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Premium mirror vs. Chinese mirror

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#51 39.1N84.5W

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Posted 15 April 2018 - 12:44 PM

There is definitely a "how many angels can dance on the head of a pin" psychology in this hobby. But with overcast skies our minds wander and our wallets open. Lol!
Personally I love well crafted objects. I'm a professional instrument maker and I know my instruments will be used and loved for decades if not centuries. It's a beautiful thing that independent telescope makers can exist in this market. And I'll invest the extra money to get the extra performance.
But at the end of the day a mass produced dob under the night sky will see 100% more than an heirloom telescope collecting dust in the garage. Just go out and use your gear!

Edited by 39.1N84.5W, 15 April 2018 - 02:02 PM.

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#52 Joe1950

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Posted 15 April 2018 - 01:38 PM

Very good posts here showing the duality of the hobby. Instruments and Observing work in concert, but in a way are dichotic disciplines.

 

We all like to feel our choices are good ones, based on what we can afford. And often they are. Ask yourself, "How good is my telescope and related equipment if I had no way of testing or comparing it?"

 

It is good to test and compare as it serves to keep standards at some amorphous benchmark - at least I hope. And I thank the stars each and every day for those who know the difficult subject of optics and the artisans who produce stunning optics.

 

I suppose it is and will remain a delicate balance where we have to make the effort to maximize our observing skills with whatever grade of instrument we enjoy, while having an appreciation of the skill and talent needed to create a jewel of an optic.


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#53 ed_turco

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Posted 15 April 2018 - 01:48 PM

When I jumped into this ATM thing face first, I was facing the same dilemma... get one off the GSO shelf or go premium... after a LOT of soul searching and getting to know me better, I went with a 10 inch Royce conical (probably one of the last he made before retiring). My reasoning was thus: If I got the GSO 10 inch, I would ever wonder if I did the right thing. Since I got the Royce, I have never wondered and sleep very well at night now.

 

Now, for the rest of the story... on most nights, I doubt very seriously that much difference could be discerned between my Royce and a Chinese mirror, at least not by me. But I have had the Royce out on a few exceptionally good nights and the image was OMG good, so good, I am not really sure how to describe it convincingly. Exceptional clarity? Knock your socks off contrast? Hold the phone Mom, you gotta come see this?

 

I will NEVER part with my Royce mirror. I am not convinced I could ever say that about a Chinese mirror.

 

And just to be clear, I do not think there is anything wrong with Chinese mirrors. Most people in this hobby use them to great satisfaction. And I could have as well... except for the wondering...

 

Keep looking up!

 

CB

If that mirror is so good, what are you doing, sleeping at night?  If so, how could you?

 

ed


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#54 Joe1950

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Posted 15 April 2018 - 02:33 PM

 

I will NEVER part with my Royce mirror. I am not convinced I could ever say that about a Chinese mirror.

CB. Never say never! George Lucas sold Star Wars to DISNEY, for $4 billion. Just sayin. But knowing you have a good mirror (or lens) is a good thing. Enjoy!

 

Hi Ed! Good to see your post. Hope you are well!


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#55 George N

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Posted 15 April 2018 - 05:42 PM

That can be an option for people who live in metro areas, not so easy for us rural dwellers. For us, CN is our astronomy club.

You need to make that once-in-a-lifetime pilgrimage to Mecca ..... or rather make that TSP, WSP, Stellafane, etc...... and hope for clear skies and friendly telescope owners......


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#56 George N

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Posted 15 April 2018 - 05:45 PM

If that mirror is so good, what are you doing, sleeping at night?  If so, how could you?

 

ed

He has to be sedated when it gets sent away for recoating......  wink.gif

 

He gets resuscitated when it comes back.....


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#57 George N

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Posted 15 April 2018 - 05:54 PM

For what it's worth (admittedly hearsay evidence):

 

A friend makes premium Dobs for a living. He will put whatever brand of optics you want in them, and has seen them all - and knows how to star test. He tests each completed telescope and will not sell you a telescope with ratty optics. He recently told me "The latest GSO mirrors we have gotten have all been pretty good!"

