Michele Scotti

3 hours ago, Astrobits said:

Yes, immersion procedure. This was way back in the late 60's/early 70's and spray silvering hadn't been developed. I don't remember using Nitric acid in the procedure and don't think it is necessary. I suspect that it was used as it was readily available then ( as a teenager I used to buy concentrated acids, Sulphuric, Nitric and Hydrochloric, from my local chemists!!! ).

The Fulminate normally appears when the mixed solutions are left standing for a period of time. I'm not sure of the exact chemical reactions that generate it. See this link: 

https://www.compoundchem.com/2017/09/06/silver-mirror/

Nigel

 

Interestingly silver spraying is avaialbe as a kit in the US (https://angelgilding.com/silver-on-glass/spray-silver.html) and a collaboration started with some ATMs - I'd say they're almost done in proving the enhanced durability of the process + antiaging agent.

My understanding is that this is modern approach to Silvering where:

• there is no hazard
• it's "easy" for ATMs
• durability is similar to Aluminium - or at least not as bad as 1y refresh needed-
• it's in general cheaper than Aluminium - at least for my personal project

Would you be able to have a look at the chemical kit? I've got the info below from the data sheets from Angel Gild website.

As far as I get it: preparation with "Tin for Silver Concentrate", spraying simultaneously "Sensitizer for 2-Part Silver Solution" + "2-Part Spray Silver Reducer"

Tin for Silver Concentrate
SnCl2·2H2O
Stannous chloride dihydrate
1 - 5 Weight %

2-Part Spray Silver Solution
Silver nitrate  AgNO3
10 - 25 weight-%
Ammonium hydroxide (Ammonia solution 25%) NH4OH 
5 - 10 weight-%

2-Part Spray Silver Reducer – C3030
Sodium hydroxide 
10 - 25 weight-%
Ammonium hydroxide 1336-21-6 
1 - 3 weight-%

*The exact percentage (concentration) of composition has been withheld as a trade secret.

22 hours ago, Astrobits said:

Yes. I have silvered a few small mirrors many years ago. I am fortunate to be an industrial research chemist so had all the chemicals needed readily available. It is fairly easy to do but can go spectacularly wrong if you are not careful and don't follow the rules. There are ( if they haven't been taken down) some recipes on the web. The main hazard is that the recipes can also produce silver fulminate which is explosive and incredibly sensitive and can detonate by itself (so no good for the usual uses of explosives). The ATM books have instructions and recipes and Stellafane.org has a page with some links.

Nigel

Hi Nigel, is the process you're familiar with an 'immersion' type?

Also, out of curiosity, where the fulminate can generate from?

Btw, I need to fetch the list of chemicals involved in this process and post it here

14 hours ago, fozzybear said:

you could try these guys

https://www.kingsviewoptical.com/precision-optics-services/optical-vacuum-coating-uk/

I'm talking about Silvering -as an alternative to Aluminizing-. Specifically spraying Silvering

1 hour ago, johninderby said:

An interesting read on handling nitric acid. Not something for amateurs.. 🙀🙀🙀

https://www.ehs.washington.edu/about/latest-news/nitric-acid-incidents-how-reduce-your-risk

Can't see the use of Nitric Acid in this process

I posted on the specific thread for the 800mm mirror but I guess it went unnoticed. Also I've searched for similar topic on this forum and found out only something from 2013.

Anyway - I'd like to go with Silvering (+anti-tarnishing agent). There is a big piece of work that has been done by a group in the US.  https://sites.google.com/site/spraysilveringtelescopemirrors/home

This is the quite interesting spraying process:: https://youtu.be/2noVRDWppII?t=312

I've contacted the company that sells the kit of chemicals (in the USA) -and that collaborated with the ATM team- but the very interesting price of ca. 130$ for a complete set was brutally met with a 250$ shipping cost mainly due to Hazardous Fees.

I wonder if anybody on this side of the pond have ever had some experience with that - looking for some advice.

Silver Nitrate in a pretty pure form doesn't break the bank by the look of it. 

17 hours ago, Rusted said:

A naked circular saw and a bent twist drill remind us all how we only survived DIY telescope making by the skin of our teeth!

That blade is the scariest this in the garage! I was surprised when I watched the video - I usually wear heavy gloves and goggles. Safety first! 😬

I was in two minds if this is going on the mirror thread or here on the project. Anyway....

Sub-frame and whiffle-tree almost completed.The frame came up pretty light as it’s all Aluminium yet sturdy – there’s a couple of details left to sort out but so far so good.

Here is the sub-frame along with the H-frame.

