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What mirror design, fast 25"

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#1 hakann

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Posted 09 November 2018 - 01:37 PM

What would be the best choice, if money not in first perspective, for deep-sky use, not planets, moon or sun.
I’m into a 25”, and look over several choices, as fast, and slow focal whit a small diagonal in thicker ( or thinner plano ) in Zerodur or Quartz.
Idea of no coma corrector( or a PII ) and there are producers that can do this mirrors very nice.

But it gets a tall construction.

 

If going bigger or max pupil observation on low power ( and high powers to ) a fast focal is needed and that also put feet on ground.

 

-Example can be ;

( not pocket milled Zerodur MRF polished, as way to expensive )

 

Ex 1, Quartz plano at 1.5” at weight around 24 kg, tested via normal test as ex Focault, but no other tolerance more than ‘trust me’.
This will has some cool-off issues if temperature change.
Few will do this under f/3, but there are producers.

 

Ex 2, Cast Pyrex at 5 mm ribs/surface and ratio at 6;1, IF tested to diffraction ( 20 nm RMS on the surface or L/4 ) and 8-10 Å surface and weight around 16-17 kg ( very stiff, and fast cool-off )

 

Ex 3, Very thin ( around 1/2” ) meniscus in fused Quartz/Silica,  Tosoh at N material star-tested thru system whit primary/diagonal/coma corrector/eyepiece, and weight under 10 kg.



#2 mark cowan

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Posted 09 November 2018 - 02:24 PM

You want the third door with IF documentation AND you want a PIII for any of these.


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#3 TOMDEY

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Posted 09 November 2018 - 02:45 PM

Well... I'd say going faster than F/3 plus ultralight, thin, milled out, etc. You've pretty much opted for all the fancy vogue stuff. At F/3 your diagonal is going to have to be pretty big, regardless.

 

If you are more interested in actual performance... something more conservative is far more likely to perform well. New Moon Telescopes has 24-inch with solid mirror and 26-inch with light weight mirror. They are fast, but not Too fast. I have one of those, and it's great!   Tom


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#4 hakann

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Posted 09 November 2018 - 04:04 PM

Mark, as I got it the PII will work great down to 2.75-2.60.
I has look into a 2.6 whit Ethos 21 whit a PII.
( did it need a PIII, can’t say )
No one outside real pro optics does what I call serious modern IF.
This test alone is very expensive and I’mean very.
I newer like it but ’trust me’ or star-test in the complete system is the only way here.
If its ok, it is.
I has a quote on a thin N material Quartz rough curved borh sides at 15K.

Tom, if one intrude the paracorr tube in primary 75 mm and has a L-distance at 13” a 5” diagonal can be used for E21 mm FS.
Real performance.
Well my guess a slower focal and a small diagonal in a real smooth primary simply just can’t be beat, but it will be a true monster.
I used those several of thoose Dobs and ones at ladder, ok, but...
My best night was in a 25” Lomo in thick Sitall at f/4.5 and a 4” dagonal less a coma corrector whit Ethos. That was a WOW. ( mid to high power )
38K mirror.

But if you ask some premier Dobson mirror grinders they say this fast performs.
As I said, seen into a f/2.6 at 91X and stars was great and a brutal wiev.

As you are into, it’s the example 1.
At OSP this year I used a Kennedy 22” at f/3.3 and it was a great scope and economy.

The ex Dream cast mirrors has not hit the visual market.
On paper they wipe it all out, but out on field.

I can answer the real performance, but the faster the bigger pupil and FoV.

#5 mark cowan

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Posted 09 November 2018 - 04:12 PM

No one outside real pro optics does what I call serious modern IF.
This test alone is very expensive and I’mean very.

 Test the mirror in the cell with ordinary IF to a few nm RMS and that's conclusive, but the IF needs to be calibrated ideally to NIST traceable reference surfaces.  It's not rocket science. lol.gif


Edited by mark cowan, 09 November 2018 - 04:17 PM.


