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

long slow newtonian + optical flat from scratch

  • Please log in to reply
34 replies to this topic

#1 GarethBarry

GarethBarry

    Sputnik

  • -----
  • topic starter
  • Posts: 49
  • Joined: 14 Apr 2019

Posted 16 August 2019 - 06:04 AM

Ok so after much deliberation I decided to go with a long slow newtonian, 10 inch, about f8 or so, as a planetary scope for the school that I work at. This is a step up from my first project in that it is both slightly larger, thinner (19mm) and plate glass. However, I don't anticipate too much trouble with this.

 

However, as a concurrent project, I would like to polish a flat to use for autocollimation and other tests. It is a 19mm thick piece of plate glass, 10 inch diameter, perforated with a 25mm diameter hole, all water jet cut. Being plate glass, of this thickness/diameter, I know this isn't the best substrate and thickness for a flat of this size. Is this project a waste of time? My plan is to start from about 600 grit with a penny tool, alternating equal amounts MOT and TOT, and just work up to 1200 grit and finally pitch.  Does this sound like a workable plan? I would perhaps use the acetone test to check for flatness during grinding-or perhaps even an engineers straight edge with a light.I plan to use the Raleigh water test for final figuring. I have tried making elliptical flats and it was a disaster (I started grinding them from an ellipse instead of a circle), so this project might be overly ambitious..



#2 siriusandthepup

siriusandthepup

    Apollo

  • *****
  • Posts: 1064
  • Joined: 14 Feb 2006
  • Loc: Central Texas, USA

Posted 16 August 2019 - 06:17 AM

I would recommend and 8" f/8 or 8" f/7 to bring down the eyepiece height. With the 10" f/8 all the students are gonna be on the ladder/step stool, which means that all the students have the opportunity to fall off. You don't want that.

 

Just buy a flat - it's not worth the effort to try and make your own diagonal for a one use telescope.

 

my 2 cents...



#3 Pinbout

Pinbout

    ISS

  • *****
  • Posts: 22995
  • Joined: 22 Feb 2010

Posted 16 August 2019 - 06:37 AM

Buy. Used comparator flat on eBay and test it. Dpac doesn’t require flat flat. Just smooth flat. No kinks.


  • Venetia2004 likes this

#4 GarethBarry

GarethBarry

    Sputnik

  • -----
  • topic starter
  • Posts: 49
  • Joined: 14 Apr 2019

Posted 16 August 2019 - 07:44 AM

Thanks guys.
Regarding the focal ratio-my limited experience so far is that my 9 inch f6 is seldomly used pointing at the zenith. I also like the depth of focus on offer with my smaller f8 reflector.
I will definitely be buying the secondary.

I am more wondering about the large flat and the process required for grinding and figuring. Anyway, i will start next week and keep updates.

#5 Pinbout

Pinbout

    ISS

  • *****
  • Posts: 22995
  • Joined: 22 Feb 2010

Posted 16 August 2019 - 08:14 AM

I have a pyrex 10in f7.8... if you want to trade something. nice figure. I made it at Delmarva with Swayze's help.



#6 Mark Harry

Mark Harry

    Cosmos

  • *****
  • Posts: 8281
  • Joined: 05 Sep 2005
  • Loc: Northeast USA

Posted 16 August 2019 - 09:08 AM

I would advise against making an AC flat out of plate glass. (just based on my experience with a 5"er I made) Pyrex works a whole lot better.


  • Oregon-raybender, MKV and Dave O like this

#7 DAVIDG

DAVIDG

    Cosmos

  • *****
  • Posts: 9088
  • Joined: 02 Dec 2004
  • Loc: Hockessin, De

Posted 16 August 2019 - 09:10 AM

 Another method to test the flat is first grind and polish the mirror for the telescope to a sphere and use the Ritchey- Common test. 

   Having made flats and I use  double pass autocollimation all the time, I would suggest that you  find a flat to use. It is going to take a good amount of time to make the flat, and I believe longer then it will take to make the telescope mirror. Especially since you have a  thin piece of plate glass. The thinness and the glass type is going to cause the figure of the flat to not  be very stable so that adds more difficulty in figuring it so it is smooth  There is also the expense of getting it coated. 

  If you want to a make a  flat  we are  all here to help,  just pointing out some of the issues 

 

                   - Dave 


Edited by DAVIDG, 16 August 2019 - 09:10 AM.

