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

6in Dall-Kirkham

  • Please log in to reply
127 replies to this topic

#26 Matthew Paul

Matthew Paul

    Mariner 2

  • -----
  • topic starter
  • Posts: 218
  • Joined: 06 Jun 2016
  • Loc: Orange County, New York

Posted 27 May 2019 - 03:51 PM

I'm looking to build a simple Newton "interferometer" 

Is a red film/ florescent light source suitable, as i have seen in one writing, or should I use a low pressure sodium light?

Also, What selectivity/transparency would you recommend for a glass two way mirror? There are several available, from 45% Reflective, 55% Transparent, 70% Reflective, 30% Transparent, and 70% Reflective, 11% transparent


Edited by Matthew Paul, 27 May 2019 - 04:21 PM.


#27 BGRE

BGRE

    Gemini

  • *****
  • Posts: 3,005
  • Joined: 21 Mar 2016
  • Loc: New Zealand

Posted 27 May 2019 - 04:45 PM

An uncoated UV fluorescent tube like those used in EPROM erasers and insect zappers that glow bluish purple (not the variety with a black coating) are better suited when used with a green filter to isolate the mercury green line. The large (6") surplus shed yellow filters are effective. Green acrylic also works. The coherence length is typically around 50mm or so. Sodium lamps produce a yellow doublet which results in very low fringe visibility for optical path differences that are separated by an odd multiple of  about 500 waves. One of  the large surplus shed plate beamsplitters can also be useful in allowing viewing the test surface close to normal incidence.   


  • Matthew Paul likes this

#28 Matthew Paul

Matthew Paul

    Mariner 2

  • -----
  • topic starter
  • Posts: 218
  • Joined: 06 Jun 2016
  • Loc: Orange County, New York

Posted 27 May 2019 - 05:11 PM

Thank you for the reply and information. Would LEDs be suitable as well (from a simplicity, cost, and attainability standpoint), specifically green if that is the spectra that is desired?


Edited by Matthew Paul, 27 May 2019 - 05:11 PM.


#29 BGRE

BGRE

    Gemini

  • *****
  • Posts: 3,005
  • Joined: 21 Mar 2016
  • Loc: New Zealand

Posted 27 May 2019 - 05:19 PM

LEDs have a very short coherence length (ie large bandwidth) and are not generally suitable unless used in conjunction with an expensive narrow bandwidth filter.

 

A suitably diffused laser source works well. One method is to use a colloidal suspension of titanium dioxide in a glass cuvette (or equivalent) which is painted matte white apart from an exit aperture. A plastic (or glass) optical fiber immersed in the liquid is used to bring the laser light into the colloidal suspension. 



#30 Matthew Paul

Matthew Paul

    Mariner 2

  • -----
  • topic starter
  • Posts: 218
  • Joined: 06 Jun 2016
  • Loc: Orange County, New York

Posted 27 May 2019 - 05:34 PM

I understand on the bandwidth and that's where I stopped comprehending what you were saying. I'm a blacksmith with no college education.. Unfortunately. I'm just looking to check a hopefully spherical secondary against a spherical test plate. 



#31 BGRE

BGRE

    Gemini

  • *****
  • Posts: 3,005
  • Joined: 21 Mar 2016
  • Loc: New Zealand

Posted 27 May 2019 - 05:53 PM

Either a sodium lamp or a uncoated fluoresecent tube with filter will work. LEDs are unlikely to work.

A standard fluorescent lamp with a green filter will also work although the fringe contrast will be reduced by the light produced by the posphor.

 

Sodium lamps emit a double line in the yellow with a wavelength difference of about 0.6nanometers With an airgap of around 250 waves (~150 microns) the 2 fringe patterns (one for each wavelength) will be out of step and the result will be low contrast fringes. This repeats for airgaps of 750 waves, 1250 waves etc or about every 0.3mm. As long as the airgap isnt close to one of these values the fringe contrast will be OK. 



