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Common mistakes when building a Dobson

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#1 Lucullus

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Posted 31 December 2019 - 08:00 PM

What are common mistakes when building a Dobson apart from the optics?
What were your personal mistakes that you'd like first-time builders to avoid and not struggle with?

#2 Oberon

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Posted 31 December 2019 - 08:44 PM

Top heavy or unbalanced, requiring the addition of counterweights.


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#3 MitchAlsup

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Posted 31 December 2019 - 08:51 PM

What are common mistakes when building a Dobson apart from the optics?

a) Confusing stiffness with strength. Telescopes are <very> often insufficiently stiff, while simultaneously being very strong and heavy.

b) Assuming that you can build to "blueprint". What you need to do is to be able to adjust almost everything.

c) Assuming that welding/brazing is difficult or expensive


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#4 ShaulaB

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Posted 31 December 2019 - 09:21 PM

Don't assume you can hand-build everything. John Dobson was a genius at making do with easily-obtained materials. I am not. My hand-made mirror cell is not so good at collimating. Moral of the story--buy what you can't build, and know when something is beyond your ability.  Dobsonian.gif


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#5 a__l

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Posted 31 December 2019 - 10:37 PM

Heavy finder at UTA (80 mm). He demanded a heavy bottom. Without adding counterweights. The bottom was calculated from the balance sheet (this is not an mistake, the correct action).


Edited by a__l, 31 December 2019 - 10:41 PM.


#6 jtsenghas

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Posted 31 December 2019 - 11:35 PM

1) Not treating balance in two dimensions instead of just along the length is an error I often see newbies make. 

 

2) Using too small altitude bearings. 

 

If a heavy finder or two are atop the tube and if the focuser also points moderately upward, the scope can balance when pointed at the horizon,  but drift towards zenith at higher altitudes. Sometimes counterweights are needed at the primary end beneath the tube to get the center of gravity close to the a point along the altitude axis,  or the altitude bearings must be moved away from from the scope axis to get good balance at all postures.

 

When altitude bearings are too small the scope is too sensitive to balance changes with different eyepieces. Larger bearings also allow for more adjustment of friction forces by giving one more ability to move altitude pads away from each other to increase friction or towards each other to decrease friction. When altitude and azimuth friction are similar in magnitude,  high power tracking is much easier. 


Edited by jtsenghas, 01 January 2020 - 08:24 PM.

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#7 Starman47

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Posted 01 January 2020 - 07:10 AM

Underestimating the total cost. Particularly if this is a first build - power tools come to mind, extra materials when the inevitable mistake happens, etc.

 

And not consulting others when your skill set ain't up to the job.


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#8 roscoe

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Posted 01 January 2020 - 08:13 AM

Using inexpensive materials, like construction-grade plywood or pine lumber,  too much chance of splitting or warping, and they don't take finishes as well as cabinet-grade lumber. 


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#9 ScottW

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Posted 01 January 2020 - 08:52 AM

Hi,

 

First mistake is to assume the first build is a one and done thing.   Dob builds are by their nature never "done".

That's my experience!

 

Have fun building,

 

Scott


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#10 Pinbout

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Posted 01 January 2020 - 09:00 AM

Not a mistake, just took time and built sub-assemblies, tested them and then refined them before using the concepts on finished build.

 

https://youtu.be/sVpjifuRRXE

 

https://youtu.be/EyfJ7qMKnoI

 

 

I took someone’s crappy build and tore it apart and rebuilt it.

 

 https://youtu.be/aAV0OKfjEIM


Edited by Pinbout, 01 January 2020 - 09:02 AM.

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#11 coopman

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Posted 01 January 2020 - 12:56 PM

Not cutting the focuser hole in the right place on the tube.


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#12 dan_h

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Posted 01 January 2020 - 02:17 PM

Top heavy or unbalanced, requiring the addition of counterweights.

So common and a real problem in a truss scope.  There is not much sense in building with a thin mirror on a light weight mirror cell  if you need to add 10 pounds at the bottom to balance the scope. 

 

dan


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#13 nirvanix

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Posted 01 January 2020 - 04:22 PM

Bearing size, friction, and counterweights interplay. if you're looking for the lightest or smallest build you have to experiment. I settled for a slightly smaller bearing size (24") to allow for easier transport, but with the lightweight mirror/cell I had to had 5lbs counterweight to the back. If I switch to 27" bearings I likely could forego the counterweight.



#14 Dale Eason

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Posted 01 January 2020 - 05:22 PM

So common and a real problem in a truss scope.  There is not much sense in building with a thin mirror on a light weight mirror cell  if you need to add 10 pounds at the bottom to balance the scope. 

 

dan

Actually there is a reason.  That is too keep it more portable by breaking it up into pieces that can be easily carried.    It may need 10 lbs of counter weight but 10 lbs is much easier to carry than a 30 lbs mirror box.

 

However if it really needs 10 lbs of counter weight then one must have designed it wrong in the first place.  Which has already been mentioned as one of the common issues.

 

My 2 cents are not realizing why the recommended AZ and Alt bearing friction methods are key to good scope movement.  Many novices think that as low of friction as possible is the key.  But of course it is not as explained in many good books on the subject.

