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  #41  
Old 07-05-2018, 07:16 PM
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Originally Posted by xelasnave View Post
I have a model in cardboard to demonstrate the idea and I will use it to talk to folk about this and that so let me nail it down a little and I will present you with rather definite.
Needless to say it will be interesting if one could build it in carbon fiber...not because I am taken with carbon fiber but I have experience with making stuff in fiber glass and it seems carbon fiber just replaces the glass and you have a similar process with a suitable resin and hardner.
But I am thinking you could make it up in balsa and over lay that with carbon fiber...sooner or later depending on how many layers it sould be as sttring as steel.
You dont need to weld and it is almost a kitchen table project.
A wild idea but you never know.
Alex
Any chance you could post the design? It sounds like it would be great for visual - could it be motorised for AP?
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  #42  
Old 08-05-2018, 08:57 AM
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Wow, rough grinding will take quite some time!
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  #43  
Old 08-05-2018, 09:31 AM
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Any chance you could post the design? It sounds like it would be great for visual - could it be motorised for AP?
The idea is that it is to be the best AP mount out there.
Overkill on bearings 4 on eack of RA and Dec☺ and in effect huge gear diameter, although gearing is via motor driven rubber wheels on a disk 4 wheels and motors for each of RA and Dec..This part needs looking into ...computer controll of stepper motors etc
. But it should be equivalent to having a 600 mm dia gear on each of RA and Dec.

I am considering if gyroscopes could be used and if they would be a benefit.

The first, now lost design, draw by a professional was entered by him in some comp. and apparently won second prize judged by some high up astronomy folk in the UK.
I have no firm plans the avaialabily of material guide my builds. Size will change depending on what bearings are available but it was to use car or truck bearings because they were an over the counter purchase. I figure four should overcome any lack of precision....but size of bearing changes things all down the build...
I will post something when I can.

Alex
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  #44  
Old 08-05-2018, 10:03 AM
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Originally Posted by xelasnave View Post
The idea is that it is to be the best AP mount out there.
Overkill on bearings 4 on eack of RA and Dec☺ and in effect huge gear diameter, although gearing is via motor driven rubber wheels on a disk 4 wheels and motors for each of RA and Dec..This part needs looking into ...computer controll of stepper motors etc
. But it should be equivalent to having a 600 mm dia gear on each of RA and Dec.

I am considering if gyroscopes could be used and if they would be a benefit.

The first, now lost design, draw by a professional was entered by him in some comp. and apparently won second prize judged by some high up astronomy folk in the UK.
I have no firm plans the avaialabily of material guide my builds. Size will change depending on what bearings are available but it was to use car or truck bearings because they were an over the counter purchase. I figure four should overcome any lack of precision....but size of bearing changes things all down the build...
I will post something when I can.

Alex
Awesome Alex, am really looking forward to seeing your design! Woud definitelty be interesting to see it in action!
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  #45  
Old 08-05-2018, 02:58 PM
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This is one photo will require 1000 in explaination.
See if you can figure out the concept..
Alex
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  #46  
Old 08-05-2018, 04:36 PM
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Originally Posted by Lognic04 View Post
Unsure whether to do normal stroke or chordal?
Hi Logan

Use a centre over centre stroke to mate the tool and mirror. Use a 1/3 centre to get to and maintain a sphere during grinding and polishing. Chordal strokes are for hogging out the centre rapidly. They are normally used only in the early stages of rough grinding.

Can you post a picture of your ceramic tile tool? It might help us give you clearer suggestions on how to adapt it.

I noticed your exchange with Alex re a short focus telescope. I would suggest you avoid very short focal lengths on your first mirror. There are some amateurs getting amazing results with ultra fast scopes - Mel Bartels is the most obvious. However parabolising a short focus mirror is very challenging. If you were successful, you would then need to pay for a high quality coma corrector (paracorr 2), an excellent focuser and high quality (ethos or similar) wide field eyepieces to take advantage of the short focal length. I suspect these additions could easily chew up a couple of thousand dollars.