 

Of course his personal telescope has a premium mirror in it from his fav maker.

 

I suppose, a good strategy might be: buy a good mid-priced Dob with 'commercial" optics and enjoy it. Order a premium mirror. When it comes, test both - star test, and planets on a good night..... if the premium mirror is better, keep it. If not, sell it and spend the money on trips to dark skies. It will cost you a few $$ and some time - but you will know what you have - and you can write it all up in a long post on CN......


Edited by George N, 15 April 2018 - 05:57 PM.

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#58 Asbytec

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Posted 15 April 2018 - 06:37 PM

The allure of a premium optic and a nice image is understandable. We do tend to want the best equipment. I think this has put some pressure on the industry to deliver better mass produced scopes. Not as good as a premium mirror, as said it doesn't get any better than premium save for some enhancements Ed Turco or a Mak Newt might employ. In the end, they don't really have to be absolutely perfect to have a pleasant observing experience. But, it is nice.


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#59 Joe1950

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Posted 15 April 2018 - 07:33 PM

I did buy a GSO 6", f/8 mirror about a year ago and it is pretty good. Thanks to Dale E, I built and learned to do the Bath Interferometer test.

 

I'm no expert with the test, but I've tested the mirror dozens of times, right side up, inside out and everything in between -  and the results closely group around 0.96 Strehl. Roughly 1/30λ RMS or 1/8λ P-V.

 

That being said, I would be very satisfied if the readings are accurate. Or even somewhat close!

 

But, the GSO mirror is physically too big (won't fit the mirror cell) and has a longer focal length than the original mirror - by a substantial amount. So, a 6" f/8 is not the same size as another 6" f/8. The focal plane of the GSO in the DOB's tube is in another Zip code off the side of the tube. And I can't move the mirror back since there is no room to do so, at all. I'd have to add tube which isn't easy to do. More than an inch and the tube hits the nut on the base.

 

I hope to get the original mirror re-figured a little so I can put that back in the tube. This issue has been going on since I got the DOB, over 3 years ago. I don't need a perfect scope, just a decent one. The brand outlet here in this country would replace the OTA if I sent it back to them in the original box with all the original parts, which I had, but they couldn't guarantee the replacement to be any better since they have no testing facilities. shrug.gif


Edited by Joe1950, 15 April 2018 - 07:34 PM.

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#60 Pinbout

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Posted 15 April 2018 - 07:49 PM

 

So, a 6" f/8 is not the same size as another 6" f/8. 

The 150’s are smaller than the 6”


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#61 Joe1950

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Posted 15 April 2018 - 08:51 PM

Right, Danny!

 

If I recall, the original mirror in the DOB has a clear optical aperture of 148mm or thereabouts. The GSO mirror is 152mm on the optical surface and more for the entire disk. It actually won't fit the mirror cell that came with the DOB.

 

I guess it depends on where they buy the glass. There is naturally some variation.

 

So to use the GSO in the DOB tube, I used a different cell, that is, of course, thicker and puts the mirror more forward in the tube and adds to the already longer focal length.

 

 

fingertap.gif  Everything I thought of doing to make the GSO fit, led to something that wouldn't work. It's a very well made tube and has a good, thin spider and secondary holder and so on, but a fit that has no leeway.


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#62 gazerjim

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Posted 16 April 2018 - 12:49 AM

My two pennies worth: penny.gif

 

An 8"  to 10" comparison is sort of an apples to oranges one for starters.

 

 A few considerations--

 

-Aperture ultimately rules.  How inferior would the 10" need to be to lose out to the 8" ?  That's a tough one.

 

-An 8" standard thickness newt mirror can usually be kept near ambient temperature with a simple cooling fan.  A 10" full thickness chunk of glass will often struggle to keep up with ambient unless active cooling (Peltiers, etc.) is employed.  A 10" conical might well do better.  As little as one degree F difference in temperature may distinguish an eye popping from a so-so view or image.

 

-Don't overlook quality in the secondary mirror.

