Calibration of our interferometer
(www.teleskop-austria.com)

1. set targets, error limits
2. Test of our appraising program
3.

OUR MENSURATE DATA
4. Appraising of numerical calibration results

On this page we are continuously informing our readers about the calibration of our interferometer. We are adopting the "Etalon piece remensuration" method, which is common in international practice. The essence of this method is that one or more characteristic optics are mensurated-around by several calibrated (confirmed) labs and we are comparing our mensuration results with theirs. Analysing the deviations we get the preciseness of our own interferometer. (Indirect calibration by remensuration an etalon piece)

History

January 2003: Procuring a 300mm and a 500mm diameter Zeiss plane-mirror (as reference surface for the autocollimation).
May 2003: Assembling our interferometer and performing its self-test.
22 January 2004: First computer analysis with the semi-professional FringeXP program which works out the Zernike-coefficients in sequence to the 16th step.
23-28 January 2004: Comperative appraising with other programs. The results are promising, but we haven't reached in all cases the percisity we had set with the above-mentioned appraising program.
13 February 2004: Procuring a new, professional appraising program. This program works out the Zernike-coefficients in sequence to the 37th step, that is Z121.
25 February 2004: Telephone consultation and asking for information with the National Office for Mensuration and the Optical labatory of BME (Technical Univerity of Budapest) about the conditions of the calibration (confirmation).
26 February 2004: We decided on the "Etalon piece remensuration" method. We started collecting etalons fom Taiwanese (GSO/Galaxy), German (ICS), Russian (INTES) and TMB sources. We are mensuration the so borrowed optics too and comparing our results with the protocols given by the manifacturers.
From 27 February 2004: Just to test our appraising program, we are running it over other officially published and appraised interferogram photos as well as over synthetic ones.
1 March 2004: Posting the f/4 mensuration piece, which is in our posession, to the lab of Wolfgang Rohr, Germany.
4 March 2004: Receiving the first reference optics which is not ours by courier mail, with which the mensuration-around of etalon pieces starts phYsically
8 March 2004: Our mensuration results of the first etalon piece (Intes Zerodur mirror) are nearly equal to the lab results of the InterCom Spacetec (ICS).
19 March 2004: Receiving the second f/4 mensuration etalon from the lab of Wolfgang Rohr. We are not informed about the mensuration results yet.
21 March 2004: Getting the interferometric protocol of a 130/780 TMB APO (Nr.016, through Markus Ludes). The new owner will lend us the lens in the near future.
2 April 2004: The appraising of the f/4 mensuration etalon is nearly equal to Wolfgang Rohr's lab results not just in its numerical data, but also considering the nature of the surface.
6 August 2004: The appraising of the 130/780 TMB APO (Nr.016, on 532nm!) is nearly equal to TMB lab results not just in its numerical data, but also considering the nature of the surface.

Set targets in the autocollimation layout related to 532nms wavelenght and wavefront-deficiency:

Its wavefront disortion photo should be nearly identical with those mensurated by other optical labs.
The error of the calculated result of its PtV value should be under 0.1 (=53nms).
Its RMS error should be less than 0.02 (=11nms).
Its central light-utilising (CL, Strehl, Definitial brightness) error should stay under 2% when performing appraising without comacorrection.
Its central lightutilising (CL, Strehl, Definitial brightness) error should stay under 3%-5% in case we have to apply comacorrection because of the inexactitude of the adjustments.
The aforesaid objectives are refering to an optic which has an f/4-f/5 light intensity, has a wavefront tightly diffractionlimited (CL=80%, RMS=0.07=1/14). As a function of the decreasing light intensity and the increasing CL (Central Light-utilising = Definitial brightness = Strehl) we would like to make the mensuration exactitude more percise.

In the mensuration-arounds we used the results of the folowing labs:

Lab1: Wolfgang Rohr's lab: http://rohr.aiax.de/

Lab2: Dr. Johannes Heidenhain GmbH (Raphael Bugiel): http://www.optikpraxis.de/

Lab3: Astrooptik Dipl. Phis Keller:

Lab4: TMB protocol: __

Lab5: Intes protocol: __

Lab6: Peter Rucks: __

The structure of the interferometer and its functioning principle:

Our instrument is a variant of the Twyman-Green interferometer. Its structure can be seen in the figure. It operates as follows:
The light-source is a HeNe gaslaser with a wavelength of 632.8nms, functioning in TEM 00 operation mode. The laserbeam emerges from it and passes through a microscope objective, than by means of a collimator objective it becomes large diameter parallel beam. An irisdiaphragm narrows down this beam to the desired size. After this the smaller diameter beam of rays passes through a beamsplitter cube. The cube reflects a part of the beam to the reference surface (plane) and lets the other part pass through towards the optic we want to mensurate. The parallel beam emerging from the cube passes through an excellent quality lens or optical system, which transfers this beam into a cone of rays with a light intensity of f/2,6.

