Gina

Oops - seems I haven't finished this. I've had a break from astro imaging but plan to get back to it this coming autumn so will probably have information to add.

Pretty dark yes, but not quite as dark as it was a few years ago when I built my observatory. Better than a lot of places though and I shall be imaging again in the autumn when it's dark enough. It certainly is a lovely view and I really enjoy it - I'm very lucky ?

Cleared the hole of stones and adjusted the odd bit of the shape to get the pond level. Started back-filling but stopped for today - done enough. Got to be careful not to overdo it. Now enjoying a much needed cuppa ?

I have a rest every half hour or so ? Having one now! Anyway, I think that is pretty much my exercise for today - no need for a nice long walk up the hill, into the woods and back.

Only a small pond but I'm not up to a lot of digging these days. Wasn't too bad though with relatively soft ground except for further down where I hit the clay. I think the hole might be just about deep enough. It needs to be a bit deeper than the pond form to allow for back-filling with soft soil and check for sharp stones. I also need to get my spirit level on it as this ground slopes and is misleading. The SW Pillar Mount (white) visible in this photo is virtually perfectly upright - I checked that a few days ago as that's where my micro dome observatory is going. This is the view from my living room window.

I'm starting with an empty area about 10 x 4 metres between sun lounge and observatory in front of living room window and facing roughly south. I have killed off most of the weeds and grass by covering with an old tarpaulin for a year or so. This has also stopped the ground from drying out and making it easier to dig. In addition to providing a path to the observatory, this will provide flowers and shrubs to see from the living room window. Plus a small pond and fountain. The latter is something I have been working on in 3D printing and plan to have something unique. Apart from the pond there will be planting areas and slabs and ornamental stepping stones. I have removed most of the tarpaulin and started digging a hole for the pond. The latter is a pre-formed pond I bought from Amazon :- Bermuda Sand Pre Formed Pond 128 x 84 cm. Here are some photos of pond hole digging and the general area.

I rather wonder if extreme micro-stepping would be accurate though bearing in mind that a microstep represents a pixel in the image and I want to use exposures up to a minute or two. Resolution is 0.002° per pixel and 1m = 1/60 of 15° = 0.25° which is 125 microsteps. So I need an accuracy of 1 in 125 (or 250 for 2m exposure) minimum. Stacking multiple frames can cope with the frames not quite lining up as long as it's not too bad. But if the mount doesn't track accurately during each exposure the image loses resolution.

Pololu do a 128 microstepper :- AMIS-30543 Stepper Motor Driver Carrier but that is delivery from USA.

Those drivers on the Duet are SMDs - I don't think I want to cope with those - they've got a lot of legs. Bad enough with 8 leg SMDs ? Ideally a unit to replace the ubiquitous A4988 drivers.

That's a thought - just a different stepper driver module, I guess. Do you know of one Dave (I don't really want to use a Duet board ).

For small timing belts and pulleys, MotionCo seem the best. There are 16t GT2 pulleys on ebay but biggest seem to be 80t - 5:1. A 16t GT2 for the final drive and two pairs of 16:80 would work. I'll have to total up the costs either way and see which is best.

Just checked the backlash in the gearbox of the 100:1 gearbox-motor and it's of the order of a degree. That won't do at all ? Think I'll have to stick with timing belts. Maybe a triple reduction. 5:1 and 6:1 in conjunction with the 25:1 final drive. Smallest MXL with 5mm bore is 15t from MotionCo and largest is 120t giving a maximum ratio of 8:1. I could use MXL belt and pulley for the final drive but already have 20t GT2 plus a length of timing belt spare from 3D printer making. MXL pitch is 2.03mm so similar but not quite the same as GT2 which is 2.00mm.

Yes, I am aware of creep. The main frame will be reinforced with metal. Retaining good PA is the main thing.