Whiffle-tree awaiting the nylon-covered bearings

Here is the video of the build:

CAD is updated now with the collimation subframe and whiffle tree at 45deg +/-20deg

 Getting on with the work on actual parts too.

Pads for polishing prepared on a full tool - we'll give it a go MOT 

As we are getting in the polishing phase I start to consider the coating. Aluminazing is out for few reasons but chiefly: cost and the peril of shipping. back and forth.

I'd like to go with Silvering (+anti-tarnishing agent). There is a big piece of work that has been done by a group in the US.  https://sites.google.com/site/spraysilveringtelescopemirrors/home

I've contacted the company that sells the chemicals -and that collaborated with the ATM team- but the very interesting price of ca. 130$ for a complete set was brutally met with a 250$ shipping cost mainly due to Hazardous Fees.

I wonder if anybody on this side of the pond have ever had some experience with that - looking for some advice.

Silver Nitrate in a pretty pure form doesn't break the bank by the look of it. 

5 hours ago, planetman83 said:

I can see here your post but I cannot find the youtube video in order to understand exactly how it works. If I understand right, we can see how rigid and well built is our construction in terms of vibrations? I am very interested in doing it. Please help me doing this experiment.

 

Yeah somehow the link is not working. Here it is again:

Yes, it's a quantitative measurement to assess the structural soundness of the telescope - this is a major design parameter when engineering big telescopes like VLT, E-ELT etc...

It's not self-explaining but you can measure the wobbling of the structure which is basically what you are experiencing at the eyepiece when touching the focuser or moving the scope or if there's a gust of wind. The reaction of the telescope it's embedded in its intimate construction parameters.

You can place the phone vertically where you have the "24" engraved with double-side tape or even with a clamp -just firm enough not to let it move. The idea being that scope and phone moves together. As an external

The X axis for the app will be left-right so hit the structure from a side - like the side of the focuser board- with an hammer. Of course doesn't need to be hard at all - you'll see with the sensitivity of the app.

Glad that you're interested - speak later

On 30/04/2021 at 10:52, planetman83 said:

It works only with iphone? I don't own an iphone.

Fount it! I tried on a old-ish Android I have and it works.

https://play.google.com/store/apps/details?id=de.bremen.btm.movalyzer&hl=it&gl=US

Sorry to pester but if you could do that it's would be of great help

6 hours ago, planetman83 said:

It works only with iphone? I don't own an iphone.

Oh damn! Didn't consider that it's only on IOS. Thanks for looking into that anyway

23 hours ago, planetman83 said:

For the record, this was the previous telescope. (As I said before, I make a reconstruction of a used telescope). 

Before:

Total weight: 138kg

Unable to transfer it

 

After

Total weight: 84kg

Able to transfer (I hope)

Hi, as it seems you have a well built big aperture scope -btw congrats!- may I ask you to take a measurement of its natural frequency? It is one of the key aspect of the mechanical performance of a telescope and would definitely help me to benchmark our project.

I loosely described it here

you have a nice flat area over the focuser where it would be easy to clamp your phone and run AnaHerz - it takes 2min.

Kalispera!

Michele

Some time ago I introduced the H-frame being a thin Aluminum bonded structure at the bottom of the mirror box. Here is a pic of the item:

Bonding was a purposeful choice to stay in line with the spirit of the project i.e. a build that is approachable by as many people as possible that may do not have access to or be skillful with welding. It goes without saying that if you can weld, by all means you should go with that.

Now bonding, especially with Aluminum makes people itchy. And for a good reason. The oxide layer can impair a good adhesion with catastrophic results - conversely if taken care of it can yield outstanding performances. Key here is that eppoxy is applied just after the surface is roughed up with sandpaper so that Oxygen doesn't virtually have time to cause harm. Not that is strictly needed, as Aluminium oxidation is not instantaneous although certainly faster than the more obvious ferrous reaction.

What I came up with is a specific, unscientific, uncontrolled stress test.

The result is ongoing: as of now 6cm^2 (ca. 1in^2) could take 25kg (60lbs) for 2 months.

On 21/02/2017 at 07:53, skybadger said:

I also read that direct drive with inbuilt encoder can do 13M ticks per rotation

That's Planewave direct drive specification - 10 ticks/arcsec. Maybe it works with less but they kind of set the standard in case the DD is coupled directly to the axis.

Hi John, you're right the stroke is dead center and the tool I think sub-radius - like 45% of the diameter. We tried a full tool but the friction was way too high. The main grinding was done with a CNC diamond tool.

I'm definitely not an expert in making mirrors - I've only done few , long time ago. For sure I'll ask about your consideration and come back to you.