        The figure shows the structure of the mensuration in case of a spherical mirror. The cone of rays reflected by the mirror we want to mensurate is being transformed back into a parallel beam and interfers with the beam reflected by the reference surface. The occuring interference image is recorded by a CCD camera on the fourth side of the beamsplitter cube. The reference surface is tiltable so the position and density of the interference lines can be changed.
        When mensurating a parabolid mirror we need the 500mms Zeiss plane-mirror. By the means of this we reach double precisity by autocollimation since the light is reflected by the main mirror twice on the "focal point- main mirror- Zeiss plane-mirror- main mirror- focal point" route.
        Because of the mechanical structure of the interferometer our lab is unique in that we have the opportunity to mensurate complete telescopes through the ocular-extension. In such cases the mensuration result considers the errors of the auxiliary mirror as well because of the "focal point- main mirror- auxiliary mirror- Zeiss plane-mirror- auxiliary mirror- main mirror- focal point" route. The auxiliary mirror can be either Newton or Cassegrain.

Mathematical bases of the RMS and CL calculated with the appraising program: http://home.earthlink.net/~burrjaw/atm/atm_math.lwp/atm_math.htm

A kiértékelő program tesztje: (strange interferogram photo, own appraising)

Optikcs	Control-mensuration	Control-mensuration	Control-mensuration	Távcső Szolg. Bt.	Max. error
Paraboloid mirror (250/1250) theoretically calculated by the radius of curvature	theoretical 546.1nm innumerable mensuration points PtV=0.00 RMS=0.00 CL=100% In greater detail	Lab2 546.1nm 278 mensuration points PtV=1/15.44=0.0648 RMS=1/78.4=0.128 CF=99.4% In greater detail		TSzBt 550nm 2158 mensuration points PtV=1/83=0.012 RMS=1/676=0.00148 CF=99.9%	0.012 0.0015 0.1% In greater detail
C8 203/2030	Lab1 550nm ? mensuration points PtV=1/3.4=0.29 RMS=1/15.6=0.064 CF=85% In greater detail			TSzBt 550nm 335 mensuration points PtV=1/2.5=0.40 RMS=1/15.2=0.0657 CF=84.3% In greater detail	0.11 0.0017 0.7% In greater detail
GSO 250/1255	Lab1 550nm ? mensuration points PtV=1/9=0.110 RMS=1/45=0.022 CL=98% In greater detail			TSzBt 550nm 104 mensuration points PtV=1/5.88=0.170 RMS=1/38.3=0.0261 CL=97.3% In greater detail	0.06 0.0041 0.7% In greater detail
Meade 250/1125	Lab1 550nm ? mensuration points PtV=1/3.4=0.29 RMS=1/17=0.057 CL=88% In greater detail			TSzBt 550nm 218 mensuration points PtV=1/3.3=0.307 RMS=1/19.9=0.0502 CL=90.5% In greater detail	0.017 0.0068 2.5% In greater detail
280/1390 Newton Stathis Kafalis	Lab1 550nm ? mensuration points PtV=1/8.3=0.12 RMS=1/40=0.025 CL=98% In greater detail			TSzBt 550nm 153 mensuration points PtV=1/4.8=0.207 RMS=1/39=0.0255 CL=97.5% In greater detail	0.09 0.0005 0.5% In greater detail
C11 280/2800	Lab1 550nm ? mensuration points PtV=1/4.5=0.22 RMS=1/28.6=0.035 CL=95% In greater detail			TSzBt 550nm PtV=1/3.64=0.275 RMS=1/25.4=0.0394 CL=94% In greater detail	0.055 0.0044 1.0% In greater detail
INTES 318/1421	Lab1 632.8nm ? mensuration points CL=95.8% In greater detail	Lab3 632.8nm ? mensuration points CL=95.0% In greater detail	Lab5 632.8nm ? mensuration points PtV=1/5.62=0.178 RMS=1/31.3=0.032 CL=96.0% In greater detail	TSzBt	In greater detail
352/1883 wrong mirror	Lab1 632.8nm ? mensuration points PtV=1/2.4=0.425? CL=39.7% In greater detail			TSzBt PtV=1/1.1=0.912? RMS=1/6.71=0.149 CL=41.8% In greater detail	-- -- 2.1% In greater detail

Test of the whole instrument: (own interferogram photo, own appraising)
Let's compare the 3D figures as well!

Optics	Lab mensuration	Távcső Szolg. Bt.	Error
TMB APO 130/780	TMB lab 532nm PtV=1/5.1=0.198 RMS=1/43.5=0.023 CL=97.9% photo (3D)	TSz lab 532nm PtV=1/4.1=0.246 RMS=1/54=0.0185 CF=98.7% photo (3D)	+0.048 -0.0045 +0.8% In greater detail
Intes mirror 250/1400	ICS lab 632nm PtV=1/6.25=0.16 RMS=1/34.5=0.029 CL=97% photo (3D)	TSz lab 632nm PtV=1/4.93=0.203 RMS=1/38.9=0.0257 CL=97.4% photo (3D)	+0.043 -0.0033 +0.4% In greater detail
GSO mirror 200/800	W.Rohr lab 550nm PtV=0.131 RMS=0.0247 CL=97.6% photo (3D)	TSz lab 550nm PtV=0.153 RMS=0.0228 CL=98% photo (3D)	+0.022 -0.0019 +0.4% In greater detail

Calibration of our interferometer (www.teleskop-austria.com)

Calibration of our interferometer
(www.teleskop-austria.com)