 

Dale


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#15 Arjan

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Posted 01 January 2020 - 05:35 PM

Too complicated to set-up. It usually is dark when (dis)assembling the scope, not a good time to fiddle with many screws or other small parts.
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#16 Bob4BVM

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Posted 01 January 2020 - 07:57 PM

Too complicated to set-up. It usually is dark when (dis)assembling the scope, not a good time to fiddle with many screws or other small parts.

I agree with this one. But as was said above a DIY scope is never really "finished"...not to say you can't use it, but just keep in mind that you may be modifying it for years to come. A complex setup is unhandy in the dark for sure, but you are free to make all those ergo changes as the needs are realized over time. I am still in the process of modifying EVERY scope I have ever built, as well as some commercial ones ive had for a long time, just to make them easier or more pleasant/comfortable to use. Good to keep that in mind.

 

To that end, you will eventually learn to incorporate ease-of-use ideas at the get-go when planning a build, but even those will get their share of fine tuning as you use them for a while.  I always keep a scratch pad handy when observing to jot down little things that could be made better on a scope. Makes for an endless source of astro DIY projects 

:)

Bob


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#17 Beeham

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Posted 01 January 2020 - 08:29 PM

Under the general headings of "poor planning", "lessons learned", and "after-the-fact improvements":

 

1) forgetting to include mass of internal plywood baffles when calculating CG - oops

2) cutting stinky FRP in the shop...the smell lingers, cut it outdoors

3) passing threaded sections of collmation bolts through holes in 3/4" plywood - the threads bind in the wood and make adjustment difficult (fixed with addition of bronze bushings)

4) not initially including a hole for a cooling fan behind the mirror cell

5) not stirring flat paint sufficiently - the flattener-stuff settles on the bottom, if you don't stir sufficiently you get semi-gloss

6) assuming "charcoal" colored paint was black (it's grey)

 

Cheers!



#18 sopticals

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Posted 01 January 2020 - 08:31 PM

Large Alt bearings "will cover a multitude of balance sins".

 

Stephen.(45deg.S.)


Edited by sopticals, 01 January 2020 - 08:40 PM.

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#19 Pinbout

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Posted 01 January 2020 - 09:29 PM

Oh oh oh... this

 

not measuring the focal length with a knife edge and just assuming f6 means 48” focal length for an 8” mirror, when it’s really not 48”


Edited by Pinbout, 01 January 2020 - 09:30 PM.

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#20 jtsenghas

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Posted 01 January 2020 - 11:33 PM

Regarding the error of assumed focal lengths from approximate f ratios,  someone without the ability to do knife edge checks could get an adequate measurement by measuring the radius of curvature with a flashlight and a white card. 

 

When the mirror returns a sharp image of the bulb of a flashlight to a card held alongside the flashlight (both the same distance from the mirror), that distance is twice the focal length. You might be able to measure that only within about 1/4", but that should be adequate for a build. 

 

Yes,  Danny,  assuming you know the focal length is a big one. 


Edited by jtsenghas, 01 January 2020 - 11:36 PM.

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#21 m. allan noah

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Posted 02 January 2020 - 07:54 AM

Heh, not the OP personally, but the biggest mistake I see is asking for advice here on CN, and then arguing with folks who take the time to respond :)

 

Smaller ones include things like balance, etc already mentioned. One that bit me on my first build was failing to include enough clearance in the rocker box for the scope to reach zenith. I was able to cut a nice hand-hold slot at that spot, and correct the issue.


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#22 Stathis_Firstlight

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Posted 02 January 2020 - 08:43 AM

1. Not understanding interplay between balance point, altitude bearings, friction and changing eyepiece weight. Results in a scope, that needs counterweights for general use or for specific eyepiece loads.

 

2. Oversizing single components while others are undersized. Results in heavy but still wobbly behaviour and unprecise tracking. Very often I see excessively strong mirror boxes, but far  too weak rocker box bottom boards.

 

3. Complicated set up and alingment

 

4. Too thick and heavy mirrors or inadequate radial mirror support with thin mirrors 

 

The more lightweight and compact the scope, the more difficult it gets, to address all points. Therefore a compact, lightweight, but still rigid and well balanced Dobsonian is more difficult to make.


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#23 Pinbout

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Posted 02 January 2020 - 06:18 PM

Regarding the error of assumed focal lengths from approximate f ratios,  someone without the ability to do knife edge checks could get an adequate measurement by measuring the radius of curvature with a flashlight and a white card. 

 

When the mirror returns a sharp image of the bulb of a flashlight to a card held alongside the flashlight (both the same distance from the mirror), that distance is twice the focal length. You might be able to measure that only within about 1/4", but that should be adequate for a build. 

 

Yes,  Danny,  assuming you know the focal length is a big one. 

Checking focal length without a ke

 

https://youtu.be/_0JlYspvQlo



#24 a__l

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Posted 02 January 2020 - 06:45 PM

Checking focal length without a ke

 

https://youtu.be/_0JlYspvQlo

What prevents checking star test?



#25 a__l

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Posted 02 January 2020 - 06:49 PM

 

The more lightweight and compact the scope, the more difficult it gets, to address all points. Therefore a compact, lightweight, but still rigid and well balanced Dobsonian is more difficult to make.

The telescope becomes unstable from accidental contact with the eyebrow (to the eyepiece) or hand (to the structure).




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