Your original design with an F5 to F6 10 inch mirror is challenging in itself but doable. You can then get away with no coma corrector, modest eyepieces and something like a GSO focuser.

Hope that makes sense, I'm really keen to follow your progress and hope to catch up one day at the ASV's Instrument Making meeting.

Regards

Rod.
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  #47  
Old 08-05-2018, 07:54 PM
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Originally Posted by xelasnave View Post
This is one photo will require 1000 in explaination.
See if you can figure out the concept..
Alex
Thanks Alex, i think i get it, the ra/dec rotates and has a small bearing that rotates on the big fan shaped object?
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  #48  
Old 08-05-2018, 07:56 PM
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Originally Posted by Rod View Post
Hi Logan

Use a centre over centre stroke to mate the tool and mirror. Use a 1/3 centre to get to and maintain a sphere during grinding and polishing. Chordal strokes are for hogging out the centre rapidly. They are normally used only in the early stages of rough grinding.

Can you post a picture of your ceramic tile tool? It might help us give you clearer suggestions on how to adapt it.

I noticed your exchange with Alex re a short focus telescope. I would suggest you avoid very short focal lengths on your first mirror. There are some amateurs getting amazing results with ultra fast scopes - Mel Bartels is the most obvious. However parabolising a short focus mirror is very challenging. If you were successful, you would then need to pay for a high quality coma corrector (paracorr 2), an excellent focuser and high quality (ethos or similar) wide field eyepieces to take advantage of the short focal length. I suspect these additions could easily chew up a couple of thousand dollars.

Your original design with an F5 to F6 10 inch mirror is challenging in itself but doable. You can then get away with no coma corrector, modest eyepieces and something like a GSO focuser.

Hope that makes sense, I'm really keen to follow your progress and hope to catch up one day at the ASV's Instrument Making meeting.

Regards

Rod.
Hi Rod, thanks for the write up. So i will do CoC right now, and that should give me an f6 curve by the time they are mated?
The fast mirror was just in theory
I was at the last IMS actually!
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  #49  
Old 09-05-2018, 02:14 AM
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Originally Posted by Lognic04 View Post
Hi Rod, thanks for the write up. So i will do CoC right now, and that should give me an f6 curve by the time they are mated?
The fast mirror was just in theory
I was at the last IMS actually!
Make sure you are close to your target focal length with the ring tool and that the curve extends to the edge of the mirror. Then move to the ceramic tiles using centre over centre strokes.

Rod
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  #50  
Old 09-05-2018, 06:52 AM
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Making a 10” F/8.66 should be easier than making an F/5 as the tolerances aren’t as strict.
Adding a Faststar/Hyperstar would be an interesting experiment but the primary mirror in the Celestron is ~F/2.2 so it would only work if you made a F/2.2 mirror.
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  #51  
Old 09-05-2018, 09:14 AM
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Hi Logan
I am glad others point out the fact that shorter the focal length the greater the challenge.
To understand the phot of the mount let me say that the "fan" bits can be thought of as a section of a large gear. The mount is designed to operate to observe 45 degrees above the horizon and with no need for a meridian flip.

The RA is supported by bearings either side of the Dec shaft running thru the RA shaft which is actually a tube with two bearings at either end.
A German Equatorial mount in effect only has bearings at one end of the RA shaft whereas this design is in effect an English Equatorial approach where RA shaft is supported either end and Dec run off in the middle.

Keep up your good work.

It certainly is best that you complete this project sucessfully before you start another and your experience gained may give you confidence to build the ultimate astrograph.

Alex

Last edited by xelasnave; 09-05-2018 at 09:27 AM.
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  #52  
Old 09-05-2018, 10:18 AM
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Originally Posted by Rod View Post
Make sure you are close to your target focal length with the ring tool and that the curve extends to the edge of the mirror. Then move to the ceramic tiles using centre over centre strokes.

