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Big mirror boxes - KB style

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#1 B9Robot

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Posted 12 November 2013 - 12:32 AM

So I am just reading KB book on "how balance affects the mirror box" p180. It has a statement - the one rule: "make the mirror box deeper than the calculations call for", in other words the top of the mirror box should be higher than the balance point. So I notice the trend has been to smaller and lighter mirror boxes - and then building cresent shaped altitude bearings. The balance point (altitude bearing centre) being somewhere in space amongst the trus rods. What are the pros and cons to the big KB mirror box versus smaller ultralight boxes?

thanks

#2 Jeff Morgan

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Posted 12 November 2013 - 09:57 AM

When you read KB you have to keep in mind two things:

Firstly, it was written 20 years ago and builders have done much experimentation and refining since then. Kriege was building these things commercially so once he had a workable design the emphasis was production, not so much experimentation.

Secondly, the KB model was built around the f/5 template. This sets up a pretty specific relationship between mirror mass and scope mass, and hence balance. Once again builders have been changing the "formula" by going with longer and shorter mirrors. Also, 20 years ago the standard mirror was 1:6 thickness and provided considerable ballast to the system. Thin mirrors were still a bit sketchy at the time both from a fabrication and support standpoint. Of course both of these things have changed today.

As far as the overall size of the mirror box - it depends upon how you are going to use the scope. If you are installing DSC's orthogonality it your primary goal and building it a little deeper will make it somewhat easier to build "straight". Deeper also gives you a little more flexibility and ease for attaching the altitude bearings and placing the mirror cell. Given the combined mass of glass and cell, moving it an inch provides a lot of flexibility for fine-tuning the balance too.

OTOH, building it "minimalist" saves a lot of weight. If you are careful about planning the balance on paper before you start making sawdust, the need for counterweights should be minimal. Likewise, planning for the attachment of altitude bearings takes a little extra care, but is not difficult. You just want to remember to add some form of connection at the end of the crescents to "close" the structure so that the mirror box/alt bearing combo doesn't behave like a tuning fork (see attached photo for example) as you push the scope.

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#3 careysub

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Posted 12 November 2013 - 10:07 AM

Great concise summary!

#4 derangedhermit

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Posted 12 November 2013 - 05:14 PM

Isn't it nice to have the alt center of rotation fall on the mirror box side for easy, solid mounting of an encoder? It seems to be a common choice to put the alt rotation point at or just below the top edge of the mirror box.

#5 Pinbout

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Posted 12 November 2013 - 06:29 PM

I would never put the top of the mirror box higher than the pivot/ balance. In their method they start with the pivot at that center of the top of their mirror box.

#6 B9Robot

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Posted 12 November 2013 - 07:21 PM

Isn't it nice to have the alt center of rotation fall on the mirror box side for easy, solid mounting of an encoder?


If you have a semi circle alt bearing the encoder attaches to the centre of the bearing, not the mirror box.

It seems to be a common choice to put the alt rotation point at or just below the top edge of the mirror box.


That is my question of this thread - why have such a big mirror box. Is it really needed? What for?

#7 hbanich

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Posted 12 November 2013 - 08:06 PM

A large mirror box has several positive attributes and few not so positive ones. Here's the way I see it:

Pros
1. Very strong and resists flexure.
2. Allows the truss tubes to be shorter, reducing vibration.
3. Eliminates the need to brace the altitude bearings.

Cons
1. Insulates the primary mirror from the night air.
2. The mirror is more difficult to install/remove.
3. Internal front surface fans can set up a circulation that degrades the image.

This is off the top of my head - no doubt I've missed several important pro and con points - but I don't mean it to be a comprehensive comparison. My point is, like every other part of a telescope, there are trade offs to consider when designing a mirror box.

#8 Pinbout

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Posted 12 November 2013 - 08:57 PM

they suggest making it a little longer than normal cause when you test for the balance point on a stick you can fine tune the pivot point and then cut the box down if needed.

but another point is to start out with a cube, lets say 12" cube for a 8in mirror.

to do the torque equation with the uta on the left and mirror on the right of the pivot - adding a 1lbs eyepiece combo 2.5ft from the pivot point adds 2.5ft*lbs of torque. your 3lbs 8in mirror that's not even 12" to the other side of the equation is only a fraction of the weight in torque, 3lbs*.8ft = 2.4ft*lbs of torque.

#9 cpr1

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Posted 12 November 2013 - 09:29 PM

As Jeff said the K&B style is somewhat dated, It can be altered to fit your needs. Thinner mirrors, light building materials, etc. The large box protects your mirror better from dew, stray light, kicked dust, small animals.

