Zygo test report for my TMB 105/650 LZOS

[Andrew_B]

1 hour ago, dannybgoode said:

This is precisely what happened with Hubble's mirror. There is a misconception that the mirror was badly ground somehow when in fact it was essentially as perfectly smooth as they could make it given the constraints of the technology but was the wrong curve. 

Indeed it was because it was so perfectly wrong (it that makes sense!) that they could make the corrector lens. 

I thought about the Hubble example last night. Very accurately figured and polished but the wrong shape.

It was such a ridiculous piece of mismanagement. The Hubble spec was changed from a 3m mirror to a 96" (2.4m) one to take advantage of the existing production facilities and experience from the KH-11 spy satellite programme, but those mirrors were all made by Kodak (now L3Harris) and still are. Kokak made the backup mirror which was perfect and is now in the Smithsonian, but PerkinElmer who were given the primary contract had never built a space-qualified mirror larger than the 26" optics produced for the KH-9 Hexagon and it shouldn't have been the biggest surprise that they made a serious mistake.

[dannybgoode]

51 minutes ago, Andrew_B said:

but PerkinElmer who were given the primary contract had never built a space-qualified mirror larger than the 26" optics produced for the KH-9 Hexagon and it shouldn't have been the biggest surprise that they made a serious mistake.

Wasn't it down to a mis-calibrated measuring tool?  Think the thing they used to measure the curve was reading wrong.  Luckily it was very accurately and consistently reading wrong hence it was the most accurate mistake ever manufactured!  A smoothness of 10nm over 2.4m is impressive!

And of course the blanks for both the PerkinElmer and Kodak blanks were produced by Corning.  Nearly everyone who has ever owned a modern mobile phone will have stroked and fondled Corning glass  

[Andrew_B]

8 hours ago, dannybgoode said:

Wasn't it down to a mis-calibrated measuring tool?  Think the thing they used to measure the curve was reading wrong.  Luckily it was very accurately and consistently reading wrong hence it was the most accurate mistake ever manufactured!  A smoothness of 10nm over 2.4m is impressive!

And of course the blanks for both the PerkinElmer and Kodak blanks were produced by Corning.  Nearly everyone who has ever owned a modern mobile phone will have stroked and fondled Corning glass  

It was the setup of the measuring tool that was wrong and the fact that P-E ignored the results of other tests performed using different equipment that showed an error which is pretty poor QC and project management  on their part to only rely on a single instrument to do the final testing of such a vital component. NASA was also at fault for a lack of oversight and allowing a deficient testing regime to be implemented.

Back on topic,  is a zygo test expensive to perform?

[dannybgoode]

5 minutes ago, Andrew_B said:

It was the setup of the measuring tool that was wrong and the fact that P-E ignored the results of other tests performed using different equipment that showed an error which is pretty poor QC and project management  on their part to only rely on a single instrument to do the final testing of such a vital component. NASA was also at fault for a lack of oversight and allowing a deficient testing regime to be implemented.

Back on topic,  is a zygo test expensive to perform?

The full clean, collimate and test is £100.  RVO have a full clean room now so saves sending it away for a test

[scotty38]

12 hours ago, dannybgoode said:

The full clean, collimate and test is £100.  RVO have a full clean room now so saves sending it away for a test

They're not far from me either so might pop my GT81 over in the new year. I know you mentioned collimation but just to be clear do they test, adjust as necessary and then test again so the report is after any tweaking?

[dannybgoode]

1 hour ago, scotty38 said:

They're not far from me either so might pop my GT81 over in the new year. I know you mentioned collimation but just to be clear do they test, adjust as necessary and then test again so the report is after any tweaking?

The lens cell does not need to be collimated to produce best results, they adjust the laser until it gives the highest possible reading.  A fuller description of the process is here:

Zygo Interferometer - Rother Valley Optics Ltd

[scotty38]

1 hour ago, dannybgoode said:

The lens cell does not need to be collimated to produce best results, they adjust the laser until it gives the highest possible reading.  A fuller description of the process is here:

Zygo Interferometer - Rother Valley Optics Ltd

Thanks, I'd already read that which is what made me not sure. I guess what I'm asking is if they test and find it's out or whatever do they adjust the scope and make it right or just tell you it's broken as it were?

