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Mirror boxes in older classical dobsonian

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#1 grzesznypl

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Posted 17 November 2019 - 05:24 PM

As I am approaching start of renovation/rebuild of my 25 year old Starsplitter and after studying designs of many modern telescopes many of them from members of this community, one thing really comes to my mind and is hard for me to comprehend. Why do older dobsonians had such a huge mirror boxes most of the time half empty (or half full - you choose). Is there any reason or rational explanation, benefit why mirror boxes use to be (and in case of some dinosaur telescope makers and still are) build so big?



#2 PeteDCard81

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Posted 17 November 2019 - 06:05 PM

The photo shows examples of the classical dobsonian telescope.

 

From Wikipedia:

 

"Two amateur-built Dobsonian style telescopes on display at Stellafane in the early 1980s."

 

 

Mark

 

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  • Two_dobsonians.jpg


#3 Oberon

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Posted 17 November 2019 - 06:06 PM

Benefits of a big mirror box:

1. A logical place to mount the altitude trunnions (especially back in the day when they were small)

 

2. They keep little and not so little fingers away from the mirror

 

3. Its traditional

 

4. It looks reassuringly dark in there

 

5. Better protection against dew

 

6. Trusses could be shorter (important when f ratios were longer)


Edited by Oberon, 17 November 2019 - 06:07 PM.

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#4 grzesznypl

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Posted 17 November 2019 - 06:39 PM

The photo shows examples of the classical dobsonian telescope.
 

Mark

Mark, I know how telescopes look like lol.gif lol.gif lol.gif 

 

Benefits of a big mirror box:

1. A logical place to mount the altitude trunnions (especially back in the day when they were small)

2. They keep little and not so little fingers away from the mirror

3. Its traditional

4. It looks reassuringly dark in there

5. Better protection against dew

6. Trusses could be shorter (important when f ratios were longer)

Thanks Oberon. 5 and 6 makes most logical sense to me. I live in dew heavy zone, so protecting against dew is of the paramount importance. Since most if not all truss dobsonian owners use shroud over truss poles does it still really matter if mirror box is big or small? I was going to significantly reduce size of my mirror box but now you gave me something to think about. 


Edited by grzesznypl, 17 November 2019 - 06:52 PM.


#5 kfiscus

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Posted 17 November 2019 - 07:50 PM

Lack of imagination? (No offense or pun intended- they were already 'thinking outside of the box' regarding motions and materials.)

Ease of building and dealing with flat-sided objects instead of cylinders?



#6 Pinbout

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Posted 17 November 2019 - 08:22 PM

F5 or slower I build like this

 

https://youtu.be/0ri3cHnmpYk

 

f4.5 or faster I build like ...

 

https://youtu.be/G3xu8CNBe0k

 

because of balance

 

the fast ultralight - I use front collimation knobs

 

https://youtu.be/yeFtBBRjuLM


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#7 grzesznypl

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Posted 17 November 2019 - 08:31 PM

Lack of imagination? (No offense or pun intended- they were already 'thinking outside of the box' regarding motions and materials.)

Ease of building and dealing with flat-sided objects instead of cylinders?

lol kfiscus. You can built 30 inch high cylinder and place 1 inch thick mirror on the bottom of it. Shape does not matter it is the high on those mirror boxes that makes me wonder.



#8 Dale Eason

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Posted 17 November 2019 - 08:41 PM

Perhaps some are mixing up names.  The mirror box is attached to the telescope tube and hold the mirror.  The rocker box is what the telescope tube fits into and contains the resting point for the altitude bearings.

 

In the pix posted by Mark there is no mirror box.  The telescope tube housed the mirror.  In a Truss style dob then the mirror housing could be called and is usually called the mirror box.

 

Then the main advantage for height is to shorten the trusses.  But that adds a lot more weight that might make the tube balance a problem.  It depends.

 

My two scopes have short mirror boxes just a few inches tall to just contain the mirror and makes packing them in the car easier after the trusses are removed.  

 

Dale



#9 tommm

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Posted 17 November 2019 - 11:12 PM

A taller mirror box also permits mounting the altitude bearings up further away from the primary mirror to balance the OTA, especially useful for taller (and heavier) UTA with heavy things like finders, telrads, binoviewers mounted on it. It permits one to make a truss dob with mirror box somewhat like the scopes in the photo in post #2, with primary mirror further back from the altitude axis.  Low profile mirror boxes don't permit this, so you typically see short, lightweight UTA with added light baffle of lightweight material on them so the OTA will balance.


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#10 PeteDCard81

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Posted 18 November 2019 - 11:15 AM

Dobsonian telescope designs has evolved over the years since John Dobson made large Newtonian telescopes mounted on an Alt-Azimuth mounts and used them to show the sky to the public.

 

It wasn't until the mid-1980's when David Kriege (of Obsession Telescopes), Ronald Ravenberg, Peter Smitka, among others, pioneered innovations that made Dobsonian telescopes functional and convenient to use.

 

 

Mark

 

Attached Thumbnails

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Edited by PeteDCard81, 18 November 2019 - 11:16 AM.


#11 MitchAlsup

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Posted 18 November 2019 - 06:13 PM

Checkout a new, modern, interpretation closer to an Ultralight than a classical::

 

https://www.cloudyni...269-mirror-box/

 

Here the mirror box is just 2 tiny pieces of wood that provide dust shield, and other parts of the assembly provide the stiffness.



