800mm Telescope Project

[Michele Scotti]

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.

[Michele Scotti]

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

[Michele Scotti]

Good progress during these vacation days.

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

https://youtu.be/Pgw69b7CzEc
 

[Michele Scotti]

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
 

Edited August 19, 2022 by Michele Scotti

[Michele Scotti]

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

[Michele Scotti]

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

 

[Michele Scotti]

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

[Guest]

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

 

[Michele Scotti]

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.

 

[Guest]

4 minutes ago, Michele Scotti said:

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 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

[Rusted]

10 minutes 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

Olde English saying: "Don't spoil the ship for [want of] a ha'penny worth of tar."

That said, the OP seems to have thoroughly examined the choice of a six pole truss.

Professional instruments would normally carry a much heavier secondary mirror & cell.
With the addition of heavy and complex instruments mounted thereon.

[Michele Scotti]

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

 

[Michele Scotti]

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!

[Rusted]

The ease with which you lifted that huge cage was ample proof of the sophistication of the lightweight materials you are using.

[planetman83]

On 09/09/2022 at 04:06, Michele Scotti said:

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

How much will the whole upper cage weight?

[Michele Scotti]

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.

[Michele Scotti]

Project update: big miscalculation!

And teaser...

[Shibby]

On 20/09/2022 at 01:28, Michele Scotti said:

Project update: big miscalculation!

You're just going to have to build a larger garage  (And maybe a larger ladder, if you do plan on doing any visual with this thing!)

[iantaylor2uk]

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?

 

[Rusted]

I once ground, polished and figured a 16" mirror to f5.
Bought the 18" PVC tube. Took it home.
Tried to stand it up in the shed.
Too tall by miles!
Dogh! 😱

[Michele Scotti]

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!

[Michele Scotti]

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

[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.

[Geoff Barnes]

No updates for a year, rather concerning. 

Hope all is well?