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127mm Mak Owners: Question

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#1 spongebob@55

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Posted 27 December 2018 - 06:36 PM

I hope you don't mind me asking you recent/new owners a question about your 127's optical performance.....I just bought a 127mm mass produced Mak. I really like it and planned on using it for doubles, etc. I've had first light +2. When I use a 17, 12, 10 and 8mm eyepiece, and I'm focused on a star/double, I see multiple circular rings around the star/double. Its very noticeable and obtrusive for me. Some have said that it 'normal' and some have mentioned that it might have a lot of spherical aberration which causes this.  Also b/c of its center obstruction, more so b/c it measures out to 120mm not 127mm.  I've observed with other Maks and SCTs and have never seen these multiple rings which are so obvious.  An observing friend has not either. 
My question is do you have these rings in your Mak too?  It becomes greater as the power increases, but is also seen at lower powers. 
Thanks very much for your response.
Regards

SB.


Edited by spongebob@55, 27 December 2018 - 06:36 PM.


#2 Jim Waters

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Posted 27 December 2018 - 07:02 PM

Do they look something like this when the star is focused?

https://en.wikipedia.../wiki/Airy_disk

 

Or are you getting these when the star image is out of focus?

 

https://starizona.co...lescope-optics/


Edited by Jim Waters, 27 December 2018 - 07:03 PM.

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#3 Bean614

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Posted 27 December 2018 - 07:52 PM

Hmmmmm.....   Well, you were informed, in a previous post, by some of CN's most accomplished observers, that not ONLY was it normal, but indicated the above average optics in your scope!  But, you don't like it.  Best advice would be to sell the scope to someone who DOES like it, and perhaps look for something cheaper that won't show those rings.



#4 KerryR

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Posted 27 December 2018 - 08:22 PM

An in-focus stellar image in a (decent) Mak shouldn't look hugely different than through other scopes of similar aperture at the same magnification on the same magnitude star.

 

We'd expect a bit more light in those first few rings than through a refractor of the same aperture and magnification, but it shouldn't be particularly obtrusive, though this is obviously subjective. The rings should be very similar to, say, an SCT, or even a small fast newt, maybe a teeny bit brighter to account for high order SA that's often said to be inherent in Gregory Maks.

It's normal to see diffraction rings on bright stars, of course, but, it shouldn't be radically different than what you're used to- I see you've been on CN for 7 years, so I'd assume you have a pretty good idea of what you're looking at.

 

Obtrusively bright diffraction rings is not something that stands out in my memories of stellar images in my 127mm Orion Mak.

I'd expect the rings to be easily observable on a bright star at the 90x your 17mm is producing (assuming 1540mm FL).

Just curious: Are you testing this scope on an artificial star?


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#5 MortonH

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Posted 27 December 2018 - 08:35 PM

Probably what you're seeing are diffraction rings around the stars.  I recently acquired a Skywatcher 127mm Mak and it is the same.  I also just acquired an older C5 and it is very similar.

 

From memory I agree that the rings seem a bit more prominent in the 127mm Mak than other Cats & Casses I've used.  This is probably a combination of:

 

(1) quality  optics

(2) small aperture, and

(3) large-ish central obstruction (relative to the aperture)

 

Only you can decide if they're objectionable or not. 


Edited by MortonH, 27 December 2018 - 08:35 PM.

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#6 spongebob@55

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Posted 27 December 2018 - 09:53 PM

An in-focus stellar image in a (decent) Mak shouldn't look hugely different than through other scopes of similar aperture at the same magnification on the same magnitude star.

 

We'd expect a bit more light in those first few rings than through a refractor of the same aperture and magnification, but it shouldn't be particularly obtrusive, though this is obviously subjective. The rings should be very similar to, say, an SCT, or even a small fast newt, maybe a teeny bit brighter to account for high order SA that's often said to be inherent in Gregory Maks.

It's normal to see diffraction rings on bright stars, of course, but, it shouldn't be radically different than what you're used to- I see you've been on CN for 7 years, so I'd assume you have a pretty good idea of what you're looking at.

