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The mirror in my Skywatcher 14

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#1 Richard Turner

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Posted 07 January 2018 - 12:50 PM

Well, I just purchased a Skywatcher 14 inch go to Dob.  After hours of cool down time, I did a star test.  Attached is a drawing of what I see.  Just inside "focus" (I use the term loosely, there does not seem to be a sharp focus), the secondary shadow is enormous.  Just outside of focus, I can't even see a secondary shadow.  You can see the significant difference in the secondary shadow far inside and outside of focus.  I believe this indicates significant undercorrection.  My mirror is too much like a sphere.  Can anyone with perhaps more knowledge than me tell me how bad (or not so bad) my mirror is?

 

Thanks.

 

Attached--drawing of out of focus star

 

Richard

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#2 Justin Fuller

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Posted 07 January 2018 - 01:10 PM

This seems indicative of spherical aberration, which is pretty common to some degree in mass-produced scopes. It's hard to say whether it's an unacceptable amount, which is subjective, though it does seem like a bad case of it. As you said, there seems to be no sharp focus, which I think may be a good argument for sending back. If there is a warranty against manufacturing defects, this seems to qualify as a defect. I would give a return a try.


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#3 jimr2

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Posted 07 January 2018 - 02:36 PM

Richard,
Hi, before sending the scope back, make sure you've let it sit outside a fair amount of time to adjust to ambient temps there wherever you are, and also make sure you're trying to do the star test on a night of relatively good, i.e., "steady" seeing conditions. If after doing all that your star test isn't any better, than yeah, probably best to send the scope back.
Good luck!
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#4 J A VOLK

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Posted 07 January 2018 - 02:37 PM

Looks like a large (unacceptable) degree of spherical aberration. If you are new to star testing, see if you can get someone more experienced to confirm. Mel Bartels has a nice primer on star testing on his website.
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#5 coopman

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Posted 07 January 2018 - 07:00 PM

Have you checked to see if the mirror retaining clips are too tight?   They should not be touching the mirror's surface.


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#6 Richard Turner

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Posted 07 January 2018 - 08:38 PM

Hi, Clay.  It's a conical mirror, supported from the center.  There are no clips.  I will check the center mounting support to see if it's too tight.

 

Thanks,

 

Richard 



#7 coopman

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Posted 07 January 2018 - 10:29 PM

contact Kevin LeGore here:
https://www.cloudyni...?hl=+skywatcher
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#8 Pierre Lemay

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Posted 08 January 2018 - 12:45 PM

Well, I just purchased a Skywatcher 14 inch go to Dob.  After hours of cool down time, I did a star test.  Attached is a drawing of what I see.  Just inside "focus" (I use the term loosely, there does not seem to be a sharp focus), the secondary shadow is enormous.  Just outside of focus, I can't even see a secondary shadow.  You can see the significant difference in the secondary shadow far inside and outside of focus.  I believe this indicates significant undercorrection.  My mirror is too much like a sphere.  Can anyone with perhaps more knowledge than me tell me how bad (or not so bad) my mirror is?

What is the focal length of the 14 inch and what eyepiece did you use to do the star test? You should normally use an eyepiece whose focal length is at least as short, in millimeters, as the focal ratio (ex: a 5mm eyepiece for an f/5).

 

Conical mirrors cool very fast. That and the fact that you let the scope cool for hours makes it unlikely that the mirror's parabolic surface was subject to a change in shape due to cooling. As you mentionned there are no edge clips so no warping (which would show up as astigmatism, which doesn't seem to be the case here according to your sketches).


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#9 Pinbout

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Posted 08 January 2018 - 08:54 PM

Under corrected

 

A high end mirror

 

https://youtu.be/DtdYBmSapio

 

Use shims to help measure defocus

 

https://youtu.be/QxUQJjjsdW4

 

What eyepiece what f# scope and

 

how much defocus

not enough can give bad readings and too much makes it less sensitive for judging SA 

 

but it looking for zones you need to defocus it like your second sketch or a little more.

