Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

24" noob dob

beginner DIY dob moon mirror making
  • Please log in to reply
90 replies to this topic

#76 brave_ulysses

brave_ulysses

    Apollo

  • -----
  • Posts: 1197
  • Joined: 19 Apr 2009
  • Loc: far outside the wire

Posted 20 November 2019 - 03:58 PM

plan b, for me, would be to mount it on a rotary table with the tool in the spindle

 

what do you have in the way of tooling or access to tooling?



#77 Steve Dodds

Steve Dodds

    Owner - Nova Optical

  • *****
  • Vendors
  • Posts: 289
  • Joined: 02 Apr 2007
  • Loc: Utah

Posted 20 November 2019 - 04:00 PM

Crazy thing about these mirrors is they are 80 lbs each. Wouldn't want to spin those very fast.
Seems to me that a concept worth exploring would be to have the blank horizontal with a high rpm cutter overhead guided by a template somehow. Kinda more like a CNC.

They have machines like that, that can diamond turn metal surfaces into any shape to about 1/10 wave.  This is how the Webb telescope was made.  They are several million each.


  • bobsorenson likes this

#78 BGRE

BGRE

    Mercury-Atlas

  • *****
  • Posts: 2549
  • Joined: 21 Mar 2016
  • Loc: New Zealand

Posted 20 November 2019 - 04:33 PM

i'd be looking  to machine something like this on a mill with the mirror spinning on the quill and the tool mounted on the table. my guess is that, among hobbyists, a lathe that will swing 24" is less common than a mill with enough x/y travel to spin a 24" mirror. not sure what the max rpm on something like this should be...

No, just mount the mirror on a rotary table and tilt the head to the appropriate angle and use a cutter in the head that is set to cut at an appropriate radius. A variant of a flycutter can be used. This is a standard setup for cutting spherical surfaces on a mill. The rotary table is used to slowly rotate the workpiece whilst the milling cutter cuts the surface. Setup details are given in Horne's Optical Production Technology and elsewhere.

 

Typically such surfaces (on an aluminium substrate) are finished on a lathe with air bearing spindles and slides etc using a single point diamond tool.


  • brave_ulysses and bobsorenson like this

#79 bobsorenson

bobsorenson

    Explorer 1

  • -----
  • topic starter
  • Posts: 52
  • Joined: 05 Nov 2019

Posted 20 November 2019 - 04:51 PM

brave ulyssis,

However interesting, the subject of milling is a side topic for me. The mirrors I have are already shaped and were quite reflective when I first obtained them 18 years ago and now suffer from some corrosion and oxidation. I will be making a tool to hand work them in the next few days using 8, 5, 1, and .3 micron alumina slurries.


  • brave_ulysses likes this

#80 bobsorenson

bobsorenson

    Explorer 1

  • -----
  • topic starter
  • Posts: 52
  • Joined: 05 Nov 2019

Posted 22 November 2019 - 09:29 PM

Found an old piece of apple wood and turned it on my lathe to create a spindle that the mirror could spin on.20191121_135117.jpg

This is the back of the mirror.


Edited by bobsorenson, 22 November 2019 - 09:29 PM.


#81 bobsorenson

bobsorenson

    Explorer 1

  • -----
  • topic starter
  • Posts: 52
  • Joined: 05 Nov 2019

Posted 22 November 2019 - 09:40 PM

Today, made the lap tool.

My theory after reading several long threads about hand figuring mirrors, is that most if not all of the difficulties encountered by the unexperienced are due to the enthusiastic use of a subdiameter tool, causing under/over corrections and "zoneiness".

 

I am going to assume that the figure of my mirrors is substantially intact other than some oxidation and minor corrosion, and if so, the most practical way to preserve this figure while polishing is to use a pie shaped tool that will equalize the abrasion across the radius, thereby avoiding the troubles that I have most commonly read about.

Here is a pic of the tool being shaped on the mirror.20191122_104941.jpg

Used short radial sanding strokes down to 1500 wet and then scored. The red surface is polypropylene and recommended by Marcos, laminated to a 2" slab of hard maple wood.

For experienced mirror makers, please consider giving me some advice or tips about the shape of the lap or methods for success!


Edited by bobsorenson, 22 November 2019 - 09:44 PM.

  • brave_ulysses likes this

#82 BGRE

BGRE

    Mercury-Atlas

  • *****
  • Posts: 2549
  • Joined: 21 Mar 2016
  • Loc: New Zealand

Posted 22 November 2019 - 11:04 PM

Actually Preston's law predicts that more wear will occur towards the edge (where the linear speed of the mirror is largest) than at the center with such a tool if uniform pressure is applied.

 

This is why Zeiss and the Finns use a uniform width flexible band tool running across a diameter. They use computer control to vary the pressure as required during figuring. These tools execute a diametrical stroke as the mirror rotates underneath. Some variant of this method could be adopted for hand work.

 

The predictions of Preston's law are more accurate if all the variables, speed, pressure etc are well controlled.