 

Obtrusively bright diffraction rings is not something that stands out in my memories of stellar images in my 127mm Orion Mak.

I'd expect the rings to be easily observable on a bright star at the 90x your 17mm is producing (assuming 1540mm FL).

Just curious: Are you testing this scope on an artificial star?

Thanks Kerry for your helpful comments.  

I haven't really seen this in another's 127 Mak, or my 6"SCT.  Or even my 8" SCT or a 7" Mak.  But I don't know how they would compare with the 127mm Mak.  I had a colleague over and he too was a bit surprised at how prominent they were compared to his refractors.  We observed with it over 3 different nights, on different magnitude stars and doubles.  No artificial star.  I was surprised how I could see rings even at such low powers and on dimmer stars; I had thought they were only visible at high power on brighter stars/doubles.   Also wondering how seeing would affect the rings.  I eliminated a dew/frost consideration that had been mentioned.  

Thanks again.



#7 Jim Waters

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Posted 27 December 2018 - 09:57 PM

Is there anyway you can take a picture and post it?  Or try to draw what you see with the star in focus?



#8 spongebob@55

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Posted 27 December 2018 - 09:58 PM

Probably what you're seeing are diffraction rings around the stars.  I recently acquired a Skywatcher 127mm Mak and it is the same.  I also just acquired an older C5 and it is very similar.

 

From memory I agree that the rings seem a bit more prominent in the 127mm Mak than other Cats & Casses I've used.  This is probably a combination of:

 

(1) quality  optics

(2) small aperture, and

(3) large-ish central obstruction (relative to the aperture)

 

Only you can decide if they're objectionable or not. 

Thanks MortonH, excellent similar experience with the same sized 127mm.  I just never have seen this in 2 other 127mm.   Just when I'm looking at a nice double, and see 2 stars with rings around them both, it ruins the view.  Thanks again.


Edited by spongebob@55, 27 December 2018 - 10:05 PM.


#9 spongebob@55

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Posted 27 December 2018 - 10:04 PM

Is there anyway you can take a picture and post it?  Or try to draw what you see with the star in focus?

I tried taking a iphone picture, but couldn't get it.   I'll try a drawing again as soon as I use it again so I get it exact at the time, and not from my memory.  Your previous post was very helpful and I printed it out also for the next session.  Thanks Jim.



#10 Magnetic Field

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Posted 28 December 2018 - 03:59 AM

I hope you don't mind me asking you recent/new owners a question about your 127's optical performance.....I just bought a 127mm mass produced Mak. I really like it and planned on using it for doubles, etc. I've had first light +2. When I use a 17, 12, 10 and 8mm eyepiece, and I'm focused on a star/double, I see multiple circular rings around the star/double. Its very noticeable and obtrusive for me. Some have said that it 'normal' and some have mentioned that it might have a lot of spherical aberration which causes this.  Also b/c of its center obstruction, more so b/c it measures out to 120mm not 127mm.  I've observed with other Maks and SCTs and have never seen these multiple rings which are so obvious.  An observing friend has not either. 
My question is do you have these rings in your Mak too?  It becomes greater as the power increases, but is also seen at lower powers. 
Thanks very much for your response.
Regards

SB.

 

1. I don't like the following site:***

 

https://www.telescop...g_telescope.htm

 

 

2. However, have a look at the Figure 4 in the above link.

 

Note: The Figure is for a 30% obstruction (denoted as o=0.3D in the Figure). Anyway, the take home message: if you see 3 or 4 rings you are probably diffraction limited. If you see more rings your optics are not so good and suffer from various flaws. For a perfect system you would see one or two rings (see Figure 3 in the above link).

 

 

3. You can use the following site of jetstream forecasts to get an idea of your seeing at your observing location. You need a steady night to see the diffraction rings:

 

http://www.chilescop...tream-forecast/

 

You can zoom in at your location and double click for some numbers.

