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NEWTONIAN TELESCOPE DESIGN PLANNER (v/IE1.3)
  • Fields bordered in GREEN are for numerical entry or choice-selected values. Click on the field to change.
  • Fields initially containing "calc'd" and/or are bordered in RED are always calulated and cannot be manually entered.
  • Fields bordered in YELLOW are either calculated (initially contain a "c/k" value) or will accept a keyed entry by clicking on the field. The "calc/keyed" mode can also be toggled by clicking on the checkbox beside the field. A visible "checkmark" indicates the field is currently configured for keyed entry. Changing a "c/k" designated field to manual entry will cause the associated dependent field to be changed from manual to "c/k" mode.
  • Fields bordered in BLUE are eyepiece specification values.
  • Unless denoted in the field description header, field units of measure are "inches" (by default) or "millimeters" as selected below:
SELECT OTA GEOMETRY UNITS: Inches millimeters >
PRIMARY MIRROR
OPTICAL DIA*: FOCAL LENGTH: f/no:
*The "Optical Diameter" of the primary mirror is the glass substrate OD minus 2 x the bevel width.
RESOLUTION (arcsec): SPARROWS: DAWES: RAYLEIGH:
SCOPE PARAMETER DIAGRAM
MIN. FOCUSER ID: SELECT FOCUSER ID:
FOC.TOT.TRAVEL FOCUSER DRAWTUBE LENGTH
PRIMARY CELL TUBE RECESS:
FOC.MIN.HT: PRIMARY EDGE THK:
SCOPE O.T.A.
FP DIST **
PRIMARY CTR. THK.
TUBE
TUBE I.D. O.D.
TUBE
THICKNESS
PRIMARY CELL THICKNESS:
FOCUSER C/L FROM TUBE REAR:
SPIDER MOUNT FROM TUBE REAR:
TOTAL TUBE LENGTH:
**The focal plane distance depicted in the diagram is from the OA fold at the secondary to the focal plane inside the focuser. By default, the focal plane is placed at the midpoint of focuser drawtube travel. If this distance is already known in advance, it should be manually "keyed in" for more OTA dimensional accuracy.

Before "Printing" the scope diagram for your off-line reference, be sure and set your browser option to enable printing of "Background Images" - In IE5.0, it's in "Tools/Internet Options/Advanced/Printers/Print Background Colors and Images".

If an eyepiece "group" average fieldstop distance from the top of the drawtube is known, a more accurate F.P. Distance value for the eyepiece group can be determined for manual entry as follows:

  1. Enter all other parameters and "calculate" an F.P. Distance.
  2. Add 1/2 the total focuser travel distance to the "calculated" F.P. Distance value.
  3. Add the E.P."group" average fieldstop distance (a fieldstop located below the top of the drawtube is NEGATIVE distance)
  4. Manually key in this new value into the F.P. Distance field.
DESIRED MINIMUM 100% FIELD ILLUMINATION
ANGLE (arcmin): DIA. (mm): Edberg(3): Berry(2):
MINIMUM DIAGONAL SIZE (MA) REQUIRED
Berry(1): Dakin(2): Edberg(3): Peters & Pike(6):
FINAL DIAGONAL SELECTION
Pick a std. size: or enter a custom size:
% OBSTRUCTION: AREA: DIAMETRICAL:
ACTUAL 100% FIELD ILLUMINATION
ANGLE (arcmin): Edberg(3): Berry(1):
DIA. (mm): Edberg(3): Berry(1): Peters & Pike(6):
DIAGONAL OFFSET
Francis(4): Dakin(2): Masson(5):
ACTUAL ILLUMINATION VS. FIELD DIAMETER TABLE
Peters & Pike(6)
mm
Dia. 		arcmin
Angle 		Illum.
% 		Mag.
Loss 		mm
Dia. 		arcmin
Angle 		Illum.
% 		Mag.
Loss
(1)Berry - "Build Your Own Telescope", Scribner's Sons, 1985, Pa. 27
(2)Dakin - "S&T", Dec. 1962, Pa. 368; correction - "S&T", Feb. 1963, Pa. 114
(3)Edberg - "Telescope Making" #18, Winter 1982/83, Pa. 20, Eqns. 1, 4 & 6
(4)Francis - "Newtonian Notes", 1979, Pa. 34, Eqn. 9
(5)Masson - "S&T", Dec. 1959, Pa. 126
(6)Peters & Pike - "S&T", Mar. 1977, PP. 220-223
EYEPIECE SELECTIONS
Below this dialog there are 3 sets of eyepiece calculations for "low", "mid" and "hi" powers. Each section provides the option to select a commercial brand and model of eyepiece as well as a Barlow power to be used with them.