 

-Everything becomes more critical as the f/ratio goes down.  Focusing and collimation are less forgiving,  and an optically  fast top notch primary is  harder to do well.  Which is why some top opticians charge a premium for them.

 

-Quality in mass production is a matter of statistics.  Hence (I think) so much  variation in personal opinion. 

 

I would say to buy what you can comfortably afford and get the best possible results from it.  That process will in itself guide future decisions in tweaking and upgrading.

 

 

 

 

-


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#63 SeaBee1

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Posted 16 April 2018 - 09:17 AM

If that mirror is so good, what are you doing, sleeping at night?  If so, how could you?

 

ed

.

lol.gif  Good one, Ed!

 

Well... I can't view ALL night, I still have an employer that recommends I show up for work, preferably awake and fully functional...

 

Getting closer by the day to retirement though...

 

Clear skies Sir, good to see you out and about!

 

CB


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#64 gwlee

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Posted 16 April 2018 - 10:37 AM

You need to make that once-in-a-lifetime pilgrimage to Mecca ..... or rather make that TSP, WSP, Stellafane, etc...... and hope for clear skies and friendly telescope owners......

It would be fun, and I might do it someday, but the total cost of the trip could easily exceed the cost of any telescope that I might buy based on the research IF the weather cooperated and IF friendly TS owners showed up with the telescopes that I want to demo.

 

When I lived in a major coastal metro area, I belonged to a large astronomy club and our star parties or a nearby a club’s star parties were a viable venue for inspecting and trying out astro gear.

 

Living in rural flyover country now, the nearest club requires an overnight automobile trip to a club with few members, so the standard, metro-centric advice to visit a “half way decent” astronomy club doesn’t always reflect the realities of rural life. For some of us, a virtual astronomy club such a CN is our only astronomy club. 


Edited by gwlee, 16 April 2018 - 02:01 PM.

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#65 gwlee

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Posted 16 April 2018 - 12:33 PM

I have owned a 1/2 dozen garden variety XT6/XT8 Dobs over the last 25 years. None of them were anything special, but they were consistently OK. I also simultaneously owned a sensibly perfect 4” APO and a custom 8” Dob with Zambuto optics.

 

The 6”f8 Dob consistently gave me better views of all objects than the 4” APO. The 8”f6 Dob consistently gave better views of all objects than the 6”f8  Dob. The 2” difference wasn’t a “wow,” but it was obvious.

 

The custom 8” Zambuto equipped Dob gave marginally better views than the 8” Synta Dob in excellent seeing, but the difference in the views was much more subtle than the difference that comes with 2” of extra aperture, which is why I would expect a typical, garden variety 10” Dob would probably give better views of anything than a sensibly perfect 8” Dob and cost much less

 

For reference, a complete XT10 costs $600 and is available off the shelf. An 8” Zambuto mirror starts at $1,100 with an 11 month lead time. A Zambuto equipped 8” Teeter starts around $3,300 with a 4 month lead time. 

 

I don’t want to deal with the weight/bulk, cooling requirements, collimation tolerances, and coma of a 10” scope, so I might be willing to pay the much higher cost of sensibly perfect 8” optics again someday, but only after making sure that every other aspect of the scope was as sensibly perfect as I could make it before addressing incremental optical quality improvements. 

 

 


Edited by gwlee, 16 April 2018 - 01:59 PM.

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#66 Joe1950

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Posted 16 April 2018 - 03:51 PM

Good comparison for perspective!



#67 CHASLX200

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Posted 16 April 2018 - 06:05 PM

I have owned a 1/2 dozen garden variety XT6/XT8 Dobs over the last 25 years. None of them were anything special, but they were consistently OK. I also simultaneously owned a sensibly perfect 4” APO and a custom 8” Dob with Zambuto optics.

 

The 6”f8 Dob consistently gave me better views of all objects than the 4” APO. The 8”f6 Dob consistently gave better views of all objects than the 6”f8  Dob. The 2” difference wasn’t a “wow,” but it was obvious.

