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Orion Optics v Skywatcher!

philsail1

By philsail1
in Member Equipment Reviews

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kev 102 42

kev 102 42

Posted
Posted

Astigmatism is a condition of the human eye which causes either blurred vision or a sense that each eye is seeing objects slightly differently. Sometimes astigmatism can be detected by an individual if he covers one eye to look at an object, and then changes to cover the other eye. By switching back and forth between covering each eye while looking at the single object or in one direction, it will become apparent with astigmatism that the object seems to move, as though each eye is seeing it in a slightly different location.

Astigmatism is not a vision problem, but a structural problem with the eye. The human eye is usually shaped in a sphere, like a ball, but with astigmatism, the cornea of the eye is curved instead into an oval shape, more like a football. When the eye is not curved properly, or has uneven curves like a football, light will not travel in and out of the eye normally. Astigmatism causes light to bounce unevenly off the flat and steep curves of the oval shaped eye, thus allowing light to more than one point in the eye. This impairs focusing ability, causing blurred vision.

There are several ophthalmological tests to determine the presence and level of astigmatism. A keratometer and a corneal topographer measure the curvature of the cornea. An autorefractor can give an estimate of the eye's ability to focus light properly. Most ophthalmologists and optometrists are able to detect even slight cases of astigmatism with just a simple eye exam.

Glasses or contact lenses can be presecribed to treat astigmatism, but they generally do not correct the condition. Often, two different lenses will help the eyes focus together, thus offsetting the uneven focusing caused by astigmatism. Astigmatism, however, can be corrected with surgery. Lasik and astigmatic keratotomy are two commonly used surgeries for astigmatism. The object of surgery is to reshape the cornea of the eye and make it more spherical.

Astigmatism is quite common, with thirty percent of the population having some level of cornea curvature. Often, mild astigmatism goes largely unnoticed, but severe astigmatism may cause, in addition to blurred vision, headaches, squinting and fatigue. Most people with astigmatism are born with it, but the awareness of it increases with age.

I believe the orignal source for this material is taken from www.wisegeeks.com - MikeP

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GazOC

GazOC

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So now we are back to where we were a couple of pages ago, the advantage of a > 1/4th scope to the average astronomer.

Seem this way everyones happy, the more expensive OO mirrors are superior if you have the vewing conditions and the Skywatchers are also great value scopes that will satisfy most people.

A 3 pointer on the pools coupon if ever I saw one... :thumbright:

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barkis

barkis

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Posted

Astigmatism is a condition of the human eye which causes either blurred vision or a sense that each eye is seeing objects slightly differently. Sometimes astigmatism can be detected by an individual if he covers one eye to look at an object, and then changes to cover the other eye. By switching back and forth between covering each eye while looking at the single object or in one direction, it will become apparent with astigmatism that the object seems to move, as though each eye is seeing it in a slightly different location.

Astigmatism is not a vision problem, but a structural problem with the eye. The human eye is usually shaped in a sphere, like a ball, but with astigmatism, the cornea of the eye is curved instead into an oval shape, more like a football. When the eye is not curved properly, or has uneven curves like a football, light will not travel in and out of the eye normally. Astigmatism causes light to bounce unevenly off the flat and steep curves of the oval shaped eye, thus allowing light to more than one point in the eye. This impairs focusing ability, causing blurred vision.

There are several ophthalmological tests to determine the presence and level of astigmatism. A keratometer and a corneal topographer measure the curvature of the cornea. An autorefractor can give an estimate of the eye's ability to focus light properly. Most ophthalmologists and optometrists are able to detect even slight cases of astigmatism with just a simple eye exam.

Glasses or contact lenses can be presecribed to treat astigmatism, but they generally do not correct the condition. Often, two different lenses will help the eyes focus together, thus offsetting the uneven focusing caused by astigmatism. Astigmatism, however, can be corrected with surgery. Lasik and astigmatic keratotomy are two commonly used surgeries for astigmatism. The object of surgery is to reshape the cornea of the eye and make it more spherical.

Astigmatism is quite common, with thirty percent of the population having some level of cornea curvature. Often, mild astigmatism goes largely unnoticed, but severe astigmatism may cause, in addition to blurred vision, headaches, squinting and fatigue. Most people with astigmatism are born with it, but the awareness of it increases with age.

Hi Kev.