Initially, an exit pupil size of 6mm (default) determines the theoretical "low" power eyepiece mm focal length recommended for the specified primary objective. The "mid & hi" power EP values are then determined based on even multiples of the "Barlow" power(s) selected. If only one Barlow is to be used with all 3 categories, enter the x value in the low power Barlow field (defaults to 2x).

Keep in mind that an eyepiece selection resulting in an exit pupil size greater than 7mm (considered the typical maximum dilated human iris) may cause some edge vignetting of the available true field provided by the eyepiece.

When the "calculate" button is clicked BEFORE selecting and "using" an eyepiece model, the model having the nearest mm size for the selected eyepiece brand in each category will be displayed in the "EP CHOICE" field. If a specific eyepiece is then selected in one or more categories (low, mid & high power) and activated with the associated "USE" checkbox, you must recalculate to obtain the correct associated values for the selected eyepiece(s). Also, if you change eyepiece brands, you must recalculate to display the nearest size eyepiece for the new brand.

The eyepiece field stop diameter (FSD), true field of view (TFOV), effective field of view (EFOV), apparent field of view (AFOV), telescope focal length (TFL) and eyepiece focal length (EPFL) are related by the following equations: TFOV=FSD*57.3/TFL; EFOV=FSD*57.3/EPFL; EFOV=TFOV*(TFL/EPFL). When the FSD is published (i.e. TeleVue), an EFOV & TFOV can be accurately calculated. When the FSD is unknown (as is the usual case), the published AFOV (which is generally inflated over the EFOV) is substituted for the EFOV resulting in a corresponding "inflated" calculated FSD.

LOW POWER EYEPIECE DATA
LOW EP FL (mm): (MAX) EXIT PUPIL (mm): ScopeX:
SELECT A BRAND: TeleVue Takahashi Pentax Meade
EP CHOICE: USE EFOV (deg): E.R.(mm):
FIELD DIA (mm): TRUE FOV (deg): (arcmin):
WITH A X BARLOW: EXIT PUPIL (mm): ScopeX:
EFOV (deg): TRUE FOV (deg): (arcmin):
MID POWER EYEPIECE DATA
MID EP FL (mm): EXIT PUPIL (mm): ScopeX:
SELECT A BRAND: TeleVue Takahashi Pentax Meade
EP CHOICE: USE EFOV (deg): E.R.(mm):
FIELD. DIA (mm): TRUE FOV (deg): (arcmin):
WITH A X BARLOW: EXIT PUPIL (mm): ScopeX:
EFOV (deg): TRUE FOV (deg): (arcmin):
HI POWER EYEPIECE DATA
HI EP FL (mm): EXIT PUPIL (mm): ScopeX:
SELECT A BRAND: TeleVue Takahashi Pentax Meade
EP CHOICE: USE EFOV (deg): E.R.(mm):
FIELD. DIA (mm): TRUE FOV (deg): (arcmin):
WITH A X BARLOW: EXIT PUPIL (mm): ScopeX:
EFOV (deg): True FOV (deg): (arcmin):
Revised: 3/23/03
(c) Copyright J. Fly, 1999 All Rights Reserved