#6 hakann

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Posted 09 November 2018 - 06:09 PM

Ok, so why are not most Dobson grinders working whit ex nasa instead, as that must be way easy if not that rocket sciance and make money plus show the real optics to the world.
After all, from what I hear, all best in world optical knowledge are in the hands of the market who think a Ethos EP is expensive and all it takes is time.
-But at this moment I’m personally ok whit a experiance star test.
Real IF is Way real expensive.
Do you has idea idea of how much ?
I know as I visited Zygo in Frankfurt and QED ex.

Edited by hakann, 09 November 2018 - 10:54 PM.


#7 mark cowan

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Posted 09 November 2018 - 07:04 PM

So just do whatever you like.  Why keep asking for opinions about the same thing over and over?


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#8 hakann

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Posted 09 November 2018 - 11:15 PM

Mark,
Well its a road ( thats real fun ) and it’s a forum right !
I’ll just ask on this examples and comment the IF test you went into.
I has understand that is way over normally peoples head in economy ( simple or not vs IF )
I went world wide on information and designs and I has no hurry at all. I has fun. ( that’s important )

-I’ll just ask where I’m right now.
All answers are concired.
My classic 18” dob got allot of help here at CN.
Next project is more advanced and more money so why not ask ?
After all, just cores are 10-20K, so a question is good.
Do that ’answer’, why I ask to ’to you’ ?

#9 EJN

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Posted 09 November 2018 - 11:29 PM

Why keep asking for opinions about the same thing over and over?

 

OCD



#10 a__l

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Posted 10 November 2018 - 06:09 AM

hakann,

When ordering a 25" mirror, write about it.
Curious On which mirror you leave your choice.
And what will be the result. Will it meet your expectations....



#11 hakann

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Posted 10 November 2018 - 09:07 AM

a_l
I will but its a long road.
I hope its ok speculate and discuss this to.
Its always good to reseach and ask, and I did that on SiC och pocket Zerodurs so then I learned pricing.
I’ll shore would like to see a Dream cast one for visual, big and fast.
As Tom was into the ’real’ performance, I has not leave focals at 4 either.

#12 Pinbout

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Posted 10 November 2018 - 03:04 PM

Why

 

buy it, if your not happy sell it 



#13 hakann

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Posted 10 November 2018 - 03:59 PM

Danny,

Do you mean the Dream cast or the plano f/4 ?



#14 MitchAlsup

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Posted 10 November 2018 - 04:34 PM

No one outside real pro optics does what I call serious modern IF.
This test alone is very expensive and I’mean very.

No one does what you want because the mirror maker does not have the final mirror cell available for the test.

As shown by PLOP, the mirror cell can make or break the figure on a large thin mirror.



#15 hakann

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Posted 10 November 2018 - 05:51 PM

Mitch,
Thats no problem for a second opinion IF test to bring the cell.
I’ll been talking about cast and machined lightweights at 6;1 and they will be very stiff so they can test it vertically to and cell or no cell, or a plano whit cell.
One can get raw data if to read it.
But this is so expensive so I has left the idea.
And a MRF polishing to a standard that at ATM will cry and IF test on that is extremy expensive and expand vs focal/diameter radically.

The Dream cellulars had been my favorite, plus price is ok but I shoure would like a peak in one if them at the difractional tolerance.

I been on slower focal for a while now but a thin meniscus in Quartz and star-test via the system and has 6 mm pupil whit the Nikon HW EP for low power.
The maker on my mirror told me a thin 25” Quartz meniscus sub 4 must be all visual peoples dreams ;-).. if now they hold up for also higher powers that might be very very hard and very few will or can do that, plus is nature force involved to say no ?
Maybe all ok for visual use.
I’m not that far to see a Quartz blank get machined out.

