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  #21  
Old 03-02-2024, 10:13 AM
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OzEclipse (Joe Cali)
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Originally Posted by Stefan Buda View Post
Thanks Joshua, my first impression is good. Of course, there is more friction than with ball bearings but not too bad, although this extra friction is expected to add a bit of hysteresis to the existing backlash in the gearing of the EAF.
Always focusing inward should take care of that.
Also remains to be seen how it will be affected by the large temperature swings on the long run - it gets very hot under the tarp in summer.
Very interesting project Stefan.

Don't worry about the sort of temperature effects you'll get on the teflon.

Teflon has very good thermal properties. Melting point 327 C.

In chemical labs, we use beakers and screw top bombs made of teflon to heat liquids that will dissolve pyrex glass and when performing leaches or digestions of some materials. Under no mechanical stress, the teflon beakers and bombs can be used directly upon a lab hotplate.

Teflon is also used as a seal in vacuum systems. It only deforms / extrudes a little when under stress from the vacuum, the seal tension and heated to 140C. It also doesn't fatigue when subjected to large shifts in temperature. I have used it in a cold trap I built for one of our instruments that cycled from liquid nitrogen -196C to heating at +120C. It did this 300C cycle 40 times a day for 20 years. When I decommissioned the instrument, the teflon piece of the trap looked like new.

Under your conditions, I think the teflon will serve you well for the life of the instrument.

Joe
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  #22  
Old 03-02-2024, 12:08 PM
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Thanks Joe,

I know that Teflon is a wonderful material as I've been using it for various things for many years, but it is also a weird material as it has a couple of phase changes where its crystal structure changes reversibly. One of those phase changes happens at about 20 degrees Celsius and I don't have information on what amount of volume change is involved with that.
The focuser is in the oven right now. I'll report back.
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  #23  
Old 03-02-2024, 06:25 PM
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OK, here's the results of the heating test:

I had the focuser in a small oven with a thermometer and after keeping it at slightly above 80 degrees for a couple of hours, I let it cool to room temperature. I found that there was a definite change after the heating. It didn't quite develop a slop, but it wasn't far from. This was at 25 degrees though, above the 20 degrees phase change for Teflon, so I put it in the fridge and cooled it to 12 degrees. This time there was a definite slop. Not much but enough to require a fix.
I think that reducing the pads thickness to less than one millimeter should solve the problem.
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  #24  
Old 06-02-2024, 08:10 PM
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I made the new Teflon pads and modified the SS inserts to match.
After reassembly I repeated the thermal test and found no noticeable change this time.
By the way, each SS insert started life as a standard M8 socket head screw.
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  #25  
Old 06-02-2024, 08:13 PM
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Great way to make your own grub screws
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  #26  
Old 07-02-2024, 09:27 PM
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I also made a new cover for the secondary. The old one wasn't 3D printed and used elastic bands to hold it in place. The elastic needed replacing from time to time as it would deteriorate from the heat under the tarp.

The blue parts are 3D printed with TPU filament and they hook onto the spider vanes. Remains to be seen how well they'll cope with the heat. If they fail, I will replace them with metal strips.
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  #27  
Old 17-02-2024, 10:41 PM
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The finder got a new mounting arrangement. The old one was quite corroded despite being anodized.
Back in the days when I made this OTA, cutting edge planetary imaging was done around a focal length of about 10m because the webcams used back then had larger pixels than today's planetary cameras. That made it very difficult to move from one planet to another as the field of view on the computer monitor was less than 2 arc minutes.
To help with that, I wanted a finder with enough magnification to make this easier but to also be compact. In the end I made my own achromatic doublet of 48mm clear aperture and 250mm focal length. That, combined with a 10mm eyepiece equipped with a custom graticule, worked quite well.
Only one more problem to be solved. On nights with hi humidity, I was getting so much dew on the objective lens that I had to use tissues to wipe it to be able to see anything through it. So a heater needs to be made.
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  #28  
Old 22-02-2024, 12:50 PM
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I had to make a cap for what was until now the AZ-axis. This part allows me to adjust the clamping force on the worm wheel.
Although this was included in the original design, I never made it because the weight of the telescope and the fork provided enough force to engage the worm wheel sufficiently to work well and still allow me to push the scope around in "Dobsonian" mode.
This axis now becomes the RA axis and, without the new tensioning part, the clamping force would be only 61% of what it was before.
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  #29  
Old 26-02-2024, 09:44 AM
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Made an equatorial wedge to suit my location. Nonadjustable for better rigidity. I will be able to fine tune polar alignment using the adjustable pier legs.
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  #30  
Old 26-02-2024, 09:57 AM
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Quite sturdy Stefan. What kind of adjusters are on your pier legs?
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  #31  
Old 26-02-2024, 10:26 AM
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Originally Posted by Joshua Bunn View Post
Quite sturdy Stefan. What kind of adjusters are on your pier legs?
If you have a look at the very first image I posted in this thread, you will see three large screws that support the whole structure. The M20 screws have locking nuts.
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  #32  
Old 26-02-2024, 12:08 PM
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Picture for Joshua. No welding involved.
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  #33  
Old 28-02-2024, 09:13 AM
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Fork installed.
I will have to hang a big counterweight from the base of the RA axis, to bring the COG closer to the pier. I provided a hole in the base plate of the wedge for that purpose.
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  #34  
Old 28-02-2024, 01:56 PM
Saturnine (Jeff)
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Hi Stefan
I have been following this thread with some interest, just love atm articles. Just one observation about the screw adjusters for the legs. Would it be better to invert the set screws so that the head is on the ground, not the threaded shank, gives a slightly larger weight bearing surface as well and the lock nut can go either above or below and still be easy to adjust.
Apart from that, great work on the rebuild.
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  #35  
Old 28-02-2024, 03:48 PM
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Hi Jeff,

