24" on up mirror projects or mirror blanks?

[Ships and Stars]

I currently am the happy owner of a 20" dob, but was looking to move up to something larger someday, simply for more aperture to view the fainter DSOs, mainly nebulae and galaxies. Financially the only real option is making my own mirror in the 24"- 28" range, ready for coating.

This morning, I was combing the web and found an older 25" dobsonian for sale at what is initially, a very attractive price. Some of you may have seen the listing. The primary and secondary mirrors alone are potentially worth close to the asking price (to me anyway), with one small caveat - it's in lovely Tenerife - which makes a lot of sense with their world-class viewing conditions and climate. I've placed a few enquiries about shipping, but will be surprised if it's under £1000 to any UK port. That pretty much ends things right there.

Just a return ferry trip from Cadiz to Tenerife is £800 if I wanted to collect the scope in my van, not to mention driving to southern Spain (hurts my head thinking about the cost). It would be a proper struggle, even by my dogged, persistent standards, to get my hands on this scope, unless a shipping miracle happens or they are willing to part with just the mirrors and cell, which admittedly is a shame for the rest of the scope and the effort the builder has put into it.

This got me to thinking if anyone out there in the UK or EU had a finished mirror or mirror blank 24" on up they might want to part with? 

- if not, who might sell them in the UK or EU?

- do you think the Tenerife 'giant' is better off finding a new home in Tenerife? (Certainly better viewing conditions, let alone clearer skies!) 

My closest other option - I've priced a finished Nichol Optics 24" mirror and secondary, and whilst Nichol is certainly reasonable and very helpful in correspondence, the 'ready to roll' 25" giant is looking a real bargain if I can sort reasonable shipping.

Any ideas?

 

Many thanks.

 

 

[Science562h]

Just buy a new one & it will arrive in 1 week, already made.

[GTom]

Joining the club with big-mirror ambitions too :), unfortunately with no mirror making experience. Watching a dozen mirror making videos and reading reports, I am tempted to grind a 20-24" piece. I see the cheapest decent 20" ready made mirrors this side of the pond sell for about €3k (TS), a grand more if you want (and I do want!) a thin design. On the other hand, grinding has costs too. I am asking around for blank costs, this one seems very affordable, about half the price I've seen elsewhere!

Problem also with bigger mirrors, that you might want to build your own Mirror-o-matic, which directly adds to the costs as this is a single-mirror project. Doing my homework on the figures, I'll go ahead with home grinding if I see the total cost for a 24" mirror will stay below €1500 AND 50 working hours starting with a pregenerated blank. This includes the grinding of a 12-16" practice mirror if it's absolutely necessary...

[Ships and Stars]

On 09/01/2020 at 10:15, Science562h said:

Just buy a new one & it will arrive in 1 week, already made.

I would if I could! That's a huge amount of money. I'd go for a Star Structure dob with Lochwood optics. Make it a 32" f3 please!

5 minutes ago, GTom said:

Joining the club with big-mirror ambitions too :), unfortunately with no mirror making experience. Watching a dozen mirror making videos and reading reports, I am tempted to grind a 20-24" piece. I see the cheapest decent 20" ready made mirrors this side of the pond sell for about €3k (TS), a grand more if you want (and I do want!) a thin design. On the other hand, grinding has costs too. I am asking around for blank costs, this one seems very affordable, about half the price I've seen elsewhere!

Problem also with bigger mirrors, that you might want to build your own Mirror-o-matic, which directly adds to the costs as this is a single-mirror project. Doing my homework on the figures, I'll go ahead with home grinding if I see the total cost for a 24" mirror will stay below €1500 AND 50 working hours starting with a pregenerated blank. This includes the grinding of a 12-16" practice mirror if it's absolutely necessary...

Oooh I think it will take me a lot longer than 50 hours to get anywhere. I'll start with an 8" or 10" then do a 16" then see if I can go bigger.  I've read that over 16" things start to get really difficult in a hurry, even some professional mirror makers struggle with figuring large mirrors, the bigger the trickier. Before I start working on one, I want to learn how to test them first. Still lots of reading to do, just picked up this book below. I love thinking about it though! I think 25-28" is perfect for what I'm looking for. Larger than 28" and I will need a trailer behind by van or park it at a friend's farm. I'd be thrilled with something in the 24"-28" range someday.

[JamesF]

I have that same book.  An excellent read.

James

[faulksy]

33 minutes ago, Ships and Stars said:

I would if I could! That's a huge amount of money. I'd go for a Star Structure dob with Lochwood optics. Make it a 32" f3 please!

Oooh I think it will take me a lot longer than 50 hours to get anywhere. I'll start with an 8" or 10" then do a 16" then see if I can go bigger.  I've read that over 16" things start to get really difficult in a hurry, even some professional mirror makers struggle with figuring large mirrors, the bigger the trickier. Before I start working on one, I want to learn how to test them first. Still lots of reading to do, just picked up this book below. I love thinking about it though! I think 25-28" is perfect for what I'm looking for. Larger than 28" and I will need a trailer behind by van or park it at a friend's farm. I'd be thrilled with something in the 24"-28" range someday.

that is the book a have, made my 20 with that

[Ships and Stars]

40 minutes ago, faulksy said:

that is the book a have, made my 20 with that

Wow! Love the stainless steel. I've just had a quick look at your build thread bit will definitely have a closer look. I do a little bit of MIG on mild steel, haven't tried stainless yet, but have been meaning to. Great to hear someone built a big dob from scratch like that, I'm a lot more comfortable working with metal than wood myself. Good stuff!

