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Bill Whitehead
member
Reged: 07/11/09
Posts: 12
Loc: Exit 135 - Garden St Parkway
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Friends,
I built an 8 inch Dobsonian and now I want to build an EQ platform. I have a description of how to do it by Warren Peters. I liked his design and so I have been using his formulas to come up with the dimensions of my platform and especially the sectors. However, after going through the calculations I dont feel comfortable with the numbers. The width of the sectors seems too small and I have to make the length of the platform bigger in order to increase the sector size. When I compare my design to others on the internet it seems too large with sectors that are too small.
So, if you are interested in helping out a math challenged amateur astronomer, take a look at my calculations below and let me know what you think.
Telescope; 8 inch Dobsonian, F6, 40 inch focal length.
A - Center of gravity = 25.5 inches
B - thickness of plywood = .75 inches
C baseboard length = 24 inches
D bottom of baseboard to reference line of tracking system = 2.3 inches
E Distance from center of baseboard to contact point of south bearing assembly = 10 inches
F Distance from center of baseboard to contact point of north bearing assembly = 12 inches
G Distance from center of drive pin to the top of the baseboard = 3.05 inches
H Distance from contact point of north bearing to the drive plate = 1 inch.
Latitude angle = 40 degrees
Radius dimensions
Rnb = ( [ (A+B+D) / {tan (Latitude angle) } ] + F ) * sin (Latitude angle) Rnb = 29.58
Rsb = ( [ (A+B+D) / {tan (Latitude angle) } ] E ) * sin (Latitude angle) Rsb = 15.44
Runner dimensions
Ln is the length of north runner is the same as C, the baseboard length.
J is the height of the runners (sectors).
Ls is the length of the south runner
J = Rnb sqrt (Rnb^2 [Ln/2]^2) J = 2.54
Ls = 2 * sqrt (Rsb^2 [Rsb J]^2) Ls = 16.97
Any help you can give me is appreciated.
Thanks,
Bill
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Gene Baraff
sage
Reged: 03/22/09
Posts: 246
Loc: Berkeley Heights, N.J.
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Hi Bill,
When we first talked (at the Sperry Observatory last spring) you mentioned planning to start this platform. I suggested that you mosey over and join the Yahoo Equatorial Platforms group (which holds the largest collection of designers and builders.)
You may have a reason for avoiding them, but if you have not already had a look at what they've got, here is the link I didn't give you last time.
http://tech.groups.yahoo.com/group/eqplatforms/?yguid=137216586
Gene Baraff
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Moki
super member
Reged: 11/06/05
Posts: 429
Loc: Under the Clouds
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I'm not familiar with Warren Peters design methodology but did take some of your numbers and worked out what the platform would look like, generally. There does appear to be some dimensional issues.
With a 29.58" radius north sector the depth of the 24" long sector under the rocker plate (top of platform) at its deepest will be 2.434". I considered that to be the front of the sector. The sagitta of the back side of the sector will be longer because of the angle of the cut.
This length is a simple sagitta calculation:
S = r^2 / 2R
Where:
S = sagitta (depth of north sector at front)
r = 1/2 sector width or 12"
R = radius of sector or 29.58"
Given the other dimensions you provided the corresponding width of the south sector will be 17.34", assuming you want the same sagitta as the north sector.
r = SQRT(S x 2R) = SQRT(2.434" x 30.88") = 8.67"(r - 1/2 the width)
8.67 * 2 = 17.34"
Usually the scope's ground board feet are placed directly over or slightly forward of where the sectors contact the rocker plate (upper board). Given the distance you show between the north and south sector (22") the scope ground board feet must be over 25" apart which is wider than the north sector.
My first guess is that the sectors are too far apart. The distance between them must be determined by the size of the triangle formed by the scopes ground board feet (or the three teflon pads directly above the feet if you're not going to use the ground board with the platform).
Also, the scope's CG falls about 2.5" below the Virtual Polar Axis. I would shoot for getting it closer to the VPA.
I'm attaching a simple drawing to illustrate what the dimensions you've provided translate to on the drawing board (and to real hardware). Take a look at it and see if it makes sense and gives you an idea about where problem lies. You can download it to view it at normal size which is clearer than what shows in this post.
I second Gene's suggestion to check out the Equatorial Platforms Yahoo Group. There's lots of info and pix in the files section, and some helpful folks.
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"In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual." Galileo
12.5" F5.3 Dob on EQ platform
8" F7 planetary Dob / EQ Platform
Current ATM projects:
Xtal-Controlled EQ platform motor drive - finished... It works!
Motorized focus for planetary (moved to the bottom of the list for now)
10" F5.5 Verylight PushTo
Edited by Moki (10/16/09 09:03 PM)
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Bill Whitehead
member
Reged: 07/11/09
Posts: 12
Loc: Exit 135 - Garden St Parkway
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Thanks Gene and Moki,
I am getting closer to the correct numbers. Your input has been very helpful. I think I will check in with the Yahoo group and let them look at my new calculations and see what they have to say. It will help me to feel confident in my final calculations.
Thanks again,
Bill
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