9885 replies to this topic

[jragsdale]

So you trust this plastic to not depolymerize and crack under the stress?

Yeah, like norvegicus said, PETG is quite durable in regards to heat, UV, moisture and stress creep, and I don't even plan to leave this outside. I also try to design parts so that they're under compression along the layer lines (Z axis, the weakest in a 3D part) so there's no tension away from the layers. With both of these parts, the 3D part isn't taking much of the load. Here's a transparent view of the top adapter plug showing the captive recess for the hex bolt I used, that's then press fit + glued into the hole so it can't rotate along with the head when tightening and it transfers all of the tension to surrounding parts of the print.

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Edited by jragsdale, 12 September 2024 - 08:27 AM.

[CHASLX200]

PETG is water bottle plastic and is UV and water stable.  His leg brackets are PETG with carbon fiber reinforcement, so extra stiff and strong.

 

The stresses here are mostly compression.  Steel bolts and the aluminum tube are doing all the shear and tension work.

 

I have printed probably dozens of astronomy related brackets and accessories and so far the only issue I've had is with stuff like a dovetail to be secured by a set screw;  plastic is too soft for that application.  Otherwise everything has proven stronger and more reliable than necessary.

 

The exception stress wise is the tube to mount adapter.  I assume that it's through-bolted with a big washer on the bottom, but the body of the adapter is bearing some torque.  However, it is big and monolithic and the stresses are spread across its surface contact with the tube and mount.  But, if it wasn't through bolted with a big washer it could suffer a layer adhesion failure at the junction.  I have made several of these and have not been able to break them, applying huge test loads way in excess of anything they see in use.  Mine all have big bolts going through them, with big washers to spread the load.

 

My pillar tube to mount adapter has been left outdoors and used often through two Upper Michigan winters so far with no issues.  It has 50% honeycomb infill and 5 perimeters, so 2mm thick walls, but is made of PLA, which can be heat and UV unstable.  It has a three big bolts with big washers going through it.

I will stick with my old school EQ's.

[norvegicus]

I will stick with my old school EQ's.

IHow that is relevant to anything I wrote?

My old school EQs have been mounted this way and they also have plenty of 3D printed accessories and parts. It is an excellent technology for producing parts for old equipment that are no longer available otherwise, as jragsdale has amply demonstrated.

[jragsdale]

I will stick with my old school EQ's.

I don't remember saying anything about Chas, nor did anyone else? You've got the wrong post ghost, we weren't talking about "What mount works best for Chas". You don't have to interject your opinion on every single post, it's pretty rude. If you have something constructive to share, we'd love to hear from you!

 

And I could bore you with comparing this grab and go alt-az pedestal with my 300lb Schaefer 12", but why would you ever compare the 2? It's like comparing a bike to a dump truck.

Edited by jragsdale, 12 September 2024 - 08:10 PM.

[starman876]

Yeah, like norvegicus said, PETG is quite durable in regards to heat, UV, moisture and stress creep, and I don't even plan to leave this outside. I also try to design parts so that they're under compression along the layer lines (Z axis, the weakest in a 3D part) so there's no tension away from the layers. With both of these parts, the 3D part isn't taking much of the load. Here's a transparent view of the top adapter plug showing the captive recess for the hex bolt I used, that's then press fit + glued into the hole so it can't rotate along with the head when tightening and it transfers all of the tension to surrounding parts of the print.

So is there a picture or drawing that shows how this is used scope and all?   I find this very interesting.

[jragsdale]

So is there a picture or drawing that shows how this is used scope and all?   I find this very interesting.

It's on the previous page, it dropped off pretty quick: https://www.cloudyni...day/?p=13680444

[Bomber Bob]

For 99% of BB Projects, There Will Be Blood...  Managed to put a 1" slice in my left thumb pad yesterday afternoon with an X-Acto Knife, using it to break the glue seal holding my BORG 76's lens cell halves together.  The cell is black plastic, similar to PVC, but much more flexible / less brittle.  I gotta get the lens out:  Previous owner left swirls from sticking fingers into that small space; and, there's a couple of mold spots between the 2 elements.  Otherwise, I'd leave it alone.

 

The glue is the color of peanut brittle, but stickier, so I'm having to slowly saw through it at the seam -- all while not cracking the plastic, or harming the lens.  My Daughter asked why I don't use a solvent:  Don't want to get any chemicals on the lens face(s).  Got less than 2" of the circumference left.  I'll git 'er dun!  Hopefully without rushing to the ER or PriMed...

[deSitter]

For 99% of BB Projects, There Will Be Blood...  Managed to put a 1" slice in my left thumb pad yesterday afternoon with an X-Acto Knife, using it to break the glue seal holding my BORG 76's lens cell halves together.  The cell is black plastic, similar to PVC, but much more flexible / less brittle.  I gotta get the lens out:  Previous owner left swirls from sticking fingers into that small space; and, there's a couple of mold spots between the 2 elements.  Otherwise, I'd leave it alone.