[saac]

If it is just the mirror set you want it may be worth just flying out. I'd speak to the carrier before to find out what arrangements they could offer regarding unusual/outsize hold baggage; you would need to arrange for suitable secure packaging. Alternatively, you could even buy the mirror set a seat and let it fly in comfort with a window seat next to you .  Seriously, I would speak to the carrier, they will generally go out of their way to help you with unusual baggage.  As for grinding a mirror that size, you have my repeat for ambition at least, it is a considerable undertaking.  My 12 inch grind of some years ago stalled at the figuring stage; it was a time sponge. Very rewarding though. Good luck with the project that is some serious aperture, I'm still getting to grips with the school's 16 inch dob. 

Jim   

Edited January 12, 2020 by saac

[Ships and Stars]

54 minutes ago, saac said:

If it is just the mirror set you want it may be worth just flying out. I'd speak to the carrier before to find out what arrangements they could offer regarding unusual/outsize hold baggage; you would need to arrange for suitable secure packaging. Alternatively, you could even buy the mirror set a seat and let it fly in comfort with a window seat next to you .  Seriously, I would speak to the carrier, they will generally go out of their way to help you with unusual baggage.  As for grinding a mirror that size, you have my repeat for ambition at least, it is a considerable undertaking.  My 12 inch grind of some years ago stalled at the figuring stage; it was a time sponge. Very rewarding though. Good luck with the project that is some serious aperture, I'm still getting to grips with the school's 16 inch dob. 

Jim   

That's not a bad idea actually. I would ideally like to buy a finished mirror set already coated but work will need to pick up considerably! If I can learn to figure and test a few smaller mirrors first then I'll look at going large. I'm not in a hurry, but a project like that would need some pace or I'd just get tired of looking at it! Like you, I'll see how I get on with the basics first. This would be a summer project when the nights are too bright for astro.  

[GTom]

3 hours ago, Ships and Stars said:

I would if I could! That's a huge amount of money. I'd go for a Star Structure dob with Lochwood optics. Make it a 32" f3 please!

Indeed, 20"+ pricing is mostly guesswork/custom request the €3k/20" starting point doesn't even work there, because no mass producers like Orion will give you even a 22" mirror. E.g. Zambuto costs 50% more. There is a maker in Bulgaria, Astroreflect, their mirrors are sold by TS below Orion pricing but even a 20" is a custom order for them, I wouldn't wait months until they figure a 24" amidst piles of routine work - and pay a €5k-ish price...

 

Quote

Oooh I think it will take me a lot longer than 50 hours to get anywhere. I'll start with an 8" or 10" then do a 16" then see if I can go bigger.  I've read that over 16" things start to get really difficult in a hurry, even some professional mirror makers struggle with figuring large mirrors, the bigger the trickier. Before I start working on one, I want to learn how to test them first. Still lots of reading to do, just picked up this book below. I love thinking about it though! I think 25-28" is perfect for what I'm looking for. Larger than 28" and I will need a trailer behind by van or park it at a friend's farm. I'd be thrilled with something in the 24"-28" range someday.

Afraid so... I don't count the research/reading/ordering part - I can do that quietly anytime, only grinding-polishing with the aid of a pre generated curve. I am a bit flexible, but have to be realistic, 100+hours on the glass would mean no scope for 3 years.

I'll go trough Rustypsplit's thread on his very first grind, a 14".

Edited January 12, 2020 by GTom

[estwing]

Jumping up from a 20"....that's a mighty leap to see a real benefit. 

[jetstream]

16 minutes ago, estwing said:

Jumping up from a 20"....that's a mighty leap to see a real benefit. 

Yes, its nice to see a magnitude gain on stars between aperture increases IMHO- it will be hard to gain real benefit in an aperture jump from a 20" unless going to 28"-32" . In my case a 36" would be a nice increase.

Just my thoughts.

[saac]

Everybody's got the fever
That is something' you all know
Fever isn't such a new thing
Fever started long time ago

Captin Smith and Pocahontas they knew it  

Jim 

[GTom]

8 hours ago, jetstream said:

Yes, its nice to see a magnitude gain on stars between aperture increases IMHO- it will be hard to gain real benefit in an aperture jump from a 20" unless going to 28"-32" . In my case a 36" would be a nice increase.

Just my thoughts.

Full magnitude jumps are of course nice. It boils down to the additional investment you have to put in both work and $$$.

My position is easier, the biggest thing I got is an unused GEM mounted 12" in a steel tube on a dead mount - not going to return to that scope. Transportability and total weight is worse than a modern 20-24" truss Dob or a c14... I need to determine the right entry point in this world. This would be the largest scope which breaks down to less than 25kg/55lbs components, and a carefully designed, thin-mirror 24" seems to sit right on that limit.

