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
ndelo
professor emeritus
Reged: 08/02/06
Posts: 528
Loc: Light-polluted New Jersey
|
|
I want to make a 10" ultralight as a dry run to converting my 18". (I have a 10" GSO mirror laying around to put to use). I have a good idea of the steps I need to take in order to make one, however, what do I need to consider in terms of balancing the scope?
When I made my traditional 18" obsession clone, I simply followed the formulas provided in the book.
I'f Im going to attempt a 18" (well 10" first) UC clone, I'm guessing that the approach I nee to take to balance it is different.
Any advice?
|
Neil Carroll
Vendor-Starbuckets Telescopes
Reged: 06/09/04
Posts: 119
Loc: North Carolina, USA
|
|
If building a UC clone, a couple factor will be:
1. Make the upper ring assembly as light as possible. The focuser is likely one of the heavier items on your upper ring. Moonlite is lighter weight than Feathertouch. Both are heavier than ideal. If you can live with it....a Kline helical focuser is very very lightweight.....and not a bad focuser.
Using 1/2 baltic birch for the ring is good. But if you can find or afford a piece of composite such as aluminum honeycomb core sandwiched with fiberglass you can save more weight. forget a finder scope and use a rigel quick finder (lighter than Telrad).
2. The larger the altitude bearings the farther forward the fulcrum will be. this is good. (my 16" UC clone uses 31" diameter altitude bearings.)
The farther apart your teflon pads the more sticktion. this will help too. (70 degrees is good).
3. If you make your cell out of steel instead of aluminum you can add a bit of ballast to the lower end without adding counterweights.
this is not easy to do and you should shave weight off of the upper end where ever you can.
Neil
--------------------
Neil Carroll
Starbuckets Telescopes
16"f/4.75 Starbucket
and whoever's I'm building at the time:)
|
MitchAlsup
super member
Reged: 08/31/09
Posts: 184
|
|
The biggest piece of advice I can give is to make sure that you can move the altitude bearings at least 1 inch (prefereably in either direction).
I found out making my 20" DOB that no mater how accurately you measure and weight and design, the final product remains different than the plan.
Light at the top is great. If the scope ends up top light, you can add more stuff up top to balance it. If the scope ends up top heavy you have to find a way to make the bottom heavier. You can move the altitude bearings up or add weight.
Edited by MitchAlsup (10/23/09 03:35 PM)
|
ndelo
professor emeritus
Reged: 08/02/06
Posts: 528
Loc: Light-polluted New Jersey
|
|
Thanks for th advice all. This has been helpful. If it's a rainy weekend, I might begin soon. I have a lot of Baltic Birch scraps left over form my 18" project. These should get me through the upper truss.
One more quick question: On a 10" scope, can the mirror cell be baltic birch? Does it need to be steel or aluminim? I know steel is heavier for counter-weighting, but I'm not sure I want to go through the process of making a cell from steel if I don't have to.
|
RossSackett
professor emeritus
Reged: 08/17/07
Posts: 692
Loc: Memphis, TN
|
|
I always built the entire optical assembly, complete with finder, then load in my heaviest eyepiece and find the actual center of balance. I locate the center of curvature of the altitude arc at the center of balance, then build the rocker-groundboard to match the altitude arc. This process actually makes the scope slightly bottom heavy because most of the mass of the altitude arcs is below the center of curvature; I like this in case I someday find a heavier eyepiece. A small UTA counterweight restores balance until them.
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
|
MitchAlsup
super member
Reged: 08/31/09
Posts: 184
|
|
Quote:
I always built the entire optical assembly, complete with finder, then load in my heaviest eyepiece and find the actual center of balance. I locate the center of curvature of the altitude arc at the center of balance, then build the rocker-groundboard to match the altitude arc. This process actually makes the scope slightly bottom heavy because most of the mass of the altitude arcs is below the center of curvature;
This was exactly what I did. However, I still ended up top heavy. After getting the scope balanced, I figured out that the scales I was weigging things on was not sufficiently accurate. Since the heaviest assembled part was 111 pounds, I had to use a scale big enough for a person.
Then later, after colimating for the first time and achieving focus, the mirror ended up 1/3" closer to the top than before getting things focused.
I finally got it, but the key is to remain flexible in the position of the CoG until the scope is actully in working order.
|
RossSackett
professor emeritus
Reged: 08/17/07
Posts: 692
Loc: Memphis, TN
|
|
I don't use scales. I tilt the assempled, fully loaded optical assembly up on the front edge of the mirror box until it balances, and using a carpenter's level mark a vertical line on a piece of cardboard taped to the box (up from the point of support under the front edge). Then I repeat tilting it up on the rear edge. The intersection of the lines marks the center of balance. I've done this with all of my homebuilt scopes, and it has always worked perfectly for me.
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
|
bbasiaga
professor emeritus
Reged: 05/10/06
Posts: 724
|
|
Ross, do you have any pictures of your balancing procedure? I'm having a hard time following your description.
