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TESTING SKYWATCHER 10-INCH DOBY PRIMARY MIRROR

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#1 RalphMeisterTigerMan

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Posted 19 March 2019 - 12:08 PM

I am curious to know if any other Skywatcher 10-inch collapsible tube dobsonian telescope owners have had the primary mirror tested by a reliable optician and the results of the test.

 

I have noticed that on the Moon and Jupiter both the contrast and resolving power is actually quite good, especially considering that the optics are mass produced. Now I am not claiming "apo" kind of views but definitely better than most SCT that I have observed with.

 

Considering my age and the fact that it is quite portable and easy for me to move around, I would like to, at least, hang onto it for a while. So, I really would like to know how good the primary is and if I get the polishing "tweaked" and probably change the diagonal for an Antares 1/30th wave secondary. After all, if I decide to keep this scope for "life", I want something with enhanced coatings and kick-**** optics. Going this way would definitely be less expensive than buying a new telescope.

 

So, has anyone who owns this scope invested in upgrading the optics and noticed a major improvement? I would like to hear your thoughts and opinions about this.

 

Thanks.

RalphMeisterTigerMan



#2 havasman

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Posted 19 March 2019 - 12:52 PM

I have noticed that on the Moon and Jupiter both the contrast and resolving power is actually quite good

That's what you really want right there. My 10" is an Orion but they're equivalent in their potential for quality mirrors. I had mine tested some years ago by John Hall and he told me he could probably make the numbers better on a test report by refiguring the primary but I'd likely never see any difference. I believed him. I put a bit of $ into a new top end including 1/16th wave Astrosystems (Antares) secondary and that made an easily seen optical improvement. That scope repeatedly puts up better images than considerably larger aperture SCT's at outreach events and star parties. A friend of mine put a Zambuto mirror in his XT12g and then replaced the secondary with an Antares. He said the optical impact of the secondary swap was far greater, not per dollar spent but in absolute terms.

 

I've made many improvements to that scope but it still has the original primary. I expect to keep it around 'til the end and it will have the same primary. You can likely find someone to refigure your mirror and take your cash but if it ain't broke...


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#3 stargazer193857

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Posted 19 March 2019 - 08:27 PM

Just point it at a high up star, early morning, after it has cooled fully. If it looks pin point in the center at high power, why refigure? Using a high power eyepiece, defocus the star on both sides. See that it is round and looks the same on each side of focus.

If you see an animated image, then either the sky or scope temp is bad. Test again later. If always this way, an better mirror won't help.
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#4 stargazer193857

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Posted 19 March 2019 - 08:33 PM

That's what you really want right there. My 10" is an Orion but they're equivalent in their potential for quality mirrors. I had mine tested some years ago by John Hall and he told me he could probably make the numbers better on a test report by refiguring the primary but I'd likely never see any difference. I believed him. I put a bit of $ into a new top end including 1/16th wave Astrosystems (Antares) secondary and that made an easily seen optical improvement. That scope repeatedly puts up better images than considerably larger aperture SCT's at outreach events and star parties. A friend of mine put a Zambuto mirror in his XT12g and then replaced the secondary with an Antares. He said the optical impact of the secondary swap was far greater, not per dollar spent but in absolute terms.

I've made many improvements to that scope but it still has the original primary. I expect to keep it around 'til the end and it will have the same primary. You can likely find someone to refigure your mirror and take your cash but if it ain't broke...


So the secondary was not so good. Was the reason you had it tested just random curiosity, or was it because you sensed something was not right?

I'm trying to estimate how many GSO secondaries are not good, and if is just the few bad ones that get tested.
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#5 stargazer193857

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Posted 19 March 2019 - 08:37 PM

I'm coming from a different direction. I'm making my own 10" dob from scratch and planning to buy a GSO secondary mirror. Jon had good luck with his.

How thick is a 62mm Anteres secondary mirror?

Edited by stargazer193857, 19 March 2019 - 08:38 PM.


#6 precaud

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Posted 19 March 2019 - 09:31 PM

Glad to hear your SW 10" dob has a good primary mirror. I've been very impressed with them. Not so with their secondaries, though. If you know someone with a good reference flat, have them test it. All 5 of the GSO and SW 2ndaries I've tested have been about 1/4 wave or worse. Even the cheapest Antares would be a good upgrade...


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#7 stargazer193857

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Posted 20 March 2019 - 04:18 AM

Glad to hear your SW 10" dob has a good primary mirror. I've been very impressed with them. Not so with their secondaries, though. If you know someone with a good reference flat, have them test it. All 5 of the GSO and SW 2ndaries I've tested have been about 1/4 wave or worse. Even the cheapest Antares would be a good upgrade...


