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#51 The Ardent

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Posted 27 October 2014 - 03:18 PM

At Staunton River I was able to view the dark Cone Nebula with borrowed (hijacked) 30" Lockwood optics. My first time seeing this. The Horsehead in the same scope was easy in comparison. 

 


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#52 Mike Lockwood

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Posted 31 October 2014 - 10:47 PM

I hope only photons were inconvenienced during the hijacking.  I have not seen the cone myself - guess I'll have to keep trying.  Big scopes make the Horsehead much more enjoyable.


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#53 Mike Lockwood

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Posted 09 December 2014 - 03:13 PM

I have finally posted a fairly lengthy article about many of the factors involved in mirror testing, including star testing.  I put a lot of time and work into writing it, perhaps more than any other article that I have written, and I think that anyone who already has a medium-size or large telescope or who is considering buying one should read it.

 

You can find it here:
  http://www.loptics.c...ourlessons.html

 

I hope you learn something from it.

 

Stay tuned for an article that addresses many issues involving mirror suppport.

 

Thank you.


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#54 havasman

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Posted 09 December 2014 - 05:38 PM

Wonderful teaching article that I'll read repeatedly. Thanks for posting it. Hope to have one of your mirrors some day! 



#55 City Kid

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Posted 09 December 2014 - 08:41 PM

Great article Mike. I just finished reading it and I learned a lot.

 

Phil



#56 Mike Lockwood

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Posted 10 December 2014 - 02:50 PM

Thank you for your feedback.  There is a lot of information and material in it, and I worked very hard collecting it all and writing it.  Please share it with your friends and fellow observers.

 

If you have questions, I prefer that you ask them on my Yahoo group.

 

Thank you again.



#57 pstarr

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Posted 10 December 2014 - 02:56 PM

Very good article Mike. Thanks for taking the time to educate us.



#58 Cames

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Posted 10 December 2014 - 09:20 PM

Mike

 

Extremely interesting reading and a real eye-opener. Thanks for sharing your impressive insights. As you hinted, many of us base our opinions on conventional wisdom and operate in word-of-mouth frame of mind. A little science goes a long way in cutting through the fog.

 

Regarding anisotropy, I take it that most glass structures end up having sort of an invisible "grain" so that they flex more readily in one direction than another.

 

Is it possible (practical) to determine the general direction of the greatest resistance to flexure when testing a completed mirror and then to mark the rear of the mirror with an arrow indicating 'this end up'? That way the customer could know the best orientation to nestle the mirror in its cell.  The mirrors would then come with the mirror center mark engraved on its front surface and the up-arrow engraved on the rear.

 

Thanks again for a great read.

----------

C



#59 Mike Lockwood

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Posted 11 December 2014 - 02:02 PM

Regarding anisotropy, I take it that most glass structures end up having sort of an invisible "grain" so that they flex more readily in one direction than another.
 
Is it possible (practical) to determine the general direction of the greatest resistance to flexure when testing a completed mirror and then to mark the rear of the mirror with an arrow indicating 'this end up'? That way the customer could know the best orientation to nestle the mirror in its cell.  The mirrors would then come with the mirror center mark engraved on its front surface and the up-arrow engraved on the rear.

 

Cames,

 

Yes, that's what we seemed to see, and it makes some sense based on how glass is made.

 

On the test stand, in some orientations the astig. is there, and in some it just goes away.  In the telescope, the mirrors show absolutely no astigmatism.  I have seen this for many mirrors.  The odds of the test stand astigmatism canceling it out perfectly are extremely small, and high-power telescope testing shows it's not there, so it must be a property of the glass or test stand.  This 28" mirror is Pyrex, but Supremax may have different properties - I simply don't know.

 

I have thought about marking mirrors in this way.  One shop may already do this, but I have not talked to them about it.  This is one reason that I am implementing phase shifting interferometry to get more data for mirrors sitting on the test stand.  Hopefully one day I will have enough data to identify the phenomena and the best orientation, if there is one.

 

Glass is certainly not perfect, and it makes sense that, with enough precision, its limits should be found.



#60 Mike Lockwood

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Posted 28 December 2014 - 10:35 PM

Tom Osypowski wrote a nice short article about a telescope that he built for a client, and gave me permission to post it.  It is a 32" f/2.8 telescope, using my optics - a monolithic primary and an 8" m.a. cast cellular secondary.

 

You can read his article here:
  http://www.loptics.c...es_32inF2p8.pdf

 

While I still don't recommend gluing mirrors, it appears that Tom has made it work for the secondary mirror.  After all, he is a professional.

 

We'll see how many more f/2.8 scopes the future holds.  That is still my lower limit for high-quality visual use, but maybe I will push it down to f/2.7.....

 

I'm looking forward to hearing from the owner after he takes delivery.



#61 Cames

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Posted 29 December 2014 - 03:48 AM

Mike,

A fascinating design for sure. An 88 inch eyepiece height has to be a record for a 32" primary.

 

Comparing the mirror cell whiffle-tree support points with those engineered by JP Astrocraft in your article above, Four Lessons in Mirror Testing, I see the contact points are domed nylon(?) instead of the roller-type supports.