I always was led to believe that covering one eye, then the other, whilst looking at a particular object, and seeing it appear to shift, is due to parallax. The same as the method for measuring some of the nearer stars using the earths orbit around the sun as a baseline. A long baseline needed for that task, but not needed to demonstrate the effect using an object quite close to the observer.

I apologise if you were referring to something entirely different, as your knowledge of this subject is quite apparent.

Ron. :undecided:

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Astronut

Astronut

Posted
Posted

Thanks for posting the results Phil and to OO for giving an honest report - seems that the Skywatcher mirror is 'good enough' (certainly alright for my ageing eyes but remember this is a sample of one - I hope my mirror is at least as good as that) but it would have been interesting to have seen the secondaries compared and tested also.

I now have a better understanding of what the Strehl ratio is all about thanks to those links posted but could someone explain in simple terms what the Astmag reading is all about - something regarding astigmatism, perhaps? :?

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philsail1

philsail1

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9243_normal.jpeg

(click to enlarge)

Above is the Zygo Chart for the Skywatcher 8" Mirror.

Blow is the Zygo Chart for the Orion 8" Mirror

9245_normal.jpeg

(click to enlarge)

I would just like to say a big thank you to all for your interest, and helpful comments in explaining all this technical stuff!

Regards,

philsail1

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wingman

wingman

Posted
Posted

Astigmatism is a condition of the human eye which causes either blurred vision or a sense that each eye is seeing objects slightly differently. Sometimes astigmatism can be detected by an individual if he covers one eye to look at an object, and then changes to cover the other eye. By switching back and forth between covering each eye while looking at the single object or in one direction, it will become apparent with astigmatism that the object seems to move, as though each eye is seeing it in a slightly different location.

Astigmatism is not a vision problem, but a structural problem with the eye. The human eye is usually shaped in a sphere, like a ball, but with astigmatism, the cornea of the eye is curved instead into an oval shape, more like a football. When the eye is not curved properly, or has uneven curves like a football, light will not travel in and out of the eye normally. Astigmatism causes light to bounce unevenly off the flat and steep curves of the oval shaped eye, thus allowing light to more than one point in the eye. This impairs focusing ability, causing blurred vision.

There are several ophthalmological tests to determine the presence and level of astigmatism. A keratometer and a corneal topographer measure the curvature of the cornea. An autorefractor can give an estimate of the eye's ability to focus light properly. Most ophthalmologists and optometrists are able to detect even slight cases of astigmatism with just a simple eye exam.

Glasses or contact lenses can be presecribed to treat astigmatism, but they generally do not correct the condition. Often, two different lenses will help the eyes focus together, thus offsetting the uneven focusing caused by astigmatism. Astigmatism, however, can be corrected with surgery. Lasik and astigmatic keratotomy are two commonly used surgeries for astigmatism. The object of surgery is to reshape the cornea of the eye and make it more spherical.

Astigmatism is quite common, with thirty percent of the population having some level of cornea curvature. Often, mild astigmatism goes largely unnoticed, but severe astigmatism may cause, in addition to blurred vision, headaches, squinting and fatigue. Most people with astigmatism are born with it, but the awareness of it increases with age.

I think I've read this somewhere before :undecided:

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Milamber

Milamber

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So if you do most of your observing from a backyard urban location get a Skywatcher. But if you can make it out to a dark site quite often, get an OO.

I should have got a Skywatcher :undecided::) :)

I suspect that sadly, this about sums it up perfectly. Sadly because in a perfect world everyone would have access to a dark site - in reality though, most people have to suffer with Skywatcher sites.

Arthur

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johninderby

johninderby

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About the results expected form the SkyWatcher mirror. The surface roughness indicated from the results is about what you would expect from a machine polished mirror.

Interpreting the results, on DSO's etc where high mag isn't used and contrast isn't so important you really wouldn't notice much difference between the mirrors.

However on lunar / planetary where maximum contrast is important then the differences will show. Also one factor that many people are not aware of is that the maximum mag possible is very dependant on the smoothness of the mirror's surface. With top quality mirrors it's not uncommon to be able to use 800 to 1,000 mag on planets. This also explains why many observers who haven't had the chance to observe with a top quality mirror think that dobs are only for DSOs.

Another factor is that smaller mirrors are easier to finish to a good standard. Once you start moving up to bigger sizes then the mass production mirrors start falling further behind.