Is that random sample, such as 5 new purchases, or is that self selected sample, such as people who sent you their secondary to be tested?
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#8 precaud

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Posted 20 March 2019 - 06:40 AM

Random. 4 from new dobs, and one 2ndary purchased new as a replacement.



#9 Pinbout

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Posted 20 March 2019 - 06:53 AM

 

I'm trying to estimate how many GSO secondaries are not good,

more bad than good from the ones I've tested

 

primaries have a better ratio.


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#10 Pinbout

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Posted 20 March 2019 - 06:53 AM

I am curious to know if any other Skywatcher 10-inch collapsible tube dobsonian telescope owners have had the primary mirror tested by a reliable optician and the results of the test.

 

I have noticed that on the Moon and Jupiter both the contrast and resolving power is actually quite good, especially considering that the optics are mass produced. Now I am not claiming "apo" kind of views but definitely better than most SCT that I have observed with.

 

Considering my age and the fact that it is quite portable and easy for me to move around, I would like to, at least, hang onto it for a while. So, I really would like to know how good the primary is and if I get the polishing "tweaked" and probably change the diagonal for an Antares 1/30th wave secondary. After all, if I decide to keep this scope for "life", I want something with enhanced coatings and kick-**** optics. Going this way would definitely be less expensive than buying a new telescope.

 

So, has anyone who owns this scope invested in upgrading the optics and noticed a major improvement? I would like to hear your thoughts and opinions about this.

 

Thanks.

RalphMeisterTigerMan

if you were coming to NEAF I could test them for you.



#11 Jond105

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Posted 20 March 2019 - 07:15 AM

I am curious to know if any other Skywatcher 10-inch collapsible tube dobsonian telescope owners have had the primary mirror tested by a reliable optician and the results of the test.

I have noticed that on the Moon and Jupiter both the contrast and resolving power is actually quite good, especially considering that the optics are mass produced. Now I am not claiming "apo" kind of views but definitely better than most SCT that I have observed with.

Considering my age and the fact that it is quite portable and easy for me to move around, I would like to, at least, hang onto it for a while. So, I really would like to know how good the primary is and if I get the polishing "tweaked" and probably change the diagonal for an Antares 1/30th wave secondary. After all, if I decide to keep this scope for "life", I want something with enhanced coatings and kick-**** optics. Going this way would definitely be less expensive than buying a new telescope.

So, has anyone who owns this scope invested in upgrading the optics and noticed a major improvement? I would like to hear your thoughts and opinions about this.

Thanks.
RalphMeisterTigerMan

Getting a 10” Sky-Watcher collapsible dob was one of the best moves I ever made. When I want to take pictures of the moon for friends and family it’s what I use. I feel the optics in mine should be quite good( never looking through any others) but to say that it’s been exceptional to view through last winter and fall would be an understatement. And the views now with a CC are even more magnificent. I have not nor will probably ever have optics checked though. But would probably change the secondary down the road based of more experienced dob observers conclusions.

Edited by Jond105, 20 March 2019 - 07:17 AM.


#12 Asbytec

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Posted 20 March 2019 - 08:44 AM

How can one tell by observing or star test whether the diagonal flat is bad. They have a reputation for producing astigmatism. I do not see any astigmatism, so I assume my diagonal is good enough. Star testing my mass produced system, including the diagonal and eyepeice of course, suggests something better than 1/4 pv waves of under correction. The moon appears sharp and limited by seeing, and focus is pretty snappy. So, the diagonal seems ok.

Edited by Asbytec, 20 March 2019 - 09:07 AM.

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#13 Pinbout

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Posted 20 March 2019 - 08:52 AM

 

They have a reputation for producing astigmatism.

if it has too much power yes it will cause astig.

 

sometimes they'll have holes or hills in the middle of what could appear as a good flat, with a lot of lines on a contact test...but only get 4lines and those errors jump out.

 

how does that relate to a star test? higher order errors of astig, not 3rd order like an overall too much power.

 

3rd order easily seen as a 90deg flip of a oval shape airy disc close to focus.

 

higher order on a 2ndry? I'm still working on how that looks in the star test. remember high order errors are smaller.

 

and astig messes up all magnifications.


Edited by Pinbout, 20 March 2019 - 08:54 AM.

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#14 Asbytec

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Posted 20 March 2019 - 09:09 AM

Thanks, Danny. Makes sense.

#15 stargazer193857

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Posted 20 March 2019 - 11:52 AM

I was about to ask what magnification reveals it, but Danny says all. Also, the Russian flats that say 70nm of wave error also say 20nm of local error.



I was going to pay $70 per GSO mirror from Agenaastro instead of $149 from Anteres. Now I'm thinking of making my own secondary mirror from plate glass and paying Spectrum $15 to coat it. I wonder how hard it is to outdo GSO.



GSO specs says 1/12 wave, and they have good reviews.

Edited by stargazer193857, 20 March 2019 - 11:55 AM.


