Re: plop auto cell 36 and 54 points bugfix [was: Re: [ATM], mirrorcell design]

["Jeff Anderson-Lee" <jonah@cs.berkeley.edu>]

Ken Hunter wrote:

>If you are trying to save cost when making a
>telescope, going to an ultra thin glass mirror is NOT
>necessarily the best way to proceed unless you put
>your time and effort at a very low value.
>
Put another way: if you want to build a scope in a hurry, use thicker 
glass and save some time.  Agreed.

However if you value the time spent working on *building* the scope in a 
positive sense (i.e you *enjoy* a challenging task) as opposed to a 
*negative* sense (where less time building is more time observing) you 
may come to different conclusions.

>Glass is really very inexpensive compared to the time
>and trouble needed to compensate for the supposed
>savings. Having to rely on a multi level PLOP
>optimized cell is going to add the additional cost of
>purchasing the cell materials and machining the cell
>components PLUS all the extra effort and time spent
>assembling and keeping the scope in an operational
>status. As for saving weight, The multi-level PLOP
>cell will negate a fair portion of the weight savings
>as well. 
>  
>
Not only that, having a thicker mirror helps balance the scope better.  
Thin mirror scopes are often top-heavy and require more complex designs 
or spring counter-weights, etc.  There are many *good* reasons for 
thicker mirrors.

>All the added cost, headaches and support equipment
>needed for mounting such a thin mirror are (in my not
>so humble opinion) not worth the time and trouble. 
>  
>
It's your opinion and may well be shared by others.  You (and they) are 
welcome to it.  I'll draw my own conclusions thank you very much.

>I guess I AM tilting at wind mills as you say but I
>still advocate for the ATM(1) - AMATEUR TELESCOPE
>MAKER as opposed to the other ATM(2) - ADVANCED
>TELESCOPE MAKER
>
>I'm looking out for the guy that wants to build and
>use a telescope, not change careers or College Majors.
>I continue to adhere to the KISS principle for ATM(1)
>beginners and recommend that they use thicker glass,
>well known and easily applied, inexpensive techniques,
>and actually build and use a scope of their own making
>before they adventure too far into the realm of
>diminishing returns. 
>  
>
Yes, if the main goal is "build to use", by all means don't be penny 
wise and pound foolish as they say.  Beginners should know that such a 
thin mirror makes for a very challenging project which will need 
persistence and care for success.

>We continually hear of folks with the idea of trimming
>some weight from the mirror to save a couple pounds.
>Then they invariably talk about the trouble that move
>has caused... having to redesign the scope for the
>changed COG or having to figure a way to attach some 
>counterweights to the mirror cell (or springs in Tom
>K's design), the change in effect when changing
>eyepieces because the eyepiece now has a greater
>percentage of the active weight and on and on...
>  
>
Very true, and some of us like a challenge.

>There are way too many ATM's that give up before
>trying because we tend to scare them away with all the
>numbers, fancy cells, ultra-light trusses, exotic
>materials etc., when in reality a great performing
>scope can be hand made with little effort or
>understanding of these advanced techniques.
>
>I ask the question... 
>
>Is it better to design a scope that is 10 pounds
>lighter (or a hundred dollars cheaper) that can't be
>easily or economically finished or a slightly heavier
>(and much simpler) scope that can be? 
>
>Of course, there are going to be many answers... 
>  
>
Of course.

>Some folks just can't lift or transport the extra 10
>pounds. For them saving weight may be the only way to
>go but it will have an added price. Others just like
>to build things and will enjoy the added complexity,
>challenge, and less likely chance of completion (I'm
>in that group).
>  
>
Exactly.

>I'm sure that Jerry wishes that he could have made his
>16 inch scope from thicker glass. ...
>
[I'll leave that to Jerry to answer.]

>We were there when Jerry first mentioned making a 16
>inch scope from thin glass and we were there as he
>progressed through all the troubled times. We are
>still here offering ideas but have we really done
>right by him when it comes to advice? 
>  
>
Let's see: Nov 15 2002.  Jerry asks for advice on his "next mirror" for 
"the largest scope I can possibly build by hand" and comments on the 
availability to him of 1in fine annealed plate glass.  Mel Bartels 
responds recommending a 16in mirror for size and 2in pyrex for the 
glass, "..it will go easier, however, 1 inch thick plate glass can be 
successfully used".