 

 

4. Can you also use your Mak without the diagonal? You would get some confirmation it is not the diagonal.

 

 

*** For most parts I cannot get on with it. There is no peer-review involved and this is the reason why it is so badly written.


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#11 Asbytec

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Posted 28 December 2018 - 04:58 AM

Check out figure 189 toward the bottom of the page. This is normal in focus and defocus MCT star test. Is this what you see?

 

If so, here are some sketches of what it looks like in my scope. And if so, it's perfectly normal behavior of waves passing through an aperture and, frankly, unavoidable with an obstruction (and to a lesser extent, even without an obstruction.)

 

Gamma Leo:

 

Gamma Leo.jpg

 

 


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#12 Dave Ponder

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Posted 28 December 2018 - 10:18 AM

Check out figure 189 toward the bottom of the page. This is normal in focus and defocus MCT star test. Is this what you see?

 

If so, here are some sketches of what it looks like in my scope. And if so, it's perfectly normal behavior of waves passing through an aperture and, frankly, unavoidable with an obstruction (and to a lesser extent, even without an obstruction.)

 

Gamma Leo:

 

attachicon.gif Gamma Leo.jpg

That is representative of my views thru my SW 127 Mak also.


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#13 spongebob@55

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Posted 28 December 2018 - 12:03 PM

Check out figure 189 toward the bottom of the page. This is normal in focus and defocus MCT star test. Is this what you see?

 

If so, here are some sketches of what it looks like in my scope. And if so, it's perfectly normal behavior of waves passing through an aperture and, frankly, unavoidable with an obstruction (and to a lesser extent, even without an obstruction.)

 

Gamma Leo:

 

attachicon.gif Gamma Leo.jpg

Thanks Asbytec,

The picture you show is very much what I see in Almach.  Perhaps the additional rings are more pronounced, but I'd have to look at it again to compare.

I'm just confused as to why I haven't seen these rings in my previous 6" SCT, and in my 8" SCT, nor in my 180mm Mak from a few years ago.  Maybe I wasn't at all interested in doubles back then so it did not register????   But I am certainly now and am crestfallen with these views.  Perhaps a 127mm Mak was the wrong scope for fast deployment doubles......but certainly fine for planets and moon.

Would these rings still be there in a 180mm Mak which has a much smaller central obstruction %/size compared to the primary?

Thanks for sticking with me on this subject!

SB



#14 Magnetic Field

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Posted 28 December 2018 - 12:23 PM

Thanks Asbytec,

The picture you show is very much what I see in Almach.  Perhaps the additional rings are more pronounced, but I'd have to look at it again to compare.

I'm just confused as to why I haven't seen these rings in my previous 6" SCT, and in my 8" SCT, nor in my 180mm Mak from a few years ago.  Maybe I wasn't at all interested in doubles back then so it did not register????   But I am certainly now and am crestfallen with these views.  Perhaps a 127mm Mak was the wrong scope for fast deployment doubles......but certainly fine for planets and moon.

Would these rings still be there in a 180mm Mak which has a much smaller central obstruction %/size compared to the primary?

Thanks for sticking with me on this subject!

SB

1. What magnifications were you using?

 

2. I have a hard time to see the airy disc and rings at 170x (6mm eyepiece) in my Vixen 110L (110mm aperture).

 

I think I need a 5 mm eyepiece.

 

3. Your 180mm Mak of course will show the airy pattern. However, larger telescopes also suffer more under poor seeing, e.g. posting #4 and #11: 

 

https://www.cloudyni...4782-airy-disk/

 

For a smaller scope you relatively need a lower magnification  to see the air disc and rings. However, for a 8" telescope you at least need more than 200x and often seeing is the limiting factor here.



#15 davidc135

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Posted 28 December 2018 - 02:02 PM

It sounds to me that you should trust your instincts (and experience) that suggest you have a genuine problem, although we can't see what you are seeing. Is it still under warranty and possible to return it and get another?

 

If not, could you do a star test on Polaris? That should show up gross errors that may explain it. Minor spherical aberration and zones should be acceptable.