--------------------
Vixen 80EDSF
Stellarvue 102ED2
Orion Atlas 11 EQ-G
15" Astrosystems Telekit w/ Discovery Optics
Lust for something Larger
Lust for something Larger than that
Past Lovelies:
Oberwerk 20x100 binocs
Meade AR5
Meade LX10 8" SCT
All sold to a good home
|
dave b
Post Laureate
Reged: 05/10/05
Posts: 3535
|
|
just balance the assembled scope on a broom stick.
at the balance point, center the bearings
--------------------
dave bonandrini
30" f/5.2 Dobsonian
President of GCAC
Astromart Moderator
|
Sean Cunneen
Let Me Think
Reged: 08/01/07
Posts: 1222
Loc: Blue Island Illinois
|
|
This is a link to my scope build here on CN where I did just as Ross mentioned.
ultralight bearing measurement The hard part is that you have to estimate your bearing mass and add that as well. In the case of an ultralight, it is always better to be bottom-heavy, so err a couple of inches in that direction. In my case, My bearings are very large and mounted at the bottom of my rocker box. You don't say what design you are following, but in any case, the basic principleis the same.
--------------------
Sean Cunneen
Blue Island, IL
12.5" Ultralight Strut Dob
127mm f/9 refractor
35Pan, 10XW, 5XO
Member of the Calumet Astronomical Society
|
RossSackett
professor emeritus
Reged: 08/17/07
Posts: 692
Loc: Memphis, TN
|
|
Dave,
Balancing it on a fulcrum works well for a solid tube scope. However, it is more difficult (and can be risky) on a truss scope, especially an ultralight with relatively thin poles. Also, there are examples where the center of balance isn't on the central axis of the scope (the case with my single-pole ultralights). My method finds the center directly.
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
|
dave b
Post Laureate
Reged: 05/10/05
Posts: 3535
|
|
ross, you are right, it sure would be hard to do on a mono pole!
--------------------
dave bonandrini
30" f/5.2 Dobsonian
President of GCAC
Astromart Moderator
|
RossSackett
professor emeritus
Reged: 08/17/07
Posts: 692
Loc: Memphis, TN
|
|
...could hang it from a string, maybe...
--------------------
"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
|
nightstalker
sage
Reged: 03/30/07
Posts: 252
Loc: Great South Land
|
|
You should be able to make something fairly simple for the 10" cell,, my 12 " sits on a ring of velcro and the cell (aluminum) bolts together.
Neils point above is a pretty valid one imo.. in that your small scope dosn't need a focuser up top that,weight wise, feels like its made from depleted uranium.
The suggested focuser
http://www.kineoptics.com/HC-2.html
is very light and works well.
cheers graham
Edited by nightstalker (10/25/09 06:41 AM)
|
TimNott
newbie
Reged: 09/10/06
Posts: 3
|
|
Balancing a scope of this type is more critical than some others- you can't shift the mirror cell or side bearings to trim it as in some designs.
As stated, you adjust the balance by changing the diameter of the side bearings, which in turn leads to the arc on the rocker, etc.,etc.
When designing my 22" UC version, I calculated everything carefully first using data form the Dobsonian Telescope or from the internet then chose a diameter that would make the scope slightly bottom heavy with everything (parracor, argo navis, 28 UWAN etc) added.
I then quadruple checked it!
|
sixela
Postmaster
Reged: 12/23/04
Posts: 10855
Loc: Boechout, Belgium
|
|
Quote:
This process actually makes the scope slightly bottom heavy because most of the mass of the altitude arcs is below the center of curvature;
For an Ultralight, it's worse: the bearings themselves have a non-negligible weight compared to the mirror box plus UTA, and they tend to shift the COG of the combination *laterally* just as much as to the bottom. This make the scope balance differently at different altitudes (close to zenith, the bearings have a tendency to make the scope nose dive, and this disappears at the horizon).
It's something you'll hardly notice on an Obsession-style Dob because the mirror box is heavier and the bearings smaller. But on an Ultralight with humongous bearings and a very, very minimal mirror box (if there is one at all), you will.
Which means that actually computing where the COG is becomes more important (I still do cross-check the COG of the mirror_box+UTA+shroud+eyepiece_train manually, as I also check the COG of the bearings manually).
That way, you can actually mount the bearings a bit more outside of the mirror box which means the mirror box is at one side of the rotational axis and the bearings COG at the other (on my scope I have to shift the bearings 15mm, but I must confess it's because I built the bearings to be STURDY - some Germans have similar bearings but have hollowed them out on the inside to avoid this issue).
It's tricky, because if you actually have a focuser tilted slightly up, that too has in influence on the lateral component of the COG placement (you can sidestep this issue by putting the focuser horizontal, of course).
You can, of course, also chicken out and try to make the oversized Ultralight bearings out of aluminium and make the rest out of heavier material  .
