[Raytrace] 3 Newtonian layouts
Michael Peck
mpeck1@ix.netcom.com
Tue, 11 Dec 2001 10:49:53 -0600
I'm going to make a small contribution to the cause with 3 Newtonian
layouts that illustrate some OSLO LT capabilities. These are pretty OSLO LT
specific, so I'm going to skip the text description and just give the OSLO
scripts at the end. I'm also afraid I don't have time to give step by step
instructions for creating these right now, but I will make some comments. I
created these in OSLO LT 6.1, but they seem to load and look OK in ver. 5.4.
VERSION 1
This is the same 10" f/6 newtonian that John sent the other day, with a few
small changes:
1) I really prefer metric units when playing these games, so I leave the
units in the default of millimeters. So the entrance beam radius is 125mm
and the radius of surface 1 (AST) is -3000.
2) I've set the field angle to 0.9 degrees. Notice in the "Aperture radius"
column the radius at the image surface is about 23.6mm, so this is just
about the maximum field size available with 2" eyepieces.
3) This is a trick I picked up somewhere that may or may not be useful. I
insert a dummy surface at the end in front of the image surface (IMS) that
I always make coincide with the paraxial focus. The main reason I
originally did this is the default behavior of OSLO is to draw out to the
last defined surface instead of drawing all the way to focus. This forces
it to draw the complete layout. There's actually a menu item to change this
behavior, so you don't really need to do this for that reason. The other
reason I do it is that in more complex designs the paraxial focus might not
be the best focus, and this allows you to manipulate image surface settings
independently of the paraxial focus.
VERSION 2
Now I'm going to add a Newtonian secondary and account for the obstruction
that it causes. OSLO isn't smart enough to realize that having a secondary
near the end of the optical train implies that incoming light is
obstructed, so this requires adding two surfaces to the design - the
secondary is surface 3, and the initial obstruction is defined in surface
1. The primary now becomes surface 2, but it still is the aperture stop (AST).
To create the secondary what I did was reason that you need about 5" for
the mirror + 1" to the tube + 2 1/2" for focuser plus back focus allowance,
or a total of about 210mm from secondary to image plane. That means the
primary - secondary spacing is 1500-210 = 1290 mm. So, I set the thickness
at the primary to -1290mm. The secondary radius is set to 0 -- OSLO
actually interprets this as a curvature of 0, ie the surface is flat. I use
an aperture solve (default behavior) to get a secondary size of 37.76mm.
Now to make this a 45 degree Newtonian flat you click on the button in the
last column labelled "SPECIAL" and choose the popup menu item
"Coordinates". This is where you specify tilts and decenters. I just need
one tilt; I enter 45 in the in the "Rotation Angle" entry TLA. I don't know
exactly what this means, but you also have to click on the button next to
the label "Tilt and bend:". It should read Yes. Click on the check mark at
the top of the data entry table to accept changes.
Finally, to define the obstruction I use data from surface 3 to define
surface 1. I don't know a smarter way to do this: OSLO calculated the
secondary size to be just under 37.8mm. What I've done is place surface 1
at a distance of +1290mm from the primary. I define a special aperture by
clicking on the button in the "Aperture radius" column and then selecting
the "Special aperture data" menu item. The "Type" of special aperture is an
ellipse, its action is to "obstruct" and the min and max values for X and Y
are set to -+ 37.8. Back in the main data entry spreadsheet notice that
I've solved for the aperture radius, which turns out to be just over 145mm.
That is the size of the unvignetted light cone at that point, which tells
us the tube needs to be at least this large.
VERSION 3
Well, OSLO just told us that if we want a fully illuminated 2" diameter
field we need a secondary with a semi-minor axis size of 37.8mm - that's
over 30% obstruction! Not so good. I'm going to guess that a 20%
obstruction is more reasonable, so I'm going to make the secondary aperture
radius and obstruction size 25mm. OSLO also isn't smart enough to realize
that a 45 degree flat should be elliptical in shape, so the other change
I'm going to make in the final version is to tell it that surface 3 is
elliptical. Again you do this by entering special aperture data. In this
case I again have a surface type of "ellipse", the action is to "transmit"
(don't worry, the light still reflects off the surface), and minimum and
maximum values for X and Y are set to -+25 and -+25*sqrt(2) respectively.
