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Old 18-05-2014, 09:11 AM
loc46south (Geoffrey)
loc46south

loc46south is offline
 
Join Date: Jan 2009
Location: Milton - New Zealand
Posts: 176
2nd Impressions of a GSO 12" Truss R/C

In my first impressions I made a few assumptions based on other peoples experiences, one thing I needed to check was if the retaining clips were contacting that mirror and causing the stress on the mirror. I disassembled the telescope and removed the retaining rings completely. The telescope was reassembled and retested - triangular stars still apparent in images 1,2 &3

Knowing that R/C's are sensitive to collimation issues I took a series of 10 images as close to focus as I could visually get and ran these through CCD Inspector - they showed that the collimation I was able to get with the Takahashi Collimating Scope was close. Camera used was SBIG ST10XME 6.8 micron pixels.

I was not surprised by this outcome. The design of the mirror retaining cell was the primary cause. Most of the articles I have read concerning telescope mirrors and telescope building say that the mirror should not be firmly anchored in the cell, yet here is a mirror that is SOLIDLY fixed within the cell - I was unable to move it. The mirror has been fixed by injecting a silicon like substance through 3 points 120 degrees apart. It has been injected under pressure and this pressure has been further increased to the point where excess silicon in the cell has been forced up and become crowned.

The increase in pressure is due to the contraction of the cell at a greater rate than the shrinkage of the mirror. The coefficient of expansion for plate glass is 5.0 - its 7.3 for steel of 12.3 if the frame is aluminium. The result is just like placing you finger in a vice and as the temperature drops just keep on winding. Telescope was assembled in Taiwan and now resides in South Otago - a diference in ambient temperature of about 20 C - The crowning of the silicon sealer is an indication of how much pressure is being transmitted to the mirror and deformation of the surface is inevitable - I just hope that the mirror has not become permanently stressed. Another thing is that this major stress point is directly above the connection/adjustment screw to the main body of the telescope so and stress caused by that connection is also transmiltted to the mirror.

This matter has been bought to the attention of GSO - what action is taken we can only wait and see.

cheers
Geof Wingham
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