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

a $26 Zeiss prism's new life

Optics Equipment DIY Accessories 3d printing
  • Please log in to reply
13 replies to this topic

#1 erraph

erraph

    Explorer 1

  • -----
  • topic starter
  • Posts: 98
  • Joined: 28 Aug 2005
  • Loc: France

Posted 14 May 2021 - 07:29 AM

Hey DIY astronomers,

 

I recently acquired an unused Zeiss 34mm 90 degree prism, multi coated, with (reflective) coated hypotenuse. For $26.

The catch: it's glued on to a metal mounting plate. 

 

Wouldn't it be fun to give it a new life as a star diagonal? In this thread, I'll post updates on how this is proceeding.

 

Step 1:  Unboxing

 

Shrink-wrapped time capsule. The original item was apparently bought for 45 German Marks at some exhibition , though I have no idea when or where. The label "gefaszt" is a typewriter-way of writing "gefa├čt", which means mounted. 

 

20210511_102400.small.small.jpg

 

Inside, a transparent zip bag, containing the prism and some desiccant

 

20210511_102518.small-1.small.small.jpg

 

That's the prism, with the metal base. The sides of the prism have a coating which shows - under certain angles - a slightly purple hue. The hypotenuse has a reflective coating, blackened from the back. 

 

20210511_102914.small-1.small.jpg

 

 

Step 2:  Separating the prism from its original metal mount

 

I tried to soften the glue with isopropyl alcohol, with limited success. (I wanted to avoid soaking the prism side for too long to avoid damaging the coating on the hypotenuse). I very slowly heated the metal base, as heat loosens many epoxy glues. Slowly, to avoid temperature gradients in the prism. I gently tried to apply some torque to the prism, wearing cotton gloves, trying to avoid touching the hypotenuse. Quite tricky. But it worked. 

 

20210511_124435.small-1.small.jpg

 

Then, with some more isopropyl alcohol and a razor blade, I removed the leftover glue from the prism, and blackened the prism sides. 

 

20210511_140119.small-1.small.jpg

 

 

 

Step 3: Preparing the new mount 

 

I found a broken second-hand star diagonal online ( though still interested in finding a second one !) , quite tricky as most star diagonal bodies are too small to house these 34mm prisms. The body of this broken diagonal is going to host the 34mm Zeiss prism.

I made a CAD drawing of a new mount for the prism which fits inside the star diagonal body, to be 3D printed by a friend. I paid careful attention that the printed mount ensures the hypotenuse to lie in the same position as did the old prism. Shims will be used to make this alignment as accurate as possible. 

 

The prism will  have 4 contact points at the edge of the hypotenuse, where I can place shims for collimation. The main contact though comes from 3 raised points from the two triangular sides of the mount. ( Thanks to the help in this thread: https://www.cloudyni...-star-diagonal/ ).

 

Screenshot_20210514_142232.jpg  Screenshot_20210514_142148.jpg

 

 

 

 

 

Currently I am waiting for the 3D print to reach me. Then I'll give more updates! Fingers crossed.


Edited by erraph, 14 May 2021 - 08:22 AM.

  • Astrojensen, jjack's, davidc135 and 3 others like this

#2 j.gardavsky

j.gardavsky

    Mercury-Atlas

  • *****
  • Posts: 2,837
  • Joined: 18 Sep 2019
  • Loc: Germany

Posted 14 May 2021 - 08:44 AM

Very good Erraph!

 

I have seen this prism at astrooptik, but already have my oversized 2" prism from Wild Heerbrugg.

 

Best,

JG


  • erraph likes this

#3 MitchAlsup

MitchAlsup

    Fly Me to the Moon

  • -----
  • Posts: 5,538
  • Joined: 31 Aug 2009

Posted 14 May 2021 - 10:37 AM

You cannot machine inside corners without a radius.



#4 Rob Willett

Rob Willett

    Viking 1

  • -----
  • Posts: 702
  • Joined: 07 Feb 2005
  • Loc: London, UK.

Posted 15 May 2021 - 10:28 AM

Please keep this thread updated. This is the type of thread that keeps me coming back to Cloudy Nights.You may not get to where you want to go, but it's always interesting to see what people are doing.

 

If you need another 3d printer, let me know. I'm based in the UK and I have an Ender 3 Pro sitting doing very little until my next mad project comes along. The last one was a combined mount plate on a Borg 76 with a Telrad base and a 30mm finder. Currently got White, Red and Black filament sitting here.

 

One think to know about 3d printing is that quite often the size you want is not the size you get. The plastic flows and you can easily get 0.1mm or more difference on prints from what you expect.

 

Other people have helped me so I'm very happy to do it for free. A print like the one above looks less than a nights job, so turnaround is quick.

