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What to use for a mirror blank? 18 inch F/6.8

DIY Dob Equipment Mirror Making Reflector
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#1 Dauntless Voyager

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Posted 30 November 2021 - 03:36 PM

Hi Everybody!

 

I've recently been interested in making an 18 inch reflector for my observatory. I currently have an 8 inch reflector but would like to scale up. We live in a relatively light polluted area of Southern Florida, though it isn't as bad as it could be towards the cities. I've moved on from observing minor objects such as the planets and moon, though I still greatly appreciate them, to more significant objects; galaxies, nebulae, etc. I feel that this can be better and more effectively accomplished by making a telescope with a greater size mirror. As of right now, I am looking for resources that could better explain in detail how the process of creating a mirror is undertaken and what it involves. 

 

Does anybody have knowledge in this area? I would greatly appreciate any and all insight.

 

Below is an attached file of a concept that accurately represents what I had in mind. 

 

 

Thank you.



#2 wrvond

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Posted 30 November 2021 - 04:15 PM

The Dobsonian Telescope by Kriege & Berry

 

https://www.obsessio...tions/index.php


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#3 perfessor

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Posted 30 November 2021 - 05:08 PM

Dauntless, your attached file does not come through.  This may be a matter of permission because you are a new poster.

 

And by the way - welcome to CN!  

 

As for your goal of making an 18" mirror - the advice you will get in the Kriege and Berry book recommended by wrvond above is - don't!  You are more likely to end up with a functioning telescope if you buy the optics and make the scope.  Of course, there are lots of folks here who have done both.  But an 18" mirror is a tremendously daunting task.

 

Good luck, and keep us posted.


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#4 Dauntless Voyager

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Posted 01 December 2021 - 03:30 PM

Hi Everybody,

 

 

Just wondering what I might able to use for a mirror blank. I looked online and found that premade blanks are ridiculously expensive and pretty much equates to a telescope of similar size. I was thinking about using circular cuts of glass from outdoor patio furniture, not sure if that'll work. (I'll try to upload a few images of what I was thinking of using), lately it hasn't been working for some reason. 

 

Any suggestions?

 

Thanks!

 

Dauntless Voyager



#5 mashirts

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Posted 01 December 2021 - 03:41 PM

3/4 inch annealed plate glass can be found on craigslist for little if you look long enough. I search using "thick glass". 3/4 is strong enough just being annealed and is not usually tempered. But you should check for stress.

I am not sure if 3/4 is thick enough for any 18 inch mirror.

I was lucky and found pieces of 1 inch annealed that I can cut out 16 inch blanks from.

The kit used to cut glass this thick is expensive and would require practice. Trepanning or rough cutting and smoothing it out with an inexpensive wet diamond blade might work. Actually cutting of glass this thick goes pretty smoothly with a wet diamond saw, but so far have only cut straight pieces. Good luck. Others should reply with more guidance.

Michael
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#6 TOMDEY

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Posted 01 December 2021 - 03:51 PM

I think that patio furniture glass must be (by law) tempered. That will explode if you try to cut or grind it. It won't explode like a stick of dynamite, but could still be dangerous... and just of no use for a telescope mirror anyway. Sometimes it's just better to buy the proper fine-annealed stuff and get on with the project. You'll be putting so much effort and care into this project - prudent to start out with a good pedigree blank.

 

[Fine-annealed is the opposite of tempered. Tempered is intentionally stressed to strengthen the glass and prevent knife-like shards if it breaks; fine-annealed is laboriously and slowly cooled to relieve virtually all of the stress... so it can be ground and polished accurately and safely. Good mirror blanks cost so much because of the homogeneity and annealing --- which processes cost way more than the bulk material itself. All glass discs are not equal, even though they may superficially look identical.]   Tom


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#7 Steve Dodds

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Posted 01 December 2021 - 06:24 PM

Since you live in Florida I would try a shipyard that dismantles old boats and look for a porthole, those are annealed, pretty thick and some are 18"


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#8 Bob4BVM

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Posted 01 December 2021 - 08:58 PM

