Gina

No.

The fork part is pretty straightforward but I need to work out the mounting for it. I don't think plastic will be suitable as it is liable to move, particularly in altitude (reducing it due to sag) so I'm thinking two aluminium plates mounted on a swivel base. The azimuth adjustment is straightforward - a large spur gear or maybe a lever with a gear rack on the end but the altitude needs more thinking about. Think I'll sleep on it

Yes, I think I'll stick to belt drive particularly for the final drive to the RA axis. I already have some MXL belting and several timing pulleys so no problem. The drive from motor to intermediate axle could be 3D printed gears though (as long as I take care of backlash). In fact I've found that fishing line cord and drums were more accurate than timing belt and pulleys on my 3D printers. The limitation of this system is that it can only handle limited motion - not round and round.

I don't think this mount needs to be as accurate as I thought I reckon I might get away with 3D printed gears. That would simplify things a lot. I could try it and see how well it works.

I guess I should be able to calculate the accuracy required given the exposure time and angular resolution. Let's see... Earth rotates at 15° per hour. One minute is is 1/60th of an hour so the angle to traverse in one minute is 15/60 = 0.25°. One pixel is 0.001 degrees of arc. Hence 250 pixels are traversed in a minute. Accuracy required is 1 in 250. This is about one tooth of the MXL belt - I think I should be able to manage that.

The RA drive will need a double step down to get the 100:1 plus rotation ratio between motor and axis. I think MXL size timing belts and pulleys will easily handle this application and I have used these extensively for various projects in the past. I have a 100t MXL timing pulley with 5mm bore plus some others - I'll have to see what I've got. The smallest pulley with a 5mm bore has 15 teeth so the first step down belt drive will be with 15t motor pulley and 100t on an intermediate shaft giving a ratio of 3:20 (1:6.67). The final drive can have a 15t pulley on the intermediate shaft and a large pulley sized to give an overall ratio of 1:100. This would have 100 x 3 / 20 x 15 = 225 teeth. The pitch diameter works out at about 145mm which seems quite reasonable. Since mount axes only have to rotate 180°, the belt could simply be attached by its ends to points on the pulley, saving the need for teeth and meaning the pulley could simply be turned from a 150mm diameter blank on the lathe. I've done this size pulley before on my little Chinese heap of a lathe quite successfully. OTOH I could 3D print a timing pulley and see how well it works - I can achieve a concentricity of around 0.2mm in 100mm with my latest 3D printer setup. This just might be enough with an exposure of around a minute or less.

Yes - direct drive via belts. The drive speed is controlled by the electronics and software that drives the stepper motors. I think I'll be able to use an INDI driver that was devised for one of the proprietary mounts by altering the drive ratio setting in the software. Then the mount will work just like any other under control of KStars/Ekos on the client computer. The hardware of the mount will need fairly precise engineering but nothing like that needed for a mount carrying a long focal length telescope and long exposure imaging.

Continuing with the DEC control, the camera resolution is 4656×3520 so allowing 10% of the height gives about 350 pixels. This would mean even a 1:1 gear ratio would work but we can do much better than that. Something like a 10:1 ratio would give a 10 pixel resolution and be easy to implement. From my experience with 3D printers fishing cord and drums would be quite adequate. A micro-stepping mode of less than 16 would be adequate too. I might stick to timing belt drive though - I'll think about it.

I'll look into the gearing and drive systems next. Longest FL lens I shall use with this mount will be 200mm and the camera resolution is 3.8microns per pixel. Pixel resolution should be fine. Angular resolution is given by tan(θ) = 3.8 / 200x1000 = 0.000019 giving θ as about 0.001°. NEMA17 steppers have 1.8° per step but using 16x micro-stepping = 1.8 / 16 = 0.1125°. I think we can call that 0.1° So to get a resolution of 0.001° we need a "gear" ratio of 100:1. A slightly larger ratio to give a workable steps per second of arc would be good. This will be suitable for the RA axis where the drive is moving to follow the target. With good PA the DEC axis should not need changing during the exposure and less accuracy will be adequate. Of course this does depend on good PA but I hopeful of achieving this. The main requirement of the DEC control will be framing the target withing the imaging frame and I think something like 10% of the frame should be adequate.

Yes indeed And this should be quite a "meaty" one

Both axes will have drive gears or pulleys for pointing the mount and traversing the sky in RA during imaging. The main axle will be mounted in bearings on platform (pier) with azimuth and altitude adjustment for aligning to the polar axis (Polar Alignment). This is critical, particularly if guiding is to be avoided. This means two more adjustments/variables apart from the RA and DEC controls for the pointing. Since the imaging rig will be enclosed in a fairly close fitting container (observatory) they will also want to be remote controlled. However, these should only need setting rarely and do not need absolute accuracy (they are in a manual control loop). I plan to use NEMA17 stepper motors for the RA and DEC controls for accuracy but the AZ and ALT PA controls could use the ubiquitous 28BYJ-48 mini stepper motor with built-in gearbox. These are not suitable for the RA and DEC controls not because they're too lightweight but because the gearbox has a peculiar ratio (a little under 64:1)

Basic fork mount. The box represents a telescope or other imaging system. The main (large) axle is RA and is aligned with the polar axis with the fork pointing towards the pole - the small axle at the end of the fork is the DEC axis.