 

David


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#16 spongebob@55

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Posted 28 December 2018 - 03:37 PM

1. What magnifications were you using?

 

2. I have a hard time to see the airy disc and rings at 170x (6mm eyepiece) in my Vixen 110L (110mm aperture).

 

I think I need a 5 mm eyepiece.

 

3. Your 180mm Mak of course will show the airy pattern. However, larger telescopes also suffer more under poor seeing, e.g. posting #4 and #11: 

 

https://www.cloudyni...4782-airy-disk/

 

For a smaller scope you relatively need a lower magnification  to see the air disc and rings. However, for a 8" telescope you at least need more than 200x and often seeing is the limiting factor here.

The magnifications I've been using are 64X, then 90X, 127X, 153X and finally 191X (8mm)  All ES, Delos or Pentax EPs.

SB



#17 Asbytec

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Posted 28 December 2018 - 06:42 PM

The magnifications I've been using are 64X, then 90X, 127X, 153X and finally 191X (8mm)  All ES, Delos or Pentax EPs.

SB

Okay, yes, you should be able to see the rings around 127x (1mm exit pupil). It has to do with the image scale and resolution of or eye to see such things. That would be 180x in your MCT and about 200x in your C8. It could be that you did not notice them before. These rings are inevitable, they would have been present in your 180 MCT and C8, as well, at a sufficient magnification. Especially when seeing does not disturb the image appreciably. 

 

As Dave said above, a star test would be nice. These rings are caused by two things. First is diffraction at the aperture, which is unavoidable (the obstruction is an aperture, as well...something in the light path causing additional diffraction). Second is aberration. Any flaws in the optics will exacerbate the brightness of the rings as rays (sic) of light do not focus at the same point causing some additional interference patterns. The result of this complex interference (phase) is usually light taken from the central Airy disc and transferred to the rings. So, bright rings can also mean there is some aberration. 

 

Star testing does have a learning curve, so I do not expect one can just defocus and tell right away. Especially in MCTs due to their complex optical design. But, there are some signs that are easy to look for. One is the shadow test where we compare the size of the secondary shadow at equal distance on both sides of focus. And fairly close to focus with just a few rings visible at high magnification. If the shadow is larger outside and smaller inside (which will likely be the case to some degree), this is an indication of some amount of spherical aberration. The shadows will likely be different and may not be cause for concern, it just depends how much. A noticeable or significant difference can indicate a problem. 

 

Another is the snap to focus test. Well corrected optics will have a certain amount of certainty when coming to focus. Poor optics will mush through focus, focus will either be uncertain or it may never focus nicely. You can check that if you wish and get a feel for how good the optics are, the more "snap" the better. If your scope is taking 191x with a nice image, I would not worry too much. No scope is perfect, some are better...but none are perfect. What really matters is how pleasing the image is at higher magnification. Good scopes can take magnification pretty well with clean images. If so, the rings are inevitable and nothing to get upset about. All scopes will show them to some degree, even a perfect refractor. 


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#18 Asbytec

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Posted 28 December 2018 - 07:02 PM

That is representative of my views thru my SW 127 Mak also.

It's kind of beautiful in it's own way. :)


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#19 Eric63

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Posted 28 December 2018 - 09:27 PM

That is representative of my views thru my SW 127 Mak also.


This is also my experience with my 127Mak, which I find to be a great performing scope.

Eric
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#20 jgraham

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Posted 28 December 2018 - 10:13 PM

The rings are normal. They are a little brighter in a scope with a secondary as diffraction around the secondary pumps a little more energy into the rings. Sharp rings usually indicate very good optics. The only fix is to switch to a scope with unobstructed optics like a refractor or a Schiefspiegler. Even these have rings, but they have more energy in the central peak.

 

Enjoy the view!


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#21 Bean614

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Posted 28 December 2018 - 10:28 PM

It sounds to me that you should trust your instincts (and experience) that suggest you have a genuine problem, although we can't see what you are seeing. Is it still under warranty and possible to return it and get another?

