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I Need Help Figuring My Fist Mirror, an 8”f/8

ATM Beginner DIY Equipment Mirror Making Optics
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#51 ClayDavies

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Posted 15 April 2022 - 12:44 AM

I thought I should update all you good folks who've been helping me out.  I wouldn't want you to think I'd thrown in the towel.  No.  No...

 

As I've tried to return my mirror to a good sphere I've had recurring TDE problems which I've had since I started polishing.  Recently I got it to a TUE (which I've had twice in this adventure) with a hole in the centre.  60 minutes of COC polishing later, alternating TOT and MOT I had a TDE again, yet some of the central hole remained.  undecided.gif

 

After that, figuring I had nothing to lose, I tested decanted red rouge without filtering it.  After mixing it up with a drop of dishwashing liquid I poured off the top half three times, adding water for the second and third decant.  The decanted slurry was a transparent, ever so slightly red liquid that I'd be tempted to drink if I were really thirsty.  It was so thin the soap would likely put me off more than the rouge. tongue2.gif

 

I polished with this thin slurry for an hour and I got about a 3/4" wide zone with no red laser scatter.  Dark room.  And I mean no scatter.  I saw nothing.  Zilch.  Nada.  Bupkis.  So yet again, I proved to myself that it can be done:  if I can get a zone "black polished" surely, with patience, I could get the entire mirror "black polished".  As a bonus, hand polishing packs muscle on this skinny old man.  But because I'm pretty much limited to polishing in the kitchen (while my wife is asleep... shhhh! smirk.gif) it's hard to get enough hours in.

 

Recently, I learned that one of my clubs has a MOM in circulation, with a bit of a line-up for accessing it.  I believe I might be able to use it within a few months - hopefully in the warm summer months in a club's "workshop" garage.  Because of the aforementioned issues, and ultimately because the focal length got shorter and shorter as I ground and polished - down to f/7.8 from the f/8.6 I would very much prefer (for favoured 0.8mm exit pupils [I'm with Texereau on this] with my favourite planetary eyepieces - that I've been thinking about starting over, regrinding with the machine, polishing it mechanically for as long as is nessicary to achieve a deep, black polish, then hand figuring to a good parabola. 

 

Hey, it's that or throwing in the towel and buying an 8"f/8.6 Fullum or Zambuto, and waiting, and waiting for these overworked masters to work their miracles in glass. 

 

While I am sure their mirrors would deliver images to the eyepiece that would satisfy me - nay, thrill me - I would not enjoy the satisfaction that the mirror that made such images was made by me. 

 

And after much navel gazing I concluded that the mirror and the telescope that would give me the most satisfaction is the telescope I had originally planned: a 200mm f/8.6 - full aperture, not masked; black polished, certainly not rough, and with a primary mirror made by me.  Even if a mirror made by a true master of the craft had a slight edge that could be seen in a star test, if the in focus image was it's equal - I would be completely satisfied with the product of my labour of love.  

 

I'm certainly not desperate to rush this scope to an arbitrary finish line - even though this mirror project is taking me much longer than I had expected.  I have both smaller and a larger scopes to observe with.  Very fine telescopes.

 

I've got a 12.5"f/5 truss scope with Normand Fullum optics.  On the best nights the optics remind me of the R.F. Royce/Protostar quartz optics I used to enjoy before I (stupidly! stupidly!) sold the scope...

 

And my Parks/Protostar quartz grab 'n' go 6"f/8 punches way above it's weight.  I mean, it's a freakin' ninja.   

 

All I could ask for is just a little more aperture to bridge the gap between the 6" and the 12.5".... just enough to deliver an optimal 0.8mm exit pupil (I'm with Texereau on this) at around 250x, which is just about right for better than average nights on planets around here, on the north shore of Lake Ontario.  ...with a few planet killing optimizations something like Ponz and the ALPO fellas discovered after decades of patient trial and error:

https://www.cloudyni...-machine/page-2

  

...an equatorial platform...

 

...and a primary mirror I've made myself that astonishes me... shocked.gif  Planets.  Doubles.  The Moon and the Sun.  And yea, I'd be very tempted to take a long drive to see what it can do under a really dark sky...

 

It's a high bar to clear, especially for a first mirror.

 

By the time it's finished it might be practically a third mirror: one mirror made three times!  lol.gif

 

Yet somehow, in spite of it's many ups and downs, advances and setbacks, the journey is almost as enjoyable as the destination.  

 

After all, sometimes at the end of a very long drive to truly dark skies - it's cloudy.

 

I've learned to laugh at that.  After all I'm an amateur astronomer.  It's all about having fun.

 

The laugher sustains me; it brings me the joy I need to persevere, to try again and again until both my own ineptitude - and the most sadistic weather gods are overcome.  

 

Then there you are, under a river of stars stretches across the sky from horizon to horizon.  And the naked eye view is almost enough to make you cry.  And the razor sharp views through the eyepiece take your breath away.  And they are forever etched into your memory.