This project is back on having virtually completed my widefield imaging rig and wanting to reserve the EQ8 mount for telescopes. The plan is to make a fork mount and matching dome based mini observatory for automatic remote controlled operation. This follows on from a previous thread relating to a mount for my triple imaging widefield rig which is now defunct.

I have started a new thread in the DIY Observatories for my latest mini observatory design. Gina's Mini Dome Observatory for Widefield Imaging Rig

The size and design of a mini observatory will depend on the mount used. I had hoped to build a fork mount for the widefield imaging rig but this has turned out to be rather heavier than originally thought and I have my doubts about the engineering of a strong and stable enough mount. OTOH I have a perfectly good GEM type mount in the form of an NEQ6 but this format requires a much greater space to rotate in. A fork is much more efficient in this respect. I guess the very wide FOV reduces the accuracy required of a mount and maybe I could make my own. I have had a couple of designs for a mini observatory in the past - a roll off roof type and a cylindrical version. I would very much like to make a dome as this would have several advantages. I have ideas for controlling a dome with INDI. Another possibility I have in mind is planetary imaging. This would involve my MN190 scope mounted on the EQ8 (probably) with a Barlow magnifier. This means several different telescopes to use on the EQ8 mount though the Esprit rig could probably use the NEQ6. At this rate I could end up with three observatories The original with EQ8 mount a mini one with NEQ6 mount and a micro observatory with the widefield rig on a home made fork mount.

I've put the Esprit on the EQ8 mount ready for when I get sorted out. The widefield imaging season is over until the autumn so I shall be imaging galaxies with much narrower FOV. Later I might be using the EQ8 for solar Ha imaging... A mini observatory might be on the books again this summer to accommodate the widefield imaging rig ready for the nebula season.

And still no imaging! It's now galaxy season so once we get a few clear nights I expect to sort out an imaging rig for that. Meanwhile I'm concentrating on other projects so not terribly bothered

Looks the like moon has it

Been looking at the fixings for the chain ends onto the roll-off-roof. The "pull it open" end is relatively easy, a block of wood with the chain screwed onto the bottom. The block would have its bottom side level with the pen mark on the draught excluder. It would stick out just a bit further than the aluminium channel that holds the window panel. The chain would then line up with the top of the large pulley. The "pull it closed" end is probably a bit more tricky as it has to clear the large pulley on the dividing wall and the chain from the bottom of that to the drive motor whilst not being too far off straight to the smaller pulley. A block of wood might do it - I need it check it out more closely. Here's a rather poor photo - I'll take a better one when weather permits and maybe from another angle as well.

Yes, I saw that - looks good - I'm already running INDI together with KStars/Ekos. Have one or two little problems - this might solve them. OTOH I'm probably already running the latest version. This is a great system for use with the Raspberry Pi.

Looked at using the NoIR RPi camera to remotely watch the imaging rig on the mount to make sure it was pointing roughly where expected and found that the INDI driver indi_v4l2_ccd works with the RPi camera so I can use this with the KStars/Ekos client. No doubt combined with the ROR control. But I could set up an RPi with camera any time now to check what's happening now that I have my widefield rig mounted on the EQ8

No no, still too cold to mess about in the observatory for more than a few minutes at a time. I shall leave any thoughts of solar imaging until the warmer weather. I shall also leave any thoughts of planetary imaging until next time there are any planets I'm interested in where I can see them. So with the widefield rig and scope systems virtually finished and waiting for the weather for final testing my thoughts are moving on to other projects. Still astro is the observatory automation which I'm looking into. I can do more planning and thinking about the control electronics - plenty to do that doesn't involve extended periods out in the observatory. Also, vaguely connected with this is my weather station which needs parts making before it is ready for installing outdoors. While I'm waiting for the weather I'm also thinking of finishing off some of my smaller or almost complete projects so that I can add to the DONE list

Here is a SketchUp model of the window crank to scale, showing partly open and closed positions of the roll-off-roof. The black disk is a wheel which acts as a roller on the longer lever arm. Green is the cord pulley.

Sunshine and showers today and relatively mild so I've been out to the observatory and checked up on the roof and window. The weather strip on the top of the window doesn't add any significant resistance to opening the roof - I can push it open with one finger The window is balanced closed until at about 5° open and by 10° will open on its own just held by the cord. A spring or weight could easily start the window opening. The length of cord between open and closed window is about 2ft so this is the distance the end of the crank must move to open the window (or close it). The uprights of the observatory framework are also about 2ft apart. I can now look into the geometry of the crank and how it can be implemented.

I've been thinking about the flap/window. The only sealing brush actually holding the window shut is the top one that brushes against the top of the roll-off-roof and once the roof is open slightly it ceases to hold the window shut. So as long as the roof opening motor is strong enough to pull the roof open against this resistance (and the other causes of resistance), the window should open easily. Possibly a weight on a lever on the axle, or a spring, could pull or push the window open against the cord. I've also been looking again at ways of allowing the window to open as the roof rolls back and the "crank lever" system shown above might work and is relatively simple.