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Replacing Newtonian primary mirror to improve quality

Astrophotography Mirror Making Imaging Optics Reflector
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24 replies to this topic

#1 skywatcherincanada

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Posted 14 January 2023 - 11:29 PM

Hi everyone 

 

I own a skywatchercanada Quattro newtonian.  I was wondering if Changing primary & secondary mirror with a better quality quartz mirror will improve quality of the images significantly? 


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#2 Jim Waters

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Posted 14 January 2023 - 11:45 PM

Yes - I have the same scope.  The question is where do you get a new certified primary mirror?



#3 skywatcherincanada

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Posted 14 January 2023 - 11:48 PM

I don't know yet . I was thinking TS optics but Would appreciate any leads.  Also if both mirrors or just primary will do the job .



#4 Jim Waters

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Posted 15 January 2023 - 12:07 AM

If I had an option between the two I would do the primary first.



#5 TOMDEY

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Posted 15 January 2023 - 12:11 AM

Well... it's most certainly an ~economy~ scope. A truly Premium Primary Mirror would generally cost more than the entire thing. Swapping both mirrors would make sense --- or wavefront testing them on an interferometer to see what you have before deciding. I used to drag OTAs into work and test them full-apaeture in autocollimation in the lab. Not surprisingly... the cheap ones were meh or worse and the premium ones were decent to fine. Only rarely did we stumble across a great cheap one or a crummy premium brand. PS: Testing the entire OTA interferometrically is a severe and most meaningful quanta/qualification, because you are looking at the whole train exactly as it will be used, all fields, full aperture, doubly-sensitized, accurate. Alas, the only available home tests are (pretty much) star testing under great seeing conditions.    Tom


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#6 skywatcherincanada

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Posted 15 January 2023 - 12:15 AM

Thank you with my knowledge and ability the most I can do is to replace the mirrors and perform simple Colima afterwards.

#7 skywatcherincanada

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Posted 15 January 2023 - 12:17 AM

Thank you . What are you using? I could only find TS optics using Google search

#8 Jim Waters

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Posted 15 January 2023 - 12:18 AM

The TS Optics 8" f/4 mirror is ~$630 USD excluding VAT and shipping.  



#9 The Ardent

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Posted 15 January 2023 - 12:24 AM

Normand Fullum is the national treasure of Canada. From my perspective that’s the only choice.


Thank you . What are you using? I could only find TS optics using Google search



#10 Jim Waters

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Posted 15 January 2023 - 12:27 AM

Steve Swayze was good at testing and refiguring mirrors but passed away.  Carl Zambuto no longer refigures mirrors and would not touch a mirror made in China.  These are the only two that I would trust working on a mirror.  I don't know much about O.W.L. mirrors and their quality.


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#11 skywatcherincanada

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Posted 15 January 2023 - 12:29 AM

Thank you I will get in touch with them ..

#12 Tony Flanders

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Posted 15 January 2023 - 05:32 AM

Do you have any reason to believe that the quality of your current mirror is inferior? I would only undertake such a project if there was a clearly identified problem that I wanted to fix.

 

Do you know how to do a star test? That's by far the easiest way to identify problems with optics.


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#13 Jon Isaacs

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Posted 15 January 2023 - 07:54 AM

Do you have any reason to believe that the quality of your current mirror is inferior? I would only undertake such a project if there was a clearly identified problem that I wanted to fix.

 

Do you know how to do a star test? That's by far the easiest way to identify problems with optics.

 

:waytogo:

 

What issues have been identified with the current optics?

 

What is the telescope being used for? Optical quality is important at high magnifications like viewing the planets but it's much less critical at lower magnifications.

 

At F/4 the two questions that come to mind are coma coma correction and collimation. These are critical.

 

Jon


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#14 MellonLake

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Posted 15 January 2023 - 08:22 AM

Normand Fullum will test your mirror and determine if it actually needs refiguring before doing the work.   To refigure a 10" mirror is going to be expensive $1000.  The views through my 16" were very poor so I sent the mirror to Normand, it had a one wavelength error.  After Normand refigured the mirror, it is now amazing.   

 

Rob


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#15 skywatcherincanada

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Posted 15 January 2023 - 09:49 AM

Thank you

#16 skywatcherincanada

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Posted 15 January 2023 - 09:49 AM

:waytogo:

What issues have been identified with the current optics?