 

barlows can add correction so don’t use one. Use a well corrected eyepiece


Edited by Pinbout, 08 January 2018 - 09:00 PM.

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#10 skyward_eyes

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Posted 08 January 2018 - 09:51 PM

This was a very odd issue that I’ve never had occur before. The factory pumps these out daily so it’s strange to have one come out like this.

He reached out to us today and we took care of it. He should have a new mirror by next week. This will hopefully resolve any issue and have him out under the stars in no time.
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#11 Pinbout

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Posted 08 January 2018 - 10:08 PM

This was a very odd issue that I’ve never had occur before. The factory pumps these out daily so it’s strange to have one come out like this.

He reached out to us today and we took care of it. He should have a new mirror by next week. This will hopefully resolve any issue and have him out under the stars in no time.

So what do you do with returns


Edited by Pinbout, 08 January 2018 - 10:08 PM.


#12 Richard Turner

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Posted 08 January 2018 - 10:13 PM

The scope is f/4.6.  I used a Celestron Ultima 7.5mm eyepiece for star testing.  Skywatcher was great on customer service.  They are sending a replacement mirror.

 

Richard 

(North Carolina)


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#13 skyward_eyes

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Posted 08 January 2018 - 10:29 PM


This was a very odd issue that I’ve never had occur before. The factory pumps these out daily so it’s strange to have one come out like this.

He reached out to us today and we took care of it. He should have a new mirror by next week. This will hopefully resolve any issue and have him out under the stars in no time.

So what do you do with returns
I’m sure a lot of people would like to know what happens to returned gear. Kind of a unknown outside the industry.

There’s a couple things that can happen to returned gear.

For gear that arrived defective that should have been caught by QA is usually returned to the factory. I’m not sure what they do with it from there.

For equipment that might be damaged in one way or another we scrap for the good parts. Once scraped for everything that’s usable we destroy and trash the rest.
Sometimes we have something returned that can be fixed up with a little effort. We either use these for demos, photographic models, reference samples or we will donate to an outreach program for education.

This 14” should have never left and will go back to the factory.

Edited by skyward_eyes, 08 January 2018 - 10:31 PM.

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#14 mark379

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Posted 09 January 2018 - 01:32 PM

I like the idea that the good parts are salvaged! It keeps from having too much waste, and allows more service parts to become available. Keep up the good practices!waytogo.gif

 

BTW, I did see the above mirror problem before at a star party. It was an Orion 14G that has the same Synta mirror in it. He had to return his mirror for another as well if memory serves.

I'm happy to say that the Skywatcher 14 Goto that I received in September is preforming very nicely, and mirror is quite fine.

I too needed a few missing parts that didn't come with it, but Kevin at Skywatcher took real good care of me and the problem was solved.cool.gif

Mark

 

 

 

 

This was a very odd issue that I’ve never had occur before. The factory pumps these out daily so it’s strange to have one come out like this.

He reached out to us today and we took care of it. He should have a new mirror by next week. This will hopefully resolve any issue and have him out under the stars in no time.

So what do you do with returns
I’m sure a lot of people would like to know what happens to returned gear. Kind of a unknown outside the industry.

There’s a couple things that can happen to returned gear.

For gear that arrived defective that should have been caught by QA is usually returned to the factory. I’m not sure what they do with it from there.

For equipment that might be damaged in one way or another we scrap for the good parts. Once scraped for everything that’s usable we destroy and trash the rest.
Sometimes we have something returned that can be fixed up with a little effort. We either use these for demos, photographic models, reference samples or we will donate to an outreach program for education.

This 14” should have never left and will go back to the factory.

 


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#15 coopman

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Posted 09 January 2018 - 09:59 PM

You're a good man, Kevin.  Thanks for taking good care of your customers.   


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#16 SteveG

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Posted 09 January 2018 - 10:51 PM

This was a very odd issue that I’ve never had occur before. The factory pumps these out daily so it’s strange to have one come out like this.

He reached out to us today and we took care of it. He should have a new mirror by next week. This will hopefully resolve any issue and have him out under the stars in no time.

