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The ED 66 Project


Stub Mandrel

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To my surprise, I find myself the proud owner of the lens cell and baffle from Skywatcher Equinox 66. Although Skywatcher claimed these lenses as 'apochromatic' they are actually 2-element airspaced ED lenses like those in the new WO 61 scope, so at best could be describes as 'semi-apo' if there can be such a thing!

That said, these little scopes had a very high reputation for sharp, high contrast views and minimal chromatic aberration. Some people report field curvature, the previous owner of this lens got some photos that don't seem to show bad effects. In the worst case, there is a Skywatcher flattener that will work with this lens.

My ambition is to make a top-class refractor with this 400mm focal length lens for grab and go visual AND astrophotography. I already have a 400mm tele lens, which frames things beautifully but has marked CA - blue haloes.

I have to decide on the type of focuser (rack and pinion or crawford) and the drawtube diameter. Do I go for a 2" focuser like the WO61, which gives a very 'robust' look or follow Skywatcher and fit an SCT-type tube that needs a 2" adapter if a flattener is to be used. The short focal length means a 1.25" focuser isn't an option for AP, unless I use a cheap 0.5x reducer/flattener but I don't want to go that way if I can help it.

So far I have gathered/found in the scrap bin:

  • 76mm(3") by 1/8" wall aluminium tube.
  • 3" aluminium bar for the focuser body.
  • 2 1/2" bar, potentially for the drawtube (originally intended for making a tripod brace but i didn't need the brace).
  • A 70mm dovetail to provide a mounting point, but I may make my own.
  • 3" internal diameter thinwall tube (ex-fire-extinguisher!) for the dewshield - currently used with the tele lens.

I've ordered some small precision ball bearings in case I go for a crawford focuser.

I'm trying to find prices for anodising the finished bits - this may be a bit steep and if so I will have a go at doing it myself.

An interesting challenge will be making a 10:1 reduction drive for the focuser.

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  • 4 weeks later...

Well, my preparations have been slower as it's taken time gathering bits together.

I have now got a better size tube for the drawtube coming. The bearings for a crawford are coming by a circuitous route. I mocked up a 1speed reducer by 3D printing, and it seems to work. It will be the most fun part to make. I have figured out how to internally thread a long tube.

Before I can start I need to mock up everything to confirm main tube and drawtube lengths. I've had some useful measurements from someone with a 66ED and a skywatcher flattener. I've decided to order with my Christmas money and when it arrives I will mock up the whole thing in cardboard or piping. I want it to work with a 2" EP, a 1.25" EP in an adaptor and with a DSLR on the flattener. Visual capability is important as my experience is that I find small scopes more comfortable to use=I see more.

I've drawn up a 3D model, quite crude at the moment, but easily edited to change the lengths of the elements.

It looks pug-ugly without any finessing at the moment, but by necking down the main tube between the ends, tapering the corners and perhaps some careful straight knurling could make it quite good looking.

I think the focuser section could be rather shorter in practice, but keeping it long gives solid mounting points for both the focuser knob and a mounting wedge. I am planning on using 5mm diameter bearings for the Crawford, fitting them in pockets 'Moonlight-style' would allow me to shape this section externally to something more graceful. Unlike Moonlight, I may put a 'cap' over the bearings. Also an O-ring to seal the drawtube would be a nice touch.

One nice touch will be graduating the drawtube like a WO scope. I may try and find something even more useful than a thermometer to put in the single-speed focuser knob.

One other idea is fitting a dew heater INSIDE the OTA to the rear of the lens element.

The ED66 was recognised as having really good baffles, in the form of a serrated cone. I've extended this with my own extra baffle, which will be 3D printed then matted (the stripy bit in the middle). A concern is that the standard baffle will vignette an APS-C sensor quite badly (the 24mm square at right, the larger one is 35mm square).

In practice I don't see complaints about the scope showing vignetting, but my thoughts are to see if it causes issues and if it does 3D print a replacement with a less severe taper but the same internal serrations.

My dewshield tube is about 1/8" too small :-( Will have to think about this.

Aside from getting the lengths right, the biggest challenges on this are cosmetic; careful thinking about finishing, anodising and other coatings will be necessary as will slow and careful work at all stages.

 

image.thumb.png.f7638d7e792bc4ef790b25281b714c9e.png

 

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  • 2 weeks later...

I've finished gathering bits.

I foolishly got far too thick tubing for the draw tube.

I now have an overlength section turned to  56.5mm externally and 50.9mm internally. Marginally smaller bore than then the 130P-DS but my coma corrector is an easy fit.

Next job, internally thread one end for the clamping ring.

Edited by Stub Mandrel
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Well, I have (aside from drilling and tapping for clamp screws and parting off) made the drawtube and its end piece.

I've just tried with my coma corrector and the 1-thou oversize I allowed for anodising on 2" isn't going to be enough. The CC is close push fit and stays in, after anodising this will be a tight fit.

I don't want to ruin the finish on the outside using a steady.

I have an idea for making a plug on the end of a 1/2" bar in a rotating tailstock chuck to support the tube while I bore out another 2 or 3 thou. The forces will be tiny but I need to ensure concentricty.

I'm about to 3D print a 2" plug with a 1/2" hole.

 

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Made the main body... but when i tried to screw in the lens cell with its baffle attached - the baffle fouled the tube!

Next job, start the blank for the focuser body, and use that to mount the body, then use an internal steady (see below) to let me skim about 0.5mm off the first 25mm of the inside).

5a73825defc7e_Specialsteady.thumb.JPG.45acc12f7a7aad9ffec430685c9dec1e.JPG

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So far today, the first two ball-bearing pockets on the focuser.

Quite stressful as the screw fixing holes have to be made at the same setting and no way of knowing they are right...

Next challenge, rotate 60 degrees then make the next two. Fortunately the exact angle is not critical.

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48 minutes ago, Alan White said:

I have been hoping you will Neil, just posting is teasing us.

Wait no longer!

So far four parts made, tube, focuser body, drawtube and drawtube flange. All still raw aluminium.

Afraid photos show up every last blemish or bit of dirt - they will be properly cleaned and finished before anodising. Also various threaded holes to add.

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5a7710d808a11_FocuserBody.thumb.JPG.d4da185b1cc7ee0ad53058bda8c4e077.JPG

 

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Wow, what a great skill set to have.
Really look forward to the finished scope. 
A good lens cell in a solid and well finished tube makes for a good scope.

Wish I had the skills to reset my ED80 cell in a smaller more solid tube, but sadly not.

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I've fitted the four ball races to the focuser body. My calculations must have been OK as the tube is central and seems to work perfectly.

Next mission - the flat for the drive spindle and the spindle mounting block.

After that the 10:1 speed reducing knob!

 

Bearings in place.JPG

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  • 2 weeks later...
  • 3 months later...

Fear not!

I had a minor disaster when making the Crayford body - my digital readout's mounting plate had come loose and I took an extra millimetre of it as virtually the last step ?

As often happens that saw it put to one side and it took a  while to pick up again.

I now have a fully working Crayford, except that I need to make the single and double speed knobs and finish the locking knob. I'll post photos when these are done.

I also need to remake the eyepiece retaining ring, the knurled section isn't quite long enough to take a compression ring.

 

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When I bought my second hand MN190 the Crayford focuser was not working so I refurbished it.  Being just a straight drive with no reduction I used a 5:1 timing belt and pulley reduction from a stepper motor to remote focus.  If you're using remote focussing there's no need for a built-in ball bearing reducer.

Edited by Gina
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