What is the telescope being used for? Optical quality is important at high magnifications like viewing the planets but it's much less critical at lower magnifications.

At F/4 the two questions that come to mind are coma coma correction and collimation. These are critical.

Jon



#17 Asbytec

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Posted 15 January 2023 - 11:06 AM

I agree with Tony. Check your mirror first. A basic star test can show you have a descent mirror set or not.

Collimate and cool your scope, first, and use high magnification on a fairly bright star. Defocus inside and outside slightly to see three diffraction rings outside the central shadow. The outermost ring will be bright on both sides of focus. If the innermost ring is also bright on both side, and the ring between them will be dimmer, then you have a descent mirror. If by three rings defocus the innermost ring is not as bright as the outermost ring on either side, then consider an upgrade.

When you scroll through focus on both sides, again to see three rings, pay attention to the darker shadow. If it's reasonably the same size on both sides, that's another sign of a descent mirror. A mirror with 1/6 wave of spherical aberration and free of astigmatism is a decent mirror.

Jon also hits on important points if your questioning your mirror. Collimation and thermal stability are the first things to master. Evaluate your seeing conditions, too. Coma at f/4 is normal even for a premium mirror, and a coma corrector can fix that. You can also check for and correct for sources of astigmatism either in the primary or diagonal mirror. Google Lockwood's article, "why aren't my stars round" for information on how to do that. If all that fails to offer pleasing images, then consider upgrading both of your mirrors.

Of course nothing stopping you from upgrading anyway. But you can do so with the knowledge you may gain from testing and working with your scope to recognize and solve issues. If you upgrade, you may have to work with your current scope structure so it does not cause, or you can recognize, problems with your new mirrors like pinching the primary or diagonal. And no doubt you'll star test it, too, for quality, pinched optics or astigmatism, collimation, and thermal stability. Only then will any mirror perform at its best.

Lastly, don't forget the human element. If you have not done so already, be sure to hone your experience and skill to really appreciate a premium mirror set and the cost of upgrading. The images will be better, but only if you are able to see it and tell the difference from your current set. Upgrading simply for peace of mind may not the magic bullet for better observing.

Edited by Asbytec, 15 January 2023 - 11:42 AM.

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#18 SteveG

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Posted 15 January 2023 - 02:27 PM

As most have noted, you need to test first. A bad secondary will make a good primary seem bad.

I spent years diagnosing my 10" scope. The first problem I has was my deteriating vision, which complicated the diagnosis.

 

End the end, I found my primary was bad. With my new knowledge I decided to try another Chinese import, which turned out to be a good option as my high-power views were improved substantially. YMMV



#19 Eddgie

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Posted 19 January 2023 - 10:49 AM

Do nothing to the mirror.

 

This scope was designed for imaging and as such, improving the quality of the mirror will have almost no effect on the quality of the view, or when using the scope for imaging

 

If you want better views at the eyepiece, buy a telescope that is more optimized for visual use. If your intention is to see more on planets, this is not the right scope and spending more money on it will be an expensive exercise in frustration.  


Edited by Eddgie, 19 January 2023 - 10:50 AM.

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#20 Tony Flanders

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Posted 19 January 2023 - 06:56 PM

If you want better views at the eyepiece, buy a telescope that is more optimized for visual use. If your intention is to see more on planets, this is not the right scope and spending more money on it will be an expensive exercise in frustration.


Why do you say that? It's true that this line of telescopes has unusually large secondary mirrors as Newtonians go, which inevitably degrades the quality of high-power images every so slightly. But the secondary obstruction is much the same as for an SCT, and SCTs make perfectly good planetary scopes.

The two things that matter most for planetary observing (aside from seeing, which is 90% of the battle, and skill, which is 90% of what's left) are aperture and optical quality. The size of the secondary obstruction is a very minor issue.
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#21 truckerfromaustin

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Posted 19 January 2023 - 07:16 PM

You might consider selling your scope and getting one that's visual friendly. F4 is really to fast for visual and requires a paracorr type 2 and well connected eyepieces. F6 is much better for visual use.

CS,
Greg

Edited by truckerfromaustin, 19 January 2023 - 07:17 PM.


