Preface

As the viewport attached the cryostat was not AR coated at the wavelength of 1550 nm but 1064 nm, there was a 10% loss for beam power at 1550 nm.
So I decided to replace the viewport for 1550 nm AR coated.
The cost, however, was a little bit expensive.
Therefore, I bought a AR coated window and a viewport without a window separately and they were supposed to be assembled.

What I did

I assembled the viewport as attached pic. 1.
Then I connected to the conversion flange though I was not confident to use M8 bolts and washers stored in the ATC.
After that I installed to the chamber and vented as a test.
Before venting, I measured the beam power before the viewport and after.

Result

First, the transmitted beam power was 2.73 mW against 2.74 mW input.
So the total loss was about 0.5% which is dramatically reduced compared to the previous one.
Moreover, the viewport does not have any serious leakages and the pressure reached less than 10^-2 Pa.