  • PrestonE likes this

#8 MKV

MKV

    Cosmos

  • *****
  • Posts: 8192
  • Joined: 20 Jan 2011
  • Loc: Florida, USA

Posted 16 August 2019 - 09:30 AM

I would advise against making an AC flat out of plate glass. (just based on my experience with a 5"er I made) Pyrex works a whole lot better.

bow.gif


  • PrestonE and Oregon-raybender like this

#9 GarethBarry

GarethBarry

    Sputnik

  • -----
  • topic starter
  • Posts: 49
  • Joined: 14 Apr 2019

Posted 17 August 2019 - 12:40 AM

Hmmm...ok thanks guys much to think about.
Pinbout i appreciate your offer but i would like the kids to be involved in the actual grinding of the mirror- in any case i doubt i have anything you would be interested in.
I might instead grind the perforated blank into a good sphere and use it for a conjugate null. I then might at a later date refigure it for a cassegrain.

#10 Pinbout

Pinbout

    ISS

  • *****
  • Posts: 22995
  • Joined: 22 Feb 2010

Posted 17 August 2019 - 09:27 AM

With the kids - an 8” is more manageable for younger hands grin.gif



#11 Venetia2004

Venetia2004

    Viking 1

  • *****
  • Posts: 595
  • Joined: 27 Aug 2005
  • Loc: Lynnwood, WA

Posted 17 August 2019 - 12:40 PM

Buy. Used comparator flat on eBay and test it. Dpac doesn’t require flat flat. Just smooth flat. No kinks.

I picked this comparator "flat" up on eBay and had it cored. It is really good for the purpose...

Attached Thumbnails

  • 16 comparator mirror (Large) (2).jpeg

Edited by Venetia2004, 17 August 2019 - 12:51 PM.

  • LarsMalmgren and Pinbout like this

#12 Pinbout

Pinbout

    ISS

  • *****
  • Posts: 22995
  • Joined: 22 Feb 2010

Posted 17 August 2019 - 01:13 PM

I picked this comparator "flat" up on eBay and had it cored. It is really good for the purpose...

shocked.gif smile.gif

 

nice hole waytogo.gif


  • Venetia2004 likes this

#13 Venetia2004

Venetia2004

    Viking 1

  • *****
  • Posts: 595
  • Joined: 27 Aug 2005
  • Loc: Lynnwood, WA

Posted 17 August 2019 - 01:18 PM

shocked.gif smile.gif

 

nice hole waytogo.gif

Tnx.Water jet cut...:)


  • Pinbout likes this

#14 Pinbout

Pinbout

    ISS

  • *****
  • Posts: 22995
  • Joined: 22 Feb 2010

Posted 17 August 2019 - 01:28 PM

Who needs a bath if, when you have a huge perf’d flat.

 

no computers,  no averaging, no fussing with actual aperature size.


Edited by Pinbout, 17 August 2019 - 01:29 PM.

  • Venetia2004 likes this

#15 Pinbout

Pinbout

    ISS

  • *****
  • Posts: 22995
  • Joined: 22 Feb 2010

Posted 17 August 2019 - 01:31 PM

Now you need. 

 

<iframe src="https://3dwarehouse....a3ebb604bcf0e91" frameborder="0" scrolling="no" marginheight="0" marginwidth="0" width="580" height="326" allowfullscreen></iframe>


  • Venetia2004 likes this

#16 Venetia2004

Venetia2004

    Viking 1

  • *****
  • Posts: 595
  • Joined: 27 Aug 2005
  • Loc: Lynnwood, WA

Posted 17 August 2019 - 03:10 PM

Now you need. 

 

<iframe src="https://3dwarehouse....a3ebb604bcf0e91" frameborder="0" scrolling="no" marginheight="0" marginwidth="0" width="580" height="326" allowfullscreen></iframe>

These is what I came up with after figuring my 6 inch Gregorian mirror in DPAC...I used a "flat" first surface mirror that I found on eBay and cut a hole in it ;DAVIDG gave me advice during the figuring...

You'll no go wrong with the DPAC since what you see is what you get.No guessing...

 

Vic

Attached Thumbnails

  • Jupiter in my 6 inch f-31 Gregorian.jpg
  • moon.jpg

Edited by Venetia2004, 17 August 2019 - 03:18 PM.