#22 Paul Sweeney

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Posted 20 January 2023 - 02:51 PM

Before you buy anything, I suggest finding your local club and let someone with lots of experience look at your scope. It could be that tweeking the collimation solves the problem. And maybe the mirrors are trash. Until you know what the problem is, you cannot solve it.
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#23 Bill Weir

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Posted 20 January 2023 - 03:12 PM

You’re in Halifax, do you belong to these folks? Feb. 4 at Saint Mary’s noon. Go.You can go to this meeting without needing to belong.  https://halifax.rasc.ca

 

I know several members and there are many there who will be able to advise you on your scope. I have a feeling there is nothing at all wrong with the scope and it just needs a bit of adjustment. At star parties there will actually be folks wearing buttons that say “Scope Doctor” so people will be able to find one if they are having issues with their scope.

 

Bill


Edited by Bill Weir, 20 January 2023 - 03:13 PM.


#24 Asbytec

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Posted 20 January 2023 - 10:52 PM

Do nothing to the mirror.

 

This scope was designed for imaging and as such, improving the quality of the mirror will have almost no effect on the quality of the view, or when using the scope for imaging

 

I suppose for imaging, I'd agree with Eddgie not to worry too much about the mirrors or the size of the obstruction. Most, if not all, astrographs have large diagonal or secondary mirrors optimized for imaging and still produce fine images with stock optics. 

 

I agree with Tony, as well, seeing conditions and skill are the primary drivers for seeing nice images visually, along with thermal stability (a form of seeing) and collimation (a form of optical quality). If you have both good seeing and skill using a scope prepped for observing, you should be seeing nice images. If not, then consider upgrading both mirrors for visual.

 

You can optimize the diagonal if you want. I shaved a few millimeters off my stock 2.4" diagonal (30%) and went with a high quality 2.1" diagonal to match the sensibly perfect primary while trying to keep total RMS /theoretically/ within the diffraction limit according to Mel's telescope designer. I guess it's "optimized" for planetary viewing but still 26% obstructed.


Edited by Asbytec, 21 January 2023 - 06:51 AM.


#25 riker1

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Posted 30 January 2023 - 09:40 PM

I don't know yet . I was thinking TS optics but Would appreciate any leads.  Also if both mirrors or just primary will do the job .

 

I have pondered this question as well recently.

 

Other than TS Optics, there are others out there as well. 

 

In the U.S. you might consider Edmunds Optics.  I believe they used to be related to Edmund Scientific a very long time ago.  They mfg a wide variety of optics.  They also offer precision parabolic mirrors which would work for a primary mirror.  They 8" seemed reasonable but they don't advertise an F4 which is what I wanted.  Anything over 8" though gets very pricey.  That said they are one of the few production facilities that will advertises a surface accuracy of λ/8.  ( https://www.edmundop...-mirrors/11895/ )

 

In Europe, I also found an outfit called Astroreflect.  They focus on building what appear to be from nice ultralight dobs and mirrors.  The mirrors they do quality testing on and state a Strehl accuracy of > 0.95.  They provide a copy of the test results with each shipped mirror.... The pricing was more competitive than Edmunds - but you might have to wait longer to get a mirror as they are a small shop ...  ( https://www.astroref...cope-mirrors/) 

 

I also found another outfit in the U.S. called Waite Research.  This is another very small shop.  It seems the owners work is likely top notch but you will wait likely > 1 year and the pricing is higher than Edmunds or Astroreflect.

 

For me, I can't justify upgrading the mirrors in my Explore Scientific PN210.  It doesn't make sense $$$ wise.  My copy was a 50% off ES Refurb special...  The mirrors seem OK but other than eyeball star tests - I can't tell how good they are or aren't.  The material though seems decent for the price point (CDGM H-PZ33 Low Expansion Glass).  I did give it a better GSO crayford focuser, a Protostar heated spider upgrade, and some flocking.  I think my biggest obstacle is poor seeing (lots of clouds in New England).

 

Edit - I also primarily use my ES N210 -38 for astrophotography and only use if for visual in rare circumstances... The ES N210 though for the most part knocks the socks off my Stellarvue 90/D for anything but the moon.  Unlike the Stellarvue, I can't pick it up easily to move around the yard.  So I am stuck where ever its setup for the night and very large oaks / poplars are sometimes in the way... 


Edited by riker1, 30 January 2023 - 09:52 PM.



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