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

8in f2.65 ross test set up help

  • Please log in to reply
33 replies to this topic

#1 macleod

macleod

    Messenger

  • -----
  • topic starter
  • Posts: 422
  • Joined: 21 Sep 2014
  • Loc: Napier, New Zealand, 4110

Posted 09 October 2019 - 10:30 PM

I have set up my 200mm f2.65 mirror and have a good ronchi view. I then inserted a plano- convex lens into the system ( generously loaned to me by Pinbout), plane side towards LED, and aligned it.OK so far. I have NOT calculated distances yet, BUT I can see through the 133lpi ronchi screen that the LED lines up with my uncoated mirror center spot and both are in the center of the 100mm diameter lens, this is in daylight. Turning on the LED and moving the lens up and down the length of the optical axis does nothing, and I cannot see the bright Ronchi Bands through the lens either.

Advice on alignment and calculations would be appreciated.Thank you, Macleod.



#2 macleod

macleod

    Messenger

  • -----
  • topic starter
  • Posts: 422
  • Joined: 21 Sep 2014
  • Loc: Napier, New Zealand, 4110

Posted 10 October 2019 - 01:23 AM

Particularly need the URL for the Ross Spacing Calculator , I can,t find it on the web. Macleod



#3 Gleb1964

Gleb1964

    Explorer 1

  • -----
  • Posts: 79
  • Joined: 13 Jul 2016
  • Loc: Sweden

Posted 10 October 2019 - 04:12 AM

You can download it here http://www.ceravolo....Ross_Null2.html

 

To calculate Ross null configuration one need to know mirror vertex radius and conic constant, lens radius (assuming one surface is flat) , thickness and refraction index on a test wavelength.

 

If you want help in calculation, you need to publish all parameters.


Edited by Gleb1964, 10 October 2019 - 04:23 AM.

  • MKV likes this

#4 macleod

macleod

    Messenger

  • -----
  • topic starter
  • Posts: 422
  • Joined: 21 Sep 2014
  • Loc: Napier, New Zealand, 4110

Posted 10 October 2019 - 04:22 AM

Thank you Gleb1964. I,m all ready to test my fast mirror, hopefully. By the way, looking at your signature , I,m from 1935 ! Macleod.



#5 Gleb1964

Gleb1964

    Explorer 1

  • -----
  • Posts: 79
  • Joined: 13 Jul 2016
  • Loc: Sweden

Posted 10 October 2019 - 04:46 AM

Macleod

 

I am quite impressed! I am following your mirror progress for a long time.

 

Gleb  



#6 Pinbout

Pinbout

    ISS

  • *****
  • Posts: 22925
  • Joined: 22 Feb 2010

Posted 10 October 2019 - 05:39 AM

1st setup the ronchi at RoC

 

then place Lens - curve side toward mirror - inline

 

then slide lens and ronchi closer toward the mirror

 

use ceravola’s software to get distance from center of mirror to center of lens’s curved surface

 

move ronchi in / out adjusted RoC  

 

even with ke at new RoC check shadows.



#7 Pinbout

Pinbout

    ISS

  • *****
  • Posts: 22925
  • Joined: 22 Feb 2010

Posted 10 October 2019 - 05:42 AM

https://youtu.be/X5yIwN9Sr8A

 

excuse my messy table

 

https://youtu.be/p27JViLwvFU


Edited by Pinbout, 10 October 2019 - 05:50 AM.

  • LarsMalmgren likes this

#8 MKV

MKV

    Cosmos

  • *****
  • Posts: 8125
  • Joined: 20 Jan 2011
  • Loc: Florida, USA

Posted 10 October 2019 - 06:47 AM

You can download it here http://www.ceravolo....Ross_Null2.html

 

To calculate Ross null configuration one need to know mirror vertex radius and conic constant, lens radius (assuming one surface is flat) , thickness and refraction index on a test wavelength.

 

If you want help in calculation, you need to publish all parameters.

Gleb, the conic value can be a known, or desired conic. The OP doesn't know the conic of the mirror. He wants to see how close or far the mirror is from a paraboloid. IIRC, the lens to be used is a 150 mm diameter f/4 Edmund Optics large PCX condenser (item # 27-511). It's specs can be found here https://www.edmundop...nser-lens/1018/

 

Notice that no mention of precision, substrate homogeneity, finish, or tolerances is made. 