Now to look again at calculations of accuracy etc. The fork is nearly 300mm wide so a 300mm diameter RA drive pulley would be appropriate. Circumference of 300mm pulley = 300 x Pi = 942mm. The pulley can be a bit bigger to give easier calculations eg. circumference of 1000mm give a diameter of 318.3mm If I use GT2 timing belt that will be 1000 / 2 = 500 teeth. I have 20t GT2 timing pulleys so ratio will be 500 / 20 = 25:1 If I were to use a NEMA17 stepper with 0.9° stride angle and 16x microstepping, each micro-step is 0.9 / 16 = 0.05625° Earlier I worked out that 0.001° would be ideal for a 200mm lens but I doubt I'll go above 105mm with this rig. so 0.002° is fine. To accomplish this resolution a further reduction is required of 0.05625 / 0.002 = 28.125 - say 28:1 Hmm... that's more reduction than the final drive - I think a motor-gearbox may be indicated. I have a NEMA17 1.8° with approx 100:1 gearbox which would give a resolution around 0.001° but it's got the wrong size shaft for the small timing pulleys I have in stock. Pulleys are available to fit the motor. The little 28BYJ-48 motor-gearbox might be powerful enough but insufficient resolution (5.6° stride angle and approx 64:1 gearbox) it would still need further gearing down. I can see why worm gears are used in mounts

With the dual imaging rig, this fork mount is turning out a similar size to that needed for a telescope but with a much lighter payload. I may leave printing until I have my Giant 3D printer working again - the hotend heater block broke and I'm getting a new one made. Probably within a week or so. There shouldn't be too much of a hurry, as long as I get this mount working for the autumn. OTOH everything takes much longer than expected so I won't be hanging about ?

The current print has served a useful purpose in advancing the design of the fork for the mount. With the dual rig mounted the Dec axis is not balanced but I'm not sure this matters. It means gear drive would be possible since backlash would not cause a problem and Dec axis accuracy is not important as operation will be manual, to centre the object in the FOV, after which the declination will be fixed during the imaging run. To improve the balance would involve an offset camera clamping bracket and longer forks and reduced rigidity.

Here is the fork model ready for printing.

Filed the bearing holes out so that the ball bearings could be hammered in which has enabled me to test the fork. Found a couple of design changes required. With the present design I was able to pull the sides out to get the camera clamp stub axles into the bearings. This shows that the sides are not stiff enough and also that the bearings need clamps to hold them in. ie. semi-circular ends to the forks and semi-circular clamp pieces so that the thing can be assembled!! More thinking and design required but that's what prototypes are for. I shall add webs to stiffen the sides. A couple other things need attention - the hole for the axle is too small and the tube was a bit too long.

Printing has now reached the bearing holes but really needs parts cooling which my Titan printer doesn't have yet.

Decided to change the design of the fork to remove the RA drive disc and add that as a separate print. The result is half the time and half the filament. Also extended the part that attaches to the axle tube to give additional support.

New fork. If I were to change the nozzle in my Titan printer to 1.2mm (largest available for Volcano hotend) it will take around 3 hours and consume over half a 1Kg reel of filament. 212m of 1.75mm filament.

Also found a piece of aluminium tube about 20mm OD and 500mm long. Aluminium is easy to cut to length. Can't remember what I bought it for but it's been lying around for years. I could print collars to fit it to the 25mm ID ball bearings I already have. Still overkill probably but nowhere as bad as those huge and heavy pillow block bearings. This would be a better axle as the shorter one is just chromium plated steel. The kit will be covered but not damp proof.

Clamp bracket added. Plus ball bearings. Drive disc will go on square cornered end.

Found another, shorter shaft I could use for the RA axle. Only trouble is the bearings I have are metric and this is imperial size and a smidgen bigger than the bearing ID. OTOH I'm not sure a ball bearing is needed - a printed Nylon bearing would probably work well enough.

3D printed a bracket for the new, larger diameter mast (5 metre high bottom section). This is on the bed of my Titan printer with 300mm square print bed to give an idea of scale. The height is required to space the mast away from the side of the observatory to clear the roof when it opens. Two 10mm coach screws will go into the middle main upright (between scope room and warm room). "Penny" washers will spread the load on the bracket. The mast will go on the north side of the observatory where it will not obstruct the view for imaging.