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The mirror in my Skywatcher 14

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#26 jring

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Posted 30 January 2018 - 07:39 PM

Congratulations - if you observe visually an image like this one, which was produced via lucky imaging aka stacking the best of thousands of frames, I'll gladly buy a 14" Skywatcher Dobs too, as it seems to come with a seeing corrector... maybe there is also a cloud filter available as an accessory ;-)

 

Joachim



#27 eyepiecedropper

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Posted 31 January 2018 - 10:55 AM

My 14" SW shows the same amount of detail, which is clearly visible and far better than marginally discernable. Blueish festoons, white ovals, no exaggeration here.

Unfortunately this happens only at sunset and in a short time window of up to 20 minutes with above average seeing conditions.

And I can tell you: I´m not blessed with above average seeing conditons.

 

 

Martin


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#28 Don H

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Posted 31 January 2018 - 12:02 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

Was that picture done with the new 14 inch mirror? It is very nice.

 

Thanks,

Don



#29 Lem Jettison

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Posted 31 January 2018 - 01:04 PM

Greetings All, 

 

If it's okay to be piggybacking on to Richard's original question after seeing many interesting responses, I recently got a new Orion 14g (my first Dob) and am inexperienced with the detection of mirror anomalies such as afflicted Richard's scope. I would perhaps only have noticed their effects and been unable to attribute their true causes had I not stumbled onto this thread.

 

First light was several nights ago in very cold temps 14 F temps. The few stars I was able to see (before it clouded over) were not as pinpoint as I'd expected despite using well corrected TV EPs of varying FLs. I did let it cool for maybe an hour + 20min with mirror fans, but, as I said, it was very cold. At the end of the session, when viewing the moon almost at zenith, I noticed very subtle heat waves. Would that have been the scope still cooling after 2 hours? Also, the seeing and transparency were quite poor, so I realize the role that may have played. However, to a Dob newbie like myself, does anyone have some essential pointers they could share? After seeing Richard's original query, I am concerned that I'm unable to properly, systematically discern whether there are unacceptable anomalies in the mirror in my scope.

 

I see some refer to astigmatisms and "over corrections", and the like. How can I check for such anomalies? Additionally, while I understand how important mirror cooling to the ambient temperature is, perhaps I don't realize just how crucial that really is? Like I said, it just seemed hard to get a super sharp focus on stars. What may cause such a situation with a 14" Dob? While I'm still under warranty, I'd like to be able to suss out any mirror flaws. I'm sure this is a well-worn topic, and apologies for tacking it on to your thread, Richard, but as our mirrors probably came from the same factory, I'd be grateful for any additional insights or advice.  Many thanks for your time (and space here on this thread)!

~Lem



#30 pstarr

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Posted 31 January 2018 - 04:53 PM

Lem,

There are many articles on star testing. Just Google the subject and go from there. Don't put too much into what you saw your first time out. Determining optical quality takes multiple tries for someone new to the procedures. You were most likely seeing distortions caused by the cold temperatures you were experiencing. You need to do testing under more favorable conditions. For now just enjoy your new scope. I'm sure it will be fine.

 

http://www.backyarda...x A-Testing.pdf


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#31 Justin Fuller

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Posted 01 February 2018 - 12:14 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


You will get much better results than that when everything clicks; seeing, cool down, etc.

#32 Asbytec

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Posted 01 February 2018 - 07:34 PM

"I see some refer to astigmatisms and "over corrections", and the like. How can I check for such anomalies? Additionally, while I understand how important mirror cooling to the ambient temperature is, perhaps I don't realize just how crucial that really is?"

 

A few quick tests when cooled and collimated in reasonably good seeing.

 

Try to evaluate the "snap" to the focus. If focus is fairly snappy, it's doing well. If it smashes through focus without any firm point where you know you're in focus, you likely have some spherical aberration. Spherical aberration does not have one focal point, it produces a small range of focus points from each zone. 

 

So you get a set of "caustic" focus points instead of one focal points. As you focus through the caustic, the image seems to mush through them without really coming to one distinct focus. You cannot say how much SA, but you can tell if it's bad. However, with SA you will have a position of "best focus." If you can find that, it'll be kind of snappy and just fine. 

 

Another clue for correction error is whether the defocused pattern is clearly defined on each side of focus. If it is or you are close, you're fine. If outside defocused pattern is more blurry, you have some undercorrection. If inside is blurry, you have some overcorrection. A little bit of undercorrection is not a bad thing, but overcorrection (like a turned edge) can be. So, if you see a difference in the sharpness of the pattern, it's better if it's outside focus. 