#16 TOMDEY

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Posted 10 November 2018 - 06:30 PM

Ummm... I most certainly favor, Strongly favor interferometry, preferably autocollimation off a good, calibrated plano. We had all that at work, so I'm hopelessly spoiled. And we had interferometers galore and even calibrated them in-house, "absolute calibration" as developed by my boss, Art Jensen!

 

I eventually built a nice 23-inch (CLAP) Test Tower at home and used that.

 

BUT: Once a mirror/mount combo are built and certified superb. At that point, the Star Test various techniques, will confirm its wonderfulness! Maybe not quantify it to the Nth decimal place... but certainly rate it as Superb!

 

And, it is true that the Star Test can direct one's figuring runs... but it sure adds to the inconvenience of the process. If you go WAY back, that's how they did it, rubbing on the speculum and all. But I believe those mirrors were generally pretty mediocre, by today's standards.  Tom



#17 Pinbout

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Posted 10 November 2018 - 06:49 PM

Danny,

Do you mean the Dream cast or the plano f/4 ?

dream cast

 

you have to spend  a lot of time with any of them to start to see the full potential



#18 Steve Dodds

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Posted 10 November 2018 - 07:05 PM

I would be real specious of a cellular mirror with a 1/8" front plate 


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#19 TOMDEY

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Posted 11 November 2018 - 12:53 AM

I would be real specious of a cellular mirror with a 1/8" front plate 

I "came into possession" of a mirror like that (20-inches) long ago. It Looked magnificent and weighed almost nothing. Then I took a knife to it. The Punch-Through was absolutely terrible. So bad, that the mirror was entirely useless. Pretty to look at, but might just as well have been a diffraction grating. I gave it away.



#20 hakann

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Posted 11 November 2018 - 05:13 AM

Steve,
They can machine surface to 1/8” if that what you mean.

It will become very expensive on a 20 or a 25” mirror.
AGI told me price will rise huge from 1/4” becsuse of time in machine and the factor of damage.
A 10 kg 20” in machined Zerodur whit FEM is very expensive, plus MRF, plus IF test, plus coatings.
Ex QED can do L8 but it’s not easy.
( here one wish a ATM plano would be tested !

Ex Dream cast to 5 mm ribs but use a complext double conical design and weight is more than needed, but instead very stiff.
There are no print troughs, se webb.
MRF will polish under 1/8” ( surface ) less print throughs.

Ok. ’If one cellular mirror was bad’, do that mean all is and no way get away from print througs ?

On the start-test.
I can’t say how good it can be but I has to stand in line as the only way as ’EP will tell’, as in ATM there are no numbers more than trust and what the EP will show.

I was close to test my mirror via connections whit Dream in Aerospace, but price was rought 3 times higher than the complete 18” mirror.
I had to drop the idea.
I did test my old 18” mirror in Germany at Rhor and I payed 75 Euro for that IF test.

Danny,
Well I can get the Dream cast 25” but we all know reputation on them and they is terrible.

#21 Pinbout

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Posted 11 November 2018 - 11:09 AM

 

Well I can get the Dream cast 25” but we all know reputation on them and they is terrible

well then...next mirror maker on your list...



#22 Dale Eason

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Posted 11 November 2018 - 01:03 PM

ror via connections whit Dream in Aerospace, but price was rought 3 times higher than the complete 18” mirror.
I had to drop the idea.
I did test my old 18” mirror in Germany at Rhor and I payed 75 Euro for that IF test.
 

Interferometery does not have to be expensive.

If your are good at building a scope then you are also capable of making a Bath intererferometer and learning to use it just like Rhor does.  The cost to make one is under $100 US.  Depending on the camera you have.  If you don't have a good camera for it you can get one for a few $100.   The Bath is self calibrating and has been proven to be so.  Thus you can be confident about your results once you have learned how to do it.  It can test a 25 F3.  Other have done exactly that.  The DFTFringe software to analyze intererograms is free.  It can analyze interferograms from any interferoemter of telescope mirrors so if you don't want to use a Bath but spend more money on other types of interferometers then you can easily do so.  

