Re: ATM 16"flexed and folded

[Ed Karch <karch@karch.com>]

>>From: "Johan Vanbeselaere"
>> > > what bothers me about a 16" flexed mirror is how it does react upon
>> >pointing
>> > > from horizon to zenit.
>> > > How thick is this 16" disk ?
>> > > Can you see any hint of sagging due to gravity ????
>> > > In my opinion there must be a superpositioned deflection by gravity but
>> >not
>> > > as much as you would simply support the mirror on it's rim without
>> >flexing,
>> > > because the spring is also shortening and thus changing the apllied 
>> force,
>> > > pulling less depending on it's springconstant. Trying to figure out 
>> how a
>> > > flexed big mirror deals with gravity just being supported on it's 
>> rim and
>> > > pulled down as Adler is suggesting ;-)
>> > > I do have for instance a mediocre 40cm f/5 that I would like to turn 
>> into
>> >a
>> > > shorter and mostly better one, and
>> > > thinking the flexing way ...
>> > >
>> > > Vanbeselaere Johan
>> > > ACG vzw Belgium
>>JOHAN:
>>   I am the designer & principal builder of "SWEET SIXTEEN".
>>Regarding your (excellent) questions:
>>1)  The mirror has a 1 : 10 thickness ratio.  (It is 16" in diameter & 
>>1.5" thick)
>>2)  The quality of the image changes with elevation angle.  (So you have 
>>to change the tension on the puller bolt)
>>3)  You also have to change the tension as the mirror cools.
>
>   4)  At low magnifications (we usually run at ~ 70 X for a wide FOV) 
> none of this is much of a problem.
>   5)  The mirror's relatively "high" "f" ratio (f/7.5) helps.  The 
> difference in figure between a sphere & parabola isn't that much, so the 
> amount of "flex" needed isn't that great.  The disadvantage of the high 
> "f ratio" is that I had to "fold" the focal length to make the telescope 
> structurally simpler & easier to use.  The "folding" also helps by 
> obscuring the central 6 inches of the primary.  Thus, only the figure of 
> the outer annulus, 5" in width, affects the quality of the image.
>
>        In general, big mirrors are more flexible than are small mirrors 
> (even when you don't want them to be).  Probably for a mirror of 8" 
> diameter, or less, the effect of elevation angle/gravity loading is 
> insignificant compared to the tension load.  For our big mirror, we have 
> use an interactive"adaptive optics" approach, with the observer also 
> acting as the feedback loop to vary the flex of the mirror.  This is not 
> a "tension once & leave it" system!
>        Additionally, the season of the year must be considered.  The 
> mirror flexing cell design incorporates foam rubber pads and 
> adhesives.  These become "stiffer" at low temperatures and behave 
> differently.  We just started having cold winter weather here, so these 
> effects can now be investigated.
>
>Hope this helps.
>Regards,
>John Avellone,  NOVAC
>
>