 

Astigmatism you will get an elongated defocused pattern that flips 90 degrees on either side of focus. The amount of elongation is indicative of the amount of error. Defocus both sides not too far from focus and see. If it's slightly elongated, no worries. If it's noticeable, you have some problem. 

 

Try to do your star testing close to focus with just a few rings, not the huge defocused pattern with the diagonal shadow in the center. Except, at this large defocus, you can check for zones and smoothness (in good seeing.)

 

Zones appear as brighter rings, brighter than the others (excluding the outer and inner most rings.) If you see a bright ring on one side of focus between the inner and outer ring, that's possibly a zone. Scroll to the other side of focus and see if that ring is darker than the rest. If so, you have a zone which may or may not be problematic.

 

A rough mirror, again in the best seeing you can get, will not be uniformly bright across the large defocus pattern with many rings and the secondary shadow. It'll actually look rough. The more uniform the larger defocused pattern is, the smoother it is. 

 

None of these can quantify your mirror, just give you an idea of whether it's good, passable, or bad.  Especially the snap to focus test. Try it on the moon, see if you can find a point on the moon you know you are in focus. If so, you're probably doing well. In the best moments of seeing, of course, and without seeing those "heat waves" if possible.


Edited by Asbytec, 01 February 2018 - 07:41 PM.

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#33 Lem Jettison

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Posted 01 February 2018 - 09:30 PM

Lem,

There are many articles on star testing. Just Google the subject and go from there. Don't put too much into what you saw your first time out. Determining optical quality takes multiple tries for someone new to the procedures. You were most likely seeing distortions caused by the cold temperatures you were experiencing. You need to do testing under more favorable conditions. For now just enjoy your new scope. I'm sure it will be fine.

 

http://www.backyarda...x A-Testing.pdf

Thanks, Paul, and thanks also for the PDF; I'll check it out...



#34 Lem Jettison

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Posted 01 February 2018 - 09:34 PM

"I see some refer to astigmatisms and "over corrections", and the like. How can I check for such anomalies? Additionally, while I understand how important mirror cooling to the ambient temperature is, perhaps I don't realize just how crucial that really is?"

 

A few quick tests when cooled and collimated in reasonably good seeing.

 

Try to evaluate the "snap" to the focus. If focus is fairly snappy, it's doing well. If it smashes through focus without any firm point where you know you're in focus, you likely have some spherical aberration. Spherical aberration does not have one focal point, it produces a small range of focus points from each zone. 

 

So you get a set of "caustic" focus points instead of one focal points. As you focus through the caustic, the image seems to mush through them without really coming to one distinct focus. You cannot say how much SA, but you can tell if it's bad. However, with SA you will have a position of "best focus." If you can find that, it'll be kind of snappy and just fine. 

 

Another clue for correction error is whether the defocused pattern is clearly defined on each side of focus. If it is or you are close, you're fine. If outside defocused pattern is more blurry, you have some undercorrection. If inside is blurry, you have some overcorrection. A little bit of undercorrection is not a bad thing, but overcorrection (like a turned edge) can be. So, if you see a difference in the sharpness of the pattern, it's better if it's outside focus. 

 

Astigmatism you will get an elongated defocused pattern that flips 90 degrees on either side of focus. The amount of elongation is indicative of the amount of error. Defocus both sides not too far from focus and see. If it's slightly elongated, no worries. If it's noticeable, you have some problem. 

 

Try to do your star testing close to focus with just a few rings, not the huge defocused pattern with the diagonal shadow in the center. Except, at this large defocus, you can check for zones and smoothness (in good seeing.)

 

Zones appear as brighter rings, brighter than the others (excluding the outer and inner most rings.) If you see a bright ring on one side of focus between the inner and outer ring, that's possibly a zone. Scroll to the other side of focus and see if that ring is darker than the rest. If so, you have a zone which may or may not be problematic.

 

A rough mirror, again in the best seeing you can get, will not be uniformly bright across the large defocus pattern with many rings and the secondary shadow. It'll actually look rough. The more uniform the larger defocused pattern is, the smoother it is. 

 

None of these can quantify your mirror, just give you an idea of whether it's good, passable, or bad.  Especially the snap to focus test. Try it on the moon, see if you can find a point on the moon you know you are in focus. If so, you're probably doing well. In the best moments of seeing, of course, and without seeing those "heat waves" if possible.

