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Truly Awful Star Test Of Coulter 17.5" In Truss Dob

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#51 Pinbout

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Posted 13 June 2020 - 04:29 PM

I prefer these nylon hex head screws.

g

they have a larger flat circular surface... the points are chamfered away from the surface.

 

I like using the length of the screws as pins to keep the rocker and or triangles in position.

 

hex head.jpg

 

 

https://www.youtube....h?v=kdMofIELVd0



#52 pastorgalactico

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Posted 13 June 2020 - 05:02 PM

Hello;
In the case of 6 support points, it is better as you say, but better with nylon blind nuts.
  In the case of 9 and 18 points of support it is better with acorn nuts.
The bearing points in a cell have to be just bearing points and not surfaces with less friction than a single point.
Regards.



#53 Starman1

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Posted 13 June 2020 - 05:19 PM

No other changes other than a point count:

Estimating 2" thick mirror, wavefront error, optimized cell:

17.5" with rear support of:

6 points--2.68nm RMS

9 points--3.36nm RMS

18 points--0.70nm RMS

So it looks like 18 points is essentially perfect and that 6 points are better than 9.

These figures are at the zenith.  The errors go up a lot at 45°.

If the mirror is thinner than 2", then the numbers go up also, but 9 points is never better than 6.

With a thinner mirror, though, 18 points becomes more critical (why I went with 18 points on my 12.5"--thanks, Rob.)


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#54 Chuck Hards

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Posted 22 June 2020 - 02:23 PM

With Coulters a good rule of thumb is:  The older it is the better it is.  As the years went by instead of raising their prices they dropped quality, by the 90's I don't even think they were testing them.

I have owned 2 Coulter 17.5" mirrors, the last one purchased around 1986 IIRC.  I took both over to Vaughn and we looked at them with the Ronchi.  Both had nice, straight bands, well-corrected and smooth.   I remember him shaking his head and saying "how can they make a mirror this good for so little money?"

 

I still have one of them, wish I kept both.  

 

I have never personally owned a poor Coulter mirror, however I have tested close to 20 over the years.  All the full-thickness mirrors were very good.  I've tested a few 13.1" mirrors that were not good.  But everything that came out of the North Hollywood shop tested good to excellent.  Can't comment on the Idylwild production except for the two 17.5" mirrors I tested.   

 

Ah, the good old days.  


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#55 Pinbout

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Posted 22 June 2020 - 07:33 PM

I have owned 2 Coulter 17.5" mirrors, the last one purchased around 1986 IIRC.  I took both over to Vaughn and we looked at them with the Ronchi.  Both had nice, straight bands, well-corrected and smooth.   I remember him shaking his head and saying "how can they make a mirror this good for so little money?"

 

I still have one of them, wish I kept both.  

 

I have never personally owned a poor Coulter mirror, however I have tested close to 20 over the years.  All the full-thickness mirrors were very good.  I've tested a few 13.1" mirrors that were not good.  But everything that came out of the North Hollywood shop tested good to excellent.  Can't comment on the Idylwild production except for the two 17.5" mirrors I tested.   

 

Ah, the good old days.  

How did you null the mirror to get straight ronchi bands?



#56 Chuck Hards

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Posted 22 June 2020 - 10:14 PM

How did you null the mirror to get straight ronchi bands?

 

That was a long time ago,  I seem to recall a huge perforated flat, we were too close for it to have been at radius, but memory is foggy after 30+ years.  


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#57 Steve Dodds

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Posted 23 June 2020 - 02:47 PM

That was a long time ago,  I seem to recall a huge perforated flat, we were too close for it to have been at radius, but memory is foggy after 30+ years.  

That was over at Vaughn Parsons, he had a 40" full thickness cervit autocollimation flat.


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#58 Pinbout

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Posted 23 June 2020 - 03:57 PM

That was over at Vaughn Parsons, he had a 40" full thickness cervit autocollimation flat.

shocked.gif shocked.gif shocked.gif



#59 suiterhr

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Posted 23 June 2020 - 04:44 PM

I have been lurking on this thread since it involves the star test, a subject close to my heart!

 

I do have a comment, however. The geometric Ronchi test, even in autocollimation, may not be the most appropriate for fast mirrors. The Coulter 13.1 and 17.5-inch mirror were f/4.5 and the estimate for a 100/inch Ronchi ruling against a 1/4-wave spherical aberration mirror showing 2.5 lines, doubled for autocollimation, is still only 9.5% distortion (Star Testing Astronomical Telescopes, 2nd Ed. p 322). That is only a little bigger than the 8% limit given there, and would still be difficult to see against the fuzziness of the line.

 

Returning to Coulter mirrors. I only owned one, a little 4-inch RFT, but tested many through the years. Most were undercorrected, but keep in mind that high resolution was not their intention or design. They were light buckets. They were just about the first large commercially-available Dobsonians, and the snow they broke cleared the way for Obsession and the others. For that they deserved our accolades.



#60 Pinbout

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Posted 23 June 2020 - 05:14 PM

 

The Coulter 13.1 and 17.5-inch mirror were f/4.5 and the estimate for a 100/inch Ronchi ruling against a 1/4-wave spherical aberration mirror showing 2.5 lines

100 lpi is kind of low... typically we use 133lpi.

