Michele Scotti

Hi, it’s about time to spec the encoders for the telescope.

The current set-up is as follows - I have a SiTech II and I deem the axis encoders essential for tracking capability and long exposure.  There will be an off-axis guide however I’d like to get the most out of unguided operation - and I have a couple of doubts to clear....

Right from the design phase, I wanted some easy way to attach a linear scale to an accurate surface rotating with the axis. The pac-man is an example of that – once it’s ground it can accommodate a linear strip.

Currently my first (*) route is to use linear incremental encoders and sourcing them as used parts from scrapped CNC machinery. Linear scales can be purchased as strips and glued, taped or somehow fixed to the trunnion.

To start with: Resolution.
The trunnions provide a ground SS strip with an approx. diameter of 1200mm. This provides a ratio of (360deg x 3600arcsec) / (1200mm x Pi) è 343.8 arcsec/mm or 0.34arcsec/um.
Linear scales with 1um resolution are pretty “standard”.
QUESTION #1: is 0.34arcsec/um or better ca. 3 ‘ticks’ per arcsec enough to track?

Accuracy:
Just like Sun Divisional Error, I’m not that concerned if adopting CNC equipment – they are usually pretty decent and unlike a CNC machine, when it comes to tracking for several minutes the portion of the scale is relatively small.
QUESTION #2: or should I be concerned??

If anyone here has some advice that’s much appreciated – I have a background on encoders but I lack insight and practicality.

QUESTION #3: also, can I directly connect the TTL (0-5V) A and B phases directly into the SiTech?

Btw, I’ve ordered a Renishaw readhead – it was just too cheap not to buy it. Looks like scales that go with that are not that inexpensive. I’ll post the image as soon as I received – so we can see if I just wasted some (more) money…

(*) I have a couple of concepts to develop a similar or better system suitable for DIY but in our club we are currently lacking somebody with Electronics experience to develop a functioning prototype.

Cheers, Michele

Before going into the next phase i.e.grinding the pac-men rails I ran a quick&dirty vibration analysis check using readily available and easily accessible techniques. With the Lower Telescope Assembly pulled together we were curious to check the ability to record and analyze the frequency spectrum of the physical modal response.

We downloaded AnaHertz free app on Apple store and double-sided tape the phone directly to the assembly in the area that feels more flimsy.

The excitation is provided by a suitable....hammer.

By no means this wants to be an exact analysis but rather and attempt to see if some info can be extracted with a tuned-down methodology that usually costs few grands.

That part of the structure - the upper part in the back- is at 5Hz which by itself doesn't meet the project target. however, this is just a warmer while the test that counts will be conducted on the whole telescope.

The front upper area returned a slightly higher 9Hz wheres the bottom is much more constrained and felt more rigid.
Also, this is possible as the mass of the phone is more than a magnitude less than the probed structure

Amazing to see what you can do today with te MEMS in your phone and a free app!.

Here's the full video:

https://www.youtube....h?v=Uy-H-QXyRwc

On 16/08/2020 at 06:00, Rusted said:

Are you aware that woodcutting circular saws, with carbide tipped teeth, can cut through aluminium like butter?
The bench type machines and table mitre saws can save hours of tedious work reducing large aluminium sections.
Downside it it makes a hell of a mess in the workshop! Swarf collection should be attended to.
It also needs very firm work-holding to avoid accidents and potentially serious injury. Plywood jigs?
Use odour free, lamp oil to reduce saw tooth clogging. It really helps.

I might actually ahve a suitable cutting disc with inserts. I'm a little bit unsure about the jig holding the piece. As of know I'm using ALuminum specific thin abrasive disks for angle grinders. The pieces are getting sooooooo hot I ned to keep spraying water on it. it's a solid 30/45 min work per piece 

Small update - small parts.

With bigger elements starting to come together it's coming the time for smaller parts needed for the final telescope assembly.

The truss heads are found at the trusses/beams ends. There are 6 beams hence we need 12 heads that will slide into the beams end and eventually secured with glue.

These end bits are made of aluminium and they have been turned into shape and hollowed on a lathe. The hollow end is tapered to make the parts as light as possible.

The following step is to cut and mill the element of the head that is mating with the flanges on the upper and lower sub-assemblies - that's an 8mm flat.

The cutting operation, which is meant to alliviate the milling effort -given that we have a mini-mill- is fairly painstaking....3 out of 12 are completed so far. 

