[Raytrace] Ross null

[Robbin Palmer]

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I set up a Ross null test in OSLO lite for a D = 22.4" (569.96 mm), f = 
94" (2387.6 mm) mirror using a D = 92 mm, f = 300 mm BK7 lens, with 
center thickness 10.4 mm and edge thickness 4 mm. I set it up using the 
"B" (lens to source distance) and "D" (lens to mirror distance) from the 
Stoltzman and Ceravolo article in Mar/1993 Applied Optics. After minor 
corrections for lens thickness and index (using green light instead of 
the red used in the article), I calculated B - 175.617 mm and 
interpolating from the table got D = 4308.351 mm.  I set up the test in 
OSLO with these spacings, and chose autofocus, RMS min mono spot size in 
the thickness field for the lens to image distance to find where the 
image plane formed relative to the object plane. I plan on using a 
"slitless" tester as described on the Stellafane website for the test, 
so need the lens-object and lens-image distances equal.  I optimized for 
total spherical abberation and got a null on the order of 1/20 wave, but 
the image plane was far behind the object plane.  When I iterated the 
object-lens distance to make get it equal to the image lens distance, 
then null dropped to 1/7 wave.  Am I doing something wrong? The OSLO 
file is attached. 

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// OSLO 6.1  9180 58816 35746
LEN NEW "Ross Null, 22\"f4.5, 300mm lens" 435.76 5
NAO  0.167462015572
ANG  0.0000572957795
DES  "tw"
UNI  1.0
SNO1 "This file uses a Melles Griot f=300mm, d=66mm, t=6.5mm lens for the nulling lens"
SNO2 "from their website, cost: $38.00. This lens should clear the return light"
SNO3 "from the mirror with a 0.5 degree angle off axis."
// SRF 0
AIR 
TH   197.29232
AP  0.00019729232
NXT  // SRF 1
GLA BK7       
TH   10.4
NXT  // SRF 2
AIR 
RD   -159.1687909981387
TH   4380.0795594864567
NXT  // SRF 3
RFH 
RD   -4780.3000000000002
PK THM  -1 0.0
CC   -1.0
DT   1
NXT  // SRF 4
AIR 
PK CVM  -2 0.0
TH   -10.4
NXT  // SRF 5
AIR 
TH   -197.2927388578441
CBK  1
WV 0.54607 0.48613 0.65627
WW 1.0 1.0 1.0
END  5
SDAD 100.0
OPDF 1.0e-08
OPOC "opcb_abs"
VAR NEW
V 1 5 0 TH 0.0 0.0 1.0 0.0033512211294
V 2 2 0 TH 0.1 10000.0 1.0 0.0033512211294
END
OPE NEW
O 1 "OCM20" 1.0 "TOTAL_SPH"
END

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