--------------------
400mm f/4.46 self made Dobsonian on Tom Osypowski equatorial platform
Orion Starblast (114mm f/4 reflector, Alt/Az)
|
RossSackett
professor emeritus
Reged: 08/17/07
Posts: 692
Loc: Memphis, TN
|
|
sixela:
That combination of lateral and longitudinal displacement is what I alluded to. Actually, it is quite small, rarely more than a cm, and helps offset the assymetry from the usual location of focuser and finder off the centerline of the scope. You are right that early in the development of ultralights we were building the upper tube assemblies too heavy, necessitating humongous altitude bearings, but I think that most now realize the advantages of a very light top end, and bearings are now usually smaller than they once were, and rarely contribute much mass to the scope.
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
|
arpruss
scholastic sledgehammer
Reged: 05/23/08
Posts: 855
Loc: Waco, TX
|
|
Quote:
the bearings themselves have a non-negligible weight compared to the mirror box plus UTA, and they tend to shift the COG of the combination *laterally* just as much as to the bottom.
I assume almost everyone knows this, but just in case not, one can take this into account when placing the bearings with a little bit of math. Let (x0,y0) be the center of mass of the OTA without the bearings with the scope horizontal (in some natural coordinates, maybe (0,0) being the bottom left corner of the mirror box with the scope facing right), and let (x1,y1) be the center of mass of the bearings relative to their pivot, with the coordinate system defined by having the bearings at the angle correspoding to the being horizontal. Let the mass of the bearingless scope be m0 and let m1 be the mass of the bearings.
Well, then you place the bearings at scope coordinates (x2,y2) such that:
(x0 m0 + (x2 + x1) m1)/(m0+m1) = x2
(y0 m0 + (y2 + x1) m1)/(m0+m1) = y2
You solve for x2 and y2 and you get:
x2 = x0 + x1 m1/m0
y2 = y0 + y1 m1/m0
In other words, you place the pivots at the center of gravity of the bearingless OTA, namely (x0,y0) plus a correction vector in the direction of the center of mass of the bearings, scaled by the ratio of the masses of the bearings to the mass of the scope. If you do this, then the contribution of the bearings to the balance will be neutralized.
If the bearings are solid semicircles of radius R tilted by 45 degrees when the scope is horizontal, then x1 = y1 = -4R/(3 pi sqrt(2)), or approximately (a very good approximation--within one part in three hundred), -0.3 R. So you just shift the bearings by -0.3 R m1/m0 towards the rear of the scope and by the same amount towards the bottom side. If m1/m0 is small or R is small, this will be negligible.
If the bearings are hollowed out in various ways, you just need to calculate or measure their center of mass. If they're symmetric, you only need to do that along the axis of symmetry--you can just use a broom handle.
Or you can use full circles for the bearings and then x1=y1=0, and there is no shift. :-)
One of my mottos is: "Don't do in hardware what you can do in software." In other words, don't switch to more expensive or flimsier lightweight bearings just to avoid a bit of math. :-)
Caution: You'll want to double check my algebra. I am assuming it's all in the book, but I don't have the book.
--------------------
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
Edited by arpruss (10/25/09 11:24 AM)
|
sixela
Postmaster
Reged: 12/23/04
Posts: 10855
Loc: Boechout, Belgium
|
|
Quote:
One of my mottos is: "Don't do in hardware what you can do in software." In other words, don't switch to more expensive or flimsier lightweight bearings just to avoid a bit of math. :-)
Rest assured, I didn't.
Quote:
I am assuming it's all in the book, but I don't have the book.
If by "the book" you mean the Kriege and Berry bible, it says to ignore it, i.e. to avoid a bit of math.
Rest qs
--------------------
400mm f/4.46 self made Dobsonian on Tom Osypowski equatorial platform
Orion Starblast (114mm f/4 reflector, Alt/Az)
|
sixela
Postmaster
Reged: 12/23/04
Posts: 10855
Loc: Boechout, Belgium
|
|
Quote:
sixela:
That combination of lateral and longitudinal displacement is what I alluded to. Actually, it is quite small, rarely more than a cm, and helps offset the assymetry from the usual location of focuser and finder off the centerline of the scope. You are right that early in the development of ultralights we were building the upper tube assemblies too heavy, necessitating humongous altitude bearings, but I think that most now realize the advantages of a very light top end, and bearings are now usually smaller than they once were, and rarely contribute much mass to the scope.
Mine are with a radius of 35cm for a 400mm mirror - just enough so that if you remove them you can slide them around the mirror box with the top downwards and not see them stick out too much (the rocker goes on top upside down for transport and follows the curve of the bearings).
They don't contribute *much* mass, but as I have a minimally sized mirror box out of poplar, a cell out of aluminium and a light mirror (it's only 33mm thick), enough to matter (as they're in birch and not in poplar, and thus much heavier per volume).
Yeah, they could be smaller (I haven't exactly underbuilt them), but they're not what limits the transportability of the scope anyway, so I had little incentive to make them lighter. So I didn't skimp on the math and just bit the bullet.
--------------------
400mm f/4.46 self made Dobsonian on Tom Osypowski equatorial platform
Orion Starblast (114mm f/4 reflector, Alt/Az)
|
|
12 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: 324
|
|
|
|
|
|
|
Previous
Index
Next
Threaded
Edit
Reply
Quote