 

The custom 8” Zambuto equipped Dob gave marginally better views than the 8” Synta Dob in excellent seeing, but the difference in the views was much more subtle than the difference that comes with 2” of extra aperture, which is why I would expect a typical, garden variety 10” Dob would probably give better views of anything than a sensibly perfect 8” Dob and cost much less

 

For reference, a complete XT10 costs $600 and is available off the shelf. An 8” Zambuto mirror starts at $1,100 with an 11 month lead time. A Zambuto equipped 8” Teeter starts around $3,300 with a 4 month lead time. 

 

I don’t want to deal with the weight/bulk, cooling requirements, collimation tolerances, and coma of a 10” scope, so I might be willing to pay the much higher cost of sensibly perfect 8” optics again someday, but only after making sure that every other aspect of the scope was as sensibly perfect as I could make it before addressing incremental optical quality improvements. 

You will still get coma even with a perfect mirror once you get below F/5.5.  It may not bother some people, but coma drives me nuts when we get close to F/5 without a Paracorr.



#68 starman876

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Posted 16 April 2018 - 09:15 PM

http://www.rfroyce.c...performance.htm

 

a good write up by Royce on Coma with fast mirrors.   The best way to quickly state what is in the white paper is the fact that coma on a planet like Jupiter will not matter as long as you keep the planet centered in the scope.    On the moon you might be able to see some coma.  I am not sure if I am stating what is the paper accurately so please correct me if i am stating this wrong.  Bottom line is that coma will come into play in a fast newtonian on objects that fill up the whole field of view.  However, it also states that a really well made mirror will keep coma under control as much as possible.



#69 mark cowan

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Posted 16 April 2018 - 10:11 PM

The coma is inherent off-axis for the paraboloid (and for under or over correction as well) so it's nothing to do with how well the mirror is made...  :shrug:


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#70 barbie

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Posted 16 April 2018 - 11:15 PM

I've owned 6 chinese/taiwanese dobs from 4 to 12 inches in aperture and every one of them was excellent.  The 10 and 12 inch were f5 and did show some coma but it was never too bothersome.  My current 6" F8 shows no coma and stars are pinpoint sharp.  It has extraordinarily figured optics and it will be my final scope for the remainder of my stargazing years.  A 10 or 12 inch dob is quite a load to handle however so I had to sell them and concede to the ravages of age and time so the 6 inch dob is what I have settled on as my compromise between large aperture and focal length.  I have not been disappointed!laugh.gif


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#71 Pinbout

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Posted 16 April 2018 - 11:31 PM

I've owned 6 chinese/taiwanese dobs from 4 to 12 inches in aperture and every one of them was excellent.  The 10 and 12 inch were f5 and did show some coma but it was never too bothersome.  My current 6" F8 shows no coma and stars are pinpoint sharp.  It has extraordinarily figured optics and it will be my final scope for the remainder of my stargazing years.  A 10 or 12 inch dob is quite a load to handle however so I had to sell them and concede to the ravages of age and time so the 6 inch dob is what I have settled on as my compromise between large aperture and focal length.  I have not been disappointed!laugh.gif

post some star tests of your scope.

 

https://www.cloudyni...355-reflectors/

 

your depriving us by not making some images and adding them to CN's star test folder...


Edited by Pinbout, 16 April 2018 - 11:33 PM.


#72 barbie

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Posted 16 April 2018 - 11:49 PM

I would be happy to once I get a new camera.  I'm getting ready to move to a new apartment in august so the camera is not a top priority right now.



#73 Pinbout

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Posted 17 April 2018 - 12:00 AM

cell phone pics would be ok



#74 barbie

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Posted 17 April 2018 - 12:03 AM

Nope!  Too low quality.  I haven't had a clear night since last October with no clear nights in the forecast till May.  I live near the lake Erie "cloud machine".lol.gif


Edited by barbie, 17 April 2018 - 12:07 AM.

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#75 Pinbout

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Posted 17 April 2018 - 12:07 AM

a lot of the images in the gallery were taken with cell phones. and there's plenty of info on each one.

 

close to focus for correction, enlarged for zones.


Edited by Pinbout, 17 April 2018 - 12:07 AM.



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