Slit Image Test
Overview Computations Setup
Baldwin On SIT Contact

SIT_conic_20.xlsm overview

Copies of the spreadsheet can be obtained by send a request to Bill Thomas

Jeff Baldwin's youtube video - taking a SIT image
Jeff Baldwin's youtube video - processing a SIT image

click to download the 24 mega byte imageJ input tif file to be used with SIT_Conic.xlsm

  • Olympus E300 Evolt
       Lens mount to CCD: 38.8 mm
       Pixels/inch: 4771
  • Canon T2i EOS Rebel
       Lens mount to CCD: 44 mm
       Pixels/inch: 5905
FigureXp results

Conic Equations (2)

Conic equations

SIT Conic Equations

SIT fixed equations

Derivation of SIT Non Linear Algebraic Equations

From the ImageJ slit values, SIT computes the Y's - the differences between each zone pair slit images in inches. SIT nonlinear algebraic equations compute Yi as a function of Z (the Foucaults). Next Excel's Solver is used to find the value of Z such that the difference between Yi and Y is nearly zero. Then Sixtests or FigureXp can be used to compute the mirror surface from the Foucault's.

Mask tests are of two verities, Moving or Fixed Source. The Lateral Wire Test is a Moving Source test. SIT is a Fixed Source test where the camera and source are separated, and as such an exact solution is required rather than an approximation.

To compute the Yi of Fig: Conic fixed source need equations for variables n and Yn of Fig: SIT n and Yn. The derivation for n proceeds using the equation of a straight line y=mx+b where slope m is slope u, the normal to the conic surface (z') and b = z's. From this then, [1]: Yu=-z'x+z's. Next to compute Yv need to know the slope of v (the perpendicular to u), which is the inverse of the slope of u i.e., 1/z' (see proof below).

Then [2]: Yv=X/z'-l/z'. Next find the intersection of lines u and v which is the solution of the simultaneous equations Yv and Yu i.e., [3]: Yv=Yu, which leads to the solution [4]: n=2(l-Kzz'^2)/(1+z'^2)-l. And then knowing n, [5]: Yn=(n-l)/z'.

Now knowing n, and Yn, can solve for Yi of Fig: Conic fixed source if we knew z. From the Conic Equations z = Z/-K, where Z is the Foucault. As stated above, Excel's Solver is used to find the value of Z such that the difference between Yi and Y is nearly zero.

The sum of the angles in a triangle is 180 degrees. The angles in the two large right triangles are the same, being the sides are the same. Angles a and b of the small triangle are the same as that of the large right triangles. Therefore the small triangle is also a right triangle and the slopes (hypotenuses) are at right angles.

SIT n and y

SIT Conic Mask Parallax











References
  (1)  
Karine and Jean-MarcLecleire, A Manual for Amateur Telescope Making. http://www.willbell.com/tm/atmmanual/index.htm
  (2)  Conics
Optical Shop Testing Edited by Daniel Malacara, Wiley and Sons, 1978, Appendix 1 page 479, 482
Conic approximation
Conic equations
Conic constant - Schwarzschild constant
  (3)  LWT

J. Francis, "Improved Test Methods for Elliptical and Spherical TCT Mirrors,", ATM J. 14, 34-39, (ISSN 1074-2697).

Unfortunately, references to the LWT on the Internet have mostly faded away. Jim Burrows author of Sixtests.exe does mention the LWT in passing at http://www.raddobs.com/atm/atm_math.htm. LWT was a variant of the moving source Caustic where instead of trying to determine a Caustic zone holes focus laterally and longitudinally precisely, the LWT measurements were done manually in the same lateral plane. Being that for the LWT the longitudinal value was irrelevant, didn't require a mask and little judgment was required to determine laterally where a zone ray crossed the lateral plane, the LWT was an improvement over the standard Caustic. However, the required lateral accuracy required was very tight. Given small room temperature and the time to take the measurements, I could not get repeatable LWT results.

  (4)  
Lens mount to CCD spacing
   Recommend saving this webpage to your computer, since it has vanished in the past.


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