 

Practical hints about the Ross Null test (RNL)

 

  1. the light source should be a pinhole. 
  2. read Ceravolo's description of the test and its limitations and understand  the theory behind the conjugate optical tests.
  3. the Ross Null lens should be of high precision because the lens is traversed twice. Ceravolo's Null lens costs $650 for a reason.
  4. theory of conjugate tests dictates that both conjugates be as prescribed by math in order for the test results to be credible and valid.
  5. the light source-to-Ross Nul lens distance (S1) is fixed 
  6. the Ross Null Lens-to-test mirror distance (S2) is fixed
  7. only the Ronchi screen can move along the optical axis relative to the light source. The light source doesn't move.
  8. the light source and the RNL are placed at S1 on a movable Sled that slides along a Guide Rail parallel to the optical axis.
  9. the Sled is then translated along the Guide Rail until theSNL is at S2 relative to the test mirror, and then fixed in that position.
  10. the Ronchi test is then moved along the axis to assess the in and out of focus Ronchi band pattern.
  11. if both conjugates are correct and the Ronchi bands are not straight then the mirror is not of the desired or stated conic value. 
  12. if the Ronchi bands are straight and both conjugates are not as required by math the null test if of a different conic.
  13. an optical null (even darkening of the mirror) can be obtained using the Ronchi screen as a knife-edge. When this happens the screen must be co-planar with the light source. If not, then the conic is not the conic desired or stated.

 

rossnullsetup.jpg

 

 

Some of the problems encountered durign the Ross Null Test

 

  1. alignment of the optical axes of the lens and the mirror is essential. Easier said than done. Usually an autocllimator is used but a laser diode can be helpful.
  2. precision tip and tilt controls of the RNL are required for good alignment. ATMs can easily make those. 
  3. optical axes of the RNL and the mirror must coincide (there should b e no lateral  or vertical Ronchi band drift when the Ronchi screen moves along the axis.
  4. it is possible to mistake the Ronchi test with the reflection of the light source of of the inner curved surface of the RNL.

 

Mladen

 

Addendum:

 

The light source needs to be monochromatic of known wavelength..


Edited by MKV, 10 October 2019 - 11:27 AM.

  • Gleb1964 likes this

#9 DAVIDG

DAVIDG

    Cosmos

  • *****
  • Posts: 9040
  • Joined: 02 Dec 2004
  • Loc: Hockessin, De

Posted 10 October 2019 - 09:02 AM

 Here is how we have done the Ross Null test for 15 years at the Delmarva Mirror Making class so this is first hand experience. The results of mirrors figured this way have been crossed check using Double Pass Autocollimation  which we also use at the class and and other test methods and the agreement is excellent. Many of the mirrors made have won optical awards at Stellafane when judged by 9 optical experts over a number of years. 

   First the Ross Lens is mounted in an independent holder. So it can be adjusted in tip, tilt and height. The bottom of the stand that holds the lens has  a straight edge. This edge can be placed up against a stop to fix the distance of the mirror to lens in place. The offset from the front edge of the base of the lens holder to the vertex of the lens is measured and is a constant for that lens/holder combination. You adjust the Mirror to Lens spacing accordingly  to take into account the off set of the holder. 

1) Using the Ross Null software calculate the distance from the mirror to the lens.  

2) Set up the mirror and your tester just like your doing a standard Ronchi test.

3) Place the tester about 6" behind were the lens would be positioned.

4) Align the mirror and tester just like your going to do a standard Ronchi test. Since your not at the radius of curvature of the mirror you'll see many more Ronchi lines then in the  standard test method. The key is to get everything lines up and so you have a symmetrical Ronchi pattern. You do not have the Ross Lens in place at this point.

5) Measure the distance from the mirror to were the Ross Lens needs to be with the correction for the Ross lens holder. Place a heavy  object like a book so the edge is at this exact position.

6) Move the Ross lens into place so the front edge of the Ross lens holder is against the stop ( ie heavy book or piece of wood etc) Something heavy enough that it won't easily move.

7) Now the lens is at the correct distance and you can  slide it side to side without  changing the mirror to lens spacing

8) Look thru the Ronchi screen and position the lens so you see the Ronchi lines coming thru the lens.  Adjust the tip. tilt and height of the lens so the lens is centered on  the outline of the mirror and the Ronchi pattern of lines you see is symmetrical up down and right left. So you can slide the lens side to side, raise it up or down and/or twist it  to get the pattern to be symmetrical. 

 9) Now move only the Ronchiscreen/light source  forward or back just like your doing a standard Ronchi test so you reduce the number of lines to 3 or 4. Now you can go from inside of focus to outside and examine the shape the Ronchi lines.  

 10 ) You want to figure the mirror so you have three straight Ronchi bands and it test just like a spherical mirror You interpret the lines like you do when testing for a spherical mirror ie a hole is hole a hill is hill etc just that your reference surface your trying to achieve is parabola and not a sphere.

 

  Remember to calculate the distance in the software using the wavelength of the light your testing at. Usually it is in the red. I use a red LED but when doing  critical testing I also look thru a narrow bad interference filter that pass 632nm. Until your down to the very  last stages of figuring  and have what looks like very straight bands the filter isn't needed. 