18 inch F/6.8   ?  You do realize that will put you at the top of a ten foot ladder when looking at the zenith ?  that is a LOT of steps up & down, in the dark :)


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#9 Dauntless Voyager

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Posted 01 December 2021 - 09:17 PM

Mercury-Atlas- Yep... We'll probably put some red lights up. But we're thinking that we might use a digital camera feed from the eye piece area to a computer. But hey, the neighbors will be shocked when they see that thing rolling out of the garage! lol.gif 



#10 Jon Isaacs

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Posted 01 December 2021 - 09:35 PM

18 inch F/6.8   ?  You do realize that will put you at the top of a ten foot ladder when looking at the zenith ?  that is a LOT of steps up & down, in the dark smile.gif

 

That's 122.8 inch focal length. This what a four different focal lengths look like: 125 8nches, 96 inches, 70 inches and 50 inches.

 

4 Dobs plus Jon.jpg
 
With a 10 foot focal length, the scope covers a diameter of 20 feet and needs more than that for clearance..
 
Jon

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#11 Augustus

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Posted 01 December 2021 - 10:05 PM

OP, I don't know why you want an f/6.8 or why you think you're going to be attaching a camera but I think you need to be doing a lot more reading before you even consider building a telescope, much less making an 18" or grinding a mirror. An 18" scope by itself is a moderately challenging project if you've bought the mirror but to make the mirror you really need a couple smaller mirrors under your belt and should probably have built at least one large truss tube instrument.

 

I am rebuilding my 24" right now. I did not grind the mirror and it is much shorter in focal length than what you are proposing. I have a shop with thousands of dollars worth of tools that I spent years acquiring and learning how to use. It is still not a project without its hiccups. I built my first large telescope, a 16" f/5, about 4 years ago - and despite what I insisted at the time it was on the verge of falling apart and worked just well enough to give me some good views before it essentially ceased to function after being dismantled for Stellafane. My subsequent 20.5" wasn't much better, as were the smaller truss tube scopes I built. It is only in the last year or two that I have been able to build a solid truss tube telescope with adequate support for the mirror, trusses that are easy and consistent in assembly and a structure that is stiff and holds collimation. And this is ignoring the mirror part! To date the largest mirror I've made successfully is a 6", and I made a 16" that had a poor figure but at least did work. 

 

I am also helping three friends make mirrors and working on one myself - a 6" f/9. My friend Rick's 6" f/4 is the only one of these that has had absolutely nothing go wrong on the first try and he still ended up getting a scratch during fine grinding. My friend with a 10" had all sorts of problems with getting a smooth sphere, then the edge refused to polish out. And don't even get me started on the 14". The 6" f/9 is going well but I haven't had time to work on it because I'm helping with those other mirrors. And while I am not as much of an expert in making things or observing I have had years of experience and the people I am attempting to teach at least have some guidance hands-on. 

 

If you want a big scope for not a lot of money, read Kriege & Berry and go buy yourself a used 16-18" mirror (and not one above f/5 if you can help it!). If you follow the book to a tee, you will get a very good if perhaps overweight and slightly bulky truss tube telescope. If you read some of the stuff here on CN while getting feedback and wisdom from many of the well esteemed ATMs on here you can build something a bit lighter, cheaper, more portable, etc. but you have to learn the basics first and always keep in mind basic engineering and optical concepts when attempting to try anything, and I mean anything new.

 

If you are dead set on grinding an 18" I would optimistically say it will take you a year or two to make at least one smaller mirror, build a telescope around it, and develop the necessary knowledge, experience and tools to make an 18" - at which point I would probably go shopping for a blank on the used market, find a porthole or get a slumped meniscus blank from somewhere. But don't even think about that yet. 

 

Astrophotography with a Dob is not really a good idea for anything other than shooting the Moon and planets. It worries me greatly that your plans consist of obtaining a glass tabletop and getting "a digital camera feed from the eye piece area to a computer" with the completed telescope. Those do not exactly inspire confidence. 


