= LCGT IFO Mirror Size =

<<TableOfContents(3)>>

== Mirror size requirements from the beam spot size ==
The minimum mirror sizes are calculated requiring less than 1ppm diffraction loss. This translates into the mirror sizes 2.7 times larger than the beam spot size.
The situation is a bit more complicated for BS. Hiro Yamamoto calculated the loss of the light power in the PRC by the finite size of the BS using FFT simulation. [[attachment:BSLoss.pdf]]. Now we have to check what is the acceptable loss level for PRC ? 1000ppm ?

||Optics||Beam Spot Size (1/e^2 radius)||Minimum Mirror Size (diameter)||
||ITM || 3.5 cm || 19 cm ||
||ETM || 3.5 cm || 19 cm ||
||BS  || 3.5 cm || 3.5*6*sqrt(2)+2*t/sqrt(2*n^2-1), where t is the thickness and n is the refractive index.<<BR>> For t=10 cm, d=40 cm. Probably this value is a bit overkill. See Hiro Yamamoto's Calculation.||
||PR3 || 3.5 cm || 19 cm ||
||SR3 || 3.5 cm || 19 cm ||
||PR2 ||less than 1 cm|| 10 cm is enough ||
||SR2 ||less than 1 cm|| 10 cm is enough ||
||PRM ||less than 1 cm|| 10 cm is enough ||
||SRM ||less than 1 cm|| 10 cm is enough ||

== Mirror Size Plans Considering Suspension Varieties ==

=== Plan A ===

For the 3rd year (iLCGT) configuration

 * All the mirrors except for BS are 25 cm x 15 cm fused silica of 16 kg. They are all suspended by the Type B suspension platform with the IP and filter stages fixed.
 * BS is 37 cm x 6.85 cm fused silica to make the weight 16 kg.

For the bLCGT

 * The ITMs and ETMs are replaced with 25 cm x 15 cm sapphire mirror of 30 kg. The suspension platforms are also replaced with Type A.


=== Plan B ===
This was proposed by D. Tatsumi.

For the iLCGT

 * ITMs, ETMs, BS, PR3 and SR3 (the mirrors which handles large beam) are all 38 cm x 12 cm fused silica of 30 kg, suspended by the Type A SAS platform.
 * PRM, PR2, SRM, SR2 are 10 cm class TAMA sized mirror. These are suspended from a smaller simple suspensions, probably in common with MC suspensions.Issues
 
For the bLCGT
 
 * ITMs and ETMs are replaced with 25 cm x 15 cm sapphire mirrors.

Advantages
 * No need to replace suspensions between iLCGT and bLCGT.
 * PRM, PR2, SRM, SR2 may be easily movable (depending on their suspension design). This is important to adjust the folding telescope in the PRC and SRC.
 * Further reduce the number of suspension types by using the same design for MC mirrors and the small IFO mirrors.

Disadvantages
 * 38 cm x 12 cm BS will have a larger loss than 37 cm x 6.85 cm one. Need to be checked.
 * PR3 and SR3 are unnecessarily large. 

To be checked
 
 * What is the seismic isolation requirements for PRC/SRC mirrors ?

== Mirror requirement summary from the IFO group ==
 * The minimum mirror sizes are as shown in the table above.
 * ITMs and ETMs should be at least 30 kg for bLCGT.
 * It is necessary that PRM, SRM, PR2 and SR2 can be moved by about 50 cm after installation to compensate for the fabrication error of the ROCs of those mirrors. 

== Mirror name convention ==
{{attachment:IFO.png||width=700}}