Michele Scotti

After the first full build there are still a number of things to implement sooo...it's detailing time.

The telescope needs to move around and to have robust feet when it's placed in its observing position.

We added wheels that are removable and are part of a system that can lift the telescope with a pedal - this is like a system for furniture I guess.
For its resting position we do not want to take shortcuts - feet are an integral part of the telescope structure and we opted for 2" threaded tubes -basically plumbing tubes and sleeve - so that we can adjust the height. There will be a mechanism to adjust the height semi-automatically - we'll describe it later on.

The position of the feet was carefully considered and the key parameter is that it needs to be as close as possible to the Azimuth rollers - this is where the load from the telescope is going through and a short distance minimizes bending and 'floppiness'.

The feet lie on a 1m diameter circle which seems more than adequate.

Here below the other 2 'legs' drilled to accomodate a 66mm sleeve -which "sits" on the bottom for better support- and a 60mm tube. Upper leg carries a driven set of bearings whereas the lower carries the driving roller, to be attaced to the AZ motor.

This is quite a milestone in making a large, affordable diy frindly telescope.

Still miles away from completion but very refreshing to see the full scale of the project.

We have to admit it turned out to be slightly bigger than we thought which caused the ceiling lamp to be removed.

Next-up: secondary mirror installation and star test

On 22/09/2022 at 14:35, iantaylor2uk said:

I looked online and found a planewave 24" OTA (https://planewave.com/product/cdk24-ota/) for $63,500. Assuming they need to make a margin on this of maybe 20% or more, this would suggest the cost to manufacture such a scope would be between  $40,000 to $50,000. You can get a 20" f3.6 mirror from TS-Optics for around $3000 (https://www.teleskop-express.de/shop/product_info.php/language/en/info/p9073_TS-Optics-20--Newtonian-Primary-Mirror-f-3-6-made-of-fused-quartz.html). I plotted a graph in Excel of approx cost of mirror (I assumed a 20% profit margin so took this off to get the approx manufacturing cost) versus mirror diameter, and by extrapolation I came up with a rough estimate of the cost of an 800mm mirror (32") to be around $5500 - does that sound about right? So, somewhat surprisingly to me, it looks as if the cost of the mirror is only around 15-20% of the total telescope cost. Or are these estimates way off?

 

 

Hi Ian, to start with, we are making our optics so apart from the cost of the blank and the grit there's only a enourmous amount of time to be accounted for - but it's DIY so time is free!

Anyway a couple of year ago before starting the operations on the mirror I've requested quotations for few businessis around the world to have an understanding of the expense in case we opted for buy - instead of make. The range for a 32" f/3.3 or 3.5 is 20k to 35k USD usually including the secondary but I'm not that sure.

To your graph I'd say that the main thing that is off is the linearity. I'd expect the curve to be exponential due to the cost of glass which is even more thatn the ^3 of the diamater (volume/weight) but its defect rate and the overall availability in general for anything over the 25mm commecially """available""". Grit grinding time and energy go with ^2 or ^3 .

I've checked the documents from some of the big observatories around the world and if I'm not mistaken the mirror itself is usuallya 10/15% of the overall investment. In a pure Dobsonian I'd expec that to be 85% but if the scope comes with proper tracking capabilities than a 20/25% is reasonable. Just a suitable encoder can set you of 3k (new)...and you need 2 of them.   

Last note: that TSO 20" f/3.6 is a huge bargain!

Project update: big miscalculation!

And teaser...

On 13/09/2022 at 10:01, planetman83 said:

How much will the whole upper cage weight?

The Upper cage as it is in the video is around 10kg.

The projected weight with focuser, derotatore, autoguider and the secondary mirror should be less than 15kg.

One thing is left before completing the assembly of the main structure.

3 brackets connect the truss poles to the upper cage as we have a 6-pole design. Here we place them following to the drawing and drill holes for the fixings.

Maybe not the most thrilling video but it shows how the placement was carried out and how we used the bracket as jigs.

https://youtu.be/HMAQDbGFReA

The positional accuracy is key for the correct position of the upper cage as the poles have been drilled with a decent level of precision - hopefully we won't need much adjustment later on!

