From: "Frank Q" 
To: "CSC" ,
        "Atm" 
Reply-To: "Frank Q" 


Hi All

Regarding the statement below, has anyone tried insulating the edge of the
disk while the mirror to see if it retains its figure ??

Cheers
Frank Q

> overcorrect as the temperature drops.  I think it's because the edge has
> more area exposed, both from the front and side of the disk and cools
faster
> than the rest of the disk.  Sort of like a temporary Turned Down Edge, or

----- Original Message -----
From: "CSC" 
To: "Atm" 
Sent: Wednesday, March 05, 2003 12:29 AM Subject: ATM star test question...


>
>
> Steve:
>
> Sounds like an overcorrection type pattern if the outside focus
diffraction
> rings are clearer.  This might be due to cooling, as mirrors tend to
> overcorrect as the temperature drops.  I think it's because the edge has
> more area exposed, both from the front and side of the disk and cools
faster
> than the rest of the disk.  Sort of like a temporary Turned Down Edge, or
> Long Radius Edge. Thermal currents off the mirror can cause strange
effects,
> often like tube currents which can look like overcorrection.  The cooler
air
> at the outside edge of the aperture is denser and refracts light away from
> the axis.
>
> Have you tried the secondary shadow method?  The secondary shadow should
> appear at equal distances inside and outside focus.  Mark the focusing
knob
> so you know how far you are actually turning it. Dick Suiter says that the
> distances should not exceed 2:1 for good optics.  It takes many nights to
> get a final idea of the figure.
>
> Important to keep viewing regardless of any abberations you suspect.  Our
> club has an old 6" f/15 Askania triplet lens that doesn't star test well,
> but the views are super anyway.  Our 16" Meade isn't all that hot either,
> but it is a very enjoyable scope! My 6" f/10 Newt is a bit overcorrected,
> and I often fuss over refiguring it or spherizing it and flexing in the
> correction.  Once I start viewing, though, I forget all that stuff and
just
> observe.
>
> Colin
>

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