  If you never used a Null test before, what your going to find it is much more difficult to achieve a null figure on the mirror then what you think so issues with getting the exact spacing, known the exact focal length of the lens, the exact refractive index don't come into play until you are  real close to having that null. It is not like if these these parameters are not known  to 3 decimal places or more  that you will  see the Ronchi  bands go from dead straight to bowing like crazy if your off by a  small amount.  

   Hope this helps. Happy to answer any questions 

 

                    - Dave 


Edited by DAVIDG, 10 October 2019 - 03:57 PM.

  • LarsMalmgren and GaryF like this

#10 Gleb1964

Gleb1964

    Explorer 1

  • -----
  • Posts: 79
  • Joined: 13 Jul 2016
  • Loc: Sweden

Posted 10 October 2019 - 09:05 AM

Mladen,

 

right, I meant desired conic, which is -1 for parabolic mirror.

You need add to your list that, because Ross Null is refraction null, it need monochrome source with known wavelength.

 

Gleb



#11 macleod

macleod

    Messenger

  • -----
  • topic starter
  • Posts: 422
  • Joined: 21 Sep 2014
  • Loc: Napier, New Zealand, 4110

Posted 10 October 2019 - 02:10 PM

Hi Danny, made a scope for uncoated mirror.At low power its terrific , great wide field, and coma is not too bad. Moon I get sharp edges of the limb and really good detail.Planets up to124x are passable but not great. Ive barlowed my ES 4.7mm ep to give me 2.35mm and 242x , and its here things get very soft. Probably a combination of the barlow / eyepiece being pushed too far, as much as the mirror. BUT I have not made this for hi power , its a low power wide field fun scanning milky way scope. My GSO coma corrector unbeleivably for a cheapie  does a good job with coma.

MY problem is I cannot open Cerevalo,s Ross Null program - my Win-Zip keeps coming up with winzip ads when I try to open. Have to try something else.Set up last night much as you describe but without measurements of course - i had an excellent Ronchi, but could see nothing except my bright LED  in the lens. Yes I,ve looked at your videos BEFORE I set up.Thank you, Macleod.



#12 Arjan

Arjan

    Viking 1

  • -----
  • Posts: 993
  • Joined: 21 Jan 2009
  • Loc: Netherlands

Posted 10 October 2019 - 02:41 PM

MY problem is I cannot open Cerevalo,s Ross Null program - my Win-Zip keeps coming up with winzip ads when I try to open.

For me the Windoze filemanager built-in zip opens up the file flawlessly.

 

Try this one: Attached File  RossNull_XP.zip   149.9KB   6 downloads

 

. . . Arjan



#13 macleod

macleod

    Messenger

  • -----
  • topic starter
  • Posts: 422
  • Joined: 21 Sep 2014
  • Loc: Napier, New Zealand, 4110

Posted 10 October 2019 - 03:32 PM

Thank you ARjan I got it to work at last ! Macleod.



#14 macleod

macleod

    Messenger

  • -----
  • topic starter
  • Posts: 422
  • Joined: 21 Sep 2014
  • Loc: Napier, New Zealand, 4110

Posted 10 October 2019 - 03:44 PM

DAVIDG - thank you for the detailed explanation, its all clear. It only remains for me to do the measurements.Sanity reigns at last ! Thank you, Macleod.



#15 macleod

macleod

    Messenger

  • -----
  • topic starter
  • Posts: 422
  • Joined: 21 Sep 2014
  • Loc: Napier, New Zealand, 4110

Posted 10 October 2019 - 04:32 PM

Problem in Ross Null XP. Won,t accept data, error message. Following is data entered:-  Mirror details

                                                                                                                                                                     Diam = 200mm

                                                                                                                                                                     ROC = 1060mm

                                                                                                                                                                     Conic = -1

                                                                                                                                   Ross Null Lens

                                                                                                                                                            source side ROC = 0

                                                                                                                                                            mirror side roc     = -206.7

                                                                                                                                                            center thickness = 10mm

                                                                                                                                                            Diam = 100mm

                                                                                                                                                              RFI = 1.553

 

So please where is my error ? XP tells me there is a data error. Help appreciated, Macleod



#16 DAVIDG

DAVIDG

    Cosmos

  • *****
  • Posts: 9040
  • Joined: 02 Dec 2004
  • Loc: Hockessin, De

Posted 10 October 2019 - 04:46 PM

 The flat side of the lens is not zero but a very large number,  try 9e99   

 

             - Dave 



#17 macleod

macleod

    Messenger

  • -----
  • topic starter
  • Posts: 422
  • Joined: 21 Sep 2014
  • Loc: Napier, New Zealand, 4110

Posted 10 October 2019 - 04:59 PM

Thank you David - I noticed that number was already entered in the program when I opened it ! I deleted it not knowing what it was, and entered " zero" 0 . Thank you, macleod.