When I upgrade to my next big dob, it's going to be a Skywatcher. I've been reading great things about them, and this customer service is outstanding!


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#17 nirvanix

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Posted 10 January 2018 - 11:47 AM

 

This was a very odd issue that I’ve never had occur before. The factory pumps these out daily so it’s strange to have one come out like this.

He reached out to us today and we took care of it. He should have a new mirror by next week. This will hopefully resolve any issue and have him out under the stars in no time.

When I upgrade to my next big dob, it's going to be a Skywatcher. I've been reading great things about them, and this customer service is outstanding!

 

I'm saving my nickels (no more pennies up here) for one of the new skywatcher 20" ultralights - they've done it right!


Edited by nirvanix, 10 January 2018 - 11:48 AM.

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#18 txmnjim

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Posted 15 January 2018 - 10:09 AM

I've been thinking of nabbing the 16" Dob while its still on sale and now I may just do that.

Thanks for the good customer support!!


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#19 Richard Turner

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Posted 15 January 2018 - 11:13 AM

The replacement mirror has arrived.  Great customer service from Skywatcher.

 

Thanks,  Kevin!

 

 

Richard Turner


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#20 stargazer193857

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Posted 17 January 2018 - 09:42 PM

14" is a good size. Not to talk to need a ladder. Not even too big to need a large vehicle. Reasonable f ratio. And good aperture. I think it is the biggest aperture to just fit the stuff I like to fit in view. And the price of a 14" is not bad either. One of these days I might get one.

#21 George Methvin

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Posted 18 January 2018 - 10:05 AM

I don't like the star test and don't use it to judge my optics. Every thing has to be just right to get a good reading using the star test. I have tested several of my scope that I know to have good optics but on most nights the star test is unreadable due to the seeing or scope not cool down all the way are not aliened well . If I were to try to read the star test in those conduction I would think I had bad optics yet on a few rare clear steady nights with the scope optic aliened and all cooled down the star test show good optics. So be careful in rushing to judgment about a scopes optic when using the star test this has just been my experience but others mileage may very.


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#22 pstarr

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Posted 18 January 2018 - 01:11 PM

I don't like the star test and don't use it to judge my optics. Every thing has to be just right to get a good reading using the star test. I have tested several of my scope that I know to have good optics but on most nights the star test is unreadable due to the seeing or scope not cool down all the way are not aliened well . If I were to try to read the star test in those conduction I would think I had bad optics yet on a few rare clear steady nights with the scope optic aliened and all cooled down the star test show good optics. So be careful in rushing to judgment about a scopes optic when using the star test this has just been my experience but others mileage may very.

It's actually quite easy to tell a good mirror from a clunker, even in mediocre seeing. A marginal mirror takes greater care. Of course any mirror needs to be completely cooled down. Now if your trying to see roughness, zones, a turned down edge or rate the mirrors wavefront error, good seeing and cooled down optics are required. IMO, Spherical aberration/correction problems and astigmatism are easy to see, even in average seeing.



#23 George Methvin

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Posted 18 January 2018 - 03:51 PM

What I do on a good steady night with the moon about half full I view the Moon with my highest power eyepiece say a 4mm with a 2x Barlow and focus in on a crater though the image will get dimmer if the image remains sharp then I pretty sure the optics are good Saturn is another good object to try this on .  That's works for me in the past I have had some poor optic that even with medina power the image would not hold up even on nights of steady.


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#24 Richard Turner

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Posted 29 January 2018 - 06:13 PM

The mirror in my Skywatcher 14 turns out to be not so bad after all.  I have attached an image of Jupiter that is a close approximation of what I see in the eyepiece.

 

Richard

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  • jupiter.jpg

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#25 acochran

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Posted 29 January 2018 - 08:08 PM

The mirror in my Skywatcher 14 turns out to be not so bad after all.  I have attached an image of Jupiter that is a close approximation of what I see in the eyepiece.

 

Richard

Really? Including the festoons in the bands? This is the new mirror? Very nice! 

Andy


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