LTA assembly dry-run.

After trimming some beams we pulled together the entire sub-assembly. That was mainly to check the robustness of all joints but also for the sake of seeing how it looks like.

The structure feels sound and light enough to be carried by a single individual if you stay inside the structure - otherwise it's very bulky. Overall it's should be around 28kg/60lbs.

Next up is grinding the Altitude bearing surfaces.

Despite the SS strips are bonded to a CNC machined surface and there are no screws or discontinuities, they do not provide at all the accuracy required by this project.

During grinding I'll take some before/after run-outs measurements.

What's following from that is a specific assembly procedure This is critical to ensure that the two rolling surfaces sit on an ideal cylinder.

At this point we have all the 4 main assemblies that took shape: UTA, "tripod", Azimith table and now the Lower "Tube" Assembly. 

Still a lot to do though.

Cheers 

Had the chance to do a couple of solid sessions on the hardware. It clearly was about time....

Anyway, focusing on the Lower Tube Assembly I've finished installing all the stud-screws. They are all pre-drilled and epoxied in place. This is a crucial element, just like any joint, the connections work with friction. And friction is proportional to the amount of load that you can torque the nut with. That is  quite a big pull the stud-screws have to sustain.  I've used M8 and M10

To assemble the H- frame I wanted to have all holes aligned on both sides. They whee pre-drilled on the CNC but in the meantime I had to change the position of one of those. Anyway - the 2 pac-men are aligned on the circumference and bolted together. Only then I drilled the hole - 3 per side., through holes. 

Here is how the Aluminium H-frame is going to be secured to the internal side on the pacman - which is covered in fiberglass whereas the outer one is CF covered. Layed up on the pacman you can see the M8 bolts - I actually going to use shanked ones, a big washer and another plate made of 3mm Al to spread the contact pressure even more and they fit snug inside the H-frame side elements. So the screw heads, washer/plate is inside and in close contact with the inner wall.   This is a design decision to maximise the joint strength. 

Last but not least, with the help of a specific cutting wheel for Al - it really does a good job!- I'm preparing all the beams and crossing elements to complete the LTA.

Hi Nigel, good question – it is probably the most popular one.

The few telescopes I made in the past indeed hinged around the optics so the mirror was the first one to focus on and to complete. Then the rest of the telescope. I agree that this is the most difficult part to sort out. With a caveat – the size. And the purpose.

This project aims at a robotized imager eventually been remotely controlled in a small observatory.

Let’s start with a consideration – if you pool the amateur telescope in the 700mm+ category there are very few scopes being used as imagers – actually, none as far as I know at a purely amateur level. And for good reasons – the engineering of the mounts gets more complicated. And focal lengths are getting bigger, making tracking more challenging thus demanding more sophisticated mounts.

That’s why we started with the mount. If the mirror is not up to the task it will be disappointing. But it will be even more if the mount is not capable to be an imager – it would fail the entire project, leaving us with a big light bucket. Still enjoyable but not serving the ultimate purpose of this project

And if the quality of the first mirror is not adequate we’ll keep trying as we did in the past.

Lastly, we have a 500mm mirror that can be adapted to mount as it has a similar FL – not the perfect situation but it will serve to debug the mount while the mirror progresses.

Not trying to convince anybody this is the "right" approach however I agree that we are adopting the same principle here which is: start witht the most critical part of the project.

Quite some time from the last post on this.

Hoever not being able to physically progress the telescope we used some time to update the cad model and run some FEA – this is actually the second run after modifying a number of ‘details’.

Modal analysis, deformation and strain energy were carried out.

Here is a check on the deformation of the telescope at 45deg -just to get the feeling at an arbitrary altitude. A mass of 2.5Kg is applied on the focuser axis 200mm outboard to simulate camera/filter-wheel/etc…

Specifically, I was interested in re-evaluating the deflection of the light beams compared to the focuser center-line.

To properly evaluate the light beam I’m evaluating where the primary is now pointing and where the secondary is aiming at.

The secondary concerned me as it’s an overhanging mass held in place by tenso-cross. And it wasn't extensivly analysed in the previous FEA run. Not really a big idea how would it react at 45deg. Would it twist? Twist and translate?? Twist, rotate and translate???

Here is the primary mirror:

Predictably, it rotates.   >>>> Disclaimer here is that I didn’t include any whiffle-tree or lateral constraint – more on that later on <<<<

So with a bit of math, the rotation is 0.017deg. Now where the primary is pointing now? At the distance of the secondary mirror, it means that the beam is now landing 0.55mm lower with negligible skew on (my) Z-axis.