 

The glue is the color of peanut brittle, but stickier, so I'm having to slowly saw through it at the seam -- all while not cracking the plastic, or harming the lens.  My Daughter asked why I don't use a solvent:  Don't want to get any chemicals on the lens face(s).  Got less than 2" of the circumference left.  I'll git 'er dun!  Hopefully without rushing to the ER or PriMed...

I always talk to my sharp things and warn them not to harm me. Before that, I was guaranteed to have several bloodbaths a year. It got to where it was more annoying than painful. And I am careful. Just didn't know my limitations.

 

-drl

[deSitter]

I disassembled an 8-element ultrawide completely to evaluate the coatings. The coating on 2 of the 10 exposed surfaces have failed, not in a completely catastrophic way, but enough to make a big dent in what is a premium performing eyepiece. Still thinking about ways to polish them away, wondering if I can make a lap from styrofoam or even just use my fingertip. The largest diameter is about 1.5".

 

It was very interesting to examine the construction, which was impressive. However, there was some debris between the innermost elements, which must have been in there since it was made.

 

It made me appreciate simple eyepieces all the more.

 

That's the first time I've experienced coating failure on anything, aside from having mirrors recoated.

 

-drl

Edited by deSitter, 13 September 2024 - 12:49 PM.

[Kitfox]

For 99% of BB Projects, There Will Be Blood... 

 

Blood has a very low likelihood of damaging either mechanics or optics. Solvents are much more dangerous 

 

I am in the process of removing a significant amount of that very hard liquid foam stuff used to fill the mount pylon on a 12” Tinsley. I am so close to getting out the gasoline…

Edited by Kitfox, 13 September 2024 - 08:14 PM.

[Bomber Bob]

You would think at my age that I'd recognize my symptoms of fatigue & STOP; or, at least, take a breather... but no!  I press on, get progressively clumsier, and get cut, bruised, etc.

 

Too late; but, I haven't even looked at the thing today.

[cavedweller]

You would think at my age that I'd recognize my symptoms of fatigue & STOP; or, at least, take a breather... but no!  I press on, get progressively clumsier, and get cut, bruised, etc.

 

Too late; but, I haven't even looked at the thing today.

Same here.

[Cavs56]

Same. I was saving dollars doing a Euro mount ona buck I got. More than $600 in stitches later I don’t think I saved much money.

[clamchip]

I had a busy morning.

Two of the 4 spider mounting screws on my new Starplitter were too long and really made a mess of things.

I shortened the two too long ones and fixed the 2 damaged vanes, all better now.

The grease in the focuser had turned to cement so I gave it a paint thinner bath and fresh grease.

I'm giving the ground board varnish plenty of time to harden. Probably by the time clear nights come back

I'll assemble and checkout the 1/20 wave mirror, yes guaranteed 1/20 wave RMS or better tested with

the use of two types of interferometers. I can't wait to see that !

Robert

 

Edited by clamchip, 14 September 2024 - 02:31 PM.

[Bomber Bob]

Got 'er dun!  Took almost 2 hours to sever the plastic - to glue - to plastic seal on the BORG K10-76, and here's why:

 

 

BORG used 2 different adhesives.  A brush-on clear / white ultra-thin layer below the other dun-colored bead at the seam.  NOW, I can use solvents to remove all that, buff out the marks from my X-Acto, and use standard tiny Allen-head grubs to hold it all together.

 

Note the cemented doublet:  The Crown has a ~ 1 mm lip that the retaining ring presses against with greater pressure than would be applied to a standard objective -- it can't be pinched.  Haven't seen that before.  The mold spots are on the back of the Flint -- no "growths" between the elements.

 

Once I get it all cleaned & reassembled, I'll put it on my DPAC Rig.  I suspect it'll be a very good achromat.  Coatings are pristine, too.

[CHASLX200]

You are way more brave than i am.

[Bomber Bob]

You are way more brave than i am.

Debbie says:  Pig-Headed, Mule-Stubborn, etc.  Haven't heard her say Brave... yet.

 

When I finally could pull the cell apart, large white flakes fell down on to the lens -- looked like really bad dandruff...

 

I can say, determined.  Altogether, about 6 hours to work that knife all the way round the cell -- and 2 blades.  Felt like digging the Erie Canal with a tablespoon!

Edited by Bomber Bob, 14 September 2024 - 04:51 PM.

[cavedweller]

I had a busy morning.

Two of the 4 spider mounting screws on my new Starplitter were too long and really made a mess of things.

I shortened the two too long ones and fixed the 2 damaged vanes, all better now.

The grease in the focuser had turned to cement so I gave it a paint thinner bath and fresh grease.

I'm giving the ground board varnish plenty of time to harden. Probably by the time clear nights come back

I'll assemble and checkout the 1/20 wave mirror, yes guaranteed 1/20 wave RMS or better tested with

the use of two types of interferometers. I can't wait to see that !

Robert

 

IMG_2673.JPG

 

I've heard that the secondary is as important as the primary. The best you can get is worse than the worst of the two.

