RE: ATM "Perfect" Tri-Schiefspiegler in OSLO

["CSC" <optic@gwi.net>]

Arthur Leonard had worked out some three element TCT's.  I have heard that
aspherizing the primary enough to take care of all the aspheric corrections
in the system is easier than trying to aspherize the tertiary a surface with
a superlong radius.

I think as the tilt gets to about 10 degrees it would seem to be a problem.
The eyepiece is tilted in the holder which moves the eyepiece along the
axis.  Longer f ratios give a greater depth of focus and might not suffer as
much, but it may be more a function of the eyepiece design than the main
system.

CSC




-----Original Message-----
From: owner-atm@shore.net [mailto:owner-atm@shore.net]On Behalf Of
Mauritz Andersson
Sent: Friday, January 23, 2004 12:40 PM
To: atm@shore.net; atm@atmlist.net
Subject: ATM "Perfect" Tri-Schiefspiegler in OSLO



Hi,

Of course take "perfect" with some irony as nothing is...
Here it just means almost free from coma and astigmatism.
The Stevick-Paul is a very nice TCT design which have this property
as was also pointed out in a recent thread.
I was exploring some more compact Tri-Schief designs (with shorter
separation between second and third mirror) and found a design that I
would like to have your comments on.

The example is a f/10, D=200 mm design.
It turns out that by the proper choice of curvature of the mirrors and
introducing some asphericity of the third mirror one can eliminate both
coma and most astigmatism. The wavefront P-V is about lambda/10 for half
a degree FOV (at a slightly curved field). See OSLO file below.

Questions of fabrication: The primary is a very mild hyperbola at
k=-1.08 (which is only surface lambda/20 from parabola) which I guess
should be no problem. Secondary is spherical but convex, so one has to
test against a matching concave or? The tertiary is concave with a conic
konstant k=32 which sounds a lot but is actually only surface
0.17*lambda from a sphere since it has alarge radius of curvature. But
does anyone have experience with fabrication of such a mirror?

The image plane tilt is 11.5 degrees. Is that within usable tolerances?
What are your experiences with different eyepiece designs and tilted
focusers?

A similar f/9 design can be made with a tilt of 12.9 deg, and an f/12
design with a smaller tilt of 9.5 deg.

Comments and questions are welcome!
Mauritz Andersson

OSLO file follows (length units are mm):

// OSLO 6.04 17390     0 18707
LEN NEW "Compact schief" -2000 5
EBR  100.0
ANG  0.25
DES  "OSLO"
UNI  1.0
// SRF 0
AIR
TH   7.5899648717e+19
AP  3.3117679452e+17
NXT  // SRF 1
RFH
RD   -2689.1352385626533
TH   -627.8392522813266
CC   -1.0827116641909
DT   1
TLA  7.6
BEN
NXT  // SRF 2
RFH
TCE  236.0
RD   -2139.8155818048399
TH   340.0
DT   1
TLA  -13.5357384929273
BEN
NXT  // SRF 3
RFH
PU   0.05
PY   0.0
CC   31.9450594977479
DT   1
TLA  -7.254778706214
BEN
NXT  // SRF 4
AIR
NXT  // SRF 5
AIR
RD   763.6612780492341
TH   -0.015535294912
DT   1
TLA  11.5308
AMO ANG
GPRT On
WV 0.58756 0.48613 0.65627
WW 1.0 1.0 1.0
END  5
DLID 1100.0
DLAP 3
DLNR  1 9
DLFP  1 -1.0
DLMN  1 -1.0
DLMX  1 1.0
DLNR  2 9
DLAS On
SDGX 40.0
SDGY 40.0
SDTF 70.0
SDSA On
OPDF 1000.0
OPST 0.0001
OPOC "tct_spot_size"