 

Just PM if you want any done.

 

Thanks

Rob


  • erraph likes this

#5 erraph

erraph

    Explorer 1

  • -----
  • topic starter
  • Posts: 98
  • Joined: 28 Aug 2005
  • Loc: France

Posted 21 May 2021 - 01:31 AM

Hi everyone, thanks for the kind comments. Here the updates!

 

The 3D prints made by a friend arrived (thanks!!), and they match my CAD drawing marvellously.

 

20210521_073429-1.small.jpg

 

The body I did get hold of belonged to a Baader T2 (mirror!) diagonal which was dropped by the previous owner. The drop chipped off parts of the original mirror, and left some scuffs on the body itself. (I at first wanted to use the diagonal with the chipped mirror, but the damage was too noticeable.)

 

Here all parts to assemble (feels like playing with LEGOs now):

 

20210521_073339-1.small.jpg

 

Some tape shims to get alignment right, and here is everything inside the ex-mirror-now prism diagonal body:

 

20210521_073648-1.small.jpg

 

And everything closed up:

 

20210521_081718-1.small.jpg

 

 

With the prism sides blackened, this makes for such a dark view inside the diagonal! Now i just need to wait for the rain to stop to give the diagonal its first light.


Edited by erraph, 21 May 2021 - 01:32 AM.

  • Diego, jjack's, photomagica and 1 other like this

#6 Benach

Benach

    Vanguard

  • *****
  • Posts: 2,482
  • Joined: 24 Jan 2008

Posted 21 May 2021 - 05:54 AM

What have you used to blacken the sides of the prism?



#7 j.gardavsky

j.gardavsky

    Mercury-Atlas

  • *****
  • Posts: 2,837
  • Joined: 18 Sep 2019
  • Loc: Germany

Posted 21 May 2021 - 06:50 AM

Hi everyone, thanks for the kind comments. Here the updates!

 

...

 

attachicon.gif20210521_073339-1.small.jpg

 

...

 

And everything closed up:

 

attachicon.gif20210521_081718-1.small.jpg

 

 

With the prism sides blackened, this makes for such a dark view inside the diagonal! Now i just need to wait for the rain to stop to give the diagonal its first light.

Erraph,

 

this is looking super! Maybe even better than the original.

 

Just on a sideline of my curiosity,

could you record a laser beam ray tracing through the Baader T2 prism,

like I have done here for the Wild Heerbrugg and Takahashi,

 

Zenith Prisms Wild Tak ray tracing.jpg

 

Best,

JG


  • erraph, jjack's and kjkrum like this

#8 erraph

erraph

    Explorer 1

  • -----
  • topic starter
  • Posts: 98
  • Joined: 28 Aug 2005
  • Loc: France

Posted 21 May 2021 - 10:23 AM

Erraph,

 

this is looking super! Maybe even better than the original.

 

Just on a sideline of my curiosity,

could you record a laser beam ray tracing through the Baader T2 prism,

like I have done here for the Wild Heerbrugg and Takahashi,

 

attachicon.gifZenith Prisms Wild Tak ray tracing.jpg

 

Best,

JG

Hi JG,

 

I tried to take a picture like yours, but did not quite succeed as neatly -- I only have a red laser which is very wide (~ 3mm), and I am unable to follow the beam inside the prism (this is a 30 second exposure). However I can state that visually, the laser seems not to scatter in any perceivable way beyond its intrinsic width of 3mm at the glass-air surfaces . 

 

I think we should be careful though not to refer to this prism as the "Baader T2 " -- it's a Zeiss prism of unclear original use inside the body of a Baader T2 mirror . 

 

Cheers

Rapha

Attached Thumbnails

  • Screenshot_20210521_171726.jpg

Edited by erraph, 21 May 2021 - 10:37 AM.

  • j.gardavsky likes this

#9 erraph

erraph

    Explorer 1

  • -----
  • topic starter
  • Posts: 98
  • Joined: 28 Aug 2005
  • Loc: France

Posted 21 May 2021 - 10:25 AM

What have you used to blacken the sides of the prism?

 

Hi Benach, I used flat black acrylic spray paint. Both spraying on and applying with a cotton bud work well -- I think it's crucial that the paint is rather liquid so it fills the tiny valleys on the rough surface of the prism edges. 


  • Benach likes this

#10 j.gardavsky

j.gardavsky

    Mercury-Atlas

  • *****
  • Posts: 2,837
  • Joined: 18 Sep 2019
  • Loc: Germany

Posted 21 May 2021 - 12:18 PM

Hi JG,

 

I tried to take a picture like yours, but did not quite succeed as neatly -- I only have a red laser which is very wide (~ 3mm), and I am unable to follow the beam inside the prism (this is a 30 second exposure). However I can state that visually, the laser seems not to scatter in any perceivable way beyond its intrinsic width of 3mm at the glass-air surfaces . 