  • tim53, Pinbout and Matthew Paul like this

#17 GarethBarry

GarethBarry

    Sputnik

  • -----
  • topic starter
  • Posts: 49
  • Joined: 14 Apr 2019

Posted 19 August 2019 - 02:07 AM

Wow those images are spectacular!
Forgive my ignorance, but what is a 'comparator' flat? Is it a machinists flat made from steel?
  • Venetia2004 likes this

#18 BGRE

BGRE

    Mercury-Atlas

  • *****
  • Posts: 2503
  • Joined: 21 Mar 2016
  • Loc: New Zealand

Posted 19 August 2019 - 02:16 AM

Machinists use optical flats to check the flatness of micrometer anvils (only a very small flat is required) and flat metal surfaces like lapping plates. Fused silica flats are commonplace as they are harder. Its not unknown for cast iron lapping plates to be flatter than 1 wave. 



#19 BGRE

BGRE

    Mercury-Atlas

  • *****
  • Posts: 2503
  • Joined: 21 Mar 2016
  • Loc: New Zealand

Posted 19 August 2019 - 02:26 AM

Optical comparators use flats to fold the projector light path. The flatness specifications for these flats may not be as tight as for the transparent optical flats used to check the flatness of the polished surfaces of micrometer anvils and lapping plates etc. Verification of their suitability in an AC setup is necessary. Fortunately that's relatively easy and cheap to do.


  • LarsMalmgren likes this

#20 GarethBarry

GarethBarry

    Sputnik

  • -----
  • topic starter
  • Posts: 49
  • Joined: 14 Apr 2019

Posted 19 August 2019 - 03:18 AM

Wow the things I am learning from this forum!

Something like this?

https://www.ebay.com...a0AAOSwubRXHMB2

Price seems almost too good to be true....


Edited by GarethBarry, 19 August 2019 - 03:18 AM.


#21 BGRE

BGRE

    Mercury-Atlas

  • *****
  • Posts: 2503
  • Joined: 21 Mar 2016
  • Loc: New Zealand

Posted 19 August 2019 - 03:45 AM

That's the projection screen it need not be extremely flat. The fold mirrors in a comparator are likely somewhat flatter. Its conceivable depending on the exact optical design the projection screen may not be nominally flat. A rayleigh water test can be used to check the flatness.

Correction for the Earth's curvature may be required if the flat is relatively large. 

When testing large flats in this way, finding a suitably large collimator objective for the illumination optics can be a challenge. Fresnel lenses and acrylic lenses have been used. Its also possible to correct for the observed defocus when a collimator lens isn't used but a computer is required to do this. 


  • Venetia2004 likes this

#22 Pinbout

Pinbout

    ISS

  • *****
  • Posts: 22995
  • Joined: 22 Feb 2010

Posted 19 August 2019 - 05:59 AM

No like this 

 

https://rover.ebay.c...tm/163816278981


  • tim53 and Venetia2004 like this

#23 DAVIDG

DAVIDG

    Cosmos

  • *****
  • Posts: 9088
  • Joined: 02 Dec 2004
  • Loc: Hockessin, De

Posted 19 August 2019 - 09:08 AM

Wow those images are spectacular!
Forgive my ignorance, but what is a 'comparator' flat? Is it a machinists flat made from steel?

 Jones and Lambson of Springfield Vermont were Stellafane is located and were Russell Porter started ATMing in the USA made a device that allows one to compare the threads of a screw to a master. Porter helped develop it and it is called Screw Comparator. Part of that devices is a large first surface mirror that can be optical flat to around a couple of waves and optically smooth. Those  mirrors  can be used in Double Pass Autocollimation as the reference flat. Here is description of the comparator http://www.jlmetrolo...om/history.html

 

                   - Dave  


  • tim53 and Venetia2004 like this

#24 Venetia2004

Venetia2004

    Viking 1

  • *****
  • Posts: 595
  • Joined: 27 Aug 2005
  • Loc: Lynnwood, WA

Posted 19 August 2019 - 12:10 PM

Comparator Mirror.jpg

Wow the things I am learning from this forum!

Something like this?

https://www.ebay.com...a0AAOSwubRXHMB2

Price seems almost too good to be true....

Mine was coated in 1979...


  • Pinbout likes this

#25 GarethBarry

GarethBarry

    Sputnik

  • -----
  • topic starter
  • Posts: 49
  • Joined: 14 Apr 2019

Posted 19 August 2019 - 12:29 PM

I do have a massive amount of mercury in the lab...maybe

-build the cope

-rest the scope vertically and upside down over a pan of mercury

-put an led and knife edge in the focuser.

-run DPAC test

-apologise to wife and parents for exposing us to mercury

 

In any case, this is all just rambling until I start the actual grinding in the next few days.




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