Gina

A couple more screenshots of the model. Pier included now.

This design of imaging rig will be alright for the smaller, lighter lenses but will be considerable front heavy for the bigger ones such as the 200mm f4 and could do with the DEC axis more towards the lenses. These aren't your modern plastic but proper glass and metal and pretty heavy.

Added DEC pulley.

Designed the dual imaging rig and added it. This is probably not the final version.

Designed and added the big RA pulley but I think I might make it smaller since I can get sufficient resolution by going for MXL timing pulley and belt rather than GT2. This means around 200mm would give similar resolution to the current almost 300mm.

Decided to put the pulley on the bottom of the RA axle. Being aluminium tube the axle should transmit the rotation fine - I can drill holes and put pins or bolts through. This means a change in fork.

Fork added. Now I have to decide whether the large final RA drive pulley goes beneath the fork or on the bottom of the axle. I think the lower position will make for easier belt drive.

RA axle and bearings added.

PA Azimuth adjustment added.

Easy enough to blow the dust off.

PA Elevation adjustment.

The "charger" is a standard 13.8v PSU, connected directly to the battery and to the power distribution box which includes fuses and switches for individual circuits - mount, imaging rig, ASC and roof control - plus digital ammeters for each circuit and an overall digital voltmeter/ammeter for the battery. Imaging rigs have a 470µF electrolytic plus 0.1µF ceramic capacitor across the power input to remove spikes or interference.

This CAD model shows the principle of the PA adjustments. The threaded rod will be much shorter and not go through the altitude plate, which will carry the fork system. The top horizontal plate will rotate on the lower, thicker plate to provide azimuth adjustment.

My roof drive motor runs off the main observatory supply of 13.8v which has battery backup. But in the event of motor failure or whatever my motor unit has a manual clutch which disengages the sprocket. I can then push the roof closed by hand.

One thing I can check before I start CAD is the PA drives. The threaded rod is single start 8mm and pitch of 1mm ie. one turn moves the nut 1mm. I'll take as a starting point an offset of 200mm (nut from altitude pivot at right-angles to screw) and that a NEMA17 stepper motor drives the screw directly, without any reduction and calculate the number of motor steps per minute of arc PA adjustment. One minute of arc = 1/60 degree or 1/(60 x 2π) = 0.0053 radians Looking at the geometry, this is a distance D at the nut given by tan(A) = D/200. Where A is in radians and D is in mm. For these very small angles tan approximates to the angle in radians, so D/200 = 0.0053 or D = 0.0053x200 = 1.06 This is just over one thread pitch = one revolution of the threaded rod . So one full step of the stepper motor corresponds to 1/200 minute. Thus this arrangement gives very precise PA adjustment. Just waiting for someone to tell me I've got this wrong! 🤣

They said they got the mirror resilvered.

If I were to use MXL belt instead of GT2 I could use a 15t MXL pulley and the equivalent final drive pulley OD would be 210mm which seems well do-able. I don't have MXL belt in stock (only GT2 from 3D printers) but the appropriate length would only be a fiver.

MotionCo do a wide range of MXL size timing pulleys and belts as posted above and I can use the same system for the DEC drive as the RA, just that the whole DEC drive system needs to rotate on the RA axis and the final drive pulley has to be smaller. Smallest timing pulley with 5mm bore to fit stepper motors is 15t and the largest is 120t giving 8:1 as in the RA drive. With the RA drive and 280mm final drive pulley a 3 stage reduction gave 0.8 pixel resolution. To get the same for DEC means either a 4 stage reduction or large final drive pulley. I don't think I can decide this without drawing a CAD model of the whole mount complete with imaging rig.

Now to the DEC drive and a new design having decided against the 28BHY-48 stepper motor with the fragile plastic gearbox. Instead I plan to use a NEMA series stepper motor, maybe one of the smaller ones such as a NEMA11. There are two options for the DEC drive depending on whether I want to allow for guiding. Now whether this mount will need guiding will depend on how well I can engineer it. It would seem prudent to allow for guiding, in which case the DEC drive would need the same resolution as the RA drive. I was originally planning to try without guiding and hence a lower resolution and step-down ratio.

I have my flap mechanically connected to the ROR using rope and pulleys. Delightfully "Heath-Robinson" 😄 Can you close the roof if there's a power cut?

Think I'll watch that episode again on iPlayer - didn't notice that they'd flocked the tube. The expertise of those people never ceases to amaze me!

Yep! That's BIG!!

My imaging computer is a Raspberry Pi and it keeps itself nicely warm and dew free without any further heating.

Excellent image - like it enormously, Probably try similar soon if/when we get some clear nights. What gain settings did you use on the ASI1600MM? I'm trying to sort out best settings. I have 200mm f4 Asahi Super Takumar lenses I plan to try. Bit slower that your 200mm but we'll see. Might be better with 135mm f2.5 but single imaging rig. TBH I wouldn't have thought of using RGB subs.

I use vintage film SLR camera lenses - Asahi Optical - Takumar, Super Takumar and SMC Takumar as were used on Pentax cameras. These can be bought cheaply second hand and are excellent quality. I use them at full aperture and hence avoid spikes caused by the aperture iris blades.