7 hours ago, Mike Q said:

It does, you went the chea..... Coat effective lol.  I can't help but notice one thing, you have spent serious dollars on material, machine work and time...and you are worried about 50 bucks for a meter of stuff?  Not knocking, just seems odd to me

An Engineering mantra goes like: if something is not there it can't fail.

It's an obsession for efficiency and respect for resources in general.

Anyway. one of the thing that I probably haven't explained in full  or elaborate on is the fact that this telescope is just a prototype. Its goal is to validate a design concept for an 'affordable' big imager. Once everything is working we will put online all drawings and cad along with processes, Bill Of Material and costs. 

I'm explaining the whole thing here a bit more in details if anybody is interested: https://youtu.be/mEisbOBj_kA

23 hours ago, Mike Q said:

Ok.... I will bite.  Why six truss poles and not eight?  If you answered this elsewhere i must have missed it. 

 

No problem, I'll elaborate a bit on this for future reference.

I try to make this kind of calls based on data/science and tune it for a meaningful business case.
From design concept I recall a study of a simplified truss system playing with few parameters. It struck me how the 6-pole was marginally worse - say a 15%- vs 8-pole in terms of deflection.

So, just by opting for a bigger pole diameter you completely recoup the gap to an 8-pole design.

How much bigger?  As the Moment of Inertia goes with the 4th power of the radius a +10% diameter means +46% "rigidity".

Anticlimatically we just wanted to be chea..err...cost-effective. So 6 instead of 8 poles, at 50£/meter for CF, means quite some money.
There's also a reduction in hardware complexity i.e. fewer brackets, flanges and fixings - the latter meaning also a shorter set-up time.

I don't see many drawbacks - apart from maybe the need to have a joining point between the trunnions - which was not a problem in our case as we need a cross member anyway.

If you look at 5m+ telescopes they mostly sport an 8-pole design which is superior by definition if the cost of trusses is negligible compared to the whole observatory investment. Big telescopes struggle to reach a good first resonance mode, so anything helps I guess. But for ATM I really don't see the need for 8 poles.

Unfortunately, I cannot find that simplified study on the internet anymore. However after the completion of the final design iteration we ran a deformation analysis which confirmed we were within the design target with a 0.3mm deflection. 

Hope this answer your question, Michele.

It's time to attach the upper cage - luckily it's light enough that can be easily handled by one single person

A rather not-so-exiting spin-off video about the temporary truss system, specifically on how we generated quick and easy tube ends.

https://youtu.be/hzeYTcbm1uQ

Another step, another update. Getting closer to a full dry-run build.

I'll post some description of how we prepared the trusses ends, the cheapo way - it might be of interest, maybe.

6 trusses, joined in pairs by L-shaped brackets that will be bolted to the lower part of the upper cage.

Hopefully the lenghts are ok and are locating the brackets in the proper position with respect to the upper cage. You gotta trust the process....

Next up: upper cage

So we now have the Aluminium poles -which are intended for the preliminary star test for the uncoated mirror and not for the final release.

They are 40x1mm with lengths ranging from ca. 1.25m to 1.7m to match a mirror with 2750mm FL.

The ends are simply generated by clamping in a vise and drilling a hole. To be honest it is a very inexpensive and practical way to make trusses - maybe not the best looking but still valid.

At this point i really appreciated the simplicity of having a 6-pole design rather than 8
 

Good progress during these vacation days.

Update: mirror is on
Next up: truss poles and upper cage

https://youtu.be/Pgw69b7CzEc
 

Quite a milestone....another step toward the final goal.

This is where this session started from:

And this is where it eneded: 

https://youtube.com/shorts/anDlPaFMFL0

I'll add a post at a later stage to go through some details but if you have any questions don't hesitate

Ok you might think: another boring update on something we don't care. We want to see it all built and running!

Well, we are getting there, slowly but steadily. I appreciate your patience.

In the meantime!