How about the secondary? Looking at 3 axis it translates by 0.29mm on the altitude plane and again negligibly on the Z-axis.

Overall: the primary tilts by 0.017deg, the secondary translates with the UTA by 0.29mm and doesn’t (appreciably) sag relative to the UTA (that’s good and slightly surprising). So effectively half of the deviation of the beam due to the primary rotation is taken by the secondary that sags with the UTA. This leaves an off-center light path of ca. 0.26mm but (more importantly) paralell to the focuser axis.

To me it's OK  although -important remark - all of this is what I can do now waiting for the build to complete and correlate this analysis.

C'mon, just a lil sketch

I suppose you want to do astrophotography - if so the Dec has to be pretty accurate too. It won't carry as much load as the RA but it's crucial for corrections either guiding manually or with an autoguider. Try and fit the larger worm gear you can - thats the secret  ...my 2 cents. I saw you techinque for gears and it's quite inspirational.

Hi Tomasz, great project and I'm looking forward to following the progress.

Do you have a skectch of the entire mount to share? Are you intending to use the HD on the AR axis and input that with a smaller worm gear?

I suppose you want to do astro-photography with the mount? I'm sure you are aware that HD are virtually backlash-free but that comes at the cost of accuracy - almost inherently in their design. Having say that on the filed it might perform great

5 hours ago, Glasspusher said:

Found this chart on the web, not sure how reliable it is but you can see that your mirror is close to the red line indicating the difficulty in using a Bath interferometer for this project.

Hope you find this of interest.

John

Thanks John, very appreciated. This is line with the feedback from the Bath group - difficult but doable

https://groups.io/g/Interferometry/topic/bath_for_a_800mm_f_3_3/74609578?p=,,,20,0,0,0::recentpostdate%2Fsticky,,,20,2,0,74609578

We'll start with Ronchi and do the optimization wwith the BAth if we can make it work. I always had thought that Bath gives that topographic feedback that makes addressing errors effectively.

On 08/06/2020 at 14:49, Glasspusher said:

Regarding supporting the convex back during working, I ground the convex back to get a smooth regular figure I then cast a support against the convex back to ensure the support matched as well as possible. Despite this and regular rotation relative to the support  I still got astigmatism. I don't think a wooden support would be stable enough for a mirror of this size. The stability and uniformity of wood is  a concern. The slightest deviation from a regular support can result in astigmatism.  I haven't asked as yet, but what sort of surface accuracy are you hoping to achieve?

What I'd look fo ris a support that mimick a flat turning table of a M-o-M. In that case you have a decently flat surface (sometimes is made of wood) and a layer of "softer" material to compensate for small irregulatiries. With the difference and complication that here I'd have a curved back. I woulnd't rotate if I can make it match properly or alternatively I'd properly rough-ground the back - again, isn't this what it is done for the back of a  standard flat-black glass?

The accuracu targegt is a tricky question. Lambda/4 at least - anything les than that and it would be more of a light buck than anything

8 hours ago, Glasspusher said:

The fire brick mould worked well and survived several firings. The 500mm mirror is plate glass; with thin mirrors like this it is not so essential to use borosilicate glass as the glass cools quite quickly being relatively thin. The mirror was finished and tested but suffered from astigmatism probably due to poor support during grinding/polishing. I should probably go back and have another go at it. The reason that I did not continued with kiln work was that the cost of running the kiln was becoming a little excessive. It was apparent that it would not represent a significant saving, if any, over a monolithic blank.  The literature available on making slumped mirrors, as you are finding, is limited compared to that on monolithic mirrors which are more ‘tried and tested’ so I decided to stay with the  monolithic route.

John

John, I'd agree that the volume/surface ratio on big thin menisci provides an opportunity to stick to float glass - I just wonder about using borosilicate. It costs more - for a 800mm, 25mm it'si around £800. vs float that I suppose would be £250(?). Borosilicate should behave better during figuring as well making it a bit easier (??). Possibly the slumping temeprature is higher - 800degC vs 650degC (???).

You see - a lot of question marks....

On the support I had a lot of thought s for quite long time - my idea would be to provide the same support as if it would be on a flat-back. What I'm trying to say is that I'm thinking about a solid support that is shaped as the back of the meniscus -like carved-out wood- and a layer of compressible material - the same that you would put under the mirror of a M-o-M . Again I'd replicate the 120deg holder on the side.

