Mountings and Controls For Widefield Imaging Rig

[Gina]

For a fork mount in a dome or clamshell observatory the diameter of the main tube is not really limited (up to 400mm could be accommodated).  No worries about the off balance like there is with the GEM version - a fork is inherently balanced.  I'm therefore thinking of simplifying the design by having all the parts inside the rotating tube (or framework).  This means a tube of 220mm diameter though I could have a hole for the USB plug for the filter wheel and reduce it to 200mm.  I favour a tube as this is inherently rigid.

Some of the parts will need my bigger "Titan" 3D printer as the maximum for my "Pilot" printer is under 190mm but I intend to get this working properly soon anyway.  Fortunately, 3D printed parts are quite light.

[Gina]

This is a photo of the basic rig with 135mm lens and DIY lens hood which acts as a dew shield.  Allowing for the plugs on the back of the camera the total length is 280mm.  The 135mm f2.5 lens is the biggest I plan to use for this rig.  Any longer lens I would use with the GEM version of the rotation rig probably on the EQ8 (I might set up a middle size observatory and NEQ6 sometime in the future).  By limiting the focal length I can reduce the requirements for the fork mount and hopefully avoid guiding.

[Gina]

To reduce the amount of design work required, I might make the rotating tube the same size as previously if this proves practical, with a hole in the side for the filter wheel.  The frame that connects to the fork will still have to be large though.

[Gina]

I've looked through my designs for the parts of the present GEM version and due to having the optical axis the same as the rotational axis there's not really any way of using the designs other than for the measurements.

[Gina]

Some measurements :-

1. Minimum internal diameter of rotating tube = 160mm.
2. Camera diameter = 78mm.
3. 135mm lens diameter = 64mm.
4. Camera support to lens support = 130mm.
5. Focus motor shaft offset = 65mm.
6. Size of rotation motor = 42mm square x 40mm long.
7. Minimum space for camera connections = 20mm.
8. Clearance needed for FW = 100mm radius or 110mm radius including RT-angle plug.

[Gina]

First model of the tube with filter wheel and camera.

[Gina]

With the optical axis being the same as the rotation axis the tube and supports will split into two equal parts.  The camera end will be one piece and include the rotation drive gear while the lens end will have the running ring which will run in three or four wheels attached to the outer cage.  The latter will have stub axles for the fork mount and the stepper motor to drive the gear on the inner tube.  One of the stub axles will be a tube through which the cables will go.

[Gina]

Gear sizes are presently 153mm and 17mm diameter hence radii of 76.5 and 8.5 with 90 and 10 teeth.  The new inner tube is 170mm diameter and the larger gear wants to be bigger so maybe 180mm or 90mm radius.  90t would give mod 2 which is fine.  The motor pinion with a 9:1 ratio would be 20mm diameter ie. 10mm radius.  This makes for the possibility of going to an 18:1 ratio with its finer and smoother control and a pinion of 10mm diameter, which I think should be easily 3D printed but I'll do a test.

[Gina]

Not suitable but would seem alright with 7 teeth instead of 5 and mod 1.4 - rather smaller than the original of 1.7.  This makes the large gear 126t and 180mm diameter and also mod 1.4 of course.  I can print the 180mm diameter gear on my Pilot printer.  The inner tube parts will fit too.  Think I'll do a test print.

[Gina]

Hmmm... think it's pushing it.  Anyway, I also need to think how the rotation stepper motor is going to fit.  It won't be sticking out the end now!

[Gina]

Looks like this is going to need a radical redesign mostly doing away with the inner tube except for the lens end.  I'm thinking of supporting the camera directly via a ball bearing to the fork frame then the lens (on the fixed part between aperture and focus rings) at the other end with a bracket for the focus motor with a ring and set of wheels.  May rotate by driving the lens end rather than the camera end.

Here is a model of the camera, filter wheel and lens.

[Gina]

This model shows my idea for supporting the camera end of the rig.

[Gina]

This model shows a disc clamped onto the fixed part of the lens (clamping method not shown) that will run in a set of wheels in a ring attached to the fork frame.  The focus motor can be attached to this disc with a quadrant gear attached to the focus sleeve driven by a small pinion on the focus motor shaft.  A large spur gear can be attached to the disc for the rotation drive with the rotation motor attached to the fork frame.  I'll show this in another model.

[Gina]

Model showing all the above plus rotation gears and drive motor.  This is showing the rotation at 90° - only 180° of  rotation is required and this is all that will be provided.  This means the filter wheel will not crash into the rotation motor and the problem of moving cables is reduced.

[Gina]

With this arrangement for the rotation motor I could have a tube between camera and lens supports so as not to rely on the tightness of the optical couplings.  Ming you they need to be tight to be sure of optical alignment.

[Gina]

This model shows part of a tube used as the fork framework cut at the back of the rotation motor to show the relative positions.  Whether I use a tube or some other framework I haven't decided yet.

 

[Gina]

Another view.

[Gina]

I have another idea that would save on loose cables - invert the rotation drive.  An inner framework would carry the rotation motor and the electronics as well as the focus motor, camera and filter wheel.  If WiFi is possible the only wires/cable that needs to bend or twist is the power.  The rotation motor pinion would engage with internal teeth on a ring gear attached to the fork frame. 

The rotation motor would be bolted onto the orange disc as would the focus motor opposite (not shown).  The rotation motor would be partly balanced by the filter wheel drive and focus motor.  This shows a 10t mod 2 pinion on the rotation motor and a 90t internal large spur gear.

[Gina]

The orange disc in the model above will provide the support at the lens end of the rig, running in (probably) four ball bearing rubber tyred wheels, as in the previous design but a bit bigger.

[Gina]

As for the electronics and control, the RPi in the imaging rig will cater for the camera, filter wheel, focuser and rotation motor.  Probably another RPi will drive the RA and Dec mount axes and the dome rotation and shutter and possibly the PA adjustments.  That's if I use a dome but if I use the clamshell (cylindrical) type roof there will be just an opening and closing operation.  Have to say a dome appeals to me more.

[Gina]

Trying to remember and sort out what astro imaging rigs I've been working on.  Been some time since I last looked into any practical imaging due to several factors including the weather and personal circumstances such as health.  I'm now looking at where I had got to with several different projects for astro imaging.  I have just acquired an Ha solar telescope so that's another rig to add into the equation.  I plan to do both observing and imaging with this new (to me) scope.  Looks like I could do with more observatories!!