DIY turret for C11

[bojan]

More than 35 years ago a friend made this turret for use with MTO 1000A (my design, material was PVC).
Unfortunately, he was not careful enough when machining the holes for eyepiece barrels, so all three are offset by ~3mm. Not a big problem for visual at low power, but because of this turret spend most of its life in the drawer.
I decided to 3D-print the more accurate version (I need to do only the rotating part), and for 4 eyepieces (it will be easier to balnce it properly)
To achieve the smooth rotating, I will finalize the inner surface on lathe.

[xelasnave]

You never cease to amaze Bojan. Great idea and an even better save.
Alex

[Wavytone]

Hi Bojan, nice idea to print one.

How will the fit onto the C11 ? Screw on ?
Also I’d suggest one of the eyepiece barrels be 2” otherwise it’s going to be a bit limited on the C11. I have a TEC turret which only takes 1.25” and for this reason I only use it on scopes smaller than my MK91 - which has a similar focal length - and I find I really do need one big 2” low power eyepiece at around 90X.

[garymck]

https://www.thingiverse.com/thing:1433034

cheers
Gary

[bojan]

I am re-using central part of existing, old turret.. it was made to fit MTO adapter (which acepts M42x1 or M39x1 thread barrels), so the hole is a bit larger than 2" thread. I already printed the interface (without thread - it is fixed (but adjustable) onto C11 2" thread by 3 screws for now, in the future I will make proper metal screw adapter).

Fitting 2" eyepiece is another matter - I have only 1.25", and I don't have suitable mirror at hand, and even if I had the larger one, it wouldn't fit inside existing barrel... so I will stick to what I have at the moment.. It is not hard to print another one anyway in the future, it only takes time (this adapter used 6 hours to print @99% fill)

Gary, thank you for the link, I knew about this design (whenever I need something to do by 3D-printing, I always check Thingiverse first - if not for suitable design, then for ideas). I was considering it but..
To fit 6 eyepiece barrels, the hub of this design needs to be quite large (and even larger for 2" eyepieces), so the optical path is significantly longer than optimal backoff for C11.. my design as it is has that distance much closer to recommended value.

[Wavytone]

Don’t think of 6 - the Unitron Unihex had 6 and it was big and heavy.

I have a TEC turret - 5 x 1.25” with a 2” frontend, and this has some sort of epoxy body to save weight and keep the size down. The eyepiece barrels have to be slim too.

With turrets size and weight are a serious issue - by the time one of these is loaded with a full complement of eyepieces and possibly a camera it could exceed 4 kg and if you rely on a typical crayford it may not hold this load - and worse it may tear the focuser off the scope.

[bojan]

No worries Nick, I am not thinking of 6, only 4x1.25".
3x was OK (apart from non-coaxiality issue), so 4x won't be too much worse.

[bojan]

I just bought suitable barrels (https://www.bunnings.com.au/holman-32mm-press-pvc-coupling_p3140837?fbclid=IwAR0QfwgI LszZyQE1cITd2iyFYh--9Vq_duDJCz6bVommZ3mpgO9n3pAp8Go) from Bunnings... so no need to print them, for that price it is not worth bothering.
I only have to make the thicker rim for screw threads, as the barrel itself is not thick enough (1.75mm only).

[bojan]

Almost done...

[Wavytone]

Oh very nice... any plans to trim the length of the tubes down ? They look a bit longer than they need to be - keeping backfocus to a minimum is a real challenge with these.

[bojan]

That is plan for today :-)
Also, they will be trimmed for individual eyepieces, to minimize the need for focus adjustment.

[bojan]

Almost finished..

I only need to add stops, 4x2mm holes in the prism holder (and spring-loaded steel ball).

[bigjoe]

Pity you can't do the whole thing to order Bojan ..I'd buy one for any reasonable price, whatever that may be for my CPC 925 SCT .
Looks very Clever.

Bigjoe.

[bojan]

At the moment I can't possibly commit to anything like this.. when I retire perhaps..
However, I will post the design here soon.

[bigjoe]

I see..Will be great to see the design for a look anyhow Bojan.
Cheers Bigjoe

[bojan]

This is central part, prism (or mirror) holder (the one I already had, made from PVC).
Prism is held in place by two nylon screws, and the diameter of the barrel is chosen to fit the prism which sits inside, precisely centered.
Collar diameter (to scope) was dimensioned to fit MTO-1000A, so I needed adapter ring to fit it to Celestron Visual adapter. If I needed to print this part, collar would include adapter.
Rotating part was printed as on image below.

[bojan]

After I finished the turret with prism (still not tested it, Melbourne weather is not helping), I decided to try another design, 3x, straight....
Eyepiece barrels are same again (from Bunnings, $1 each)....
The calotte looks acceptable outside, but it didn't turn out perfect on the inner side (bridging), which is more important from the functional point of view. So I am thinking of doint it in two (thinner) parts, and glue them together, to avoid excessive post-machining.

[bojan]

A bit improved C11 adapter...

[bojan]

playing with different versions and assembly...
Unfortunately, my 3D printer started to behave erratically... so progress on this project is erratic as well.

[bojan]

It turned out better than I expected....
Gluing was done with ABS dissolved in acetone.
Now I only have to make stops and the thing is finished.

[bojan]

Now about stops or "clicks"....
I decided to start with hex spacer with M3 thread.
I have steel balls ø3.2mm, so all I need to do is to widen the hole to ~ø3.3mm ~6mm deep, then cut M6 thread ~3mm long and cut the spacer to 10~12mm.
The assembly as pictured below (without ball and spring) will be screwed into platic holder at apprppriate position, then it will be used as guide to mark the position of the rotary part with drill so the ball can click into the recession and the eyepiece tubes are properly aligned.

[Wavytone]

Ah... Bojan I think these will be OK at first but with use the plastic will rapidly wear an the click stops will go soft and possibly a bit off-centre.

Someone muttered that its possible to print now in an aluminium alloy of some sort (my guess its sintered or something to get it to bond properly). That would produce a better result esp if you could sooth the surface a tad - a careful hand job with grade 600 wet & dry paper (used wet) would probably produce a reasonable finish.

[bojan]

Good point... but printing with aluminium (or any metal) is way beyond the capabilities of my DIY 3D printer....
I saw such printer in CSIRO Clayton, Lab22 (https://www.csiro.au/en/Research/MF/Areas/Metals/Lab22)... amazing experience.



However, now that you put a bug in my ear, I can place metal shim on the ball path to avoid the scenario you descibed.

One thing I do differently (I think..) from other 3D-printers is I always try to combine techologies to mitigate inherent weakneses of printed ABS (printing time, mechanical strength..).

As another example (apart from ball-spring assembly), I am using ø6mm PVC rod as shaft (in 3x turret), inserted into machined hole and secured (and glued) with M3 rod (cutoff from m3 screw). Initially I wanted to use brass standoff, which is still in consideration as this 3x turet is preliminary model.. turned out too well not to be used for a while.

[bojan]

Printer works again (problem was with dodgy temp sensor connector), now experimenting with printed stops.... looks very promising.

And easy to replace if broken.

[Wavytone]

Bojan... here’s another DIY effort FYI https://www.skyatnightmagazine.com/how-to/build-a-rotating-eyepiece-turret/