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#12 Augustus

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Posted 01 December 2021 - 10:16 PM

18 inch F/6.8   ?  You do realize that will put you at the top of a ten foot ladder when looking at the zenith ?  that is a LOT of steps up & down, in the dark smile.gif

A 122" focal length with an 18" will translate to a taller instrument than the 25" f/5 Obsession like Jon's, which a nightmarishly large thing that really stretches the concepts of "portable" or "one-man setup". The Obsession is 3" longer in focal length but also quite a bit fatter so it is actually a few inches shorter in height as a result. The eyepiece will be right around ten feet above the ground at zenith.

 

Anything with an eyepiece height that high has a real chance of putting you in the hospital if you fall off the ladder. On wet nights I have slipped off the whopping 3 foot high step I need to look through my 24" f/3.5 at the zenith and that could easily lead to a sprained ankle. Yes, ladders are quite safe in the daytime but in darkness and with the excitement of observing and damp terrain or shoes the risk of injury is a lot higher. Yes, there are big heavy duty ladders like the kind you see in warehouses that will be quite safe - but that's another thing to buy, transport etc. and if you are designing your scope from scratch why would you figuratively and potentially literally handicap yourself for no reason?

 

There is absolutely no good reason for a telescope larger than 12" to be longer than f/5 - and other than cost, there is no reason to be much above f/4 either.


Edited by Augustus, 01 December 2021 - 10:28 PM.

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#13 Jon Isaacs

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Posted 01 December 2021 - 10:26 PM

Anything with an eyepiece height that high has a real chance of putting you in the hospital if you fall off the ladder. On wet nights I have slipped off the whopping 3 foot high step I need to look through my 24" f/3.5 at the zenith and that could easily lead to a sprained ankle. Yes, ladders are quite safe in the daytime but in darkness and with the excitement of observing and damp terrain or shoes the risk of injury is a lot higher.

 

 

waytogo.gif

 

I'm in my 70's and observe alone about 50 miles from the nearest hospital.  If I fall, i could really get hurt. $100 ladders and $10,000 telescopes don't cut it.

 

Not This:

 

 

5910770-Obsession and Jon at Jewel Valley.jpg
 
This:

5873860-Obsession with Rolling Ladder 2 CN.jpg
 
I sold that scope about 5 years ago and downsized to the second one, a 22 inch F/4.4. I use a ladder on wheels with railings.
 
Jon

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#14 brentelgeuse

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Posted 02 December 2021 - 09:21 AM

8 to 18 seems a big jump. undecided.gif

 

Jon, if you turn your steps around you could just walk down the steps as you lower your altitude! laugh.gif

Seriously, I love those observing steps.


Edited by brentelgeuse, 02 December 2021 - 09:22 AM.


#15 Bob4BVM

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Posted 02 December 2021 - 02:37 PM

8 to 18 seems a big jump. undecided.gif

 

.....

Although i would not recommend the OP start grinding an 18" for his first project, i would definitely encourage him to acquire the biggest scope he can handle...

Back in 1990 i make the same 'big jump', going from an 8" SCT to a 17.5" Dob. 

 

The Dob is probably the reason i still go out on those cold damp nights-

Where the 8" showed stars, clusters, galaxies OK, the 17.5 took the image from silver spots painted on black, to gleaming electric lights on a black velvet backdrop.

Nebulae went from grey fluff to intensely bright swirls of detail, some with shades of blue, green, & pink.  Galactic details i could only guess at with the 8" became in-my-face obvious.  

 

All the mirror advice given above i agree with. 

But nothing wrong with dreaming big, and possibly finding a way to make the dream happen !

CS

Bob


Edited by Bob4BVM, 02 December 2021 - 02:41 PM.

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#16 dustyc

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Posted 02 December 2021 - 02:57 PM

18" F6.8?

The only person that might bookmark this thread is CHASLX200 ('cause he's a planet junkie) but a "planet killer" scope like this is not the tool for general DSO observing. Thumb's up on F5 at the maximum. 

Sure, the longer focal lengths are easier to grind but impractical for larger sizes. 



#17 dustyc

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Posted 02 December 2021 - 03:00 PM

I'm not the seller and don't know them but there is a 17.5" Coulter mirror in the classifieds right now. F4.5 too. 

Build around that one. Maybe Telekit for the structure?


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