20" is readily available off the shelf "cheap" from mass production (ES, Skywatcher), but I don't like the reviews I keep seeing on them AND of course the affordable ones weigh as much as a CF construction 24". Latter is a steep, 4x jump in price.

Edited January 13, 2020 by GTom

[Cosmic Geoff]

This post may provoke howls of rage and derision😀 but, why construct a massive Dob when with EEVA you could see equally faint objects using a scope of a fraction the size and bulk?

Using a 102mm achro refractor for EEVA I found that I could image in near real time fainter objects than I could see with an 8" SCT. 

[Peter Drew]

These were also my thoughts but not as brave as Geoff to post them!      😀

[andrew s]

45 minutes ago, Cosmic Geoff said:

This post may provoke howls of rage and derision😀 but, why construct a massive Dob when with EEVA you could see equally faint objects using a scope of a fraction the size and bulk?

Using a 102mm achro refractor for EEVA I found that I could image in near real time fainter objects than I could see with an 8" SCT. 

I had the same initial thoughts but then we all enjoy this hobby for different reasons and in different ways.  You pays your money and takes your choice.

Regards Andrew 

Edited January 13, 2020 by andrew s

[JamesF]

Perhaps one might as well ask:

"Why use EEVA and have to cart about power supplies and electronic equipment that can go wrong when all you really need is a big mirror?" :D

Personally I can see myself using EEVA in the future.  But I also fancy owning a stonking big dob :)

James

[Rusted]

Cons: Sheer size, weight of literally everything involved, sheer bulk, viewing height, danger of falling, bad back, covering the beast, recovering the beast after another storm, moving it about, number of clear nights, planning permission for a barn conversion, etc. etc.

Ready made? Don't forget to add the cost of the air fair for the forklift driver's seat for the primary alone. Or cost of borrowing one at each end.

Making a 24" mirror starting from zero? Nah. Several years minimum. Even if you are a "natural." Practice at the dinky sizes doesn't scale well, IME.

Mr Lockwood himself recently mentioned lots of interest in using "light amplification" even on on his larger apertures. Bingo!

[GTom]

1 hour ago, Cosmic Geoff said:

This post may provoke howls of rage and derision😀 but, why construct a massive Dob when with EEVA you could see equally faint objects using a scope of a fraction the size and bulk?

Using a 102mm achro refractor for EEVA I found that I could image in near real time fainter objects than I could see with an 8" SCT. 

No rage from my side at all, use that EEVA with a 24" Dob! I am actually seriously thinking on 1-2 minute subs deep sky photography.

Quotes started to arrive in my mailbox, apparently the basic budget for 24" mirror making will be closer to €2500 than to 2000, main expense being the €1500ish blank.

[Ships and Stars]

Hi everyone! The EEVA route is appealing I must say. Recently saw images of someone using a relatively small refractor from a suburban/urban LP zone - the results were simply astonishing. Was that you @Cosmic Geoff?

I originally bought a 55mm TV plossl to adapt to an intensifier tube, but never got that far and sold the plossl. Can you even get Gen 3 or 3+ in the UK? I think that's the top end of the spectrum as far as I know. I'm guessing £3k if I'm lucky?

 

 

[andrew s]

You don't need to go that extreme why not a Watec 910HX for £500.

Regards Andrew 

[GTom]

Back to the mirror topic, I'd be interested on blank suppliers. The €1000++ per blank doesn't sound very encouraging.

[Peter Drew]

I have a 30" Dob and EAA equipment (not intensifier tube type).  Used in conjunction with a 12" SCT at F3.3 it easily outperforms the 30" used visually on DSO's.

We have a 42" pre- generated ceramic blank, that cost nearly 5K several years ago!  

[scarp15]

There was someone on here a few years ago, I think went by the name; crash test dummy. As I recall he went from a 12" shop bought dob to a 16" (I think) mirror grind. He meticulously kept an ongoing blog recording the stages from a blank. It seemed to go on for ever and then just seemed to inconclusively completely fizzle out with no further updates. I assumed he had lost interest in the polishing and subsequently dropped out of the forum all together. I can only think that a 24" plus would become a very dedicated quest.

It is certainly great to be inspired by aperture, yet also perhaps being able to use what you have, repeatedly over a succession of years, will develop your observing skills and familiarity with the equipment you are using, as well as keeping costs which can start to get a bit insane in this hobby more reasonable. Indeed a 20" mirror taken to the Cairngorms if transparency is good will be exceptional I would expect. Above all else there would seem few opportunities to fully appreciate venturing to the best dark sky location you can get to when transparency is favourable. In those rare circumstances- of very good transparency, taking what you have, with the knowledge and ability to gain the most from this, whilst investing using the best range of filters and eyepieces is highly engaging.  

That said it is good to know that there are a few who are inspired and go for these monster dobs.    

 

Edit: 22" mirror grind by former member

Edited January 16, 2020 by scarp15
Correction