Hopefully you see what I mean 🙂

[dannybgoode]

5 minutes ago, scotty38 said:

Thanks, I'd already read that which is what made me not sure. I guess what I'm asking is if they test and find it's out or whatever do they adjust the scope and make it right or just tell you it's broken as it were?

Hopefully you see what I mean 🙂

There are two separate processes.  The Zygo test simply measures the optical performance of the objective and the objective cell does not need to be collimated to do this.  If the objective measures poorly then there the options to correct it are pretty limited.  You could dismantle the cell altogether and try and play around with the alignment of the individual elements but a) that could make things worse and b) is outside of the scope of this particular service.

Once the guys know the optimal measurements they can then collimate the cell as a whole to try and ensure best performance at the eyepiece however it is easily possible to collimate without a Zygo report if you have the right kit.  So the Zygo is more a diagnostic tool.  If you know the objective is poor then you know that fiddling around with collimation will only get you so far.

[scotty38]

1 minute ago, dannybgoode said:

There are two separate processes.  The Zygo test simply measures the optical performance of the objective and the objective cell does not need to be collimated to do this.  If the objective measures poorly then there the options to correct it are pretty limited.  You could dismantle the cell altogether and try and play around with the alignment of the individual elements but a) that could make things worse and b) is outside of the scope of this particular service.

Once the guys know the optimal measurements they can then collimate the cell as a whole to try and ensure best performance at the eyepiece however it is easily possible to collimate without a Zygo report if you have the right kit.  So the Zygo is more a diagnostic tool.  If you know the objective is poor then you know that fiddling around with collimation will only get you so far.

Ah ok thanks. As far as I'm aware my scope is fine and to be honest the measurements mean not a lot to me so I was more looking for a test (confirmation/reassurance) and fix (if it needs it) type of thing 🙂

[dannybgoode]

1 minute ago, scotty38 said:

Ah ok thanks. As far as I'm aware my scope is fine and to be honest the measurements mean not a lot to me so I was more looking for a test (confirmation/reassurance) and fix (if it needs it) type of thing 🙂

I am sure the guys would be happy to check the collimation and adjust if necessary.  Drop them a message and see if there is any benefit to having it on the Zygo as well.  I wanted the report as all LZOS cells came with one but as I got my scope second hand the original certificate was missing so I was interested to see how it measured.  Of course as they took the cell out to test mine it had to be re-collimated when they put it back in anyway  

[Andrew_B]

21 hours ago, dannybgoode said:

The full clean, collimate and test is £100.  RVO have a full clean room now so saves sending it away for a test

That's not bad actually.

So the collimation service only relates to the alignment of the lens cell to the telescope? I'd been under the impression that collimation also adjusted the position and/or spacing of the lens elements but it makes sense that it wouldn't involve something as complex as that.

[dannybgoode]

2 hours ago, Andrew_B said:

I'd been under the impression that collimation also adjusted the position and/or spacing of the lens elements

It can do and on a doublet it’s not such a difficult procedure. Triplets though can be hugely complex and time consuming and are often best left un-touched.

If there were a known issue I’m sure you could pay for someone to try and sort it though…

[StarryEyed]

On 22/12/2021 at 17:24, dannybgoode said:

@Space Hopper - in the meantime have some outright objective porn.  This is my scope on the testbench...

 

 

 

I have never owned but always admired this refractor not just for the optics but the mechanics as well. Its a design classic, looks fantastic. Never thought I would see one caught with its pants down though. 

[dannybgoode]

2 hours ago, StarryEyed said:

I have never owned but always admired this refractor not just for the optics but the mechanics as well. Its a design classic, looks fantastic. Never thought I would see one caught with its pants down though. 

To be honest I shouldn’t have bought it as it was way way beyond budget however likewise, it was the whole history of the scope; the engineering, the fact it is a an early Thomas Back one, and yes of course the optics also. Oh and it’s true, Kruppax tubes just don’t dew. No idea why but it doesn’t matter what the temperature is - the objective simply refuses to few up

I just had to have it