The relatively heavy K&B style boxes allowed for the way the UTA was designed. Bearings, UTA height, Mirror box height, thickness of the plywood. Most people add finders, some DSC, dew heater, power distribution, etc. So the taller boxes add weight below the COG and make it easier to add things to the upper assembly. And it would seem to be easier to add encoders. Shorter truss poles are less likely to vibrate or flex. The longer poles required for a short mirror box will likely flex more. The K&B alt bearings don't need to be removed for transport because they are compact. I am sure they could have went with a short mirror box larger bearings but the bearings would likely need to be removed to transport a big one.

Plus when K&B made these coming from solid tube scopes, these were extremely portable and still are. That may be some of the original reasons for the taller box.

Most premium commercial scope builders still use this design to this day. Some builders make their own tweaks to the design to improve it, but they are still based on the good ole dated K&B design. Which works well by the way.

Are you building a compact scope? I like both designs. The compact scopes for their light weight and the K&B for their sturdiness.

#10 B9Robot

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Posted 12 November 2013 - 09:48 PM

Thanks for the replies. I don't know if I would call what I am building "compact", but the mirror box will only be 4" high. I am working to keep weight down, above and below CG. So far I don't see any compelling for a big mirror box. Encoders, no problem. Flex in truss tubes, not going to be an issue. Bearings too big? well maybe, but I am making them removeable.

#11 Pinbout

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Posted 12 November 2013 - 10:21 PM

well i don't have a mirror box...

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#12 Jeff Morgan

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Posted 13 November 2013 - 12:30 AM

Thanks for the replies. I don't know if I would call what I am building "compact", but the mirror box will only be 4" high. I am working to keep weight down, above and below CG. So far I don't see any compelling for a big mirror box. Encoders, no problem. Flex in truss tubes, not going to be an issue. Bearings too big? well maybe, but I am making them removeable.


You know, I built a scope just like that, a 16" f/7 Newtonian true Serriuer truss (the "Gen 1" photo attached). And to make it light weight, the much of the scope was a composite of 1/16" Finnish Birch plywood skins and end grain balsa wood cores - the mirror box weighed less the 5 pounds before installing the mirror and cell. And it had removable alt bearings. And ServoCAT.

So how did it work out? Lots of assembly in the field, 45-50 minutes worth due to ServoCAT. And removable alt bearings and thin skinned components made orthogonality for DSC's problematic. Perhaps I could have worked the issues out in a second or third iteration (Dave Kriege once said that it took him three attempts to get the Obsession template), but a wood scope that totally disassembles is not the best for a DSC installation. As a purely manual star-hopping scope it would have been dynamite though.

After a few outings I decided that ServoCAT was such a force-multiplier that I built another structure in a more conventional (and heavier) design to accommodate ServoCAT. On the new scope set-up time is now 10 minutes at a relaxed pace. Using wheelbarrow handles negates the weight. That Archimedes was a pretty smart fellow.

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#13 gatorengineer

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Posted 13 November 2013 - 05:51 AM

Jeff - what do you mean by force multiplier?

#14 careysub

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Posted 13 November 2013 - 07:49 AM

...So the taller boxes add weight below the COG and make it easier to add things to the upper assembly. And it would seem to be easier to add encoders....


And make it easier to add other stuff to the mirror box. If you want to add any of a variety of fan cooling arrangements you can, and probably tuck a battery pack for them in their also if you want a little extra counter-weight. If you want light baffling behind and in front of the mirror you can do that too.

#15 careysub

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Posted 13 November 2013 - 07:59 AM

Jeff, is this your second double truss scope?

http://www.cloudynig...hp?item_id=2571

I would be interested in hearing more about this build, could you post a write up on this forum about it so that we can learn from your experience? It incorporates a number of unusual or innovative features (the double truss itself, construction materials).

I would have thought that the lengthy set-up time would be due to the double truss instead of the ServoCAT.

#16 NewMoonTelescope

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Posted 13 November 2013 - 09:11 AM

The KB book is an excellent resource for building beautifully performing scopes. As mirror makers keep bucking the norm and going faster and faster, the large mirror box becomes unnecessary (and too heavy). As stated before, the balance point of the scope can be anywhere above or below the top of the mirror box (and why not shave off several pounds and make it above). Regarding the encoders, just make an adapter that extends to the central point of rotation. I make them out of aluminum and then powder coat them flat black, but any material could be used.
Beautiful scopes btw Jeff!

Darker skies,

Ryan

#17 Pinbout

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Posted 13 November 2013 - 09:13 AM

As mirror makers keep bucking the norm and going faster and faster, the large mirror box becomes unnecessary (and too heavy).



Also thinner and lighter effects the torque equation. :grin:

#18 kfrederick

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Posted 13 November 2013 - 09:48 AM

Not hard to test put in the laser move the telescope all around it does not shift be happy [For a dob ].Short and fat telescopes can be made lighter . Not too many under built dobs The Coulter 29 could be used as a storm shelter .Marks 36 structure will weigh close that a 13 Coulters structure .

