Click here if you are having trouble logging into the forums
Privacy Policy |
Please read our Terms
of Service | Signup and
Troubleshooting FAQ | Problems? PM a Red or a Green Gu.... uh, User
InkDark
Carpal Tunnel
Reged: 10/29/07
Posts: 1839
Loc: Montreal, Canada
|
|
Hi,
I'm not a regular on this forum and I know very little about DIY telescope projects.
The Orion dobs have a triangular shaped ground board. OTOH, Sky-Watcher has a circular ground board (hopefully I'm using ground board for the right part here) but has only three pads (legs) (so also placed in a triangular pattern). Is there an advantage to having a triangular or circular ground board? Would four pads be more stable than three?
Thank you.
--------------------
Jimmy
If you could stop time, for how long would you stop it?
"...since that time, I have not complained about the weather one single time. I’m glad there is weather." – Alan Bean, Apollo 12
|
Jim Moscheck
Carpal Tunnel
Reged: 01/08/07
Posts: 1624
Loc: Under the SE Michigan lightdom...
|
|
Quote:
Is there an advantage to having a triangular or circular ground board? Would four pads be more stable than three?
No, the three pads(feet) form a tripod and will be stable over most surfaces. Four feet could cause it to rock.
--------------------
Jim Moscheck
Ford Amateur Astronomy Club
ATM truss dob
Stevens 14.5" f4.3
"A mind is like a parachute. It doesn't work if it's not open." - Frank Zappa
|
Achernar
Postmaster
Reged: 02/25/06
Posts: 5029
Loc: Mobile, Alabama, USA
|
|
Four points will not be more stable than three, which by the way defines a plane. In my experience on uneven ground four points can get very wobbly. It is not important if the ground board is round, triangular or square, but you should use three feet space 120 degrees apart. Use a compass, protractor and straight edge to position the feet, and always place the Teflon pads right over the feet. Otherwise the telescope will be wobbly and the ground board can act like a trampoline.
Taras
--------------------
15-inch F/4.5 Dob under construction
10-inch F/4.5 Discovery Dob
6-inch F/8 Homebuilt Dob
4 1/4-inch F/4 Homebuilt reflector
A whole bunch of eyepieces, filters and other accessories....
Two curious cats
|
epee
professor emeritus
Reged: 11/30/06
Posts: 697
Loc: Suh-van-nuh, Jaw-juh
|
|
Three feet for stability. Beyond that the only advantage I can think of might be that the triangular ground board, being the smallest possible profile, weighs less and is less likely to come in contact with destablizing objects on the ground.
--------------------
Jim Girardeau
Orion XT12 Intelliscope
Celestron 11X80mm binoculars
|
InkDark
Carpal Tunnel
Reged: 10/29/07
Posts: 1839
Loc: Montreal, Canada
|
|
Thank you guys! Makes sense.
--------------------
Jimmy
If you could stop time, for how long would you stop it?
"...since that time, I have not complained about the weather one single time. I’m glad there is weather." – Alan Bean, Apollo 12
|
arpruss
scholastic sledgehammer
Reged: 05/23/08
Posts: 858
Loc: Waco, TX
|
|
My 13" has a triangular ground board, and I am not completely happy--it is a bit more tipsy as a result. Still, my 8" travel scope has three legs, but it is very low to the ground so tipsiness is not an issue. :-)
I've found the four legs on my Coulter 8" surprisingly fine on cement driveways and parking lots, and also fine on grass as, I assume, the legs sink in a bit and stabilize.
--------------------
Coulter Odyssey 13.1" split-tube
Coulter Odyssey 8"
Home-made 7.8" F/4 dobsonian travel scope
Home-made 68mm F/5.3 achro (typically used as finder on 13.1")
Skymaster 15x70
BPTs4 8x30
32mm Plossl, 30mm Rini, 27mm Kellner, 13mm Hyperion, 6mm TMB/BO Planetary, Owl 2X Barlow
Palm TX with AstroInfo and RescoViewer
|
rboe
Reged: 03/16/02
Posts: 45411
Loc: Phx, AZ
|
|
You'll need to push your three pads further out to make it less tippy. That may introduce more vibrations though.
A circle is easier to fabricate - circle cutting jig and your done. A triangle will use less material. Kinda depends on your priorities.
--------------------
Ron
NS11GPS
Pronto
16" dob
15X70 Obies
|
HandyAndy
professor emeritus
Reged: 01/11/08
Posts: 652
Loc: West Midlands and around
|
|
Hi,
Four legs on a circle are less tippy than 3.
On my 16" Dobo I put 5 screw down legs on the original square ground board.
This means I could directly compare 3 and 4 legs in the same square limitation.