#12 dave brock

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Posted 18 November 2019 - 07:09 PM

A taller mirror box also permits mounting the altitude bearings up further away from the primary mirror to balance the OTA, especially useful for taller (and heavier) UTA with heavy things like finders, telrads, binoviewers mounted on it. It permits one to make a truss dob with mirror box somewhat like the scopes in the photo in post #2, with primary mirror further back from the altitude axis. Low profile mirror boxes don't permit this, so you typically see short, lightweight UTA with added light baffle of lightweight material on them so the OTA will balance.


Also makes the pivot higher off the ground which makes lower altitude observing more comfortable.

#13 Kunama

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Posted 18 November 2019 - 07:27 PM

If you're going to go with a shallower mirror box design it needs to be stable enough to keep the altitude trunnions in place so there is no side to side swaying which would make for a difficult time tracking objects..

I built my previous 18 with a 'normal' tall mirror box but due to other restrictions (getting the scope into my vehicle through a 15" high hatch) I had to come up with another plan for my new scope...

 

Welded aluminium box section with gussets, reinforcing inserts and mounting pucks in stainless steel, alt trunnions cut from 1/2" thick aluminium plate.....

 

 

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  • MIS_5818.jpg

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#14 grzesznypl

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Posted 18 November 2019 - 11:37 PM

Matt I know your scope, well not really but seen it before and have few pics of it save on my HD in ATM design folder. This is beautiful telescope and exquisite piece of ATM engineering. Wish I had tools and expertise to work with metals unfortunately I do not. I have tools and I am proficient with wood so I have to follow that path at least with my incoming job. My problem that prompt me to renovate/ rebuild my 15" is somehow similar to yours:

1. Telescope does not fit into my sedan unless I buy SUV or trailer and not buying neither, for 20" or 25" if I have one of those one day maybe but not for 15" one.

2. Mirror box with bearings + mirror and cell weights astonishing 110. I am confident I can shed 40-50 pounds to make it more compact and wieldable by 1 person
3. Bearing are sticking to high so loading it to car trunk is impossible. 
4. 2 bearings itself weight 32 pounds (16lbs each) agrrrrrrr thumbsdown3.gif 
5. Astronomy should be fun and not an attempt to break my back. 

Hence my question about mirror boxes. I am sure I have few more questions. Its a learning process and you guys are best sources of info bow.gif



#15 Oberon

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Posted 19 November 2019 - 06:53 AM

Make the trunnions removable. Here is my 16” in the trunk of an ordinary car, with room for more.

 

gallery_217007_4913_87064.jpg


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#16 grzesznypl

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Posted 19 November 2019 - 09:57 AM

Make the trunnions removable. Here is my 16” in the trunk of an ordinary car, with room for more.

Either removable or foldable like UC Obsession series for example.

16" in the trunk of sedan with room to spare, amazing!!!



#17 tommm

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Posted 19 November 2019 - 12:42 PM

Also makes the pivot higher off the ground which makes lower altitude observing more comfortable.

Yes, and as a result reduces the range of eyepiece height with altitude. 

 

A short mirror box made of wood can be made very stiff using the same construction K&B scopes use - gussets in the corners, sides bolted to metal "tailgate" frame, etc. No issue there. A short, lightweight mirror box assembly puts somewhat severe constraints on the UTA assembly since it then has to be very lightweight for the scope to balance. A lb added there is typically like 3 - 4 lb added at the mirror box, depending on radius of the alt bearings (height of the alt pivot axis).  This is many times addressed by making larger radius alt bearings, but for large scopes there is a trade off in making the alt bearing radius large to help balance the OTA, and fitting those big alt bearings into your car without removing them.

 

You may not want to make the alt bearings removable on a larger scope, as it can be a pain to have to mess with installing/uninstalling those each time you want to use it when the mirror box, mirror, and mirror cell assembly weighs 80 lb or more. A 16" scope is quite light in comparison to a 25", though the relatively new thinner mirrors help (my 22.4" diam, 2" thick f/3.6 mirror weighs 57 lb). Any added step to using the scope is an impediment.

 

In case you think I may be biased because I have a K&B type scope...that is the way I made my first 22.4" scope - exactly by their book. It worked great, very smooth movement, and enough friction so that balance with everything from a small 1 1/4" ep to a 2" Powermate with DSLR attached was fine.  I recently rebuilt it to a short mirror box. And I recently bought a hatchback that I would like to fit the scope into.  That's why I am well aware of the trade offs.  Those big alt bearings make it balance, but I'll have to chop off 4.5" of them to fit it in the hatchback rather than hauling it in my truck.

 


Edited by tommm, 19 November 2019 - 12:58 PM.


#18 Kunama

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Posted 19 November 2019 - 02:44 PM

Thanks Greg, building in wood was my forte also, I used to make furniture and rocking horses among other things.  

I have seen some nice low profile timber Dobsonians from New Moon Telescopes as well as one from SDM in Australia.

With the 15" F5 primary you have the key is going to be keeping the UTA and poles as light as possible without compromising stability.

 

Low slung mirror boxes usually mean large radius altitude trunnions, for transport the trunnions should be easily removable.


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