So to sum it up, if your are looking for something in the 8" to 12" range and your primary interest is DSO's then you aren't going to see a lot of advantage in paying the extra for a 1/10 PV OO mirror. However if lunar / planetary performance is important then yes it's worth paying the extra for the OO mirror.

Wonder what OO would charge for polishing and re-coating (and Hilux) of a SkyWatcher mirror?

John

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ngc2403

ngc2403

Posted
Posted

It's hard to tell from the scale but the Skywatcher look ok in the middle, however the fact that it starts to rise at the edges and the middle would worry me.

mass produced would then imply that the mirror could have these sorts of problems, and it looks like the extra 3mm on the edge of the skywatcher causes alot of problems i.e. it is raised up. and that the middle has no been finished well either.

however all in i think this is a very good test of skywatch mirrors and i will probably make the 1/4 wave mirror at OO disappear.

Thank you Phil, and OO for the test

ally

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George

George

Posted
Posted

The only thing that would "worry me" is the view through the eyepiece... :)

Well I'm quite happy I have the scope for my seeing conditions :undecided:

TBH at the moment the 250 is being used more for guiding than the ED80 but thats only because I'm imaging big FOV targets.

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kniclander

kniclander

Posted
Posted

what a great thread -entertaining and informative and I for one have learned lots of things I didn't know before. Well done Philsail and thank you :hello1:

not wanting to open further debates, I did notice that 4 tests were done on the skywatcher mirror and the best result (the second test I think) wasn't chosen - the one that was used was either second best or third best (out of 4). What we don't know is how many tests were done on the OO mirror and which result was chosen.... :scratch:

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philsail1

philsail1

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"Kniclander,"

Barry from OO said that they'd carried out 4 Zygo passes on the Skywatcher, as it had shown great fluctuation on the first two. They said that the mirror (being made out of ordinary (window pane) type of glass (on much thicker of course) took a longer time than the Orion's to reach an ambient temperature. First test was way out (poor), Second was near Perfection (as mirror cooled), third test was way out (poor) as mirror went past its optimum point of cooling (and glass was still moving). Fourth test was done under mirror's optimum cooled down stage.

"Arthur" & "Tiny"

Yes, I think you are right in saying that if one is observing from a "back yard" (or any urban, light polluted situation) then perhaps you are not going to get the best from an Orion Mirror, whereas the Skywatcher will be a little more forgiving.

Regards,

philsail1

BTW this was the initial photo I posted last night!!:-

9263_normal.jpeg

(click to enlarge)

(Even though it was only meant as a joke, TBH, it was the best shave I've ever had!!)

Skywatcher mirror is safely back in scope, and it easily collimated perfectly as usual. (Just need a clear night to test now!).

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John

John

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Posted

Thanks for the original review and the subsequent postings philsail1 - all very interesting :undecided:

This is probably an unfair question but having been through all this, do you feel that the additional purchase cost (about £395) of the Orion Optics OTA over the equivilent Skywatcher OTA is justified overall ?

John

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philsail1

philsail1

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Well, John & Gaz,

Looking purely at the cost comparisons - if given the choice of buying either the Skywatcher or Orion "Tube only" options (assuming I already owned the mount, I would have chosen the Orion, as this scope does come with a Crayford as standard, the better quality mirror, and an excellent straight through finderscope. Also it's a slightly shorter tube, and it's lighter, more easily handled than the Skywatcher, and it will be more portable. Also it seems to sit quite happily on the lighter Celestron CG5 motorised mount with just one 7lb counterweight.

I would have bought the Orion (tube only) largely on its advertised superiority over the Skywatcher.

However, with that marvellous quality called "hindsight," I would definately have chosen the Skywatcher!

To be honest, in a way, I wish I had not bought the Orion at all! Not because it's of poor quality (which it isn't), but more so, that as the scope (at present) seems only "marginally" better than the Skywatcher, it has now thrown up the dilemma of me having to let one of the scopes go. (I've only got one pair of eyes!).

I have a historical (and bit of an emotional) tie to my Skywatcher. It's sort of been there almost from the start (and has never let me down) of my deepening interest in Astronomy, and as such I am loathe to part with it (for the new kid on the block!).

I feel though, that I must look beyond the sentimental, and think purely in terms of the practical. In this respect, keeping the Orion is the best option - for the reason's stated above.

The final comparison test with the Skywatcher - under clear skies, and after having "flocked" flocked the OO scope, will convince me one way or the other!

Regards,

philsail1

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