[clamchip]

From what I've been reading about RMS is 1/20 wavefront RMS is about 1/6 wave PV, a pretty good mirror.

RMS is an accurate optical test usually done by interferometer.

I've never heard of RMS, new to me and might explain the high claims made by some manufactures back

in the 60's for mirrors.

1/14 wavefront RMS is 1/4 wave or diffraction limited. 

 

Robert

[cavedweller]

From what I've been reading about RMS is 1/20 wavefront RMS is about 1/6 wave PV, a pretty good mirror.

RMS is an accurate optical test usually done by interferometer.

I've never heard of RMS, new to me and might explain the high claims made by some manufactures back

in the 60's for mirrors.

1/14 wavefront RMS is 1/4 wave or diffraction limited. 

 

Robert

Yes, and for those more familiar with electricity, 120 V AC is an RMS measurement and the peak voltage is about 170 and peak to valley is 340. Similarly, 1/20 RMS is a much smaller number than 1/6 peak to valley.

[jragsdale]

From what I've been reading about RMS is 1/20 wavefront RMS is about 1/6 wave PV, a pretty good mirror.

RMS is an accurate optical test usually done by interferometer.

I've never heard of RMS, new to me and might explain the high claims made by some manufactures back

in the 60's for mirrors.

1/14 wavefront RMS is 1/4 wave or diffraction limited. 

I'm no optical expert, but RMS and PV don't have a direct correlation to each other, but generally, because of how mirrors are made, you see a much lower RMS measurement. Both require interferometer with multiple point testing. RMS is the average of the whole mirror (root mean squared) and PV is the difference between the lowest and highest measurements (Peak Valley). So you can have a 1/200 RMS mirror with a 1/2 wave PV (typically would indicate a great mirror that had a small touch of damage or a scratch) or even a 1/10 wave RMS and 1/10 wave PV (unlikely) but would indicate a possible rough polish but good figure. But typically you see a correlation like you mentioned above but there are exceptions. Also surface error vs wavefront error is 1/2 the error, so a 1/8 wave surface mirror is actually 1/4 wave wavefront. A lot of the old claims about 1/20 wave probably involves RMS and surface measurements.

Edited by jragsdale, 15 September 2024 - 09:12 AM.

[clamchip]

Wave front RMS seems like a useful test with accurate results.

I would like to sit down and read more about it.

Another facet of our wonderful hobby to explore.

 

Robert

[jragsdale]

Wave front RMS seems like a useful test with accurate results.

I would like to sit down and read more about it.

Another facet of our wonderful hobby to explore.

Yeah, you kind of need to take both measurements into account. I don't think any single test can tell you everything about a mirror, each test has its own pros/cons. Also whoever is running the Zygo (or interferometer test) is also important. Lots of testing parameters they can adjust to manipulate the results; masking the outer 1-2mm to exclude a turned edge, masking the central portion excessively using the secondary shadow as an excuse, using insufficient sample points or rotating the same points to ignore an excessively high peak or valley, doing multiple tests and cherry picking the best, nulling astigmatism when it is present in the surface figure, etc, etc, etc. Here's a test of an Antares 3.5-inch "1/30 wave" Secondary I have in my 20" Zambuto dob. Shows PV 0.033 (1/.033 = 1/30.3 wave) and RMS 0.004 (1/.004 = 1/250 wave RMS).

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[Kasmos]

Yeah, you kind of need to take both measurements into account. I don't think any single test can tell you everything about a mirror, each test has its own pros/cons. Also whoever is running the Zygo (or interferometer test) is also important. Lots of testing parameters they can adjust to manipulate the results; masking the outer 1-2mm to exclude a turned edge, masking the central portion excessively using the secondary shadow as an excuse, using insufficient sample points or rotating the same points to ignore an excessively high peak or valley, doing multiple tests and cherry picking the best, nulling astigmatism when it is present in the surface figure, etc, etc, etc. Here's a test of an Antares 3.5-inch "1/30 wave" Secondary I have in my 20" Zambuto dob. Shows PV 0.033 (1/.033 = 1/30.3 wave) and RMS 0.004 (1/.004 = 1/250 wave RMS).

So.... basically take the human element out of it.

[fullthrottle_man]

Last weekend, I had the Dynamax 8 out for a bit. It just didn't feel right. The focus felt tight, collimation was off a tad still, and the drive was a hair loose (could wiggle the fork side-to-side a little bit).
Today, I deforked it to check the alt slow motion adjustment -- all was great there. I also pulled the bottom off the drive base and adjusted the main shaft so it was tight. Then, out came the corrector so I could pull the primary carrier out to double-check that the primary mirror was centered on the carrier. Once the carrier was back in, I adjusted the focuser feel to where I like it. I finished reinstalling everything else and brought it outside. The Golden Pyramid for it was still set up in the yard, so on it went so I could do the hall of mirrors alignment. It only took a couple small tweaks to get the hall concentric. Now I need to wait for a clear night so I can touch up collimation with a star test.

Edited by fullthrottle_man, 15 September 2024 - 03:07 PM.