A D.455mm steel ring is used to carry the encoder grating tape -still to be glued on- and it's installed on the tripod. The read-head is rotating on the Azimuth table and to do so we need to cut a sizeable hole to be able to do fine tuning adjustment for the readhead as well as the ring.
As a matter of fact the ring is secured with 3 screws and oversize hole so that it can 'float' around and be torqued down when the pretty tight tolerance (+/- 0.1mm) is met.

I'm just waiting for that - it's gonna take a lot of time and discipline....

Anyway the good news is that NEXT STEP is to start some assembly dry-run!

As usual, any thought, suggestion, critique is welcome.

New upload and update:

!!! WARNING: excuse the crickets and my EngRish !!!

A quick tour of where we are at right now followed by a lengthy and frankly boring description of the DIY-friendly apparatus that can be built for less than 100Euros / $ / £.

Such apparatus is in line with the spirit of the project i.e. it's cheap!

Ehr...I mean value for money. Getting serious this is an attempt to generate a big -1.2m wide- bearing rail for the Azimuth movement similar to a Lazy Susan.

Well, so far so good....we'll see shortly how is working.

UPDATE! Motor drives for Azimuth & Altitude movement nearly completed!

It is quite some time from the last update - we've progressed a lot of things and the final assembly date doesn't look too far in the future now.
However sometimes it takes more to collect the proper footage/pics to make a meaningful post.

Arrengments for both AZ and Alt are pretty similar.

For the Azimuth the motion is transmitted by a friction drive between a d.21mm roller and the rail on the bottom of the Azimuth table.

For the Altitude the friction drive is achieved between the axis -a store-availabe precision ground bar-, d.20mm and the pacman/trunnion ground perimeter.

A refurbished Harmonic Drive provides a 1:100 reduction and its input connects via GT2 belt and pulleys to a modified 12V Pittman motor which has an additional 500 CPR (count per rev) encoder integrated on its back.

The motor - from SiTech system- has seen its gearbox taken out -too much backlash- and replaced with an extenstion 5mm shaft that carries the smaller pulley. On the other side of the shaft a small bearing is helping to take the load of the belt tension that otherwise would compromise the tiny shaft out of the motor.

So all in all this should provide a nearly backlash-free movement with an overall ratio around 1:9000 that can be tuned with different sets of pulleys. Hopefully!

Hi everybody, we've just completed the assembly of the mirror cell and here is a VERY lengthy video of the process: - but at the end we try the big mirror on!

It's a 27-point cell with contact made out of Delrin 14mm flat pads. I'm tempted to slightly dome them off but I'll pass it for now. Eventually the cell will support a meniscus hence a smaller contact surface - which I don't even know if it's really desireable.

A note on the strange/fancy/unusual shape of the triangles: they are optimized trading off cost, flexure and weight. The 3 main ones are mild steel, blued for some rust resistance. They are 5mm thick with cut-outs to match the deformation to the secondary Aluminium triangles.

The 3 big triangles would be pretty expensive if therey were made of 8mm Aluminium. I've used FEA to size the cut-outs to match also the weight of a 8mm Aluminium, 580gr or slightly more than a pound.
They are all laser-cut.

The hinging or pivoting mechanism is based on spherial bearings - these are PTFE lined to provide smoother operation. The bearing sits against a modified washer that is secured with screws. Gravity makes it work or at least rest in place.

I think I have a video about steel blueing but not for the triangles hinges.

There are still one or two clearances to fix but overall we're happy with what we've got.

Also the mounting 'pillars' for the whiffle trees are set to accomodate a thicker mirror - they need some adjustment to properly sit on this thin mirror.

Clear sky,
Michele

sorry the previous link was broken

Wow I really haven't posted in a while! Guess a mix of work, covid other stuff and a dash of laziness maybe.

Anyway here's a quick update on the status of the project: https://www.youtube....h?v=pmk9X52cC0A

We made progress on few areas -I have pics and footage to post at some point- however the biggest endeavor was to find a solution to grind the trunnions rolling surface to a good level of accuracy - more to come on that.
 