 

Just to be clear.  Good accurate interferometry does not have to be expensive.  Many are doing it.

 

Dale


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#23 hakann

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Posted 11 November 2018 - 02:13 PM

Dale,
I'm not a into either glass polishing ( or test figure ) or build a telescope really, but like to learn tech as that’s a big part in my astronomy hobby.
Right now, as not that long in visual astronomy I has not the experience yet or time off my company either, but I has time do some trip’s for research and observations.
That’s why I leaved my first real serious idea of a pocket milled 20” f/3 in a CF chassie and did order a plano mirror and a wood scope from people/shops whit good reputation.
Plus research in what observing I did like, what EP, and collimation, sky conditions etc.

I don't say you are wrong, but at US 100 + 100, so why do ex Zygo and ex 3D exist.
When I seen test in this so call low budget or really old IF systems I'll seen ++L/20 in way bigger than 20" and as I been to 4 aerospace company’s and they told me what kind of test lab needed and how hard it is to reach 10 nm RMS/Surface, so I must say has very hard to believe this.
At L/8 ( actually L/7 at 632 nm ) that is what most in ATM see as a decent tolerance, a tolerance Aerospace just can't test vs nature force involved.
The answer I get back on that topic’s was this company’s has not the time as ATM has, as if so it would be to expensive.
Here I see a chance for ATM that might could do space mirrors on lower budgets ( and also better )

-I think I leaved high end test in IF and stand in line of what the EP tells me.
Some EP talk to me, some not.
People ( knowledge here at CN ) has told me in private the IF is also a very lousy test as the cheap old visual test is way better as there instead of few data points ( actually millions ) they see this by eye and that is more senility for visual use.
And practically the only thing the IF exist is get company pay big buck’s that need data in documents.
( it don't get the EP look better.
I told this into Aerospace, but to keep a serious way one has to ask this polities’, and their answers is in articles and optical fackts ( I got so many I can swim in them )
-That don’t mean private people or shops whit experience can do great mirrors at home.

Danny, I did not said cast cellular’s, ex Dream was removed on my list, just said their reputation is more than terrible at ATM.
It’s allot of experience here, so maybe there are huge problems whit them as print through, to low tolerance etc.
So it’s a big chance get one and maybe also be a test pilot.

Edited by hakann, 11 November 2018 - 05:51 PM.


#24 Dale Eason

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Posted 11 November 2018 - 07:03 PM


I don't say you are wrong, but at US 100 + 100, so why do ex Zygo and ex 3D exist.


People ( knowledge here at CN ) has told me in private the IF is also a very lousy test as the cheap old visual test is way better as there instead of few data points ( actually millions ) they see this by eye and that is more senility for visual use.
And practically the only thing the IF exist is get company pay big buck’s that need data in documents.
( it don't get the EP look better.
I told this into Aerospace, but to keep a serious way one has to ask this polities’, and their answers is in articles and optical fackts ( I got so many I can swim in them )
-That don’t mean private people or shops whit experience can do great mirrors at home.

 

There are several reasons why Zygo exists.  Probably the first is they can do the work quicker than an ATM can with a Bath.  However as I have shown here recently the Bath is just as accurate when compared to a Zygo.  

 

Those that claim IF is also a very lousy test are misinformed or ill informed  about the capabilities of the Bath interferometer and the modern software used with it. 

 

You said in an earlier post you had Mr Rhor test you mirror.  He uses a Bath interferometer as well as other tests.  But any of his test he does you could also do within a reasonable budget. 

 

For you then if you don't want to build then all you can do is decide what you want then pick a builder and mirror from those you trust.   

 

Dale



#25 Dale Eason

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Posted 11 November 2018 - 07:08 PM

I should also add that there are some things a Bath interferometer can not measure easily or at all.  However it can do a 25 F3 Telescope mirror and most mirrors an ATM would make.  Commercial shops that have and use a Zygo need to be able to measure many different things that a Bath can not.

 

Dale




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