 

I think we should be careful though not to refer to this prism as the "Baader T2 " -- it's a Zeiss prism of unclear original use inside the body of a Baader T2 mirror . 

 

Cheers

Rapha

Hello Rapha

 

what is needed is the green laser pointer. I cover its head with an alu foil (from the kitchen of my wife) and make a pinhole with a needle. Herewith the laser beam has just a minimum diameter.

 

The test I have posted shows a clean retroreflection on the Wild Heerbruug, as expected from something manufactured under the Mil standards.

 

Best,

JG


Edited by j.gardavsky, 21 May 2021 - 12:19 PM.


#11 kjkrum

kjkrum

    Mariner 2

  • *****
  • Posts: 263
  • Joined: 08 Mar 2019
  • Loc: Tucson, Arizona, USA

Posted 22 May 2021 - 10:28 AM

JG,

Is that photo of the Tak prism meant to illustrate more scattering than the other prism?
  • j.gardavsky likes this

#12 j.gardavsky

j.gardavsky

    Mercury-Atlas

  • *****
  • Posts: 2,837
  • Joined: 18 Sep 2019
  • Loc: Germany

Posted 22 May 2021 - 11:40 AM

JG,

Is that photo of the Tak prism meant to illustrate more scattering than the other prism?

Yes,

 

I have been surprised about the scattering and about some multiple retroreflections on the Tak.

Therefore, it would be interesting to see the laser ray tracing on the Zeiss prism for comparison.

 

Prisms for the tactical optics are specified and manufactured under the military specifications, like the Wild Heerbrugg for the Swiss Army.

If not found as used or among the surplus parts, such a 60mmx60mm prism would cost today more than a nice refractor.

 

There used to be the Manfred Pieper's shop in Germany, specialized on repurposing of the tactical optics for the hobby astronomy.

What is still left is the astroshop, offering some tactical optics, but not repurposed for the astronomy.

 

Best,

JG



#13 LU1AR

LU1AR

    Vostok 1

  • -----
  • Posts: 189
  • Joined: 04 Mar 2020

Posted 24 May 2021 - 02:56 PM

Prisms are great for doing other projects. Right now I'm building a Herschell Wedge for solar observation, using the long catetus of a prism, salvaged from a broken binocular.
To center the line of sight and ensure that it passes through the center of the objetive, I used a homemade laser collimator that you can see in the photo.
To get the laser in the middle of the objective, I had to touch up the prism, which -at this stage- is mounted with epoxy putty.
After doing that test, I was curious to know how the commercial diagonals centered the beam. I was surprised that not all of them focused on the center of the target.
Using one of these diagonals will make us use one side of the lens or the mirror and the quality of the image will not be what we could obtain, if these diagonals had a centering adjustment.
My inexpensive refractor has been upgraded with baffles and aperture reduction mask. By using a correct diagonal, the image quality is very good.
If you are going to make any diagonal -either with a prism or a mirror- I suggest that you have some kind of adjustment to optimize it.
Diagonals with a prism are usually problematic in telescopes of F / 7 or less.
They are due to the fact that the short focal length does not ensure that the light enters strigh, but passes through the prism in a different way between the lateral and central rays.
This causes interference patterns that degrade optical quality.
If you plan to use high magnification at short focal lengths, you should use a mirrored diagonal.
I hope my experience serves someone.
Edgardo

Attached Thumbnails

  • Prisma de Herschell 06.jpg
  • Prisma de Herschell 05.jpg

Edited by LU1AR, 24 May 2021 - 03:00 PM.

  • j.gardavsky likes this

#14 j.gardavsky

j.gardavsky

    Mercury-Atlas

  • *****
  • Posts: 2,837
  • Joined: 18 Sep 2019
  • Loc: Germany

Posted 24 May 2021 - 03:34 PM

A very nice setup, Edgardo, and thank you for sharing the pics!

 

You are right, that the prisms are better suited for the slower refractors.

This has been the reason why I have over the years selected some eyepieces, which don't make chromaticity problems with a prism on my F/5 refractor.

 

Other test have been made by Wolfgang Mohr in:

http://astro-foren.c...asweg-verwenden

http://astro-foren.c...g-ein-super-apo

http://astro-foren.c...g-sehr-farbrein

 

Otherwise,

I have next to the zenith prism, the BBHS mirror, and the dielectric mirror (all in 2").

So there are three options to choose from. The 1.25" Tak prism is in reserve and for the time being in a drawer.

 

Best,

JG

 

 

 


  • LU1AR 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: Optics, Equipment, DIY, Accessories, 3d printing



Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics