14 replies to this topic

[Ed Jones]

This is a project I finally  got some free time to work on, a 10 inch F/5 Chief.  It’s partially obstructed because I doesn’t do well fully unobstructed.  However only haff the secondary obstructs and there is still improvement to the diffraction disk.  It’s a work in progress.  Got my primary back from the coater so time to get on with it.

 

I use a tripod support, a 1.5 inch aluminum angle (7) and two  ¾ X1/8 supports  giving a decent damping time.  The upper assembly screws on to a hardwood block (not shown) screwed to the aluminum angle using a thumb wheel (4).  It is prevented from rotating by plate (5) and nut (6).  A helical focuser (1) is fastened to the lens barrel (2) which has a machined  groove in which Delfrin clips (3) ride and allow it to rotate, locked with a plastic tip set screw not shown.

Attached Thumbnails

• 
• 

[Ed Jones]

The two lenses (17) are held in a wedged doughnut (18) and rotate about pivot (19).  The tilt of the lenses is changed using a string winder (14) pulling a piece of parashoot string over a piece of Teflon tape (15 and opposed by spring (15).  So the lens holder can rotate about Z and tilts in Y. 
  The 2 5/8 in. secondary is mounted to the secondary holder (10) with 3M mounting tape is held with nut (11) for rotation and screw (12) for tip.  A .054 in. thick sheet rubber baffle (9) covers the secondary eliminating all stray light (better than any Newt).  
The aluminum angle is clamped in place so the tripod is removeable.

Yet to do
1. Modify the concave lens (17) curve slightly. Lap is done and lens is accessible Without disassembly
2. get stronger magnets and replace washers with black ones using epoxy not contact cement
3. replace side clamps with steel
4. alignment, first lught…see if this was worth it  LOL

Attached Thumbnails

• 
• 

[Chris Westland]

Can you diagram the optical train ... is this a corrected Newt?

[Ed Jones]

No this is a partially obstructed Chief like this one,

[hamishbarker]

Beautiful design and execution Ed!

[Dick Jacobson]

Beautiful! This could also reduce dew/frost buildup on the secondary.

[Ed Jones]

The next thing I had to do is tweak the radius on the concave lens.  It’s a Newport  lens is KPC067, a 2 inch diameter plano-concave lens with a radius of 129.2 mm , the same radius as the plano-convex lens  KPX205.  The design calls for a radius of 144.46 mm.  So I made a 2.5 inch disk of ¼ inch glass, stuck it on a post for my polisher with 3M ultra high strength tape and generated the 129 mm radius on it.  I made a 2 inch OD plaster cast of it, varnished it then epoxied ¼ inch squares of double strength window glass as shown.  I would have used microscope slide glass but I was out.  I lapped it smooth down to 20 micron grit and recorded the sag measured on the spherometer.

Attached Thumbnails

• 

[Ed Jones]

I covered the flat side with clear packing tape and made a 1.25 OD Delrin holder and put a steep counter sink on it.  I then taped it to the lens with the 3M tape checking with calipers that it was perfectly centered so as to not develop wedge in grinding.  I ground a groove in it because anytime you grind two solid disks together you need a channel for the grit to circulate and avoid possible sticking.  I set up the stroke shown in this video where the edge of the lens just crosses center (required) and started lapping with 20 micron with light pressure.  Since the lens is steeper it starts grinding from the edge in.  It goes pretty quick and you need to reduce the offset as it does to help stop driving the lap steeper.  It did any way by +25 microns so when the ground ring was about half way across I fixed the lap with the plaster lap and made it about -10 microns.  BTW you have to measure the radius of the lap not the lens because you can’t measure the lens until it cleans up completely and I wanted to be done by then (0 thickness removed).  Also you want to measure the width of the ground edge as it increases to be sure there is no wedge developing.  I didn’t so I didn’t have to correct it by hand working on the thinnest side.  When there was only about 1/8 inch left in the center I switched to 9 micron grit and moved the offset out a little to increase the sag a few microns to read the same initial sag.

 

 

 

 

 

Attached Thumbnails

• 

[Ed Jones]

Next I made an oversize pitch lap by my pizza method.  With an oversize lap you can find a neutral stroke that will not change the radius but an oversize lap is trickier to make because you have to keep moving the part around and pressing.  It cooled pretty quickly so I used a hot air gun for just seconds to warm it up (easily overdone!).  After a lot of pressing I finally got it right and went on to polishing.  I tested it with a Ronchi and it was great (not set up to shoot it).  The packing tape did save me in an accident but left a sticky mess when I peeled it off needing kerosene to clean it up.

Attached Thumbnails

• 

[duck]

not your first rodeo

[hamishbarker]

Thanks so much for the detailed posts Ed!

Edited by hamishbarker, 06 August 2024 - 03:06 PM.

[Ed Jones]

I haven't been to a rodeo but I have made a few lenses. LOL   Actually it isn't very difficult if you have a machine, not so easy if you don't I suppose.  A motorized spindle with a manual oveerarm should work and east to make or maybe a mineature mirror-o-matic.  

[hamishbarker]

how many rpm?  little electric mini pottery wheels are available on aliexpress for less than $100. (listed as 0-1500rpm! whew 1500 would really be throwing a pot!)

[Ed Jones]

I would thing 240 rpm max would be sufficient but i have used a lot higher at work.

[jgraham]

It’s always fun to watch an Ed Jones project. I was wondering if we’d eventually see a fast Chief. I have all the parts for my 8” f/8 Chief, now all I need is the time!

 

Fun stuff.