 

Don't you think that the nylon domes might tend to constrain the primary a little too much as the frame of the mirror cell shrinks over the course of the observing session? I'd be concerned with induced astigmatism unless there is a built in degree of freedom that isn't obvious in the photo. Maybe the whiffle-tree rockers are supported by linear bearings like some of the sling supports? Wouldn't linear bearings still be only a partial solution, though? 

 

Reading the report, there is no hint of a problem. It seems you have outdone yourself once again and created an optic capable of unrivaled performance. Congratulations!

--------

C



#62 Bob S.

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Posted 29 December 2014 - 08:15 AM

Tom Osypowski wrote a nice short article about a telescope that he built for a client, and gave me permission to post it.  It is a 32" f/2.8 telescope, using my optics - a monolithic primary and an 8" m.a. cast cellular secondary.

 

You can read his article here:
  http://www.loptics.c...es_32inF2p8.pdf

 

While I still don't recommend gluing mirrors, it appears that Tom has made it work for the secondary mirror.  After all, he is a professional.

 

We'll see how many more f/2.8 scopes the future holds.  That is still my lower limit for high-quality visual use, but maybe I will push it down to f/2.7.....

 

I'm looking forward to hearing from the owner after he takes delivery.

Congrats Mike on being part of a special telescope build. When you have the likes of Tom O. and Tony Hallas, two very experienced and critical observers giving a thumbs up to a ground breaking build, it is high praise indeed. It appears that Fast Mike's 28" f/2.75 has met its match and then some.

 

In terms of gluing mirrors, it appears that Pierre Desvaux of the Dobson Factory, a scope builder in France who has made scores of large/fast telescopes, reported that gluing versus more conventional ways of mounting secondaries has not had any noticeable effects on the performance of his scopes. You can find his response here to my concerns: http://www.cloudynig...y/#entry6360766 His empirical findings suggest that our concerns about gluing secondaries may be overblown. I also think about the fact that all of my 12.5" f/5 Zambuto-mirrored Portaballs had the primaries glued to the mirror cell with no apparent deleterious effects. Of course, the primaries are much larger than secondaries but the practice has gone on for a long time and continues for PB's to this day.

Bob S.



#63 Mike Lockwood

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Posted 29 December 2014 - 11:56 AM

Comparing the mirror cell whiffle-tree support points with those engineered by JP Astrocraft in your article above, Four Lessons in Mirror Testing, I see the contact points are domed nylon(?) instead of the roller-type supports.
Don't you think that the nylon domes might tend to constrain the primary a little too much as the frame of the mirror cell shrinks over the course of the observing session? I'd be concerned with induced astigmatism unless there is a built in degree of freedom that isn't obvious in the photo. Maybe the whiffle-tree rockers are supported by linear bearings like some of the sling supports? Wouldn't linear bearings still be only a partial solution, though?

I recommend rollers, and linear bearings would work too.  All cells are built slightly differently, with different materials, so the magnitude of any non-ideal effects will vary from builder to builder, cell to cell, and mirror to mirror, depending on things like the smoothness of the edge of the mirror.

 

It takes time for manufacturers to understand and implement changes.  Most products made in the world are in a continuous process of improvement, mirror cells included.  I have talked to the cell builder (and it is not the telescope builder), and he understands why I recommend rollers and I believe he will use them or something similar in the future.  This cell was built before we talked about that, but it could easily be retrofit if that were deemed advantageous.  Judging by the description of performance and the qualifications of those who have used the instrument so far, there is no major issue, but if there is a minor issue it is easily addressed in the future.

 

I have been blamed for mirror mounting issues in the past and even recently, which is why I have tried to educate telescope builders and owners about how to properly support mirrors.  It is the reason that I put a tremendous amount of work into the article that you mention, "Four Lessons in Mirror Testing".  It is also the reason that I wrote "Why aren't my stars round?", an article based on about a decade of telescope debugging experience, which can be found here:  http://www.loptics.c.../starshape.html
 

Congrats Mike on being part of a special telescope build. When you have the likes of Tom O. and Tony Hallas, two very experienced and critical observers giving a thumbs up to a ground breaking build, it is high praise indeed. It appears that Fast Mike's 28" f/2.75 has met its match and then some.

 

In terms of gluing mirrors, it appears that Pierre Desvaux of the Dobson Factory, a scope builder in France who has made scores of large/fast telescopes, reported that gluing versus more conventional ways of mounting secondaries has not had any noticeable effects on the performance of his scopes. You can find his response here to my concerns: http://www.cloudynig...y/#entry6360766 His empirical findings suggest that our concerns about gluing secondaries may be overblown. I also think about the fact that all of my 12.5" f/5 Zambuto-mirrored Portaballs had the primaries glued to the mirror cell with no apparent deleterious effects. Of course, the primaries are much larger than secondaries but the practice has gone on for a long time and continues for PB's to this day.

 

Thank you Bob.  This is indeed a special telescope, and another glimpse at what may be to come in conveniently-sized large instruments that perform to high standards of quality.  I believe that big scopes should be capable of superb performance, and should not be "light buckets".

