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  #381  
Old 06-08-2020, 11:33 AM
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Sunfish (Ray)
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Thanks for that.
I was directed to this site by the ATM who gave me the blanks and this blog describes working with a TDE .

http://atm-workshop.com/mirror14-log.html

Perhaps it is possible to be over corrected in the centre as per the stellafane table yet still have TDE. I did start off through the whole polish with short strokes But not much W . So I can try adding more W pattern stroke with the trimmed up lap.

I have an 8 inch to go on to which is down to f7 which I would like to deepen to f4 with a tile tool and 80 grit.

Quote:
Originally Posted by Spacecat View Post
Sunfish - the best strokes to get rid of TDE are very short 1/4 to 1/5 length W stroke with mirror face up , with some randomness thrown in to maintain or head better toward spherical .


Something like a 200mm mirror is much better fit for the hands and more stable , so you are setting yourself a task with a 100mm . The back should be ground flat or slightly concave. If you can see the mirror rock at all while you are polishing tool on top , this can contribute to a TDE .

Last edited by Sunfish; 06-08-2020 at 12:02 PM. Reason: Spelling
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  #382  
Old 27-12-2020, 09:34 AM
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How do you cut thick glass blanks?

Hello,

I am looking at trying to take my blanks from a large sheet of soda glass and I am after help from those who have done the same.

So before I re-invent an old wheel any pointers or tips?

Here's what I thought I could do:

Below is a youtube video showing a great sales pitch for cutting thick glass, while the video shows the product being fool proof, the price is not within this fools reach. The kit listed gives a 404 error but I've sourced the hammer at about $500 USD.

I thought I could modify a "Automatic Centre Punch Adjustable Spring Loaded Metal Drill Tool" (ebay) which allows for interchangeable springs for an adjustable striking force. Then all I need is a similar head component with a washer and blunted striker head. The hole punch can be had for under $10.

But I am sure better minds than mine have been here before. So links, tips, tricks and traps and any other advice would be great.

I don't have access to a slow speed drill press for a DYI glass cutter.


Thanks
Steve
Youtube link:
https://www.youtube.com/watch?v=x-84Sdq7jaE
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  #383  
Old 27-12-2020, 01:37 PM
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What's the size of the sheet? and thickness?

Stephen.
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  #384  
Old 27-12-2020, 03:16 PM
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Size of Sheet

Approx. 1300x690x25mm Slab of soda glass - ie green at the edge from an previous ATM so I assume non-toughened etc.
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  #385  
Old 27-12-2020, 03:39 PM
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Rainmaker (Matt)
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Prestige Metal Works in Fyshwick can cut it into discs for you using their waterjet.....
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  #386  
Old 27-12-2020, 03:47 PM
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Quote from ACT Steel works

ACT Steel Works quoted $770 for 3 420mm, with 4 220mm and one 340mm cut from another sheet. Prestige Metal Works would not give a quote till they had cut a blank to work out the time and costs.

The waterjet saving is not having the disc plus 10% waste as the video showed, which is very nice. I have added extra as I am pretty sure I will have TDE and I'm just looking to bank/baffle the mirror down to 16", 12" and 8" etc.

PS. Matt(Rainmaker) I sent a pm regarding the estate purchase
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  #387  
Old 28-12-2020, 11:03 AM
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Had 4 sheets float (2.4m x 1.2m) waterjet cut back in 2014 by Aquajet NZ (Napier) sheets thicknesses from 34.1mm to 37.1mm. Cost was NZ$4000 to cut 39 blanks ranging from 12.6"-33" diameter.
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  #388  
Old 28-12-2020, 02:41 PM
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nice glass

Do you know/remember if there was a limit on how close the discs can be together or from the edge of the sheet?



Steve
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  #389  
Old 28-12-2020, 05:05 PM
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Checking original cutting plans I see I allowed 1" from edge. Probably could have reduced that to less though. Some of the sheets had some edge damage so allowed a margin for that.
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  #390  
Old 05-02-2021, 08:55 AM
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starting the grind (hopefully)

Hello,

Well I tried Supaglass Industries out in Fyshwick and they were the cheapest quotes I could find in the Canberra region. $25 for the 8" cuts and $50 for a 16" cut on the quotes, best in Canberra by a long way.