Thanks for the observations.

I thought that the screws would be easier to adjust from the top as there is no danger of grazing my knuckles on the concrete if I slip up.
The M16 screws, not M20 as I wrote before, can support several tonnes weight and I think this must be the lightest 16" telescope in the world.
I even considered rounding the ends of the screws to give them a bit more of a bite on the concrete, but we don't get too many earth tremors here.
Another reason for having the screws as they are, is to make it harder for the rainwater to get into the threads, which are loaded with grease by the way.
And lastly, I think my toes suffered less damage over the years by not having the screw threads poking out on the top. I'm sure I would've had to implement some toe protectors.

Any comments or suggestions are welcome.
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  #36  
Old 29-02-2024, 09:04 AM
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I extracted the primary assembly from the mirror box.
Lots of sanding and painting to be done.
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  #37  
Old 29-02-2024, 12:18 PM
sfarndell (Scott)
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I continue to watch with interest and fascination. Your metalworking skills are outstanding!
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  #38  
Old 01-03-2024, 09:28 AM
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Thanks Scott. Here are some more metal work pics for you.

I will try to not use the side support levers any more. All the planetary imaging I do is above 45 degrees altitude, so I might be able to get away with just the central support and the 9 point flotation approach.

I'll retain the heat pump which worked very well.
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  #39  
Old 01-03-2024, 09:57 AM
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Quite an interesting mirror support there, Stefan.
Were the lateral supports on a cam adjustment to get the right spacing to the mirror edge?
I'm trying to understand the layout of the last 3 pictures, in the middle photo, are the heat pumps not installed between the plates, just doest look like there is room...
Also, are the 3 sets of 3 supports, independent from the other supports, and do the collimation adjusters act on those supports or the mirror support plate as a whole?
A thought on the m16 leg adjusters, I think a point would be better than flay, on the ground. A rotating flat bolt has a tendency to wonder when turning it, which may alter PA in an undesirable way.
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  #40  
Old 01-03-2024, 09:17 PM
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As my friend Diego says, there are two ways of making things: adjustable or right. I prefer the later, so the side supports were not made adjustable. The lead weights are able to swing several millimeters in any direction and that is equivalent to less than half a millimeter movement at the top - the fulcrum of the levers being close to the top. The mechanical advantage is about 8, so each 150g of lead weight can apply a force of up to 1.2kg on the mirror. Because the top part of each lever was bonded to the mirror, the weights were not able to swing but they all worked regardless of orientation or location around the mirror.
Regarding the heat pump, there are details that are not visible on the photos and that makes it a bit hard to understand. The big black disc is the cold plate of the pump and the three big radiators are the hot side where the heat is released into the air stream before getting vented out the back.
Between the cold plate and the radiators, you can see the main structural element that I call the wobble plate. It is made from two laser cut SS discs, with many aluminium spacers in between and a SS hub. On the top of the wobble plate, but hidden by the cold plate, we have the 9-point flotation support. The flotation is made so the there is a 1mm gap between the mirror and the cold plate and it is not adjustable.
The mirror box has a large sturdy SS hub with a spherical rim a the top end, which fits into a cylindrical section of the wobble plate hub.
Each of the three SS pins, at the bottom side of the wobble plate, go through the back of the mirror box and terminate in lockable differential screw mechanisms. The differential screw adjusters can tilt the wobble plate a couple of degrees in any direction, as well as move it along the optical axis for adjusting the mirror spacing.
Regarding the pier leg adjusters, you are right about rounded being better than flat, but this telescope is too big to be moved anywhere, so unlikely that it will need frequent tweaking.
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