 

but still I tested my 13.1 with a 100lpi

 

https://www.youtube....mSieQr&index=32


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#61 dave brock

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Posted 23 June 2020 - 07:08 PM

And on a star 250 lpi is usable.

#62 Starman1

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Posted 24 June 2020 - 07:15 PM

And on a star 250 lpi is usable.

That's what I use--10 lines/mm



#63 Jimmy462

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Posted 20 August 2020 - 04:55 PM

Re cutting silicone blobs, I've had good luck doing it with a high E guitar strings and two wood handles. My Coulter, the mirror was on neoprene foam pucks, glued to the mirror with Duco cement and something like coarse cheesecloth. I cut through the foam with a wire, then removed the Duco with acetone.

Dental floss wrapped around two pens (for handles) worked-in-a-pinch (I highly recommend wearing work gloves ;) ) when separating my 14" Discovery mirror from the massive silicon(?) blobs in its cell. FWIW, that mirror threw up gorgeous star tests despite its, um, primitive cell. It'll be returned to its cell using the same, er, "plop" design. Haw! No sense messing with what already works.



#64 pierce

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Posted 03 September 2020 - 06:42 PM

ugh,  while packing my scopes in a hurry for storage in a safe location when I had to evacuate my house two weeks ago, I put my wheelbarrow handles on backwards, and the rocker box tilted and fell off the base, causing the mirror to come unglued.    no mirror damage, but now I need to remove the silicone blobs and figure out what to do about reattaching it.     this is a 17.5" f/5 so its a rather big piece of glass for the 3 blob mounting method.



#65 Bob4BVM

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Posted 03 September 2020 - 08:34 PM

ugh,  while packing my scopes in a hurry for storage in a safe location when I had to evacuate my house two weeks ago, I put my wheelbarrow handles on backwards, and the rocker box tilted and fell off the base, causing the mirror to come unglued.    no mirror damage, but now I need to remove the silicone blobs and figure out what to do about reattaching it.     this is a 17.5" f/5 so its a rather big piece of glass for the 3 blob mounting method.

 

ugh,  while packing my scopes in a hurry for storage in a safe location when I had to evacuate my house two weeks ago, I put my wheelbarrow handles on backwards, and the rocker box tilted and fell off the base, causing the mirror to come unglued.    no mirror damage, but now I need to remove the silicone blobs and figure out what to do about reattaching it.     this is a 17.5" f/5 so its a rather big piece of glass for the 3 blob mounting method.

So how were the images with that glued cell ?

I cannot believe the difference in my 17.5 since replacing my old , crude, 1990 wood cell (NOT glued mirror at least) with a real PLOP designed and precisely built cell with roller edge support.  Difference was huge.

You might consider doing yourself a favor by treating your mirror to a full flotation cell.

CS

Bob


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#66 pierce

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Posted 07 September 2020 - 02:14 AM

the Discovery 17.5" f/5 mirror is pretty thick...   I think the big coulters were thinner.   Optically, the DT 17.5 was excellent in my opinion.  

 

i don't have the machine tools I'd need to make a decent floatation cell, and my friend that did has sold them all when their arthritis got bad.   and I don't have the money to buy one, either.   the scope has been for sale for over a year, with very few nibbles, I hate to pour money into it that i won't get back.


Edited by pierce, 07 September 2020 - 02:15 AM.


#67 Tom Duncan

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Posted 16 September 2020 - 11:02 AM

Turns out there is no astigmatism issue with this scope, the misshapen image was caused by a slight mis-alignment of the scope such that the light cone was trimmed on one side, making it look like astigmatism. My error. I took any personal eye issues (I'm 70) out of the equation by using a video eyepiece and a small monitor to look at the image. 

 

Scope will need a top to bottom alignment optimization but that's routine with these, something a new owner can do.  

 

Tom 


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#68 Bob4BVM

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Posted 16 September 2020 - 01:23 PM

the Discovery 17.5" f/5 mirror is pretty thick...   I think the big coulters were thinner.   Optically, the DT 17.5 was excellent in my opinion.  

 

i don't have the machine tools I'd need to make a decent floatation cell, and my friend that did has sold them all when their arthritis got bad.   and I don't have the money to buy one, either.   the scope has been for sale for over a year, with very few nibbles, I hate to pour money into it that i won't get back.

both my coulters are 1.5" thk.

 

You are welcome to my old cell if you want it for the cost of shipping, sent you a PM

 

CS

Bob



#69 calypsob

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Posted 16 September 2020 - 07:10 PM

I prefer these nylon hex head screws.
g
they have a larger flat circular surface... the points are chamfered away from the surface.

I like using the length of the screws as pins to keep the rocker and or triangles in position.

hex head.jpg


https://www.youtube....h?v=kdMofIELVd0



Seems like a finish washer would be easy to center

Clipsandfasteners Inc 50#14 Nylon Finishing Washers - Black https://www.amazon.c...i_WKQyFbK20XBWA


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