3 replies to this topic

[mihaiG]

Hi all,

 

  I am preparing the environment to start my 20"/f3.6 1.1/4" thickness mirror grinding.To avoid astigmatism induced by stand during measurements I was thinking of using wiffle-tree at CoG,due to smallest RMS  calculated using mirror edge calculator (https://www.cruxis.c...ator.htm).Would be this a beneficial ideea or just an unnecessary complication?For the former 16" f/5 1" thickness I was using 90 degrees edge bearing at CoG on the stand and no visual astigmatism detected.

 

Thanks

 

Mihai

[Stathis_Firstlight]

Hello Mihai, greetings from snowing southern Germany.

 

How do you plan to meassure the astigmatism? 

 

I measure it with an interferometer using the well known procedure: Measure first position, rotate the mirror, measure second position, rotate back the pictures and average the measurements. This way you cancel out the astig from the test stand and the measurement device and get the real astig in the glass.  

 

With this technique it is most importand, to get a high repeatabilty. You want to get allways the same forces and same deformation at changing positions of the mirror. It is much less importand, to minimise the absolute value of deformation. I use 2 supports in 2x45°made of large washers, that are placed exactly at the center of gravity. This way the mirror hovers on the supports almost free of axial forces from the back or from the front side. 

 

So, I see no benefit in using 2x2 wiffle tree or even more radial supports. 

Edited by Stathis_Firstlight, 30 November 2023 - 08:09 AM.

[mihaiG]

Hi Stathis:),

 

Very interesting way of using large washers, thanks for the ideea.Was using the same principle, but with 2 small rollings. Your stand seems better, because one can get closer to CoG using washers than rollings.

I am not using interferometer.I made Ronchi photos in different mirror position,15 degrees one from another,up to 360,checking the deviation from 90 degrees Y axis on each photo.Plotting deviations(if there are any) I can approximate the astigmatism plan orientation and take corrective actions.Of course, using Ronchi is less sensitive than interferometer, but on the final star test, using this metod,the astigmatism was not visible.

 

Later edit:forgot to mention that stand induced astigmatism is a constant value,on different mirror positions,and this one will be substracted on plotting.

Edited by mihaiG, 30 November 2023 - 09:12 AM.

[Dale Eason]

When using ronchi and Foucault it does not see astigmatism that is aligned parallel or at 90 deg to the ruling or knife.  Thus if the mirror support only generates astig in those directions it will not be seen be the test.   Of course you still need to rotate the mirror and test again to see if it shows up but was hidden by the first rotation.   So with careful testing and mirror placement on the stand you should be ok and not induce astig with the stand.  But of course that is in the perfect world.  

 

However when testing with an interferometer it of course can be seen in all rotations.