#16 stargazer193857

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Posted 20 March 2019 - 11:57 AM

more bad than good from the ones I've tested

primaries have a better ratio.


You've tested several. What usually prompted you to test this ones?

#17 SteveG

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Posted 20 March 2019 - 02:50 PM

more bad than good from the ones I've tested

 

primaries have a better ratio.

I had my GSO secondary tested. At least 1/10 P/V wave or better.

 

I have had 2 GSO 10" primaries. The one I had tested showed 1/6 wave P/V, but was horribly astigmatic.

I purchased a new 10" GSO that is so good I haven't bothered to have it tested.


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#18 Pinbout

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Posted 20 March 2019 - 07:46 PM

 

The one I had tested showed 1/6 wave P/V, but was horribly astigmatic.

It can’t be 1/6~ and badly astig. Only in its best orientation can be 1/6 but overall can’t be like that if astig is the dominant error.

 

plus im not saying all gso 2ndry are bad just more than not.

 

if they are good I’ll use them. 



#19 Pinbout

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Posted 20 March 2019 - 07:47 PM

You've tested several. What usually prompted you to test this ones?

I test every loose 2ndry my hands touch. Have test flat will travel 


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#20 stargazer193857

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Posted 20 March 2019 - 08:17 PM

I test every loose 2ndry my hands touch. Have test flat will travel



What is the typical reason they are loose and get to your hands?



As for 1/6 wave error, it is good if it builds gradually across the whole surface, and bad if it several hills within that bound. 1/100 wave is bad if it is a 100 wave washboard. Just like 6 point cell vs 18. The 18 has less absolute error but more hills, so the improvement is not as dramatic as the raw numbers say.

#21 SteveG

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Posted 21 March 2019 - 04:01 PM

It can’t be 1/6~ and badly astig. Only in its best orientation can be 1/6 but overall can’t be like that if astig is the dominant error.

 

plus im not saying all gso 2ndry are bad just more than not.

 

if they are good I’ll use them. 

Agreed. I never got a clear explanation from OWL on this. They also gave it a fairly struehl number.



#22 25585

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Posted 22 March 2019 - 09:54 AM

What type of glass is best for secondaries? Smaller minor axis mass produced ones seem to have plate, but larger are made from Pyrex or quartz, for example.

 

Should secondaries be as good as star diagonal mirrors, or at least the primary in quality?



#23 precaud

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Posted 22 March 2019 - 11:48 AM

What type of glass is best for secondaries? Smaller minor axis mass produced ones seem to have plate, but larger are made from Pyrex or quartz, for example.

 

After I couldn't find a higher-quality replacement of the right size a couple years ago, Terry Ostahowski kindly (though reluctantly) consented to rework my 10" GSO dob's 2ndary. He was quite vocal about what a pain it was working with their glass, partly due to its higher TE coefficient. Heat which built up quickly while lapping would distort the surface and require large amounts of time between laps for it to equalize. Less glass cost, but much higher work time, and greater outcome uncertainty.

 

The same Tdelta distortions also occur during use in temp extremes.

 

Should secondaries be as good as star diagonal mirrors, or at least the primary in quality?

 

Since star diagonals are not of uniformly-good quality, I don't think that's a reasonable measure. My predisposition is to have the diagonal be better than the primary, though again comparisons are inexact, because the errors they impart are different. But the fact that 2ndaries are measured on-axis but used 45º off axis, thereby magnifying the error (not always in predictable ways), contributes to that conviction.

 

Most of the 2ndaries I've measured have asymmetrical domes or ridges at or near the center. (Example: the before/after composite IF's of the GSO rework.)

post-214527-0-35515200-1546994198.gif

 

With this surface topology, the errors can actually become smaller at higher magnifications, because a smaller area of the glass is being used. Astig and scattering visible at low mags gradually lessens at higher mags. So it can be difficult to characterize the visual impact.


Edited by precaud, 22 March 2019 - 11:54 AM.


#24 stargazer193857

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Posted 22 March 2019 - 12:27 PM

What type of glass is best for secondaries? Smaller minor axis mass produced ones seem to have plate, but larger are made from Pyrex or quartz, for example.

Should secondaries be as good as star diagonal mirrors, or at least the primary in quality?


Glass type needed depends on glass thickness. The edges change temp faster than the center, unless you insulate them. This becomes noticeable in the figure when they get thicker. So lower CTE glass is used on the bigger ones. Some flat black paint around the edge might provide some insulation.

#25 stargazer193857

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Posted 22 March 2019 - 12:33 PM

True that at high power, the edges are not used nearly as much.

As for a 1/10 wave primary vs secondary, the secondary has that error over a smaller diameter. The same sphere curvature error gives a better wave number to the secondary. But the secondary is also closer to the eyepiece, where errors have less effect due to less time to propagate.


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