 

If not, could you do a star test on Polaris? That should show up gross errors that may explain it. Minor spherical aberration and zones should be acceptable.

 

David

??????  Are you actually suggesting that the OP, who has already declared that "I'm just confused as to why", return a telescope that Obviously has optics of outstanding quality???  Why would you suggest such a thing?



#22 Asbytec

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Posted 29 December 2018 - 12:36 AM

??????  Are you actually suggesting that the OP, who has already declared that "I'm just confused as to why", return a telescope that Obviously has optics of outstanding quality???  Why would you suggest such a thing?

I am not sure it's conclusive one way or another whether the OP has good or not so good optics. All we know is he see's rings and is somewhat surprised by them. Diffraction rings are, of course, normal with any aperture, brighter with an obstruction, and a little more pronounced with aberration. But, we just do not know anything about any aberration he may or may not be experiencing based solely on seeing rings. Without seeing the rings, the defocused patterns, and the image in focus for ourselves, we have no way to judge whether the scope is adversely impacted or performs nicely. Except we can assume some, if not most, MCTs are at least good, or we can assume all mass produced scopes are terrible and this one is no different. My hunch is, based on my experience with MCTs, a lot of anecdotal reports, and until more evidence is presented, the OP's scope is likely just fine. Seeing rings is normal at higher magnifications. 


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#23 MortonH

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Posted 29 December 2018 - 04:51 AM


I'm just confused as to why I haven't seen these rings in my previous 6" SCT, and in my 8" SCT, nor in my 180mm Mak from a few years ago.

I haven't often seen these rings in my C8 despite it having good optics.  But now that I think about it I've generally only used it in the winter where the seeing is less stable.  In summer it's often too warm at night and it doesn't get dark till almost 10pm so I don't bother.  However, since I acquired both a C5 and 127mm Mak plus a few eyepieces recently I've been observing more than usual for the time of year (I'm Southern Hemisphere so it's summer here) and the diffraction rings seem more obvious than usual.  So it's possible that your previous scopes weren't actually displaying the rings as prominently depending on the conditions when you used them.



#24 Dave Ponder

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Posted 29 December 2018 - 08:26 AM

I haven't often seen these rings in my C8 despite it having good optics.  But now that I think about it I've generally only used it in the winter where the seeing is less stable.  In summer it's often too warm at night and it doesn't get dark till almost 10pm so I don't bother.  However, since I acquired both a C5 and 127mm Mak plus a few eyepieces recently I've been observing more than usual for the time of year (I'm Southern Hemisphere so it's summer here) and the diffraction rings seem more obvious than usual.  So it's possible that your previous scopes weren't actually displaying the rings as prominently depending on the conditions when you used them.

I remember from several years ago that I thought my C8 was defective because I could not see any diffraction rings”....problem was I was not using enough magnification.  I rarely had an opportunity to get to 200X and for the C8, you need at least 25x per inch of aperture to begin seeing the rings as a rule of thumb. Once I had an opportunity to crank up the power I could see the rings! Now I use an artifical star to check collimation and can really go to high mags.  I like the rings!  They are quite helpful.



#25 davidc135

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Posted 30 December 2018 - 07:28 AM

??????  Are you actually suggesting that the OP, who has already declared that "I'm just confused as to why", return a telescope that Obviously has optics of outstanding quality???  Why would you suggest such a thing?

Often, for a time, customers are free to return goods for whatever reason. As the OP seemed to be having an unsatisfying experience with this Mak, that possibility seemed a reasonable one.

The OP has experience of similar telescopes; scts and 180 Mak and especially a friend's 127 Mak and compared this scope unfavourably to them. 'Very obtrusive even at low power' which doesn't suggest exceptionally good optics although, on the other hand, he was very pleased with views of the moon.

 

Well, as has been said, we don't know just how prominent these effects are and this scope may well be typical of it's type. Reduced aperture and increased c.o % plus residual HSA are (usually acceptable) deficiencies that may or may not have combined with other faults to cause a genuine problem.

 

David


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