Stacking some (several) boards like well seasoneds pallets would be quite inexpensive. The a curved jig/rail to guide a router to carve-out the 'pit'. I woun't be quick but I feel it's inevitable. IF the meniscus back face is irregular - whcih is usually the case- I suppose one can use the meniscus itself to mould a layer of any material (plaster/sawdust+glue/etc) to have a cozy rest. I reckon I'd need to sketch it out to make sense of it.

Oh btw, I've read that Mel Bartels uses towels as a support....

"I should probably go back and have another go at it"     ----->   Oh yeah!  

On 04/06/2020 at 14:17, Glasspusher said:

Yes that was Dave Thompson who was part of the group that built the 30 inch scope.

The problem in using a Bath interferometer is that the resulting interferogram will have crowded fringes due to the size and speed of the mirror making analysis difficult. You will need to obtain good clean high resolution igrams with such a fast mirror.

It has been a few years since I did kiln work. I made the mould from kiln fire bricks glued together with kiln cement as shown in the pictures. I sanded in the curve using a glass disk with the approximately the same curve as required.  A circular self adhesive sanding disk was stuck onto the glass which was about 50% the diameter of the mould. The process is just like grinding a mirror and was continued until the correct curve depth was reached as measured with a spherometer. I have attached some images, first one is of the finished mould. Next three are the finished 20inch diameter by 25mm thick meniscus mirror. The forth one is the mirror stress test, even after a second annealing run irregular stress still present. With kiln work it is essential to do all heating and cooling at the slowest rate particularly when annealing. The final image was my attempt at fusing a 20 inch mirror using 15mm thick plate glass. In the end the cost of running the kiln was getting silly. I reverted to the tried and tested monolithic blank. Hope some of this helps.

John

Fire bricks sound like an excellent idea - did it last all runs in the kiln? We used some dental plaster - worked fine but was expensive although I know there are suitable alternatives now.

So were you able to complete the mirror and test it? Did you use float glass or borosilicate?

The last experiment it is indeed what Fullum and Hubble Optics are doing - very interestinf - you weren't happy with the quality?

Again thanks for sharing experience and pics - it's really appreciated. Annd thanks for your patience in getting back to all my questions...

On 30/05/2020 at 18:58, Glasspusher said:

As a mirror maker with many years experience I wonder if I might share some thoughts on your ambitious project. When making a big mirror the first thing to consider is how do I test it? If you can’t test it don’t make it. Why go through all of that work in the hope that testing will be achievable?

When testing mirrors the difficulty increases with larger diameter and shorter focal ratio. So a 300mm F5 mirror can be figured in a few hours or less by an experienced optician. An 800mm F3.3 is an entirely different proposition.  All forms of testing require a degree of skill which is acquired over years of experience. What level of accuracy are you hoping to achieve? It is unlikely that you will end up with a high resolution planetary telescope; a low to medium power deep sky telescope is a more likely outcome. Thinking about testing you might consider:

The Foucault test. The mirror could be tested at the centre of curvature with a zonal mask but this test becomes less reliable with increasing diameter and speed of the mirror. Reading the shadows is a subjective business and requires years of experience to get it mastered.

http://www.jeffbaldwin.org/figure.htm

The Ronchi test. Again this could be used. The test is basically qualitative but Mel Bartels amongst others, has developed a semi-quantitative version. The observed Ronchi patterns are compared to computer generated images of an ideal Ronchi pattern. The mirror is worked until the observed Ronchi pattern matches the computer generated pattern. This is done for a number of positions both inside and outside of the centre of curvature of the mirror.

https://www.atm-workshop.com/ronchi-test.html

https://www.bbastrodesigns.com/ronchi.html

Star testing. The best method of testing a mirror is on the stars. You need a tube assembly to pop the mirror in between figuring spells. Mirror could be figured using the matching Ronchi test to get is as near as possible, final figuring being done using the star test.

https://www.bbastrodesigns.com/JoyOfMirrorMaking/StarTesting.html

Other ‘less common’ tests include:

Double Pass Autocollimation test (DPAC). An excellent null test used extensively by professional optical shops the world over.  You need a high quality optical flat, usually the same diameter as the test optics. A smaller flat might be used with care. Optical flats are very expensive items.

