sets:

Yes indeed, I can vouch for their quality. I have a pair in my mount. The RA being quite old, probably 1980ish, but the Dec was bought new last year, not exactly cheap, but an elegant way of doing a simple widefield rig

H

[Gina]

Beacon Hill Telescopes still make worm wheel sets:

http://beaconhilltelescopes.org.uk/pricelist.html

I used one of these on my original 1980's home made portable mount!

Thanks for the info   Rather too expensive for my cheap DIY mount though.  I've decided to go for the easiest solution of timing belt drive.

[Gina]

Gradually getting the designed refined

 

[Gina]

I've decided what I'm doing about the drives.  If the mount is set up so that it is in the HOME position, the RA will only need to go 90 degrees in either direction.  This would point the telescope down to the horizon.  We will never want to go below this so the RA drive only needs to rotate the mount by + or - 90 degrees.  With the sizes of large pulley, small pulley and centre distance the total range is about 270 degees ie. + or - 135 degrees which is more than needed.  This means that the timing belt can be fixed on the large pulley at the opposing position to the small pulley.  A belt tension adjuster could also be placed here avoiding needing a jockey pulley or other sort of tensioner.

 

[Gina]

Another re-design

[Gina]

A slight rethink on the drives too.  I'm sticking with the 150mm/240t large pulleys - in fact I've ordered two 150mm diameter x 10mm thick acrylic discs.  I already have a 16t timing pulley so plan to order one more.  That gives a 15:1 reduction.  motionco are out of stock of 15mm plastic timing pulleys (that can be drilled out to just under 5mm to provide a tight fit on the stepper shaft) but they have 12mm ones that I think will still be alright.  I'm planning to team that up with 60t aluminium pulleys rather than the (huge) 100t ones - giving 5:1 reduction.  Total physical reduction is therefore 75:1 which I propose to team up with 2:1 microstepping to give the required resolution.

[Gina]

Some of the aluminium plates used in the latest design :-

1. 100mm x 50mm x 8mm - 4 off for the altitude PA adjustment
2. 200mm x 75mm - base plate
3. 100mm x 75mm  - 2 off - top plates
4. 150mm x 75mm - 2 off - uprights
5. 75mm x 50mm - upright
6. 150mm x 100mm - 2 off - side plates
7. 100mm x 50mm - 2 off - side plates

[Gina]

This design uses more components than the previous one - not sure whether it's better or not.  The RA axis has better spaced bearings - 150mm apart rather than 75mm.  Maybe this latest design would be better as a single rectangular box, 200mm x 150mm x 75mm.  Still thinking

[Gina]

I'm thinking about wood for at least some of the construction.  I have an electric bench mitre saw and prefer working in wood.

[Gina]

I have plenty of wood left over from building the "fancy green shed with the rolling roof" both solid and 6mm plywood (11mm OSB too but I think that's too rough for the mount).  I'll do another design for wood

[Gina]

Lightweight mount in wood.

[Horwig]

Looks good, but I'd still be tempted to make it into a fork, not that I'm biased you understand      .

But seriously, a fork gives great stability, you don't need a counterweight, and it's easy to make out of wood.

Huw

[Gina]

Thanks   I'll give it some thought - see if I can work it out.

[Gina]

Well, I've a good play around with a fork system in SketchUp and so far everything I've tried looks a lot more difficult to make

[Gina]

Here's the PA altitude adjuster converted to wood.