Rod
Hmm, i hogged it to 1.5mm with the ring tool, and i gave up on it because the tile tool seems to be MUCH faster. any ideas as to why?
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  #53  
Old 09-05-2018, 04:43 PM
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Hmm, i hogged it to 1.5mm with the ring tool, and i gave up on it because the tile tool seems to be MUCH faster. any ideas as to why?
Hi Logan

I suspect it's the stroke you are using with the ring but really if the tile tool is working for you keep going. I was mainly concerned that you not wear through the tiles. If it's a second hand tool, like the one I passed on, then they might be quite thin. How thick are the ceramic tiles?

Rod
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  #54  
Old 09-05-2018, 07:04 PM
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Hi Logan

I suspect it's the stroke you are using with the ring but really if the tile tool is working for you keep going. I was mainly concerned that you not wear through the tiles. If it's a second hand tool, like the one I passed on, then they might be quite thin. How thick are the ceramic tiles?

Rod
About 4mm now. (did you pass on a 10 inch f6 tool? if so, i am using it now! )
I am at 2.1mm sag right now btw. Is there a way to check the FL other than physically and without the sun?
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  #55  
Old 10-05-2018, 02:22 AM
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About 4mm now. (did you pass on a 10 inch f6 tool? if so, i am using it now! )
I am at 2.1mm sag right now btw. Is there a way to check the FL other than physically and without the sun?
Yes that sounds like it could be mine. I used it 20 years ago to make a 10 inch F6. I would need to see a photo to be sure.

You can test the focal length approximately with a torch. Place the mirror on a stand. Hold a torch parallel with your eyes. Wet the mirror. Locate the torch reflection in the mirror and bob your head up and down. When the torch reflection moves in the same direction in the mirror you are inside the radius of curvature. When the reflection moves in the opposite direction, you are outside the radius of curvature. The point at which the torch reflection does not move is the radius of curvature. Halve that and you get the focal length.

Some people do a couple of wets with 120 grit and then 240. This helps give a clearer reflection and the water seems to stay on the surface longer.

Hope that helps.

Rod
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  #56  
Old 10-05-2018, 06:08 AM
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Hi Logan,

...Don't let it dry out. If it gets stuck, really hard to release.

Joe
It can get stuck even when wet..
It that case try to immerse the whole thing in hot water, than cold, then hot again.. it will sepatare suddely, so don't drop it.
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  #57  
Old 10-05-2018, 06:49 AM
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Mirror and tool, plus 10 cent coin (2mm) fitting under the steel ruler!
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  #58  
Old 10-05-2018, 03:49 PM
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Hi Logan

Yes that's my old tool.

Assuming a sag of 2.1mm and a diameter of 250, your focal ratio is currently 7.4 according to the stellafane calculator:

https://stellafane.org/tm/atm/mirror...g2fl-calc.html

So there is a little more hogging to do, but I am impressed by how quickly you are getting to your target!

I am a little nervous that you may grind through those thin tiles at some point. I would feel better if we glued some new tiles over the top. At 80 grit new tiles will conform to the curve you are grinding very quickly. I could bring some epoxy (jb weld) and tiles to the meeting next Saturday if you would like to do that.

Happy grinding!

Rod
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  #59  
Old 10-05-2018, 05:49 PM
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Hi Logan

Yes that's my old tool.

Assuming a sag of 2.1mm and a diameter of 250, your focal ratio is currently 7.4 according to the stellafane calculator:

https://stellafane.org/tm/atm/mirror...g2fl-calc.html

So there is a little more hogging to do, but I am impressed by how quickly you are getting to your target!

I am a little nervous that you may grind through those thin tiles at some point. I would feel better if we glued some new tiles over the top. At 80 grit new tiles will conform to the curve you are grinding very quickly. I could bring some epoxy (jb weld) and tiles to the meeting next Saturday if you would like to do that.

Happy grinding!

Rod
Nice to know who used this tool before! I am at about 2.3mm now. I dont hink the tiles will actually get that thin, its at about 3mm thick still.
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  #60  
Old 12-05-2018, 12:31 PM
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Hi Logan
I am glad others point out the fact that shorter the focal length the greater the challenge.
Alex
I've come in late here but I see nobody has mentioned the issue that F/2.2 would need a 45-50% secondary obstruction.

Dave
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