Four legs are less 'tippy' than three when you are pushing the top of the Dobo tube. On concrete or tarmac 4 legs are stable if they are torqued up the same. I use large bolts with a captive nut in a hole on the underside of the ground board. Through the top of the I put a thin steel bar as a handle. This is about 4" long and gives enough leverage to torque the bolts up by feel to pretty much evenly take the weight.
On soft ground there are 4 small 3/4" feet at the corners and they sink into the ground enough to be stable. One can always screw a bolt down where required as well.
Cheers. Andrew.
--------------------
Monarch 8x42, Zeiss 10x50 WA
10mm F2, Pentax 60mm F5
City: 7" MN78: MK4#2, 10" F6.3: MK4#1, 16" F5 ParaCorr
Country: 8" VISAC: (GP2)
Car: 6" F5 MPCC: SP, 5" 127mm F7.5 (GP2)
TV 55mm, Paragon 40mm, UO Pretoria 28mm
B&L 32 Pl, Clave's 25, 8, 6, 2x
Hyperions 5, 8, 13, 17, 24, 31
Nagler1 9mm, Meade 14mm 4000 UWA
Antares 1.6x, 0.7x, 0.5x
|
greenglass
professor emeritus
Reged: 01/22/06
Posts: 540
Loc: Ontario, Canada
|
|
You can use an axel in a hole but thats easier to do on a tripod
--------------------
10" f/5 dob unf.
7x50 Tasco binos
|
RossSackett
professor emeritus
Reged: 08/17/07
Posts: 692
Loc: Memphis, TN
|
|
Purists may quibble about having four feet rather than three, but there are certainly stability advantages. You can always rotate the groundboard to level the feet on uneven ground as I found with my grinding stand with four feet.
Ross
--------------------
"A craftsman relies on science when the state of knowledge allows it, tradition and experience when it does not, and makes art whenever he can."
12 scopes from 4.25 to 18" and a 24" in progress. 12 ATM awards. Webpage: http://stardazed.com/ Some more scope pix at http://www.flickr.com/photos/8315630@N04/
Anagrams: Amateur astronomer = A mature moon-starer; Dobsonian maker = Debonair as monk
|
PitchHitter
member
Reged: 09/23/09
Posts: 23
Loc: Southern Sierras
|
|
It is a geometrical known. Four points will find a plane but three points will define a plane. That means with four points you may rotate the base until the base will make contact on all four legs but three will always make contact so that is easier. Now if the three points are tipsy then either they were not far enough apart or the ground is not firm as it should be. A wider base would help with both conditions. Are the pads as far apart on the ground board as they can be covered by the friction plate above?
Best,
Rick
--------------------
About 20 telescopes from a 200mmAPO up
Fav is Meade 10" f:4.5 using my 3" 40mm 96°AFOV
BUT! In Progress:
14.5" f:5 grab n' go with DSCs will offer 2.4 degrees at 45 power and 8mm exit pupil (Light Bridge Buster)
16.25" f:7.5 to refigure
(20th wave ruined in a fire at the coaters)
22" f5: New mount almost done
(old all aluminum mount was stolen)
16" Varian Vapor Deposition Coater.
Looking for larger chamber.
Edited by PitchHitter (09/29/09 08:31 PM)
|
David L
sage
Reged: 11/12/07
Posts: 278
Loc: Lee County Iowa
|
|
A few years back, I discovered for myself that I could make four feet contact by picking up the base and rotating it, but I could never find out a mathematical theorem that guarantees this. To get the same stability with three feet, you need a rather larger footprint. Note that they don't make three-wheelers anymore, and you don't often see a three-legged stool. With a 16-inch Meade Lightbridge, a 16-inc-square base is quite adequate to prevent tippiness. A square is also quite easy to cut out on a table saw, although I have a couple different router compasses I use to cut out circles. I also noticed that Tom O.'s equatorial platforms use four feet. Dave
--------------------
16" Lightbridge, 10" F4.5 Homemade with Orion optics, 6" F6.5 Antares 1529 refractor, 6" F8 Homemade with Orion optics, 100mm F5.6 finderscope, 22X100 Oberwerk binocs, 8.5X44 Swift Audubon binocs, Orion Atlas mount, equatorial platform, various tripods and altaz mounts
|
|
6 registered and 7 anonymous users are browsing this forum.
Moderator: Don W, Mike I. Jones
Print Thread
|
Forum Permissions
You cannot start new topics
You cannot reply to topics
HTML is disabled
UBBCode is enabled
|
Thread views: 248
|
|
|
|
|
|
|
Previous
Index
Next
Threaded
Edit
Reply
Quote