#32 Arjan

Arjan

    Apollo

  • -----
  • Posts: 1,263
  • Joined: 21 Jan 2009
  • Loc: Netherlands

Posted 28 May 2019 - 12:01 AM

A green DPSS laser has a well defined wavelength of 532nm. These are attainable for under $10.
I have deviced a simple diffuser from a white plastic bag and some tissues or so. Many things work, you just need a more or less egally lit plane facing the glass.
  • MKV and Matthew Paul like this

#33 DAVIDG

DAVIDG

    Cosmos

  • *****
  • Posts: 9,779
  • Joined: 02 Dec 2004
  • Loc: Hockessin, De

Posted 28 May 2019 - 09:47 AM

I'm looking to build a simple Newton "interferometer" 

Is a red film/ florescent light source suitable, as i have seen in one writing, or should I use a low pressure sodium light?

Also, What selectivity/transparency would you recommend for a glass two way mirror? There are several available, from 45% Reflective, 55% Transparent, 70% Reflective, 30% Transparent, and 70% Reflective, 11% transparent

 If your building a Fizeau type "newtonian" interferometer, then the light source goes were the eyepiece would  be and slightly off axis. So what you want is a 110 volt neon bulb. They are small enough to mount off axis in a wooden plug that goes were the eyepiece is located and a hole in the middle to look thru.

   If just want to build a light box with a piece of glass mounted at 45 degrees then a standard CFL bulb and a diffuser  made from  a white a piece paper will work fine.

 

                  - Dave  


  • Matthew Paul likes this

#34 Matthew Paul

Matthew Paul

    Mariner 2

  • -----
  • topic starter
  • Posts: 218
  • Joined: 06 Jun 2016
  • Loc: Orange County, New York

Posted 28 May 2019 - 11:09 AM

 If your building a Fizeau type "newtonian" interferometer, then the light source goes were the eyepiece would  be and slightly off axis. So what you want is a 110 volt neon bulb. They are small enough to mount off axis in a wooden plug that goes were the eyepiece is located and a hole in the middle to look thru.

   If just want to build a light box with a piece of glass mounted at 45 degrees then a standard CFL bulb and a diffuser  made from  a white a piece paper will work fine.

                  - Dave  

Thanks Dave. I'm just looking to build the simple light box type. Hopefully the mirrors and test plate will be polished out in the next couple of weeks and Ill be able to start on figuring them. 

I'm not sure how much it matters that there be a narrow band of light, but I found a Rosco Permacolor 2"x2" Borosilicate filter that has over 90% transmission in the 610-620nm range and zero percent from 590nm down. There will be a little bit of the yellow, but it is quite a low output from that bulb anyway, and with this filter should be about 50% of what it is. It should work perfect with an Ecosmart Daylight CFL bulb to isolate the 610-620nm light from the blue and green spectrum. The filter is $12 and the bulb is just a couple of dollars. That is, if it matters for what I'm doing. 

I'll attach the two images of the spectra. Also for future reference I found that Rosco has software with data on all of their filters showing wavelength transmission, etc. which could be useful. It can be found here: https://us.rosco.com/en/mycolor

ecosmart daylight.png rosco filter 5900.png


Edited by Matthew Paul, 28 May 2019 - 11:14 AM.


#35 DAVIDG

DAVIDG

    Cosmos

  • *****
  • Posts: 9,779
  • Joined: 02 Dec 2004
  • Loc: Hockessin, De

Posted 28 May 2019 - 12:19 PM

 All you need is  a simple plastic filter. I have used clear green or red candy wrappers or cover sheets from  " book report" binders from Staples. Also you don't have to filter the light source you can just look thru a filter like an eyepiece filter to improve the contrast. Even if you don't use a filter the fringes are pretty easy to see with a CFL bulb.

Here is  a picture of the fringes from the  CFL  bulb in my desk lamp that I use all the time to check small flats and the centering of the elements in lenses. 

  The test plate doesn't need a full polish. Just enough to test to be sure it a sphere.  If you used the tool that you used to grind the convex secondary, the problem you might have is getting the radius to match close enough so you don't see  20 or more  fringes when you test. That is too many to have the needed sensitivity. You need around 5 fringes. Even though you ground the pieces together there is still a difference in the radii between the pieces and  fully polishing the convex surface may increase the difference. So I would flash polish both surfaces and test them by interference and see how close they match. 