So, I drove out there in a flash last Friday as I have been reading "How to make a Telescope" by Jean Texereau for way too long without anything to work with, had my grits since Christmas after an awesome estate buy.

Just got a call today, the 203mm is cut and the 220mm is on the machine deck. Cut those from my smaller bit of glass to test the services, the 220mm is my tool.

Going to try a 8"F9.x for a test run on a 1850/1900mm or so dob, and the F9 sits well with Rayleigh's rule. With a great deal of luck my difference from sphere to parabola will be next to nothing.

Happy chappie!
Till I hit the first of many expected hurdles no doubt...
Steve
Ps. Smiling till reality sets in at least
Pps. Hopefully Covid restrictions in Sydney will be eased and I can get my dental stone powder at the end of this month. Got a Tassie holiday lined up and the shop front is only a 5k detour, which SWMBO has approved.

Quote: Got the final pricing for the two 8" cut and they had mucked up the quote. Its now $42.50 each for the 8" and $85 for the 16" after they timed it on the machine. They're matching the original quote for the two 8" blanks, which is very nice, even at the higher prices they are still the cheapest option around the Canberra region though.

Last edited by mura_gadi; 05-02-2021 at 11:08 AM. Reason: Added "Quote:"
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  #391  
Old 05-02-2021, 11:12 AM
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Sounds good. Great and economical blanks. I hope grinding goes well. I have stalled at the final polishing stage of my little F4 4 1/2 inch. I might have to follow your lead and move on to the 8 inch blank.

I am interested to find out how ATMers are planning to get a mirror coated. That seems to be an issue.
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  #392  
Old 05-02-2021, 11:58 AM
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Good question,

It all depends if Wayne Sainty from www.saintech.com is still in business or has managed to onsell the business to a new owner. wayne@saintech.com is the contact, I'm off to get my blanks.

I'm a fair way away from needing to know atm, but if you do get a response or answer to that question, please post. I know others are keen to find a astronomical mirror coating services here in Oz.


Steve
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  #393  
Old 05-02-2021, 12:08 PM
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Quote:
Originally Posted by mura_gadi View Post
Well I tried Supaglass Industries out in Fyshwick and they were the cheapest quotes I could find in the Canberra region. $25 for the 8" cuts and $50 for a 16" cut on the quotes, best in Canberra by a long way.
That's bloody cheap. What type of glass and is it waterjet cut?
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  #394  
Old 05-02-2021, 01:34 PM
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Woohoo, blanks look really good! Great way to finish the week...

Sunfish - Those prices are priced at the cut only I supplied my own glass.

Sydney Glass (Ph 02 97902830) supplies and cuts 25mm glass, they might be worth a try.

A.E.C in South Australia who supply all the ATM'rs needs also offers mirror coating services last I read, so they might be worth a try as well.

I can supply you with a nice 6" Pyrex blank - to meet Rayleigh's rule/criteria that would be a 6" at F8.2 or little under 5' with mirror cells etc if you're looking to mount on a tripod that might be a better option.

Two arguments for that:
1) Why both going to all the hassle when you can get a 6" F8 anyway
2) You can get a 6" F8 very cheaply second hand, that means all the build requirements for the scope are taken care of, tube, base, mirror cell and secondary mirrors holders etc.

While everyone says Pyrex is better due to better thermal properties, soda glass actually takes a better polish/finish than Pyrex does. I am hoping that the better finish will be worth the little extra wait in cool down times.

Last edited by mura_gadi; 06-02-2021 at 07:15 AM.
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  #395  
Old 05-02-2021, 01:41 PM
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Quotes

Multiweb

I have updated the quote price in a PS. but still very cheap, my own glass supplied and yes water jet cut, real nice finish - I think anyway. No bevel on the edges etc though, that's my step one.

See my comment about Sydney Glass Pty Ltd below.

SupaGlass Industries Canberra
Quote: Got the final pricing for the two 8" cut and they had mucked up the quote. Its now $42.50 each for the 8" and $85 for the 16" after they timed it on the machine. They're matching the original quote for the two 8" blanks($25 each), which is very nice, even at the higher prices they are still the cheapest option around the Canberra region though.