Waineo test. This uses a spherical mirror to perform the null; the mirror can be smaller than the test optic. Again additional optics are needed on this occasion a spherical mirror of good quality. The test goes by several alternative names.

https://www.bbastrodesigns.com/waineo.html

Interferometry. The Bath interferometer has become popular with amateur telescope makers in recent years. Unfortunately it is not suited to large fast mirrors.

http://rohr.aiax.de/Using a Bath - EN.pdf

 

Optical testing: https://www.telescope-optics.net/testing_optical_quality.htm

The Secondary mirror. A large flat secondary mirror is a very expensive item and not one which is easily made. Most telescope makers prefer to buy a secondary mirror which will be a very expensive item.

I have experimented with both fused and slumped mirrors over the years. You would need access to a large (and expensive) kiln for a project such as this. There is a steep learning curve which can be very expensive due to the cost of electricity and breakage of glass. Annealing takes much longer than anticipated which means running the kiln for a numbers of days. Expensive! I think a monolithic blank would be better, as to the thickness, the thicker the better unless you have had experience in controlling astigmatism in thin mirrors. The problem with astigmatism is that in its more subtle forms it is easily missed until you come to examine a star image with the telescope.

Sorry that much of this sounds a little negative but undertaking a project like this is no trivial task, many have tried, a few have succeeded.

Good Luck.

John

Hi John, thanks for taking the time to write - by no means I take this as negative. It's rather a realistic approach.

As we speak I'm going through the resources you shared - same new, some already known.

To start with I agree on the testing - I'm not the 'optician' of the group but I pushed some glass in the past and at least I appreciate where your concern is coming from. 

May I ask you why you reckon Bath is not suitable for a big+fast mirror? I recall some discussions in the Bath Interpehromertry group but I not sure it was conclusive.

About slumping: so far our group has slumped up to 400mm, thin mirrors - it was quite some time ago and it was dotted with quite some shattered glass....
We are investigating where to go 'nearby' to get an oven big enough - and available for such things. I'd really like to avoid building one from scratch though.

I agree on menisci (aww, that looks and sounds terrible!) haven't proven yet to be a fully viable way - it's nonetheless pretty attractive because for instance in our case the alternative is a more standard but hefty 4k blank.

Btw, ...Thompson....isn't he the guy that built some 800mm in Liverpool years ago? That group had a radical approach to using the Bath as far as I remember.

Lastly, thanking you again for pitching in, have you wrote anything about that slumped 20"/25mm? That would be a golden experience.

Regards,
Michele

5 hours ago, davidc135 said:

I'd contact Stathis Firstlight in Germany regarding Borofloat glass. His web page says he can supply  discs of low expansion glass in thicknesses from 5mm to 57mm with order times up to 2 months for the big stuff. Up to 20 ins in stock but larger blanks need to be ordered.  David

PS I should add that Stathis seems a person very much after your own heart and I should think will be very keen to help your project.

I had a quotation from Stathis some couple of months ago - it seems that he isn't able to provide an 800mm borofloat disk more than 25mm thick, 

Yeah, Stathis is top bloke  

On 27/05/2020 at 07:59, Chriske said:

A M-o-M (Mirror-o-matic) is a very good concept. The mirror rotates and the 75% diameter tool swings back and forth. There's lots of information on the net about the technique 'how-to'. I have two of these machines that can easily handle 20" mirrors. I'm busy right now grinding a 20" f/4. Carbo is fed by a peristaltic pump. To avoid contamination every grid has it's own separate system. These machine's have a autonomy of about 1.5 hours. When the containers, filled with a mix of carbo/water(stir constantly), are nearly empty the machine halts automatically.  The courser grit is fed by hand. There was no way to feed carbo 80 through that peristaltic pump.

In fact one of these machines is Marc's. Side by side we use to grind our own mirrors in my workshop. We had lots of fun building them and grinding mirrors with it. Almost every year we tried to make a new scope, every time a different scope. Target and goal was open door at our local observatory. Very often a weird scope that has been built only once by an amateur. This time we were busy building a Stevick-Paul. A few month's back Marc past away at age 57, and I never touched our machines again from that day on.
Sorry to go of topic...😧

At about the same time we made our MoM's there was that guy Gordon Waite building his mirror mirror grinding machine. He uses a fixed post grinding machine. iow the tool is stationary.
He has a set of movies on YouTube. There's even a set of movies on very fast mirrors.
If we would do it all over, we probably would build a 'Waite version'. Simple to build. But these MoM's can easily grind in 'fixed post' modus too.
 