Many thanks, Norme. This is very interesting and informative. I'll endeavor to see what these methods will reveal with my new scope. Additionally, I just ordered a Hubble Optics "Artificial Star", so perhaps I can do some of this testing in a more controlled setting in-doors. I hope it all turns out okay; it would be a real bummer to find some flaws that exceed acceptable levels. Thanks again!



#35 Asbytec

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Posted 02 February 2018 - 06:10 AM

Yes, you can do it indoors, but you introduce over correction. Maybe best in the field. You dont need a full blown star test to validate performance. You will know when images are soft or nice and sharp. For the soft images, you may want to know why: bad mirror, seeing, collimation, or thermals?

Snap focus is a good indicator and the other methods tell you something about the mirrors when seeing, cooling, and collimation are ruled out. The star test is very sensitive, so if you see something it's not necessarily bad. You know you have a good optic when your images of the moon are nice at 50x/inch and in good focus, for example.

An artificial star indoors is good for collimation, which you want to attend to. It reduces coma and improves performance on and near axis. So, collimate, then go snap focus on the moon or any object, really, at a good distance. If you get a decent snap, that's all you really need to know for an acceptable mirror. IMO, anyway.

Edited by Asbytec, 02 February 2018 - 06:14 AM.

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#36 pstarr

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Posted 02 February 2018 - 09:55 AM

One thing to remember when doing your testing. Commercial mirrors are figured to 1/4 wave at best and always have been.  A 1/4 wave defect is quite noticeable in the star test. Don't expect to see a near perfect star test in a modestly priced scope. That's not to say 1/4 wave is bad. A true 1/4 wave mirror will perform quite well when the seeing permits. It would be a rare night in the northeast when the seeing would permit a 14" scope to perform to the limits of it's aperture. Your scope will most likely be limited more by the atmosphere than it's optical quality 99% of the time. Good luck in your testing and remember, ALL mirrors have some error. I have never seen a perfect star test from any manufacture, premium mirrors included, very close but not perfect.


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#37 Lem Jettison

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Posted 02 February 2018 - 09:02 PM

One thing to remember when doing your testing. Commercial mirrors are figured to 1/4 wave at best and always have been.  A 1/4 wave defect is quite noticeable in the star test. Don't expect to see a near perfect star test in a modestly priced scope. That's not to say 1/4 wave is bad. A true 1/4 wave mirror will perform quite well when the seeing permits. It would be a rare night in the northeast when the seeing would permit a 14" scope to perform to the limits of it's aperture. Your scope will most likely be limited more by the atmosphere than it's optical quality 99% of the time. Good luck in your testing and remember, ALL mirrors have some error. I have never seen a perfect star test from any manufacture, premium mirrors included, very close but not perfect.

Thanks, Paul, for the insights.



#38 Lem Jettison

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Posted 02 February 2018 - 09:04 PM

Yes, you can do it indoors, but you introduce over correction. Maybe best in the field. You dont need a full blown star test to validate performance. You will know when images are soft or nice and sharp. For the soft images, you may want to know why: bad mirror, seeing, collimation, or thermals?

Snap focus is a good indicator and the other methods tell you something about the mirrors when seeing, cooling, and collimation are ruled out. The star test is very sensitive, so if you see something it's not necessarily bad. You know you have a good optic when your images of the moon are nice at 50x/inch and in good focus, for example.

An artificial star indoors is good for collimation, which you want to attend to. It reduces coma and improves performance on and near axis. So, collimate, then go snap focus on the moon or any object, really, at a good distance. If you get a decent snap, that's all you really need to know for an acceptable mirror. IMO, anyway.

Thanks, Norme, for these points. For what it's worth, despite the subtle but visible heat waves I'd mentioned, the scope's view of the moon was, all things considered, nothing short of stunning, so perhaps that's a good preliminary omen. 



#39 jeffmac

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Posted 02 February 2018 - 09:17 PM

Richard, did you get a chance with your first mirror to let Jim Presley take a look at it?



#40 jjgodard

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Posted 03 May 2018 - 09:10 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.

Don't know if you remember, but I have the same issue of not being able to achieve focus with the same 14" mirror. 



#41 Pinbout

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Posted 03 May 2018 - 08:06 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 starpost-65519-0-14855400-1515347403_thumb.j

 

Richard

gallery_106859_355_6324.jpg



#42 jjgodard

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Posted 03 May 2018 - 08:09 PM

gallery_106859_355_6324.jpg

OK, so can you walk us through this, which are which, what's important etc.?



#43 Pinbout

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Posted 03 May 2018 - 08:22 PM

mel does it better

 

http://www.bbastrode...tarTesting.html

 

I walk thru this star test

 

https://www.youtube....&index=7&t=110s




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