#19 Víctor Martínez

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Posted 13 November 2013 - 01:01 PM

For me, a compact mirror box design for a telescope from 16 ", it has more advantages over the K-B classic mirror box, especially less primary box weight and acclimatization. My mirror box.

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#20 gatorengineer

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Posted 13 November 2013 - 01:29 PM

Beautifully done Victor

#21 Jeff Morgan

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Posted 13 November 2013 - 11:32 PM

Jeff - what do you mean by force multiplier?


Meaning it vastly increases your observing efficiency per unit of time.

Firstly, there is the tracking aspect. On recent threads some folks have enthusiastically stated it is like adding aperture. I understand the sentiment, but a more accurate thing to say would be that it lets you fully utilize your existing aperture, vs. manual guiding that effectively knocks you down a step. It's a pretty big deal for detail viewing, although probably not as big for 3F viewing (Faint, Fuzzy, Featureless). And the ability to take a break from the action, come back and have the object centered in the field is golden. Outreach potential is great too.

Secondly, the GOTO portion. For the last five years one of the friends I observe with has been using ServoCAT. So I was aware of it, but not wholly impressed with the Argo user interface. While rugged and simple, (being limited to two buttons and a dial) it involves a lot of steps through menus. I could operate nearly as fast via star hopping, provided it was over a small area of sky. But the advantage of the GOTO system grows over the night as human fatigue sets in. So while I had to admit my friend was seeing more targets every night than I was, spending the $$$ was not yet justified.

What really changed it all for me was SkySafari to handle the interface duties. Fast, flexible, and graceful. I can use planning software (AstroPlanner in my case) to develop lists during my "off" times and export those lists to SkySafari. (SkySafari's internal list generation, search, and sort capabilities are nonexistent, but it is very effective at using a list like a script.) And of course it eliminates paper charts and references (20 pounds less stuff to schlep).

Once the scope is set up I launch SkySafari, connect wirelessly to the scope and pull up a list of say, 50 objects. Tired of challenge objects and want to take an eye candy break? Or your buddy wants to confirm an observation? No problem, I can slew across the sky in 5 seconds and have some fun. When it is time to get back to "work" (that is, my observing plan for the night), I'm back to that obscure 4 arc second 14th magnitude planetary nebula next on the list, all in about 5 seconds.

Or for the free spirits, you can just tap on interesting map targets. ServoCAT does the rest.

So overall, the gain in productivity made replacing the structure a worthwhile venture.

#22 Jeff Morgan

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Posted 13 November 2013 - 11:59 PM

Jeff, is this your second double truss scope?

http://www.cloudynig...hp?item_id=2571

I would be interested in hearing more about this build, could you post a write up on this forum about it so that we can learn from your experience? It incorporates a number of unusual or innovative features (the double truss itself, construction materials).

I would have thought that the lengthy set-up time would be due to the double truss instead of the ServoCAT.


Yes, my second double-truss. Maybe I'll put together a write-up over the winter. There was some good things there, and some mistakes. It was very much a test bed.

The original plan was to use it on an equatorial platform, which would have been fine. ServoCAT was a late change in specification. Since ServoCAT proved very much worth keeping and the system was really conceived for Obsession-style scopes the path of least resistance was to re-build that way. It could have taken me a couple of more rebuilds to adapt the system to double-truss. When you have a nice Zambuto mirror sitting around, you feel the time pressure.

In terms of set-up time, the extra truss set really only adds about 3 minutes (just like a normal set x 2). But the total set-up time was the sum of many steps since the scope totally dis-assembles. I was even installing the primary mirror every time! It all adds up.

One error I will point out right away to all builders - note the orientation of the Moonlight truss block clamps - outwards. The clamps are stainless steel 1/4-20 rods threaded through a thin Delrin section. (In the article you linked to, notice the hand knobs all face inwards!) The Delrin blocks are very susceptible to handling and transport damage should those thumb knobs bang into anything. I ruined several blocks in this fashion. I like the Moonlight system, but that is a weakness to be aware of.

#23 Jeff Morgan

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Posted 14 November 2013 - 12:00 AM

An elegant beauty indeed!

#24 B9Robot

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Posted 14 November 2013 - 01:34 PM

For me, a compact mirror box design for a telescope from 16 ", it has more advantages over the K-B classic mirror box, especially less primary box weight and acclimatization. My mirror box.

Posted Image


Victor - very nice build. A question about your Alt encoder. It looks like it is supported only by a single flat bar attached to the alt bearing. If so does the bar movement cause issues with accuracy?

thanks

#25 Víctor Martínez

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Posted 14 November 2013 - 03:12 PM

There isn´t movement. If the center is well calculated, no problem with accuracy. If you look closely the bar, this is fixed to the Alt bearing with two screws, preventing tilt up.






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