Thanks for openly sharing! You separate frit pump system seems very clever.

I'm very familiar with Gordon's videos - among my favorites! I like the concept of fixed post as it simplifies the constrcution and/or make it more robust. However I do not get what is the disadvantage of that

13 hours ago, Peter Drew said:

I have no experience with making large thin mirrors but I do have a 750mm F4.1 professionally made mirror which I have built into a traditional style truss tube Dobsonian.  The mirror is plate glass, flat backed, 35mm thick at the edge and with a 75mm perforation.  The mirror cell is a composite of 6mm aluminium discs separated by a radial arrangement of square tubing, the edges are closed off with a peripheral band of aluminium.  The mirror is supported by a 75mm central spigot with a retaining disc to stop the mirror riding forwards.  There is no conventional mirror support system, interspaced between the mirror and the backplate is a disc of heavy duty bubble pack type material providing 750 points of support.  The inventor of "PLOP" would have a fit, but it works.  Initially there was clearance between the central spigot and the perforation in the mirror, this was to allow an edge supporting strap to take the weight of the mirror.  This actually introduced astigmatism at low angles due to the mirror "potato chipping", it was cured by wrapping tape round the spigot until it was a good fit in the mirror perforation and then dispensing with the strap.

Seeing conditions and mirror cooling are the main influences on its performance.     🙂 

Peter, thanks for sharing the details of your optics - it sounds like a monster of a telescope! Are you able to do some tracking or is it just visual?

35mm thickness is really the minium I would dare to go - ending up with 20ish mm in the center.

4 hours ago, Astrobits said:

I'd scrub option 2. If you are going to slump it then low expansion will be a safer bet than float. Low expansion can be cooled a bit more quickly than float. With the development of toughened and laminated glass the thickest float glass now commonly produced is 19mm although 25mm is listed by some Chinese suppliers. Really thick stuff can still be had when aquaria replace their very big glass tanks with acrylic and occasionally when old ships are broken up.  Unfortunately most of these sources have already gone, so these are not very easy to come by but there might be some available from someone who just grabbed some when the going was good.

You will need to experiment with the support during grinding/ polishing as I am sure that you will get some astigmatism at first. I tried supporting a 40mm thick 500mm dia mirror on bubble wrap and got triangular astigmatism because I didn't rotate the mirror adequately ( I was in a hurry and bubble wrap has a triangular pattern of bubbles).

As for testing I think that you will need to make a tester specifically for this mirror. There is plenty of literature on test equipment.

Nigel

 

I agree - if we have to slump  then Borosilicate would be better and at a marginally higher cost. Btw I've just enquired today Galvoptics in Basildon - they have borosilicate 25mm...they need to check the inventory for thicker material..

On testing I have all the bits to pull together a Bath - however I reckon our mirror maker memeber is going to prefer his Ronchi test.

14 hours ago, Chriske said:

That's a challenge indeed 800mm f/3.3...😳

The largest I've made so far is 20" f5.6.( by hand). Two identical disks to make a bino. Almost polished out. On hold because I've promised...   right now there's again a 20" f/4 on it's way(for a friend), but this time it will be made using a M-o-M. Most of the time I use these M-o-M's fixed post.
All these mirrors will be supported 27-point (PLOP)

My problem is : there are only 24 hours in one day...😬

We like challenges, don't we?

The mirror making machine is something I'm curious about - how does yours look like? Can you recall a thread were various kind of machines are discussed? That topic is something I just know I need to dive in sooner rather than later.. 

Hi everybody - I'm humbled that some of you are following the thread on the 800mm telescope. I thought it was worth starting a sub-topic specifically related to the mirror making.

So as the works on the mount got halted by the lockdown we had some time to virtually meet-up and discuss the optics for this project.

The onset was pretty straightforward with an 'if we are doing this we are making the mirror'. And that sorts out the make vs buy, I suppose.

As of now, the only two things decided so far are the diameter -at 800mm- and the f/3.3 - of course we can accommodate some variance.

Some aspects of the making are pretty unchartered territories for our club so I'd like to seek some good advice from anybody in terms of direct experience or rather point us at some resources/threads.

