Dual Widefield Imaging Rig

[DaveL59]

I'd more suggest using aluminium foil for shims than paper/card. It won't compress if it absorbs moisture nor hold moisture to create other problems. Easy to just add another fold to get a little more shift in angle being it is so thin. I did the same when shimming prisms in an old JB bino to get the prism tilt just-so since there was no other way to adjust the alignment, tediously long-winded task that was but now rock solid and hasn't shifted at all.

Micro-adjuster screws tho would be so much easier and way less frustrating  

[Gina]

There isn't a problem, Chris.  I'm now 3D printing the base part of the camera and lens mounting.  I am going to try a fixed setup to start with and see if that is accurate enough.  If it isn't I can go on to design adjusters but if I can get away without adjustment it will save a lot of work.  A small amount of misalignment of Ha and OIII images can be accommodated in the Star Alignment process but too much loses image data round the borders.

Here is a screenshot of the model.  This will be mounted on a Losmandy style dovetail bar.  The main weight is taken by the right hand side in this pic (lenses).  The extra depth is to accommodate the filter wheel (ZWO EFWmini).

Edited May 18, 2020 by Gina

[Gina]

To recap, this is the C mounting in this photo.

[MarkAR]

The rendering looks good Gina, should be able to re-use the existing top caps.

Just out of interest, where did you shim to get better alignment. Was it under the base or between the camera body and printed mount ?

If it was the base you could add a "jacking screw" in each corner to add a little torsion if necessary but not sure how well the printed plastic would take to small threads.

[Gina]

Between camera body and mounting.  I used a very thin piece of metal.

Edited May 18, 2020 by Gina

[Gina]

While waiting for the 3D printer I thought I'd have another play with the shim.  This is with a piece of aluminium foil.

Edited May 18, 2020 by Gina
typos

[DaveL59]

wow almost there Gina  

[MarkAR]

One little shim on the side by the looks of it and you're done.

[Gina]

I couldn't persuade it to move sideways (X axis) but I have plenty of tolerance in that direction.  If by any chance the sky is clear tonight I might put the rig on the mount and see how it does.  Only manual focussing so far but the focus on a distant comms mast seems pretty good so it might be adequate for nebulae.

[MarkAR]

Maybe take a bit of sand paper to the opposite side of where it needs to be shimmed.

[Gina]

DRAT!!!  Print failed!! At 80%!!!

[DaveL59]

rather than sanding off bits of the mount, perhaps undo the screws holding the plate in the base of the lens and shim it there. That way its easily reversible and no future gotcha's when you forget you modified it be grinding a bit down  

[Gina]

I'm not sure what you mean - AFAIK the lenses are not made like that.

[DaveL59]

where the lens fits to the camera, isn't there a metal plate afixed on the lens that then mates into the camera?

I guess I'm assuming its a regular camera bayonet-type lens fitting so I could be wrong, not knowing the lens you have.

[Gina]

No, it's a screw fitting.  Pentax type. 

[DaveL59]

ahh ok, the old Pentax I had was K-mount IIRC. Pity as that'd have been a neat solution, tho frustrating to do given dismantle, shim, try, rinse and repeat...

[Gina]

Polaris.

[Gina]

Although the present mounting on cameras only with a shim gets pretty close to alignment, I really feel that this is not quite up to par.  I like to do the best I can on any project and I enjoy building things so I'm looking into both the longer base support system and providing adjustment for alignment. 

The guider picks up plenty of stars so that doesn't need any pointing adjustment.  For imaging it's only the relative alignment that matters because the Ha imager will be pointed to target with plate solving so we just need to align the two imagers horizontally and vertically.  This may be achieved with either both adjustments on one imager or one on each.

The longer base is already designed based on the C type support system so this just leaves the adjustment.  Two decisions need to be taken for this :-

1. Moving lens or camera.  There's more room for adjusting the lens but this is what takes virtually all the weight so I think I'll adjust the camera.
2. Two adjustments on one camera or one on each.

[MarkAR]

Sounds like you'll be moving into the realm of scope rings type affair. You'd really need 3 or 4 adjustment points on either lens or camera. 

It might be easier room wise to do it at the front lens side of things. Wonder if you can just drill and tap your printed parts, one in the base and 2 on the top clamp giving a Y form adjustment like a scope ring.

[Gina]

I see your points.  I was also thinking in terms of a scope rings type arrangement.  I do use threaded holes in the printed parts but make sure the stress is kept within limits.  For instance, the part holding the pair of cameras and lenses is attached to the dovetail with 6 M6 screws with 10mm long threads. 

Each imager weighs over a Kg with the lens much heavier than the camera and I wouldn't want that much weight on a few screw threads in plastic.  Embedded metal nuts are a possibility.  The amount of force needed to hold the back of the camera is quite small with all the weight on the lens mounting which was why I thought adjusting the camera would be easier.

Edited May 19, 2020 by Gina

[Gina]

The arrangement on a finder-scope is two screws and a spring on the third leg.  I was considering doing the same but springs and plastic don't go well together as a continuous force can cause plastic to deform so maybe three screws and loosen one while tightening another. 

[DaveL59]

sounds like an upgrade to a CNC milling machine should be on the wish list Gina  

You're going to risk some deformation even with tilt screws aren't you, especially if things get hot in the day?

For sure better than springs and threaded plastic but just wondering - I guess if the parts are thick enough you might be ok though.

[Gina]

Something like this might be good but I think I can do it with 3D printed parts and metal nuts (eg. T nuts).

[MarkAR]

By the look of the thickness of your parts I think M5 or M6 threaded holes will be no problem.

The adjustment screws will only be taking a little pressure, I remember hearing that maximum load can be placed on a M6 bolt using just 3 full turns. Call it 6 for plastic, so 6mm thick material will be plenty.

I see that the print failed at 80% yesterday, is this just scrap now ? If so drill and tap a couple of 6mm holes and see if you can rip them out.

Just thought, if you used the screw adjustment method then you should allow some slack in the ring diameter. probably 1 -2 mm will be plenty especially if you do it for both lenses.

Edited May 19, 2020 by MarkAR

[Gina]

I have managed to pull M5 screws out but only with getting them very tight many times.  I agree that for this application, threaded holes should be fine.  These adjustments will not need to be very tight.

I expect the amount of adjustment to be no more than 0.3-0.6mm from experience with the current rig so a mm or 2 should be fine.   I see no reason to do it with both lenses.