#83 BGRE

BGRE

    Mercury-Atlas

  • *****
  • Posts: 2549
  • Joined: 21 Mar 2016
  • Loc: New Zealand

Posted 22 November 2019 - 11:31 PM

If the radial motion of the tool makes a significant contribution to the relative speed of the tool and the workpiece the above prediction is modified. The radial motion of the tool will make a larger relative contribution to the relative speed towards the center in which case either a change in pressure or width of the tool towards the center will be required to achieve uniform wear over the surface of the workpiece.



#84 bobsorenson

bobsorenson

    Explorer 1

  • -----
  • topic starter
  • Posts: 52
  • Joined: 05 Nov 2019

Posted 23 November 2019 - 12:04 AM

thanks BGRE,

an engineer I'm not confused1.gif ....Prestons law applies to a stationary mirror?

 

If I work around the mirror in a circle with short radial strokes and even pressure across the tool, the pie shaped tool won't remove material evenly? In my cowboy mind, this is what makes sense to me.

 

I certainly can modify the shape if I can accurately determine where the wear is going to be. Maybe easier said than done.


Edited by bobsorenson, 23 November 2019 - 12:06 AM.


#85 BGRE

BGRE

    Mercury-Atlas

  • *****
  • Posts: 2549
  • Joined: 21 Mar 2016
  • Loc: New Zealand

Posted 23 November 2019 - 01:31 AM

 It applies to both a moving and a stationary mirror and tool as well. Wear is proportional to pressure, time and relative speed of the tool and the workpiece.

 

If the mirror is stationary and you work slowly around the mirror then the radial motion of the tool dominates and a pie shaped sector tool is approximately the correct shape for even wear with uniform pressure provided the stroke length isn't too large.

 

The exact shape and or pressure distribution for uniform wear can be calculated if the velocity is known as a function of time so the appropriate averaging can be done. 

 

If the mirror rotates as well it becomes somewhat more complicated to calculated the optimum tool shape.

 

Preston's law has been successfully used to accurately model the effect of bias polishing flats including the development of a kink (abrupt change in surface slope) if the point of application of the load etc are incorrect.

 

Zeiss and others have successfully Preston's law to guide figuring of large and small surfaces. 


  • bobsorenson likes this

#86 bobsorenson

bobsorenson

    Explorer 1

  • -----
  • topic starter
  • Posts: 52
  • Joined: 05 Nov 2019

Posted 23 November 2019 - 10:53 AM

BGRE, thanks for taking the time to explain.

 

Of course, the proof will be in the pudding and while I have started building a foucault tester I suspect it will be some time before I can become proficient enough to be confident about the accuracy of the mirrors figure.



#87 marcosbaun

marcosbaun

    Explorer 1

  • -----
  • Posts: 98
  • Joined: 01 Jan 2012
  • Loc: Rio Claro / Brazil

Posted 23 November 2019 - 12:42 PM

At this link you can download the Polsim software. : http://www.astrosurf...achines_eng.htm

 

With this software it is possible to have an approximate prognosis of the work of the tool in the mirror.



#88 bobsorenson

bobsorenson

    Explorer 1

  • -----
  • topic starter
  • Posts: 52
  • Joined: 05 Nov 2019

Posted 25 November 2019 - 08:46 PM

Hello,

Could use some advice of kinds of Ronchi screen(s) best suitable for testing a 24" F/3. I am starting on my foucault/ronchi tester construction and want to do it as right as possible.

 

TIA!



#89 Pinbout

Pinbout

    ISS

  • *****
  • Posts: 23030
  • Joined: 22 Feb 2010

Posted 26 November 2019 - 07:48 PM

Hello,

Could use some advice of kinds of Ronchi screen(s) best suitable for testing a 24" F/3. I am starting on my foucault/ronchi tester construction and want to do it as right as possible.

 

TIA!

Depends on what you want to look for

 

but ronchiscreens.com is so inexpensive, and their line are very clean under high magnification.

 

100 lpi for help looking at the edge, but ke rules

 

133lpi for gross looks at correction comparing it to diffract program.

 

Get the lines outside RoC flowing smoothly then use ke to test correction.


  • bobsorenson likes this

#90 Steve Dodds

Steve Dodds

    Owner - Nova Optical

  • *****
  • Vendors
  • Posts: 289
  • Joined: 02 Apr 2007
  • Loc: Utah

Posted 27 November 2019 - 08:40 AM

Ronchiscreens.com is having supply problems.  The 85 is all that is available. 



#91 bobsorenson

bobsorenson

    Explorer 1

  • -----
  • topic starter
  • Posts: 52
  • Joined: 05 Nov 2019

Posted 30 November 2019 - 12:57 PM

Thanks Steve, Ronchiscreens has 65 only so I ordered a selection of three from Willbell.com.

I have read in CN that their screens count both lines and spaces (?), so what they call a 200 LPI actually has 100 LPI. I'll find out when they arrive.

 

Good progress made on making the foucault tester. Excited to start testing.


  • brave_ulysses likes this


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics





Also tagged with one or more of these keywords: beginner, DIY, dob, moon, mirror making



Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics