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

As you are a Mac user do you have a separate monitor. 

If you have get a Mac Mini M1 and that will be massively faster than what you have now and is a cost effective upgrade (as these things go).

I prefer to work locally so this would be my route to faster processing

4 hours ago, malc-c said:

Not sure what grease SW now use in these mounts, but a few years ago a lot of people would strip an HEQ5 / EQ6 down straight after the 12 month warranty period just to replace the tar like gunk with a better lubricant, often lithium grease

It's not terrible stuff in mine however it's far from the best and it's very gloomy and sticky. 

Will report back on any improvements

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  

7 hours ago, Andrew_B said:

It strikes me that you could have a lens that's been figured and polished to an incredible level of smoothness, but it could also be the wrong shape to produce a good image

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. 

1 minute ago, Captain Magenta said:

Interesting. My one is not far from yours in the production run: yours is No 048, mine is 026.

Nice. Yes I saw the For Sale post for yours and it is an original Thomas Back CNC also with the outrageously engineered focuser. Lovely thing to use

Mine isn’t going anywhere!

5 minutes ago, tooth_dr said:

I was reading your post, loving the testing you had done.  After using my 105/650 for a few months I was impressed at the images despite it relatively small aperture, and ended up getting a second one!

A pair would be very nice. You could set them up as the most outrageous pair of binoculars going

Aye, getting the full Zygo test and a clean and collimate has been well worth it. Just need some clear nights now...

44 minutes ago, Space Hopper said:

That seems a lot ??

Mine is closer to 7kg  if i remember correctly.

Mine is the early Thomas Back CNC one and they weigh a fair bit more than later ones, mainly due to a focuser that would survive a nuclear blast.

5 minutes ago, Andrew_B said:

It’s impressive that the Strehl was so high in res light given that you’d expect the lens to be optimised to deliver its best performance in green light. 
 

I doubt the little peak near the edge has much effect on overall performance. There’s a reason very high end optics always have their surface accuracy specified as the RMS value rather than P-V. 

I found this on that very subject from Thomas himself:

---

   Strehl ratio is defined as the illumination at the center
of the Airy disk for an aberrated system expressed as a
fraction of the corresponding illumination for a perfect
system (i.e., 1.00). It is not calculated by P-V, but by the
RMS value. As my friend Valery Deryuzhin of ARIES
INSTRUMENTS Co. is fond of saying, if a drop of dew
forms on your lens, what is the P-V of the systems now?
And would you see any performance difference? No,
because very small localized errors have no affect on
overall performance.
   As we sample a few thousand discrete points over 100%
of the aperture, in a three element, six surface lens, the P-V
measures relatively low as compared to smaller sampling or
knife edge tests. It is the RMS wavefront measurement that
really defines the quality of the optic, and thus the Strehl
ratio. As{*filter*} Suiter says, an optic with a Strehl ratio of .88
to 1.0 is "excellent to perfect." Zeiss' standard for its highly
regarded APQ triplet apo objectives were .95 or better. We
agree, and a lens with a Strehl of .972 is certainly a planetary
lens.    
Thomas Back
TMB Optical

---

Nice. I'd want one with an LZOZ cell though. Although in any event my buying the 105/650 nearly led to divorce. A purchase like that would seal the deal 🤣

I have one of my dream scopes in my TMB 105/650 LZOZ. A bit like @John a good part of my interest in this scope is the background and mine is a Thomas Back built one as opposed to a later APM one and I enjoy the history that goes with that.

Plus it's just a joy to look through and use. That huge, massively over-engineered focuser, enormous CNC rings, the choice of Kruppax for the tube and of course that LZOZ objective can. A commercial failure because nothing but the best would do. A total no compromise approach. The only downside to it is weight. Nearly 10kg for a 105/650!

Until I got this scope though a frac had been of limited appeal but now I can't see me spending serious money on anything else and a 130 Tak of some description or a longer 4" is highly tempting. Of course if a 130 TMB came up...

Maybe a Mewlon as well if I were going to have something spendy with a mirror.