Sydney Glass Pty Ltd:
Quote to supply 25mm clear float Disc 420 dia , $290.00 + GST + Each
Quote to water jet own glass $65.00 + GST + Each

So, its a lot cheaper if you can source your own glass and the wetjet at Sydney is slightly cheaper than my best price in Canberra.


Ps. Barnes in Sydney for Hydrostone TB at around $70 for 25kilo tub, hoping to make that detour later in the month. HQ is only 5k's off the route but currently closed due to Covid, Newtown store is still operating though.

Last edited by mura_gadi; 05-02-2021 at 04:57 PM.
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  #396  
Old 07-02-2021, 10:27 AM
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If you want thicker float, I still have a few pieces left. Contact me if interested.

Stephen.
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  #397  
Old 19-02-2021, 03:37 PM
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Popped my glass pushing cherry

Hello,

Thicker glass sounds nice, with my 25mm I'm happy to go to 12", but the 16" at 25mm has me doubtful... When I get half an idea I may feel differently.

I have just popped my glass pushing cherry, got the table built and the glass tool beveled to 1-2mm. After nearly 3 hours I have a nice little bubble of trapped air in the center of the glass which I am pretty happy about.

Hopefully now that I am starting to make a wider contact between surfaces it will progress a lot quicker. On #120 grit it does seem to take a long time to get started as a complete newb.

But happy I've started, see what the next 3 hours gives me, hopefully a measurable curve!!!


Steve
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  #398  
Old 05-05-2021, 07:26 PM
legswilly (Werner)
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oval or round secondary

Hello. This is my first post and I have two questions.
Is there a mirror maker in the Port Macquarie/Wauchope area? I hope to get come information on testing equipment and tests.


Why are the secondary mirrors on reflectors made an oval shape? The reflection from the primary mirror creates an oval shape on the seconday, it being at 45deg angle. Could a round mirror, or even a rectangular or square one of sufficient size be used and would it be cheaper to make? Has anyone tried to use a piece of a normal mirror like a bathroom one?
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  #399  
Old 06-05-2021, 06:18 PM
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Quote:
Originally Posted by legswilly View Post
Hello. This is my first post and I have two questions.
Is there a mirror maker in the Port Macquarie/Wauchope area? I hope to get come information on testing equipment and tests.


Why are the secondary mirrors on reflectors made an oval shape? The reflection from the primary mirror creates an oval shape on the seconday, it being at 45deg angle. Could a round mirror, or even a rectangular or square one of sufficient size be used and would it be cheaper to make? Has anyone tried to use a piece of a normal mirror like a bathroom one?
Welcome. Not sure of where any makers are and perhaps others will answer. In answer to your second question:

Any shape mirror will do but the elliptical if correctly sized is most efficient at producing a fully illumination circular field and without introducing odd diffraction effects in the process. The reason is that an elliptical secondary will show a circular reflection through the focuser when intercepted at 45 deg. which if sized correctly will show you the circular primary when you look through the focuser (no eyepiece).

When buying a secondary you need to know the minor axis of the elliptical since this will in effect be the diameter of the reflection at 45 deg. and equal to the minor axis. The major axis of the elliptical is 1.4142 x the minor axis; 1^2+1^2=1.4142^2 but it is not necessary to know this for any calculation involving the secondary.
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  #400  
Old 07-05-2021, 08:49 AM
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Normal mirror glass will not work as the mirror surface is on the back of the glass. Telescope mirrors are first surface so the mirrored surface is in the front of the glass. Technically difficult but necessary.

Go to any amateur mirror making site such as Bartels or download a book and the geometry will be made clear.

Quote:
Originally Posted by legswilly View Post
Hello. This is my first post and I have two questions.
Is there a mirror maker in the Port Macquarie/Wauchope area? I hope to get come information on testing equipment and tests.


Why are the secondary mirrors on reflectors made an oval shape? The reflection from the primary mirror creates an oval shape on the seconday, it being at 45deg angle. Could a round mirror, or even a rectangular or square one of sufficient size be used and would it be cheaper to make? Has anyone tried to use a piece of a normal mirror like a bathroom one?
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