By the way, if you're wondering about extraneous digits of precision in the
Y values, it turns out you can do simple calculations when entering numbers
in OSLO data sheets. Instead of whipping out a calculator or multiplying 25
by 1.4 in my head and getting 43 I just typed in 25*sqrt(2) in these boxes.
Comments:
1) Since a flat doesn't do anything optically you'd rarely need to set one
up in a telescope design. If you're interested in the effects of an
obstruction on MTF or something you can just define an obstruction. On the
other hand if you're interested in, say, curvature tolerances on "flats"
it's helpful to be able to add one to a design. Now you can enter non-zero
curvatures into the surface specification and see how a curved Newtonian
secondary creates astigmatism.
2) Most of this analysis is probably easier in one of the many Newtonian
design programs that you can get for free. In this case the main extra you
get for the effort with OSLO is wavefront analysis. There are also some
basic vignetting analysis tools - they are under the main menu item
"Optimize -- Support routines" in OSLO 6.1.
3) Except for this I've done exactly one telescope design that used tilts,
so don't expect a lot of insight from me into how to create general TCT
layouts.
*****newt_250f6.len******
// OSLO 6.1 14289 0 64702
LEN NEW "Newtonian 250mm f/6" -1500 3
EBR 125.0
ANG 0.9
DES "OSLO"
UNI 1.0
// SRF 0
AIR
TH 1.0e+20
AP 1.5709255324e+18
NXT // SRF 1
RFL
RD -3000.0
PY 0.0
CC -1.0
NXT // SRF 2
AIR
NXT // SRF 3
AIR
CBK 1
WV 0.58756 0.48613 0.65627
WW 1.0 1.0 1.0
END 3
SDAD 41.05
SDSA On
****end *****
***** newt_250f6_s.len *****
// OSLO 6.1 23681 0 64702
LEN NEW "Newtonian 250mm f/6" 1500 5
EBR 125.0
ANG 0.9
DES "OSLO"
UNI 1.0
// SRF 0
AIR
TH 1.0e+20
AP 1.5709255324e+18
NXT // SRF 1
AIR
TH 1290.0
APN 1
AY1 A -37.8
AY2 A 37.8
AX1 A -37.8
AX2 A 37.8
ATP A 1
AAC A 2
NXT // SRF 2
RFL
TCE 0.0
RD -3000.0
TH -1290.0
AST
CC -1.0
NXT // SRF 3
RFL
PY 0.0
DT 1
TLA 45.0
BEN
NXT // SRF 4
AIR
NXT // SRF 5
AIR
CBK 1
WV 0.58756 0.48613 0.65627
WW 1.0 1.0 1.0
END 5
SDAD 41.05
SDSA On
****end****
***** newt_250f6_s2.len *****
// OSLO 6.1 43729 0 64702
LEN NEW "Newtonian 250mm f/6" 1500 5
EBR 125.0
ANG 0.9
DES "OSLO"
UNI 1.0
// SRF 0
AIR
TH 1.0e+20
AP 1.5709255324e+18
NXT // SRF 1
AIR
TH 1290.0
APN 1
AY1 A -25.0
AY2 A 25.0
AX1 A -25.0
AX2 A 25.0
ATP A 1
AAC A 2
NXT // SRF 2
RFL
TCE 0.0
RD -3000.0
TH -1290.0
AST
CC -1.0
NXT // SRF 3
RFL
PY 0.0
AP 25.0
DT 1
TLA 45.0
BEN
APN 1
AY1 A -35.3553390593274
AY2 A 35.3553390593274
AX1 A -25.0
AX2 A 25.0
ATP A 1
AAC A 4
NXT // SRF 4
AIR
NXT // SRF 5
AIR
CBK 1
WV 0.58756 0.48613 0.65627
WW 1.0 1.0 1.0
END 5
SDAD 41.05
SDSA On
****end****
_________________
Michael Peck
email mpeck1@ix.netcom.com
Wildlife photography page http://home.netcom.com/~mpeck1/index.html
Amateur telescope making http://home.netcom.com/~mpeck1/astro/astro.html