Btw I've cobble up another video - not advisable if you're seasick ;D

https://www.youtube.com/watch?v=w4HIsp1-zXw

Work continues with #800grit:

Aiming at #1000 then #1500 to prepare a pitch lap and check if we have something that resambles a sphere.

9 hours ago, Chriske said:

This is how I do it with large mirrors. Hogging out is done with this 'machine' the rest is done with my MoM.
It took me about 1.5 hours hogging out this 20" f/5.6 mirror using a angle grinder with a diamond disk. The surface is rather bumpy and to remove these irregularities it took me a 2 hours(or so) using a MOM(mirror-O-matic) and carbo #120. It is not futuristic looking I know, but this device saves me lots of work.
When I need another ROC, I make me another set of curved side panels(using a router), that's all there is to.

I saw this several times and it is saved in my favorite yt folder. Did you try to use any water or was it messy?

This is another good example:

https://www.youtube.com/watch?v=eRdr86gXBLI

Although it started like this....:

https://www.youtube.com/watch?v=lonAulU-kWA

On with #200 grit...

Cheers

An update on the mirror side of the project as a part of the club has been working on the mirror for a few months now. The bulk of the material has been ground off at a machine shop that works on granite.
Subsequently, the hogging was done with a half tool to smooth everything out with #80 grit.

You "may" notice that the route we took is the ....most impervious...but cheaper and readily available. Also not very recommendable which is a flat  25mm float glass.

All in all, I'd consider this a practice run for the part of the team who works on the mirror - we'll gain some experience.

Since we are not expencing to reach lambda/4 we are still exploring some alternative solution for a potential 2nd mirror. I'm getting more and more convinced that everything taken into consideration a 25mm thick float (?) slumped meniscus is the way to go. The big deal is that it looks pretty hard to find a suitable place in Europe to slump the glass - it's probabaly me not looking hard enough.

Currently we are looking at 2800mm FL 

Well done - much of satisfaction when things turn for the best.

Would you be able to make another video just like those before to check what it's fixed and what are the residual contributions.

Btw is now the motor/worm assembly fixed solidly and not sprung anymore, right?

The SiTech controller accepts 5v pulses as 'telescope encoder' so that gives an easy way to directly wire industrial encoders to it.

In the last few months I managed to salvage parts of a Renishaw encoding system - now 'obsolete'.  I suppose they are coming from some CNC machinery. 

Here I'm testing one of the 3 read-heads which has a 1micron resolution although the final system needs 0.1micron resolution. This equals ca. 30 pulses each arcsecond - hopefully it should be plenty for tracking accuracy.

This is a very crude test just to see if the salvaged hardware is working properly. The grating ruler is 1000mm long and is intended for the trunnion outer diameter to sort out the Altitude position.

The next step is to test the two RGH22 read-heads with 0.1micron resolution.

If you're curious I'm using a DSO138 oscilloscope kit - first time for me working on such an instrument but it was quite straightforward after a couple of tutorials. It's like 20$/£/Euros but I mean...it works for me.

I opened a thread on encoders some time ago - I'm planning to add some findigns there for whom is interested to adapt this type of encoders to their scope.

Hi Rusted, sorry I missed some of the last messages.

Did you identify the source of the substantial wheel movement as per your vid here: https://youtu.be/bWKQBCOkrNw?t=14 ?

Also, I recall you ordered angular bearings - were you able to instal them? Did you notice any improvement?

On 30/11/2020 at 14:44, Horwig said:

Yes, it's a strip of 0.2mm stainless steel, as long as possible, constrained by stronger strips on both sides, so it can only bend evenly with no twist along its length,

something like this in cross section:

Hi Huw, can you explain this sketch? Apologies if it's explained somehre else but couldn't find anything.

2 hours ago, Peter Drew said:

There still seems to be excessive "rock" between the wormwheel teeth and the worm, as though the mesh needs adjusting, I would expect no visual movement if all was well. 

If the camera is on a tripod on the floor what I see is that the wheel is moving away from the wormgear assembly.

I almost have the fealing that the entire RA shaft is pushed around by the reaction on the teeth. On the video from the right side you clearly see that the housing and the mounitng plate experience small deflection.

From the 'front view' video instead the deflection of the wormgear assy is more pronounced. I'm a bit baffled - would you agree with the observation?

For peace of mind I'd take a vid from the left side too and another one where you can check whether the main RA shaft and its housing are moving relatively to the main pier.

5 hours ago, Rusted said:

After hours of searching very unhelpful bearing websites I found some angular contact bearings in the matching size to the original deep groove.
12x28x8mm for a straight swap.

Angular bearings are a good decision - you nee to have a strategy to pre-load them though. It should be feasible with the housing you have though by adding a small plate.