Dec 10 finds Jerry looking for a source of thick pyrex blanks -- reply 
off-list.  Dec 11 finds an offer of a 3in blank that could be split in 
two.  Dec 12 finds advice on how to cut it in two with a wire.  [Hmm. 
Did no one mention Newport Glass?  Or did he hear tales of woe?  At 
least he's looking into pyrex.]

Feb 24, Jerry is asking for advice on hogging tools.  [Sounds like he 
already bought his glass.]

Feb 25 begins a thread on "Too big, too thin" regarding a 36x3/4in 
table-top as mirror?  Ken responds with the troubles involved, but adds 
"OK, now that I've burst your bubble, GO out and DO IT!"  Richard throws 
down his gauntlet at 11+in for 3/4 plate.  Lost of discussion on 
slumping, thinnest I've heard of, etc.

Feb 27, I start a thread on supporting thin mirrors.  Lots of discussion 
ensues.  Jerry joins in with good thoughts.  Ken joins in on saving $25 
on the blank and spending $500 on support. Seems like we've been here 
before.

Ok.  So all in all, he asked for advice, was recommended to use 2in 
pyrex and ultimately choose to use 1in plate.  Should Mel's comment have 
been less positive on using the plate glass?  [Should Newport have 
responded to Jerry's e-mail?]  Should the archives be more full of 
"danger, beware"?  Or should people have the right to bite of more than 
they can comfortably chew?

Without the list, Jerry might never have tried to make a larger mirror.  
With it, he picked the road he did and has had lots of companions on his 
journey.  Without it, he would not likely have gotten this far had he 
started down that road at all.

>PLOP sounds nice and I'm sure the numbers are accurate
>without question... but Jerry is still struggling to
>get his scope finished. I applaud his desire to stick
>with it and to make that mirror work. I know from
>private conversations with him that he's wanted to
>give up several times. I've offered to try figuring
>the mirror for him and I've prodded him to keep at it.
>I'm now not so sure that was the right thing to do.
>  
>
Ultimately, it's his choice regardless.  I would hope he has kept going 
for his own good reasons.

>In spite of all the setbacks, he's kept moving along,
>made his own secondary mirror and has taken that thin
>main mirror to his testing limits. The whole project
>would have been much easier with thicker glass and
>we've let him down by telling him there's a MAGIC
>MIRROR CELL that will take care of all of his
>problems. 
>  
>
Testing his limits or testing his mettle?  He certainly has chosen to 
put more into it than I hope I will have to.  I for one don't plan to 
make a secondary.  And my thin 16in is still at 400 grit while I finish 
the twin 8s.

>You can believe in those numbers if you want to but it
>still isn't working in Jerry's world and I've yet to
>see anyone that's made a multi level cell (in the real
>world) publish results that would make me feel
>comfortable designing a telescope where the success or
>failure of the instrument depended on the mirror cell.
>  
>
There are *lots* of two level cells (10-27 points) in the world that 
work.  28-81 points brings you to three levels and that is another 
kettle of fish as it were.  Not for the faint of heart indeed.  Jerry is 
still at 27 points which should (in theory) be enough if you space the 
points well, measure precisely, eliminate stiction and have good edge 
support.

>Knowing Jerry, he'll eventually have a thin 16 inch
>mirror mounted in a scope that will always need an
>extra measure of care in it's handling to perform
>well. It may well include a multi-level PLOP optimized
>cell but I don't believe it is the total answer to his
>thin mirror problem.
>  
>
If multi-level means more than nine points I think you would cross out a 
lot of larger dobs by drawing that line.  Yes his scope may always need 
some care, but hopefully he will have the satisfaction of knowing he has 
done what few others have dared to do.  (Had he known that would be the 
case, he might have chosen differently however.)

So since this thread morphed into being about 36 and 54 point cells, 
let's return briefly to that point:

Such beasts do exists, they are not for the timid or those not wise in 
the ways of manufacturing.  They are not recommended for "beginners" or 
even a "second scope".  If you think you might need them, reconsider 
what you plan before buying your glass.  Are the possible? Yes.  Are 
they a challenge? Very much so.  To be lightly undertaken?  Not at all.

Is that fair enough warning?

Jeff Anderson-Lee

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