Author Topic: How difficult is Ross Null Test?  (Read 367 times)

artufanchess

  • Active Astronomer
  • ***
  • Posts: 116
  • Activity:
    0%
  • Reputation: +0/-0
    • View Profile
How difficult is Ross Null Test?
« on: December 30, 2017, 07:09:05 PM »
Please forgive and don't hesitate to correct me, I'm way out of my element here.
I have a ross null lens, actaully I have all the parts from Cervalo to build an interferometer. Anyway, how hard is it to set up a null test? I imagine if not properly done you can easily get misleading results. Maybe a guy should put everything on rails to maintain alignment and ease set-up?
It sure would be nice though when parabolizing to just shoot for straight lines on a Ronchi test, at least I think that's how it works?



selfjomargast

  • Jr. Astronomer
  • **
  • Posts: 93
  • Activity:
    0%
  • Reputation: +0/-0
    • View Profile
Re: How difficult is Ross Null Test?
« Reply #1 on: December 31, 2017, 10:11:57 PM »
Setting it up
https://youtu.be/X5yIwN9Sr8AEffects of misplacement
https://youtu.be/p27JViLwvFU

If you have a gimble for the lens - easy peasy lemon squeezy

No need for rails.

James Bradmon

  • Active Astronomer
  • ***
  • Posts: 125
  • Activity:
    0%
  • Reputation: +0/-0
    • View Profile
Re: How difficult is Ross Null Test?
« Reply #2 on: January 08, 2018, 04:03:53 AM »
You need to know how good your lens is, and you need to keep separations to within required tolerances. You better off with a Ceravolo interferometer. You'll the error quantitatively.

With conjugate tests, you can get a null with just about any configuration and lens, but it won't necessarily be the conic you want.

Chris Smale

  • Active Astronomer
  • ***
  • Posts: 129
  • Activity:
    0%
  • Reputation: +0/-0
    • View Profile
Re: How difficult is Ross Null Test?
« Reply #3 on: January 09, 2018, 09:33:34 PM »
Quote
You need to know how good your lens is, and you need to keep separations to within required tolerances. You better off with a Ceravolo interferometer. You'll the error quantitatively.

please...that's not what he's asking and he states he has his lens.

adrajacte

  • Active Astronomer
  • ***
  • Posts: 121
  • Activity:
    0%
  • Reputation: +0/-0
    • View Profile
Re: How difficult is Ross Null Test?
« Reply #4 on: January 10, 2018, 10:51:06 AM »
Quote
Quote

You need to know how good your lens is, and you need to keep separations to within required tolerances. You better off with a Ceravolo interferometer. You'll the error quantitatively.

please...that's not what he's asking and he states he has his lens.
He said he has parts not necessarily the Ceravolo lens. In any case you have to know the lens quality otherwise it's a shot in the dark.

Sean Meyer

  • Active Astronomer
  • ***
  • Posts: 138
  • Activity:
    0%
  • Reputation: +0/-0
    • View Profile
Re: How difficult is Ross Null Test?
« Reply #5 on: January 15, 2018, 01:38:08 PM »
Quote
otherwise it's a shot in the dark.
not really. the biggest problem is getting the mirror shaped correctly, then its just correction. you can null it then star test it to verify, or ke test.

but if you want to check the pcx for zones and ish, star test it with a ronchi screen. it will tell you if its smooth.

and for the pcx, if its 80mm @ 1/4~ SA and your only using 1.3" dia of it [the program will tell you the working dia of the lens for the light cone your working with,] the error added will be much less.

Robert Spencer

  • Active Astronomer
  • ***
  • Posts: 127
  • Activity:
    0%
  • Reputation: +0/-0
    • View Profile
Re: How difficult is Ross Null Test?
« Reply #6 on: January 15, 2018, 07:57:44 PM »
Just so you guys know, it isa genuineCeravalo lens. In that case, I should probably try and set up a nulltest, yes? I have a holder for it too, I had forgotten about that. I have a whole shoebox of optical goodies!

moiquadachus

  • Active Astronomer
  • ***
  • Posts: 105
  • Activity:
    0%
  • Reputation: +0/-0
    • View Profile
Re: How difficult is Ross Null Test?
« Reply #7 on: January 16, 2018, 03:19:46 AM »
We use the Ross null along with double pass autocollimation at the Delmarva mirror making class. The test is not a problem to setup. You only have to worry about getting the spacing dead on when your at the very finally stages of figuring. If you have never made a mirror using a null test you'll soon discover that just getting close to a null is the tough part. Also if your off by a bit in the spacing and achieve a clean null the result will be a slightly over or under corrected mirror that is smooth and most likely good to around 1/8 wave. That is far better then what I have seen time after time when someone makes a mirror using just knife edge readings.
Double check your results against other methods and make sure they agree. If not find out why they don't and don't assume one method is right since the one you think you have been doing correctly for years is the one that could be wrong.
       Dave

latorika

  • Active Astronomer
  • ***
  • Posts: 116
  • Activity:
    0%
  • Reputation: +0/-0
    • View Profile
Re: How difficult is Ross Null Test?
« Reply #8 on: January 17, 2018, 12:13:04 AM »
Quote
Just so you guys know, it isa genuineCeravalo lens. In that case, I should probably try and set up a nulltest, yes? I have a holder for it too, I had forgotten about that. I have a whole shoebox of optical goodies!

yes

its as simple as doing the ronchi setup then just move everything a little further up.

you have to be aware if the optical axis is tilting up, as you move forward you may have to move the grating down a little.

at first you may be thinking a reflection is the mirror but you'll soon get the hang of it.

when performing the test make sure inside and outside are both straight. sometimes one side will look straight and the other side isn't. don't give in there make sure both agree.