Cheers, Michele

6 hours ago, Astrobits said:

I will be interested in using two separate bottles, one for each hand. I can foresee that the two hands approach might give some variation in quantity of material dispensed and possible inhomogeneity to the coating but worthwhile to try that. Certainly the cheapest way.  

The recipe in ATM book 2 includes glucose and silvers the mirror suspended face down in the final mix ( obviously not practical for 800mm mirrors ). Also the Material Safety Data Sheet ( MSDS ) for the Angelguilding reducing mix ( buried on p9 ) shows that it contains an unidentified ingredient which may well be glucose or something to do the same job:

 

 Nigel

Good catch Nigel - I'm kind of surprised that there's nothing available in Europe - a real pity.

About the spraying consistency - interestingly enough the procedure calls for try out on a some measuring cup to adjust the nozzles. I guess one needs to practice a bit - pumping those things takes some stamina 

4 hours ago, Astrobits said:

Your post mentioned "the bundle" and I assumed that you were referring to the whole kit and not just the chemicals. When (if) you purchase the chemicals you will have far more than needed for one mirror so you will be able to experiment on window glass to get the concentrations for one coat. My suggestion of starting with the lowest concentrations was simply to minimise the waste incurred in any trials that fail. Clearly, the American ATM's are using the product as supplied by Angel Guilding who have done these trials and determined the best concentrations for one coat. By starting with the lowest concentrations you will be able to get an idea of the concentrations that should provide a one coat solution. If you need to spray twice then doubling up the concentration should get you near there, if you need three coats then three times as much etc.

I'm interested in what equipment you choose for this project as I might want to go the same route.

Nigel

I'm not really keen to spend 250USD on shipment -there is something being 2x the value of the material that goes against my ethos.

I've opened this thread to figure out a way to do that here in Europe.

Btw I don't think they go down the 'Tollen's reagent' route - as the previous link you kindly shared. My understanding is that is good for a second surface mirror because the 'sugar' leave a residual which is unwanted for a first-surface astronomical mirror.

Here below the 'equipment' I have in mind: £5 spray bottles + gloves + tilting jig.

20 hours ago, Gasman said:

Have you tried local universities?  A nice chat with the lab tech at my local uni got my 8inch home ground mirror aluminised many moons ago , if I recall it could have been coated in whatever I wanted even gold 😁. Just cost me a nice bottle of single malt!

Steve

I have an 800mm piece of glass - it's a bit out of the std research equipment.

Also, I really think it's time to reconsider Silvering - I remember the first time I've heard of it and I was like 'nah, that's old stuff'....not anymore apparently

11 hours ago, Astrobits said:

On their "How to" video they use multiple coats if necessary. Therefore I would go for the low concentrations and expect to need to do multiple passes to get the thickness. However, you will need accurate scales to measure gram amounts.

I don't see the kit for $130. The two spray bottles and wand nozzles alone are priced at $208?

Nigel

This is what you need:  https://angelgilding.com/2-part-spray-silver-concentrate.html

+ "Tin for Silvering" . The antiging is another 15USD.

I would not buy bottles and nozzles that I can easily buy here.

The ATM group is applying one coating - they've published some study in terms of reflectivity too

19 minutes ago, Astrobits said:

I saw that Angel Guilding article in Sky & Telescope January 2020 and have considered doing it with my mirrors ( when I get round to making them, or my current scope needs re-coating). I don't know if the spray bottles are available here or will need to be obtained from the States. The raw chemicals are common and should be available fairly easily although I have not tried to purchase any yet but should be cheap enough to do some experimental silvering to get the desired effect.

Nigel

Well, I've enquired them directly - the bundle incl. antiaging is ca. 130USD......shipment to Italy (where the mirror is) is ca. 250USD which I find outtrageously high although it's dirven by 'hazardous material fee'

As a consequence I'm trying to figure out the process to reproduce it. I'm not a chemist so I can't figure out the % of the mixed involved which are kept vague in the datasheet as "trade secret".

Other 2 reasons why I'm very interested in silvering:

- no shipment concerns/risks

- over 500mm I understand it's increasingly more difficult to find places that can Aluminize