 

 

                     - Dave 

 

  lens interference pattern CFL bulb.jpg         


Edited by DAVIDG, 28 May 2019 - 12:41 PM.

  • tim53 likes this

#36 Matthew Paul

Matthew Paul

    Mariner 2

  • -----
  • topic starter
  • Posts: 218
  • Joined: 06 Jun 2016
  • Loc: Orange County, New York

Posted 28 May 2019 - 12:32 PM

Thank you VERY much Dave. Do I even need the two way mirror and all that for what I'm trying to do?



#37 DAVIDG

DAVIDG

    Cosmos

  • *****
  • Posts: 9,779
  • Joined: 02 Dec 2004
  • Loc: Hockessin, De

Posted 28 May 2019 - 12:39 PM

Thank you VERY much Dave. Do I even need the two way mirror and all that for what I'm trying to do?

 I would say no you don't need the mirror just  look straight down from about 2  feet above the pieces being tested and  you'll be fine. I think the radius match is going to be what you should look into first. 

 

               - Dave 



#38 Matthew Paul

Matthew Paul

    Mariner 2

  • -----
  • topic starter
  • Posts: 218
  • Joined: 06 Jun 2016
  • Loc: Orange County, New York

Posted 28 May 2019 - 12:53 PM

Fantastic. I'm going to have to buy you a vacation to Disney Land as a "Thank You" by the time I finish this telescope, haha. 



#39 MKV

MKV

    Cosmos

  • *****
  • Posts: 9,065
  • Joined: 20 Jan 2011
  • Loc: Southeast, USA

Posted 28 May 2019 - 01:30 PM

Here is  a picture of the fringes from the  CFL  bulb in my desk lamp that I use all the time to check small flats and the centering of the elements in lenses. 

Here's a simialr image in a brightly lit room with a CFL bulb. The only requiremenet is dark background for the fringes to stand out.

 

new fring.jpg


  • tim53 and Matthew Paul like this

#40 Arjan

Arjan

    Apollo

  • -----
  • Posts: 1,263
  • Joined: 21 Jan 2009
  • Loc: Netherlands

Posted 30 May 2019 - 07:22 AM

A green DPSS laser has a well defined wavelength of 532nm. These are attainable for under $10.
I have deviced a simple diffuser from a white plastic bag and some tissues or so. Many things work, you just need a more or less egally lit plane facing the glass.

Here is an example of the contrast you can achieve when using such a laser.

 

300f8-39.jpg

 

One more thing is to make sure the support of the lower piece is very flat and a somewhat compliant. I use a slab of stone covered with black flocking paper, but found that I needed to support the tile carefully as well, to prevent deformation of the reference glass. Once you're there and use only a few fringes you will find that the test is very sensitive. I have used the rotation/derotation technique with DFTFringe (like with Bath interferometry) to average the measurements and analyze the fringes.

 

Good luck!


Edited by Arjan, 30 May 2019 - 07:22 AM.

  • Matthew Paul likes this

#41 BGRE

BGRE

    Gemini

  • *****
  • Posts: 3,005
  • Joined: 21 Mar 2016
  • Loc: New Zealand

Posted 30 May 2019 - 08:29 AM

Such cheap green DPSS lasers can be hazardous due to strong IR emission at 1064nm as detailed in NIST technical note 1668. They are best avoided for such visual use.


  • brave_ulysses and Matthew Paul like this

#42 MKV

MKV

    Cosmos

  • *****
  • Posts: 9,065
  • Joined: 20 Jan 2011
  • Loc: Southeast, USA

Posted 30 May 2019 - 11:26 AM

Here is an example of the contrast you can achieve when using such a laser.

 

 

One more thing is to make sure the support of the lower piece is very flat and a somewhat compliant. I use a slab of stone covered with black flocking paper, but found that I needed to support the tile carefully as well, to prevent deformation of the reference glass.

The US Bureau of Standards publication suggests the reference glass be supported at the critical (0.7071) of aperture radius for minimum deformation. Not the towels, bubble wrap, foam, etc. The publication is dated and the Bureau has been renamed since then, but the psysical principles remain the same.