#18 Pinbout

Pinbout

    ISS

  • *****
  • Posts: 22925
  • Joined: 22 Feb 2010

Posted 10 October 2019 - 05:41 PM

 

Planets up to124x are passable but not great. Ive barlowed my ES 4.7mm ep to give me 2.35mm and 242x , and its here things get very soft.

1st - I learned how to test and work a mirror at Delmarva

 

a f2.65 mirror will beat up eyepieces and especially Barlows. It would be better to use 2.5 / 3mm eyepiece with moderate eye relief and angle of view.

 

whats a star test look like with your 4mm eyepiece using (2) 2mm shims for inside / outside focus.


Edited by Pinbout, 10 October 2019 - 05:43 PM.


#19 macleod

macleod

    Messenger

  • -----
  • topic starter
  • Posts: 422
  • Joined: 21 Sep 2014
  • Loc: Napier, New Zealand, 4110

Posted 10 October 2019 - 06:22 PM

Hi Danny, star test needs way less than 2-3mm shims !! Just a touch of the helical focuser, probably 1/2mm and you get secondary breakout. Looks pretty even inside and outside of focus - certainly no major differences. Way better than my f3.7 I did, which did show big difference. Sorry no pictures. Thank you for help, too. I,m just Ross testing, setting up.Afternoon here. Oh, I did try the DPAC over oil bath the other day, and wasted whole day trying to get it. Hopeless, most I could get was an occasional flash as I tipped the oil bath. Kind regards, Stew.



#20 macleod

macleod

    Messenger

  • -----
  • topic starter
  • Posts: 422
  • Joined: 21 Sep 2014
  • Loc: Napier, New Zealand, 4110

Posted 10 October 2019 - 07:55 PM

Managed to get these photos - they are rubbish really, nothing like I see!  Outside roc:- post-233319-0-93972200-1570755289_thumb.

Attached Thumbnails

  • DSC_0002 (Custom).JPG


#21 macleod

macleod

    Messenger

  • -----
  • topic starter
  • Posts: 422
  • Joined: 21 Sep 2014
  • Loc: Napier, New Zealand, 4110

Posted 10 October 2019 - 07:56 PM

Inside roc :-post-233319-0-64417200-1570755369_thumb.

Attached Thumbnails

  • DSC_0009.JPG


#22 macleod

macleod

    Messenger

  • -----
  • topic starter
  • Posts: 422
  • Joined: 21 Sep 2014
  • Loc: Napier, New Zealand, 4110

Posted 10 October 2019 - 07:58 PM

Outside roc the lines look dead straight, even a single one. However Inside roc is where the problem lies curvature. I know the images are poor, but not easy to photograph. Macleod.



#23 Pinbout

Pinbout

    ISS

  • *****
  • Posts: 22925
  • Joined: 22 Feb 2010

Posted 10 October 2019 - 08:18 PM

Outside roc the lines look dead straight, even a single one. However Inside roc is where the problem lies curvature. I know the images are poor, but not easy to photograph. Macleod.

I use a 40mm plossl in a 30x78 doublet scope (something that fits into plumbing tubes) to shoot the images with a camera.

 

cant tell what’s happening with any of those images

 

if you can’t take a good image, sketch it

 

how does the lines bow, in or out.


Edited by Pinbout, 10 October 2019 - 08:19 PM.


#24 Pinbout

Pinbout

    ISS

  • *****
  • Posts: 22925
  • Joined: 22 Feb 2010

Posted 10 October 2019 - 08:23 PM

If inside is bowing out - your over 

 

https://youtu.be/p27JViLwvFU

 

but if one side looks straight and the other side bows your almost there


Edited by Pinbout, 10 October 2019 - 08:24 PM.


#25 macleod

macleod

    Messenger

  • -----
  • topic starter
  • Posts: 422
  • Joined: 21 Sep 2014
  • Loc: Napier, New Zealand, 4110

Posted 10 October 2019 - 08:27 PM

I find the Ross Null a bit confusing.I can get perfectly straight lines accross my mirror at one position, very critical positioning.The lines look like " jail bars", no bulges kinks, or whatever.So I move the Ronchi/light unit closer to the lens, and as I go I see like a couple "eyes" like I do in a Ronchi test. Then closer in to lens the test deteriorates into what looks like a distorted Ronchi pattern. Thought I was going inside roc ??

Anyway the fact I can get dead straight lines at one point I think I,ll call it completed, even if its only 1/2 correct. Getting a bit OTT for me now, and the mirror works in reality anyway.Star test was not too bad at all. Macleod.




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