Create your cassegrain with autocad 2008
Nederlandse versie
First you have to create a parabola, I found this script.
Download http://www.caddzone.com/ascpoint.lsp as well ant put all those files after unpack in the support directory of autocad, in my case here
D:\Program Files\AutoCAD 2008\Support
In autocad give the command by typing "appload", next chose geomcurv.lsp and ascpoint.lsp and click "load".
Or seperatly if you wish.
Watch down in the window "Successfully loaded 2 files." and you can close this now.
In this sample i create almost a 4 meter dish, focus lenght is 2100mm, subdish with a diameter of 300mm with the focus in the middle of the focuslength of the parabola.
The white line is the focus of the parabola with the lenght of 2100mm.
After installation of the downloaded lips files gave "gcurv" from the autocad prompt, click left on "Parabola"
Click then left of the end of the line, make sure snap is on, next on the right.
Autocad ask now "point spacing distance", enter 1, the parabola exists outof shorte right lines, in this case they are 1mm long.
The number of lines (number of points), 4 meter dish, 2 meter is de half, 2 meter is 2000 x 1 mm, so 2000.
This is the result:
After some dimensioning which you need later in excel to calculate the subdish:
The distance of 1050 is just random, this is the focus of the subdish.
Download Here Excel spreadsheet for excel 2007.
Download Here Excel spreadsheet for older versions.
These spreadsheets 'belongs' to this sample.
Original is from Here, from G3PHO, thanks Peter !
In column B, those 5 bold values do you need to calculate the subdish.
Now you need all values in column I and J under "Sub reflector"
Select from cel I4 until J1544, 1500 rows, these are the coordinates of the subdish.
Copy this in wordpad, NOT in notepad or word, because they are too slow for the next step.
Except if you have dots for a decimal in excel, then you can copy direct the values in notepad and save it as 'subschotel.txt'.
Autocad 'swallows' no comma's so all comma's must be changed in a point (dot).
Go to the begin and give CTRL-H
Search, type a comma, change in, a dot, Click "change all".
This takes a while (5 seconds or so), dependent of the performance of your PC.
Save this as a text file, in my case 'subschotel.txt'.
Go to an 'empty point' in the drawing and zoom in as far as you can, if you move your mouse the coordinates in autocad will change almost nothing.
Why this ? I don't know but it's important, i found out that this wil work properly.
If you doesn't zoom in so far, the subdish will not be acceptable because it seems that the script will skip some coordinates.
From the autocad prompt give "ascpoint", next chose the file you just created, subschotel.txt.
Now AutoCAD asked, comma or space, gave S, end then a L.
Now je have the subdish in autocad.
Result like this in the next image after zooming out, it's a half subdish:
By the command "pedit" you can join all 1500 lines of the subdish to create one object., is easer instead of 1500 lines.
Nou put the subdish on his place, cell B19 of the spreadsheet there is the value from the focuspoint of the parabola, in this case 156,8363mm
The halve dish mirrored and some dimensions for that everything is okay, other color for fun.
Now it's time to check all of these, from the choosen focus point, the half of the focus of the parabola in this case, draw 3 lines (or more if you wish) that far, just over the subdish.
Now you have to zoom so far that you can see every line of the subdish due to click on the subdish.
Next draw from a random point a line perpendicular on the line where the other line you just created which cross the line of the subdish.
Move this line to the intersection of the crossing lines, and mirror the first line like in the next image.
After mirroring extend the line to the parabola.
Trim all yellow lines 'behind' the subdish.
If all yellow lines are joined with "pedit" you'r finished, all three lines have the same lenght.
Click here to download this sample.
Good luck !
With these parameters of an 32 meter cassegrain i have checked my procedure.
73, Theo PA3DSS
Created, Tuesday, 10 December 2008, minor changes Monday 4 May 2009.
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