Back-ground: to make it short, our senior member had three 500mm f/5 done years ago. He did of course faster optics up to f/3 among many other mirrors - I actually never asked him how many, uh! He has always worked with full tool and has no experience with slumped glass. Btw we do have some experience in slumping glass but -weirdly enough- not in machining.

During our initial discussions we boiled the scenarios down to 3:

• float glass, 35/40mm thickness considering 15mm of sagitta. This is a thermal challenge with its big outward mass, it's going to make the machining more of a challenge and the stabilization time longer
• float glass, 25mm (which seems more of a commercially available), slumped. Trading the thermal challenge with the slumping
• borosilicate, 25mm, slumped. Tbh this is just a better version of the previous point at a cost that is not prohibitive

What are the sources of glass in Europe for thickness over the bog standard 25mm? 

Disclaimer: this is surely an ambitious endeavor and by no means we are underestimating that. Not only the bigger diameter is a step-up; the fast optics is a challenge too. 

To start with we have some questions about slumped glass. It looks like an attractive, modern approach to mirrors that exceed a given diameter. I think I saw already some threads specifically about the slumping itself - that's golden.

However it's the grinding/finishing/parabolizing that is puzzling us. Are there specific techniques or is it the same as the flat back glass?

Also, how do you support the mirror? Would a support that replicates the telescope mirror cage be appropriate - a 27-point in our case? Or is it a matter to build a concave support that holds the back of the mirror? How accurate/solid should that support be?

Any experience out there??

I reckon that's enough as a start - thanks everybody in advance for your contribution.

Stay safe! Michele

Hi Geoff thanks for checking out.- as a matter of fact the astro-club and the assembly is taking place in Cremona which was the epicenter in Italy.

I'm actually based in the UK whereas all the guys supporting the project are doing well although the activity is halted as the lockdown is going to be partially lifted from today.

To make the most out of this situation we are having call confs to decide the strategy over the making of the mirror but we can't really progress on the assembly.

Cheers and thanks for following,,

Michele

These videos seem to support Gina's view on different spec TECs disguised under the same dimensions.

https://www.youtube.com/watch?v=ix_hqW5kfS0

https://www.youtube.com/watch?v=iVarAi4qFpM

However the 5degC delta T looks too poor even for a fake TEC. I'd encourage to try to power the peltier directly form the 12v source as well as the source itself can provide at least 6A which is quite some current.

17 hours ago, R26 oldtimer said:

Hi Michele.

You've mentioned that you intend to make this an imaging scope rather than visual. If that's the case, then wouldn't be easier to skip the secondary mirror and place the camera directly to the upper cage assembly?

This would give you far less weight, up there, so the cage could be designed to be substantially lighter, and a round body camera will give you a smaller secondary obstruction. My guess is that it would also be easier to colimate and the only downside would be that probably you could not use a filter wheel.

Kalimera! We considered this in the early stage of the project and that's one of the reason why the spider has a big cilindrical section to let the cone of light go thru it. 

However we decided to drop that - an additional add-on section and the cabling can be sorted out - no big deal.

The showstopper is the focusing and de-rotator mechanism - they are part of the project in the standard configuration but in that position they'll add significant complexity.

As a final note -  we would really want to experience the magic of 'guessing' the colour of some DSOs - feature that only this kind of diamater can untap.

New CAD model update, now version #13 with few improvements - mainly:

- model aligned with actual built parts
- new lower part sub-assembly
 

Next step is another run of FEA for modal analysis and displacement.

After that we are going to modify the structure from the original f/3.75 to the currently assumed f/3.3. Consequently the focal will be reduced from 3000mm to 2640mm.

On 10/02/2020 at 16:28, Rusted said:

Thank you both for your posts.

Michele,

The milling machine example of hobbing shows a typical problem of clearance along the tangent of the wormwheel.
The chuck is almost rubbing on the largest wormwheel.  Some sort of rigid and accurate extension is necessary.
It must help to clear the chuck as the wormwheel diameters grow larger while still providing the drive to the tap.
If a lathe is being used then the tailstock is usually much narrower and an MT center can provide extra clearance.
A typical 3-jaw chuck would be even larger and force the need for an even longer extension.

I always fancied giving it a go - I might actually need to do that for the derotator wheel.

Ideally I think that lathe+tailstock -as you confirm- it's the must sturting point. I reckon that using the gear already on its on bearing i.e. in its final confirguration, will improve the result in terms of run-out. On top of that, running on the same set-up I'd do the lapp[ing after swapping the tap with the threaded shaft