 

Yea... that's what I live for.  That's what keeps me going.  

 

And going, and going...

 

Clear, steady skies!

Clay


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#52 ClayDavies

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Posted 23 January 2023 - 11:25 PM

Howdy Y’all!

 

I’m back in the saddle again, trying to figure my first mirror back to a sphere.  It’s an 8”f/7.7 which was about f/8.3 when I started grinding #120. 

 

I’ve made concrete tools of 8.5”, 6.5” and smaller.  I’ve got a larger wood tool and an assortment of smaller ones in incremental steps.  I have a plaster 8” tool. 

 

Lately I’ve been using a borrowed Mirror-O-Matic with cerium oxide on Acculap Standard.  I have read that Acculap is great for polishing to a sphere.  At one point by hand polishing I had a gorgeous sphere - apart from the red laser sparkle and TDE.

 

After almost a hundred hours of mostly hand polishing I never completely eliminated red laser scatter.  I ran out of cerium oxide and I’ve replenished it. 

 

I also have red rouge and black rouge.

 

At one point I got a TUE with cerium oxide, so I switched to red rouge with the goal of getting a better polish as Jean Texereau suggested.  

 

After six hours of spin polishing with thin, decanted red rouge every last trace of red laser scatter was gone, gone, GONE!  Even in a completely darkened room I could detect nothing more than the occasional speck of dust on the surface of the mirror.  The only sparkle I could detect was on the back of the mirror.

 

But the TDE had returned, and I was getting a conical hill in the centre.

 

I used sub-diameter tools and cerium oxide to knock down the high zones.  As things were nearing a good sphere again I switched to a 6.5” tool to smooth out the zones and finish the edge.  The wonky Ronchi lines got straighter and straighter and the TDE was forced outward every time I checked.  Things were looking better and better until suddenly I got a big surprise:

 

https://www.flickr.c...th/52645954598/

 

The first video and the next four images are current.  The remaining images are ancient history.

 

I’m using a 100 LPI Ronchi screen from Edmund.

 

KE is on the right. 

 

COC appears to be at 123 3/4” / 3144mm.

 

How does the mirror look to you?  How's the edge?  And what do you suggest I do next?

 

I hope you’ll be patient with this stubborn, moronic, newbie mirror maker… 

 

Clear, steady skies!

Clay



#53 dave brock

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Posted 23 January 2023 - 11:55 PM

Check that your image details are correct. You say Foucault knife comes in from the right but that doesn't match the Ronchi if the inside/outside are correct.
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#54 Dale Eason

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Posted 24 January 2023 - 12:24 AM

If your Foucault image is correct with the knife on the right.  Then for starters you are way way over corrected.  Something like 8 times or more the desired conic of -1.  


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#55 ClayDavies

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Posted 25 January 2023 - 02:43 AM

Oops!  You're right, Dave, I got inside and outside mixed up.  I've corrected the captions on Flicker.  
I'd post here, but every time I try CN says the file is too big. 

Anyway... 

Yep, way over corrected.  Zig-zag lines from zone work were getting straighter and straighter after each 20 minute session aiming for a good sphere with a good edge, and all of a sudden the tool wasn't spinning right and this happened in about 20 minutes of spin polishing.  I was hoping that machine work would get the TDE fixed quickly.  

Now I'm not so sure about machine work.  I'm thinking about assembling parts I cut a few months ago for a "barrel" so I can do hand work comfortably and efficiently and maybe taking it back to a sphere before parabolizing.  Or would it be easier / better just to take it back to a parabola?



#56 ccaissie

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Posted 25 January 2023 - 08:07 AM

I'm not a machine figuring guy, so I think f/8 is a good candidate for hand work, using the motorized spindle vs. walking the barrel.  It should respond in a reliable fashion using logic, testing and feel.   Foucault and measurements should be your guide, with Ronchi for evaluating smoothness.

 

The old methods of going to a sphere gives you the skills to manage zones and fixes the edge.  Then gentle parabolization techniques preserve the edge and puts in the asphere..gently at f/8.


Edited by ccaissie, 25 January 2023 - 08:09 AM.


#57 Dale Eason

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Posted 25 January 2023 - 11:48 AM

When images are too big to post (> 400K) I use Irfanview (windows image editor) and In the save option for saving .jpg format there is a feature that lets you specify a size.  I set it to 300K or less.  



#58 Steve Dodds

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Posted 25 January 2023 - 12:08 PM

You can also resize pics with Windows paint



#59 cosmicfrank

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Posted 25 January 2023 - 07:22 PM

My experience with 3 mirrors (8” f/10, 12.5” f/6, and currently 12.5” f/4.3) is that the fastest way to correct an over-corrected (or wonky in any way – TDE, TUE, etc) mirror is return it to a spherical surface with straight COC strokes (maybe an hour of work), then start parabolizing again. The Ronchi test needs to show straight distinct bars edge to edge when you reach the spherical figure. Too easy! (especially with hand polishing on a Parker barrel.) Then start over parabolizing.


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