 

As for gluing mirrors, there are many variables involved, such as 1) glass type, 2) characteristics of the material that the mirror is glued to, 3) type of glue used, 4) dimensions of the glue bond area, 5) temperature range that the telescope is used in, 6) distance separating areas of glue, 7) thickness/design of the mirror, and 8) orientation of the mirror with respect to gravity.

 

So, to say it simply, it is complicated, and should not be done unless one understands all of those variables for their particular situation.

 

My experience with gluing mirrors involved a 5" m.a. secondary that we glued to a piece of astigmatism.  This produced wildly astigmatic images when temperatures dropped, and this is my reason for not recommending it UNLESS the person doing it knows what they are doing and knows how to detect the effects of the mirror being warped.  There are forces applied to mirrors glued to PLOP-designed cells that are not accounted for by PLOP.  Distortion caused by these may or may not be apparent in observing.  When a telescope is aimed low these non-ideal forces do exist and are at their maximum, but the steadiness of the atmosphere is at a minimum, so they may not be visible, which is fortunate.

 

I have done edge support testing in my shop, which has superb seeing.  This shows far more than pointing a telescope low under the real sky ever will, and I can see what is going on.  Lessons learned from this testing, as well as advice for those mounting their secondary mirror in the traditional shell-type holder, are the subject of another article that I am working on now and hope to post fairly soon.



#64 Mike Lockwood

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Posted 28 January 2015 - 03:43 PM

I posted a new "In the Shop" article describing some new test equipment that I now have operational:
  http://www.loptics.com/intheshop/shop48.html

 

My "Winter Star Party 2014" web article is included in the current issue of "Amateur Astronomy", page 46-52.  (It's missing from the table of contents, though.)

 

Finally, I also recently posted a few images of Comet Lovejoy shot from my deck.  They're not as good as others' efforts, but I had fun getting them.

  http://www.loptics.c...hop/shop47.html



#65 pstarr

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Posted 28 January 2015 - 05:07 PM

Congratulations on the new equipment. Hope it make your job somewhat easier. I always enjoy reading your articles. Thanks for posting them.



#66 City Kid

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Posted 29 January 2015 - 03:56 PM

Mike, just want to say how much I (and no doubt others) appreciate all the time and effort you put into educating all of us on various aspects of optics. 

 

Phil



#67 Mike Lockwood

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Posted 30 January 2015 - 07:05 PM

Phil,

 

Thanks for saying that.  There is a lot of (unintentional) misinformation out there that I would like to put straight.

 

If people are using the info and teaching their friends, I'd like to hear about it.

 

I should also say that I am also putting a lot of time into educating *myself* about optics and all the nuances of testing, mirror cells, mirror cooling, etc., and some of that has already paid off and will pay off in the future.  Always keep learning.



#68 Mike Lockwood

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Posted 06 April 2015 - 11:01 AM

After many years of work regarding mirror cells, and after having been blamed for the shortcomings of certain mirror cells that distorted/bent my optics, I have finally posted my article about supporting primary and secondary mirrors.  You can read it here:
  http://www.loptics.c...rorsupport.html


None of my recommendations are made lightly.  There are certain mirror support methods that simply work better.  This article represents years of work and development.

Please share with your friends.


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#69 Mike Lockwood

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Posted 07 April 2015 - 11:19 PM

This post has been deleted so that it is not duplicated in another thread.


Edited by Mike Lockwood, 09 April 2015 - 10:40 AM.


#70 Mike Lockwood

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Posted 18 May 2015 - 11:50 AM

I've just posted my 2015 WSP article, and you can find it here:
  http://www.loptics.c...15/WSP2015.html

Carl Zambuto joined me for the star party, and you can read the article to see some photos of the fun.  Hope you enjoy it.



#71 rockethead26

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Posted 18 May 2015 - 08:18 PM

Nice article, Mike! Not the best WSP, but it still looks like you had fun, at least until you got home.



#72 Mike Lockwood

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Posted 19 May 2015 - 11:30 AM

I can never complain because it is always going to be warmer there, and the seafood is excellent.

 

It was great to meet Carl, to introduce him to the southern sky a bit, and to talk with him about many topics all week.



#73 Jeff B

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Posted 22 May 2015 - 09:06 AM

I love that transition from Carl enjoying the tropical crab to the last picture!

 

Looked like a lot of fun.



#74 crazyqban

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Posted 22 May 2015 - 09:02 PM

Great article Mike (as always). I had a great time observing with you, Carl and Howie. See ya next year.



#75 Mike Lockwood

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Posted 03 June 2015 - 09:42 AM

Jeff, yes, that transition was fun, but far more traumatic when you experience it for yourself!

 

Sergio, thanks, and likewise I always enjoy observing with you and your friends.

 

Next star party is Okie-Tex (http://www.okie-tex.com/) with very dark skies and lots of space to set up in, and I hope to see some of you there.


Edited by Mike Lockwood, 03 June 2015 - 09:44 AM.



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