The focal length of the scope divided by the focal length of the eyepiece.  So a 650mm FL scope with a 10mm eyepiece will give you a 65x magnification.  The max theoretical magnification of the scope as a whole is 2x the aperture in mm - so a scope with a 105mm aperture will have a theoretical maximum magnification of 210.

In reality this figure may be lower or in some case much higher though.

1 minute ago, Space Hopper said:

It looks a beauty. 

Fascinating stuff. I bet it was interesting to watch it being done.

Couldn't watch. This is was all done in their clean room. They just took some pics to share

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

Just now, Space Hopper said:

Someone else with Bortle 10 billion skies....??!!

I thought i was unique here in Derby.....😃  A 30s DSLR exposure of my back garden here in complete darkness literally turns night into day.

Good to hear your optic is back to its best : any chance of any pictures of the scope ?

I will always indulge in pictures :).  Will get it out and take some snaps.  I am not far from Hillsborough football ground in Sheffield with a patio facing a row of houses, each with huge security lights etc so as well as just the general city light I also have localised light pollution also.  Still, I get some images and now the scope is back to its best, I have finally fully completed everything I need to get things set up quickly each time etc I am hoping to do more work.

7 minutes ago, Space Hopper said:

Of course, all these figures are just figures. So many other things come into play as well.

Absolutely however as I don't have the original report I was keen to have one just for interest really.  Given my Bortle 100000000000 skies and pretty much permanent cloud cover the figures are indeed meaningless  !  Plus it got cleaned and collimated also and the objective is now looking really good.

@John Specifically the red generated by a helium-neon laser  (I looked that up BTW, I am no laser expert).

@John - RVO test at 632.8nm ...

1 minute ago, John said:

Do you know what wavelength of light your latest tests are done in ?

I don't but will ask the guys...

The guys thought it looked good for sure.  Just that little peak right at the very edge ruining the P-V  .  I never had the original certificate and Markus couldn't track it down for my cell so now RVO have the Zygo thought I may as well see what the numbers are :). 

Could the test rigs be a little more accurate these days than when the optics were first tested perhaps?

Very nice.  Am finally getting round to having my 105/650 all ready for some proper imaging.  I have had a bit of a play but for one reason or another not spent serious time with it yet which is criminal really...

Thought this may be of interest to some.  Took my scope into the fine chaps at Rother Valley Optics the other day just for them to give it a quick once over as I've been paranoid about lens fungus.  They assured me it was fine which was great news and since I was there I left it to make use of their Zygo test service as well as I was interested to see how it measures.  This is an original Thomas Back built scope and is my baby and I am ultra paranoid about anything happening to her!

And here is the report - looks a really nice optic :).  They also gave the cell a good clean and then recollimated it as well.  Overall a first class service and well worth the money

And of course my scope is now ready for me to get down and actually start doing some imaging and observing....

First snag hit.  Yes the RA locking nut is a pig.  I have found this though - looks the right size so ordered.  When I reassemble I will not quite tighten it as much and then bent nose pliers should do the trick but it ain't shifting with a locknut tool.

Sealey SMC22 Motorcycle Swingarm Lock Nut Socket 4-Pin 50mm | McCormick Tools

5 minutes ago, newbie alert said:

Inner bearings as the main bearings, or the ones in the worm housing ?

The ones that are pretty firmly embedded in the actual casing,,,

9 minutes ago, david_taurus83 said:

Has anyone ever been brave enough to try and remove those inner bearings? 

Ha - I am not.  I suspect with the right tool it would be fairly straightforward but then of course there is getting them back in again as well.  May well need a bearing press to do that...

4 minutes ago, Jonny_H said:

Thanks Danny - much appreciated.

It does look very straight forward. The only thing I have heard that can be a real pain is the RA locking nut. Did you manage to undo yours yet?

 

Thanks,

Jonny

Not yet! You just need the right tool or to be able to at least fudge the right tool. I’ll have a rummage later for something to use. The grease doesn’t come until Friday so taking my time with this one