I think its one of the easiest test. not as accurate as a dbl pass test but for me its easier.

if you find out you don't like it....pm me.

chionewssesu

  • Active Astronomer
  • ***
  • Posts: 130
  • Activity:
    0%
  • Reputation: +0/-0
    • View Profile
Re: How difficult is Ross Null Test?
« Reply #9 on: January 20, 2018, 01:00:19 PM »
Quote
The test is not a problem to setup. You only have to worry about getting the spacing dead on when your at the very finally stages of figuring.

This is true but the accuracy of setting up is also influenced by the diameter and speed of the mirror under test. The larger and faster the mirror, the more accurate the spacing must be. Furthermore, the larger and faster the mirror, the more accurate the mirror's radius of curvature needs to be known to accurately calculate the lens to mirror spacing. Not a major issuer for an 8 inch f/6 but important for a 20 inch f/4.

What size and speed of mirror are you anticipating testing with your Ross null apparatus?

Scott Etrheim

  • Active Astronomer
  • ***
  • Posts: 129
  • Activity:
    0%
  • Reputation: +0/-0
    • View Profile
Re: How difficult is Ross Null Test?
« Reply #10 on: January 21, 2018, 07:04:47 AM »
A longer focusing pcx helps relax tolerances with faster primaries.

finatissau

  • Active Astronomer
  • ***
  • Posts: 120
  • Activity:
    0%
  • Reputation: +0/-0
    • View Profile
Re: How difficult is Ross Null Test?
« Reply #11 on: January 21, 2018, 02:43:28 PM »
Quote
Quote

The test is not a problem to setup. You only have to worry about getting the spacing dead on when your at the very finally stages of figuring.

This is true but the accuracy of setting up is also influenced by the diameter and speed of the mirror under test. The larger and faster the mirror, the more accurate the spacing must be. Furthermore, the larger and faster the mirror, the more accurate the mirror's radius of curvature needs to be known to accurately calculate the lens to mirror spacing. Not a major issuer for an 8 inch f/6 but important for a 20 inch f/4.

What size and speed of mirror are you anticipating testing with your Ross null apparatus?
12" mirror, about F5.8.

Stanley Elliott

  • Active Astronomer
  • ***
  • Posts: 132
  • Activity:
    0%
  • Reputation: +0/-0
    • View Profile
Re: How difficult is Ross Null Test?
« Reply #12 on: January 25, 2018, 04:58:30 PM »
Raytracing can easily demonstrate sensitivities to alignment, and tip/tilt errors, substrate inhomogeneity, and a variety of other factors. Unless you have a quality lens with known specs, testing a plano-convex lens to high degree of accuracy is difficult without sophisticated equipment (interferometers such as Zygo, etc.). The usual freebie optical raytrace programs used by ATMs are limited in their ability to do tolerancing computations and Monte Carlo analysis to establish precise parameters. It can also easily be demonstrated that any figure of revolution close to a paraboloid can be nulled with a Ross null test and not be a paraboloid. So, obtaining a null cna be misleading.

What Danny and Dave suggest is reasonable: test the mirror by more than method, be it a star test, autocollimation, knife-edge, etc. A decen Ross null lens will get you close to your goal if you keep the configuration setup within stated tolerances for separations. Tips and tilts are a different story. I personally find it difficult to do so freehand, so I use carefully aligned translation stages and optometric equipment to hold components in precise alignment.

Personally, I think it's much easier and more informative to test your mirrors using a simple interferometer (such as Bath or Twyman-Green), and available free software to analyze the results. The software creates an ideal artificial reference surface for a digital null, produces a 3d wavefront surface picture with quantitative errors, the Strehl and RMS errors, and a slew of other information tidbits. It ca also produce a very realistic rendition of the surface as it would appear under a knife-edge, as well as Ronchi grating and a start test, all based on the obtained interferometric mirror or lens data.

Mladen

Jaye Agting

  • Active Astronomer
  • ***
  • Posts: 122
  • Activity:
    0%
  • Reputation: +0/-0
    • View Profile
Re: How difficult is Ross Null Test?
« Reply #13 on: January 26, 2018, 07:14:39 AM »
What I find in using a null test like the Ross is that you see were the mirror departs from the conic you want. Having taught optical fabrication for years that is very powerful in giving the person figuring the surface direct and instantaneous feed back as what is right and what is wrong. The result that I keep seeing time after time is much better figured optics. If you achieve a clean null the result is an optical surface that is smooth and if you double check your setup, one that is going to be very close to theoretical.

   Dave

dextcinthrervest

  • Active Astronomer
  • ***
  • Posts: 114
  • Activity:
    0%
  • Reputation: +0/-0
    • View Profile
Re: How difficult is Ross Null Test?
« Reply #14 on: January 31, 2018, 09:24:11 AM »
So, combined with Foucault and Ronchi, you can get a real good idea of what shape your mirror is in. If they all seem to agree, chances arevery good you have a great surface.
Just what I need, I'm no expert at Foucault or Ronchi, I'm getting better. My problem is, I'm a perfectionist, without the means or skillsto achieve perfection! What a dilemma.....
Most of my hobbies Ican just keep fiddling around until I am satisfied, mirror making is more demanding. You cant see what you have done until after you have done it. I have yet to figure out what does what for surewhen it comes to figuring....
Having fun though!