 

https://www.nist.gov...ions/75-975.pdf


  • Matthew Paul likes this

#43 MKV

MKV

    Cosmos

  • *****
  • Posts: 9,065
  • Joined: 20 Jan 2011
  • Loc: Southeast, USA

Posted 30 May 2019 - 11:26 AM

Such cheap green DPSS lasers can be hazardous due to strong IR emission at 1064nm as detailed in NIST technical note 1668. They are best avoided for such visual use.

Bruce, what if such a laser is < 5 mW?



#44 brave_ulysses

brave_ulysses

    Apollo

  • -----
  • Posts: 1,374
  • Joined: 19 Apr 2009
  • Loc: far outside the wire

Posted 30 May 2019 - 12:07 PM

any issues with pump laser 808nm emission?



#45 Matthew Paul

Matthew Paul

    Mariner 2

  • -----
  • topic starter
  • Posts: 218
  • Joined: 06 Jun 2016
  • Loc: Orange County, New York

Posted 30 May 2019 - 04:04 PM

I adjusted some of the specs on the mirrors and gave myself an additional 2in of backfocus, as I had it set for 8in from the start, and now I will have 10in. Spot diagrams look great still. Primary mirror ROC is now 47.5 and secondary is 13.375. System ihas  FL of 124.4"



#46 Arjan

Arjan

    Apollo

  • -----
  • Posts: 1,263
  • Joined: 21 Jan 2009
  • Loc: Netherlands

Posted 30 May 2019 - 04:52 PM

Such cheap green DPSS lasers can be hazardous due to strong IR emission at 1064nm as detailed in NIST technical note 1668. They are best avoided for such visual use.

Good warning; you should not need to look into the light, but use a camera. Also, before use the light is diffused. The nice thing about these lasers is their well defined wavelength.
  • Matthew Paul likes this

#47 BGRE

BGRE

    Gemini

  • *****
  • Posts: 3,005
  • Joined: 21 Mar 2016
  • Loc: New Zealand

Posted 30 May 2019 - 05:10 PM

The NIST publication shows how you can check a particular pointer for IR emission using a camera without an IR filter (eg webcam with filter removed) and a cheap diffraction grating (eg a CD).

The problem is most acute when the IR blocking filter in the DPSS laser is left out to reduce costs.

In the NIST paper the power in the 808nm pump beam present in the output was much lower than that of the 1064nm beam. A low visible output power is no guarantee of low IR emission. NIST actually found that a green DPSS with low output at 532nm had a very high output at 1064nm. Link to download page for Technical Note 1668:

 

https://www.nist.gov...-pointer-hazard



#48 BGRE

BGRE

    Gemini

  • *****
  • Posts: 3,005
  • Joined: 21 Mar 2016
  • Loc: New Zealand

Posted 30 May 2019 - 05:53 PM

Schott BG18 and KG5 filter glasses can be used to block (transmission < 0.01% if used together) the 808nm and 1064nm beams.

https://www.schott.c...jun-2017-en.pdf

 

https://www.schott.c...jun-2017-en.pdf

 

IIRC BG18 was used in the Johnson  V filter used in photoelectric stellar photometry.



#49 Matthew Paul

Matthew Paul

    Mariner 2

  • -----
  • topic starter
  • Posts: 218
  • Joined: 06 Jun 2016
  • Loc: Orange County, New York

Posted 02 June 2019 - 07:52 AM

So, like a dope I did more than a flash polish on the test plate. I wound up chasing it back and forth all day with the lap trying to get it back to a sphere. Do you think that I would be better off hitting it with 5 micron to bring the plate and secondary back in contact, to a sphere, than trying to do it with the laps? 



#50 MKV

MKV

    Cosmos

  • *****
  • Posts: 9,065
  • Joined: 20 Jan 2011
  • Loc: Southeast, USA

Posted 02 June 2019 - 10:38 AM

So, like a dope I did more than a flash polish on the test plate. I wound up chasing it back and forth all day with the lap trying to get it back to a sphere. Do you think that I would be better off hitting it with 5 micron to bring the plate and secondary back in contact, to a sphere, than trying to do it with the laps? 

Depends how far off sphere you are.


  • Matthew Paul 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






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics