Star sizes refractors versus reflectors

[gregbradley]

I thought this was a very informative post by Roland Christen of AstroPhysics fame:

Reflector - refractor, what is the difference?
Under real good seeing of course there is very little difference between reflector and refractor of same aperture size. Such seeing as we get in Chile at Las Campanas which is usually under 0.5 arc seconds. However, here in the middle of the country, we often have the jet steam above, so seeing is always compromised.

It is somewhat complicated, but I will try to make it very simple as a first order approximation. In a refractor the theoretical strength of the Airy Disc is 84% of the energy, with 16% in the diffraction rings surrounding the central Airy Disc. The first diffraction ring is the brightest, which makes the diameter of a star of long exposure to be approximately 2 times the theoretical resolution. When the image wiggles due to atmospheric instability it "paints" a star diameter somewhat larger than 2x the theoretical resolution, depending how much the star wiggles. Let's say that a 7" aperture can resolve 4.5/7 = 0.64 arc sec, so under really perfect seeing you might get 1.3 arc sec star disc, with maybe 0.9 arc sec FWHM value. The shape of the star brightness is a Gaussian distribution with a base diameter of 1.3 arc sec and a peak diameter of 0.64 arc sec. This would be the absolute limit for that aperture.

In a reflector, you always have some central obstructions. Large obstruction occur in fast instruments like the R-H F3.8 astrograph exceeding 50% by diameter, and perhaps 40% for an F8 Astrograph. The obstructions are necessary in order for the system to cover a wide field. You get similar obstructions in fast Newtonian astrographs, something that is unavoidable if you want to cover a large chip. So let's regard a 40% obstruction mirror system. The immediate effect is that the central Airy disc drops to around 60% of the energy with 40% remaining going to the surrounding diffraction rings. In fact, the second ring is now brighter than the first and has considerable energy. Thus the star image that gets "painted" in a time exposure is now some 3 times larger than the theoretical resolution limit versus 2 times for an unobstructed aperture. There is also considerable energy further out, so in terms of raw resolution you may see even in perfect seeing somewhat larger star sizes for the same aperture. When the seeing is not really good, all these rings begin to paint their own diameters and you get poor resolution.

Of course reflectors generally are larger in size, so the actual arc second resolution may still be larger than a much smaller refractor, especially when the seeing is excellent. In our case, the 12" F8 astrograph down in Chile experiences seeing of less than 0.5 arc seconds, and the best images I have recorded in tests have been on the order of 0.9 arc sec FWHM for a 10 second exposure. It might have been better but we are limited by the 9 micron pixel size of that setup.

Rolando

I think this is also why we see some star bloat with bright stars with some CCDs. Small wells mean the outer airy discs can saturate quickly given how much of the energy in some scopes goes to them. Its not a matter of pixel bleed as CCDs are usually anti blooming but merely the outer airy discs saturating the small pixels charge carrying capacity.


Greg.

[Slawomir]

Interesting read Greg, thank you for sharing. I am still however uncertain about what all of that means in practice for those of us imaging with less than perfect seeing. Have I understood correctly, that for less than perfect seeing refractors could perform better for the given aperture, but since people own reflectors that are usually significantly larger in aperture, the difference is kind of nulled or could even be reversed?

As for star bloating, wouldn't small wells also mean smaller pixels leading to lower sensitivity? Perhaps I simplify things a bit but I think that stretching images makes pixels around bright stars brighter anyway thus possibly leading to bloated stars?

Having said that, I think what you are saying about stars bloating is very true when comparing images acquired with cameras with varying well depths but at the same resolution in arcseconds per pixel.

[Shiraz]

hi Greg.
In Australian conditions, the star shape and size for all but smaller refractors is determined almost entirely by the atmospheric seeing. With anything above about 6 inches aperture, you will never get anywhere near the optics diffraction pattern. ie the stars from my 250f4 and your CDK17 will be exactly the same shape and size as those from the AAT in typical conditions. It woud be nice to have seeing of 0.5 arcsec, but I tend to jump up and down if the seeing gets below 2 arcsec and I think that even the best Australian sites rarely get down to 1 arcsec.

I am not altogether sure what you mean by bloat, but if you mean that the stars look bigger, the two main reasons for this are: 1. that the pixels are small (eg the stars from your Trius will be 4x as big as those from your 16803) 2. you may be stretching the image to get to deep features (you see more of the star skirts as you go deeper - ie they look bigger).

If you are managing to saturate the stars excessively, then the simple answer is to expose for shorter subs - not just a bit shorter, but a lot. The optimum sub exposure varies almost with the square of the read noise - at the same aperture and pixel scale, you only need subs about 1/4 as long with your Trius as with your 16803. There is a tendency to overexpose with the new cameras, because that is how we had to do it with the older Kodak ones - those cameras have relatively high read noise, so you need long subs to overcome it - and then of course you need deep wells to handle the extra signal you get from a long sub. If appropriate subs are chosen, the results from both classes of camera should be identical.

Edit: as Slawomir said - I posted before reading his summary

[gregbradley]

Yes as a general rule I notice refractor images tending to have smaller star sizes (with exceptions) than reflectors although I am often amazed at how small stars look in some Newtonian images. Especially the ASA type scopes. I take it that is more from the excellent corrector they use more so than the Newtonian scope.

The bloated stars I am referring to are what Roland is saying here. With a reflector perhaps 50% of the energy is in the outer airy disc not the central bright dot of the star. Now if you have small wells and high QE like the KAF8300 and the Sony ICX694,814, 894 sensors then those outer airy discs are going to overexpose quicker than with a refractor where less of the energy is in the outer airy discs.



Yes Ray you may be right there. I need to experiment with exposure length more. I am now settled on just using the 2 scopes - the AP RHA and the CDK with or without the .66X reducer. I see little difference between a 5 minute sub and a 10 minute sub (usually tighter stars with 5 minute subs due to less tracking errors) in terms of brightness, noise and detail. A bit more in the 10 min. I have been using 5 minute subs with the RHA and 10-20 minute subs for narrowband on the RHA (usually 10).

For example I imaged Corona Australis 5 minute subs and I noticed the main stars were overexposed. 5 minutes. Wow.

I am also thinking maybe 2 sets of subs like you do for M42. One for the bright areas and one for the general field and combine them in layers using Photoshop.

On the CDK with reducer 5 minute subs are usual and sometimes 10minute ones for fainter objects. That at 17 inches aperture and F4.4 with 77% QE so its pretty fast.

Greg.

[Slawomir]

Makes sense. By the way, you have some really nice astro gear Greg :)

[Peter.M]

Be careful all of that jumping doesn't offset the good seeing!

[gregbradley]

Thanks. Yes I do. I have "simplified" down to 2 scopes from 4!.

Greg.

[Eratosthenes]

....I think the important thing for optical quality in a refractor for example is whether the optical resolution is "diffraction limited" rather than limited by glass imperfections etc....

Diffraction also occurs in human vision. The diffraction spread of light at the back of the retina is of comparable size to the spacing of the rods and cones which sense the light - pretty handy isnt it? Millions of years of evolution can do this. (there are several dozen examples of "sight" evolving independently in different species - must be something important in sensing light in your environment one would think??)

[gregbradley]

I would disagree Peter.

Having used many scopes the few that were way up there in their Strehl ratio and that were way beyond diffraction limited way out performed lesser figured scopes.

Diffraction as a barrier is only part of the picture. Light scatter, throughput, the extent of the spectrum that is in focus at the same point, anti reflection coatings reducing ghosting, increasing micro contrast etc all play a part. Even focusing a high end instrument is a lot easier as there is more of a snap to focus effect. Star sizes off axis, correction for the 6 optical aberrations all play a large part in making a high quality image.

I think diffraction limited has been pushed so hard so as to justify not being willing to or able to go the extra mile so they make out that there really is no need anyway. Its a marketing justification for an inferior optical system.
Greg.

[Slawomir]

Hi Peter,

I was quite disappointed when I looked through a telescope for the first time. Eons of evolution and only a few percent of Quantum Efficiency? That's one of the main reasons why so many of us in here utilise silicon retinas - CCD and/or CMOS, for converting light into electrical impulses and then the signal gets amplified by a computer screen so our limited rods and cones can detect it, ultimately allowing us to more fully admire the heavens ;)

[SpaceNoob]

I see this first hand, very obvious and pronounced disc structure.... I sample at 0.44" with a 12.5" @f/8. When I run focus routines I can see the airy disc itself with the bright core where most of the energy resides and the outer rings. Sampling at this level tends to really clearly show issues such as tube currents etc. With smaller wells, the washy airy disc can quickly register a sizeable portion of the available dynamic range. It seems to further amplify the effects of the CCD micro-lens. The washy disc causes a "shimmer" over the lensing, very tiny diffraction spikes which further spread from the disc structure. - I have watched this live at 100+ fps using another sensor with similar sized pixels to my main 8300 sensor.

There are some cool advantages - I have revealed some pretty tiny structures within some targets but bright point sources of light tend to suffer greatly, i.e stars themselves and I am yet to find a way to suppress it. You can do short subs, but other parts of the target will suffer.

Oversampling excessively also results in significant loss of sensitivity and any forms of aberration present, either optical or environmental become extremely obvious.

A few examples:

Oversampling with long subs:
Antennae Galaxies, core structure (https://flic.kr/p/rR1ZjY)

Shorter sub composition for High dynamic range targets:

Core of the Sombrero (https://www.flickr.com/gp/92067259@N06/8Q7n3m) (note the strange diffraction spikes.... thats a @!#$ spider building a web in my OTA that i didn't discover until the following morning)

It's a nice thing to have data present that would otherwise not be seen when sampling less, but I find 80% of my time is dealing with the adverse effects. Bigger camera should be here soon so I'll be looking forward to less fuzzballs...

[Eden]

When considering star size in photography -- especially long-exposure photography -- it is important to account for ultra-violet starlight, particularly between 300-400nm.

Light at these wavelengths is not absorbed by the Earth's atmosphere (in fact, the atmosphere does not completely absorb light between 280-300nm) and is more susceptible to atmospheric diffraction compared to light at longer wavelengths. This very fact alone will cause a star to become "bloated" over the course of a long exposure.

For most observers, this light is not visible to the human eye but is readily picked up by unfiltered monochrome CCDs and indeed color CCDs not equipped with a UV blocking window.

Choice of CCD sensor will almost certainly have an impact on star size when these wavelengths are not properly restricted. For example, a KAF16803 has an approximate QE between ~25% (350nm) and ~40% (400nm), whereas an ICX694 is much more sensitive to UV light, with sensitivity ranging from ~30% (350nm) to ~65% (at 400nm).

[alpal]

I've been on dozens of group star observing nights.
That gives everyone a chance to view the same target through up to 8 or more different scopes.

Those with the highest quality refractors always had better stars
than those with reflectors.
The top refractors also had better contrast on planets.

cheers
Allan

[gregbradley]

Yes same. Best views were always with a refractor. Stars can be pinpoints and better contrast and cuts through the seeing better.

Eden, that's a great comparison graph you've got there. Thanks for that.
Where did you get that from?

Gee the 11002 is very insensitive to Ha. Also I thought the QE of the 8300 was more like peak at 60%. That's what FLI rates theirs at.

Greg.

[Camelopardalis]

But isn't this just an illusion? That the scope lacks the resolving power to actually see the disturbances in the atmosphere?

Don't get me wrong, I :love: refractors...

[Eden]

You're welcome, Greg. I thought that you might find it to be of some interest.

The QE graph attached was done by Philippe Bernhard (http://astroccd.eu) however I sourced it from http://blog.astrofotky.cz/, which has some excellent CCD related articles.

Having looked at many such graphs, both independent and vendor-supplied, I have found that QE sensitivity tests done by independent 3rd parties tend to give a much more realistic "real world conditions" assessment of CCD performance. A lot of this has to do with the fact that many camera vendors simply provide the default QE graph supplied by the CCD sensor manufacturer and if you read the fine-print which accompanies these sensor manufacturer QE graphs you will find that the test conditions under which they were generated not only vary between sensors, but also deviate wildly from the typical astrophotographical conditions in which these sensors are often used. Sensor manufacturer QE graphs also fail to account for other factors such as the coatings found on refractor lenses and camera windows.

This is where the CCD sensor comparisons provided by Point Grey are particularly reliable -- they use industry standard test conditions which are consistent right across all test articles and give you an accurate assessment of the performance of an actual camera, not just the sensor.

If star size is of concern in a photographic setting, I would definitely suggest at least investigating UV suppression. I have observed a noticeable decrease in star bloat after switching from a Baader filter with relatively low UV suppression to an Astronomik filter with high UV suppression. This is with an OSC camera with relatively poor UV blocking at the sensor window. Obviously in a narrowband setting the luminance channel would be the culprit.

[Eden]

In order to answer this properly, I think some evaluation of the optical coatings used on the refractor in question would be required. The number of elements used in the optical path (also resulting in more coatings between the light source and your eyeball) would have some impact also. Obviously there's a lot more going on there compared to having light reflect off a couple of mirrors.

[gregbradley]

\

I know what you are saying that a long focal length scope will show the disturbances better. But as the article from Roland shows, refractors have tighter stars. Also better contrast as the secondary mirror robs contrast.

So tighter stars and better contrast = the better view albeit at a wider field look.

Also less effect of thermal currents and boundary layers.

Greg.

[Camelopardalis]

Thanks Greg. What I'm struggling with is why they'd deal with thermals/layers better if the contrast and resolution is better :confused2:

[alpal]

http://www.garyseronik.com/?q=node/55

[Camelopardalis]

Ah get it, thanks :)

[clive milne]

If refractors are a panacea, why aren't they commissioned in a contemporary professional context in apertures exceeding 4 inches?

[gregbradley]

I think the simple reason for that is cost and size. An excellent 6 inch APO refractor costs around USD$12,000. An 8 inch one jumps to USD$30,000.

Plus an 8 inch refractor is getting to the permanent observatory type size and weight.

A good 8 inch reflector is a fraction of that.

Greg.

[Camelopardalis]

So the same as for most of us average amateurs then :lol:

Thinking about this, there must be an overlapping, blurred zone where practice takes precedent over theory and real atmospherics make the differences irrelevant, or tips the balance one way or the other :confused2:

For example, between 6 and 8" in average seeing west of the Dividing range?

Is this what is meant by "diffraction limited" ? And what stage is this actually noticeable to the eye, rather than with instrumentation?

[glend]

That quote by Gary can cut both ways in its relevance, yes a refractor objective is at the front and exposed to the night air - but so is the corrector plate of a SCT (dew magnets that they are), and are not all refractors equipped with dew shields these days, and they all (refractors) need dew heaters. Newts however, mostly all have fans, and have a long tube that acts as a dew shield, and being open tube do not have cool down issues that SCTs and refractors do (minimal tube currents) and for truss dobs no tube currents at all.

My point is many of these pronouncements can be turned right around by looking at all aspects of the design and performance. There is a thread over on Cloudy Nights called "Are APOs better than Newts" and of course it was stuck in the Reflector forum by a refractor guy looking for a bit of entertainment. That thread has over 300 posts now and shows no sign of dying off. I own both refractors and reflectors and while the refractors are nice, especially on planets, contrasty, no diffraction spikes, etc, they cannot keep up with a good coma corrected newt in the deep sky light gathering category. My closest hybrid scope is my new MN190 Mak-Newt which seems to combine the best of several worlds at a reasonable cost, and while being the equivalent of a 6" APO in performance (at way less $), it cannot give me the deep reach that the 10" imaging newt does. Aperture, as always, is king; and in the analysis of aperture per $, there is nothing that can match a newt. So it comes down to what aspects of performance are important to you, if it's only uncorrected star shape, and contrast then yes a large refractor will get you going, but if your after deep reach then only a reflector can give you that at a cost way less than any larger APO refractor.;)

[Slawomir]

In the last two years, since I got into astrophotography, I never had dew on my refractors (imaging in Sydney and Brisbane), nor had any cool down issues :)

[alpal]

Hi Glend,
I Agree & I only use Newts. because I like imaging deep sky targets.
Newts. give the best bang for the buck in that area.

A Schiefspiegler Newtonian telescope would be interesting as there is no obstruction
but I've never looked through one.
There is a good article here on obstruction & design of Newts:

http://www.alpo-astronomy.org/jbeish/Newt_Sec_Mirror.pdf


cheers
Allan

[glend]

Thanks for that link Allan. I wonder if the calculations can apply to my mak-newt as well, which has a rather small central obstruction of 26% and no spider as itts mounted directly on the corrector plate. The mirror in the mak-newt is a spherical and not parabolic. I will hwve to read that article carefully.

[Eden]

Looking forward to seeing what you can bring down with that new Mak-Newt of yours, glen. It sure looks like a top instrument and I am surprised there aren't more of them out there and in larger aperture sizes (maybe there are? I couldn't find any, just the Skywatcher offerings.)

Between your Mak-Newt and Greg's Riccardi-Honders, we're all but doomed this winter :D
Brett

[alpal]

Yes the formula applies to all obstructions:

Contrast factor = CF = 5.25 - 5.1x - 34.1x²+ 51.1x³

where x is the obstruction ratio.

So the maximum contrast factor you can have is 5.25 for any telescope.


cheers
Allan

[Alchemy]

Fascinating topic,

given all seriously large professional observatories are mirrors then the questions already answered, once you get so big that atmosphere plays a part then adaptive optics and computer tricks are required so it's no longer just the scope.
Given ground based observatories are now producing Hubble esq images, anythings possible, just depends how deep your pockets are.

HOWEVER for me I have had both reflectors and refractors, for simplicity of use I prefer a refractor, no collimating, no reapplying coatings, minimal flexure ..... Grab and go then enjoy.

[glend]

After reading Jeff Beish's paper on Newtonian Design Calculations (supplied by Alpal), I have run the Contrast Factor calculations for each of my Newts:

With a theoretical perfect score of 5.25 (assuming zero obstruction - admittedly not going to happen with a newt anyway):

MN190, with 0.26 obstruction ratio, gives a Contrast Factor of 2.5168.

10" Imaging Newt, with 0.25 obstruction ratio, gives a Contrast Factor of 2.6412

16" Dob, with a 0.216 obstruction ratio, gives a Contrast Factor of 3.0773.

What can be shown from these numbers:

Firstly, the two imaging scopes have larger secondary obstructions due to the fact that they are imaging scopes and thus run larger secondary mirrors than what is required for a purely visual setup as in the 16" dob.

Secondly, using the Central Spot Energy table it seems that over 70% of the energy is concentrated in the centre spot for all these scopes. Theoretical maximum Central Spot Energy is 84% but again that would be with no obstruction, which no newt can deliver.

Thirdly, the MN190 and 10" Imaging Newt are very close in terms of contrast factor but very different in terms of diffraction - something not addressed in Beish's paper. The 10" newt has a traditional spider holding the secondary where as the MN190 has no spider and thus no diffraction spikes (beause the secondary is mounted on the rear of the corrector plate).

Finally, I am not sure how Spherical and Parabolic primaries affect the image and contrast, if at all. The MN190 has a spherical primary while the others have traditional parabolic primaries. Spherical abberation is corrected of course by the Corrector plate, and is the typical design used in SCTs and Maks. Parabolics are figured to correct spherical abberation.

Conclusions, well from a visual point of view big dobs are clearly the kings of contrast factor as long as you keep the secondary size under control as you go up in size. However, there would be a question as to how this translates to image quality as conventional imaging newt design recommends larger secondaries to provide the correct spot size for imaging sensors.

It is important to remember that these calculation are related to relative obstruction area in the pupil (see the reference paper below), and thus may have limited validity in imaging scope comparisons (especially in comparing imaging against visual scopes).


For those seeking an even more indepth look at central obstruction math here is a link for you that is beyond my aging brain:

http://www.telescope-optics.net/obstruction.htm

[alpal]

Hi Glend,
nice post below.

I think if you can get the central obstruction below about 18%
you would hardly notice the difference between a refractor & a Newt.
Then it boils down to how good your eyepieces are.
Some eyepieces have poor contrast because they scatter the light
either by poor choice of glass or bad coatings.

It's hard to get the central obstruction down to 18%
just by the design of the Newt. as the larger the Newt is
the bigger the secondary mirror must be to obtain all
of the light cone from the primary & reflect it to the eyepiece.
To do so you would have to get the eyepiece into focus with a low profile focuser
as close as possible to the telescope tube.
In that case the telescope would be only usable for eyepiece viewing &
no good for a camera which requires more back focus as it
works at prime focus.

Therefore - you need one Newt. for imaging & another one for viewing.
If you're rich you could have an exotic oil spaced refractor
with top eyepieces & do better than a Newt. on planets.
Oil spacing gives less scatter although with the latest technology & coatings this is debatable.


cheers
Allan

[glend]

Thanks Allan. Those central obstruction ratios and Contrast Factor calculations, really cannot be used to compare different scope designs IMHO. If you were comparing two similiar sized visual newts to make a purchase decision, then yes they might be good to know.

When I look through the MN190 it is a wonderful view compared to looking through the 10" Imaging Newt, with pinpoint stars, great contrast, good shape, coma free to the edge, etc where as the Contrast Factor numbers say the 10" newt should be the better. That's the problem for me. It's not apples to apples. I'd be happier comparing the MN190 to a equivalent clear aperture APO (Something like a 6" APO), because the view I am seeing is closer to what that APO provides. Trying to get a central obstruction down to 18% in a newt to reach refractor central spot parity is only relevant where your are comparing equivalent apertures. IF you accept that you need to have more aperture in the newt to negate the obstruction differences then your getting to the heart of the matter. Re the Mn190 verse the 10" I can only assume the difference lies in the mak corrector front end, and the rest of the optical design that is not brought into consideration in that paper's Contrast Factor determination, which is a newt to newt comparison factor.

[glend]

I have completed initial imaging setup for the Skywatcher MN190, and wanted to demonstrate my last comments on Contrast Factor numbers.

Here are two images of the same object: Eta Carinea.

http://www.astrobin.com/full/180326/0/

http://www.astrobin.com/full/185615/0/


The first is taken with my 10" imaging newt with a Baader Coma Corrector on the camera (this is my lightweight f5 newt).
The second is taken with the new Skywatcher MN190 f5.3, which of course has no coma corrector on the camera. Both sub taken at 209" with the same Canon 450D Full Spectrum camera with minimal processing ( i think I did more processing with the first one, the second only has auto levels done). The first was taken at Bretti the second taken in my backyard observatory. The first was the NEQ6 on its tripod, the second the mount head was on my pier.

Love the edge to edge coma free field of the MN190. Sure it is wider field of view to the 10", and the 10" gets in a bit closer; however, I much prefer the contrast and stars in the MN190. Of course the 10" has a higher Contrast Factor - which is my point - that number doesn't tell you much at all.

Note: Submissions to Astrobin are jpegs for the public gallery.

[Slawomir]

Hi Glen,

The photo taken with MN190 looks heaps better, especially that the one taken with 10" Newtonian looks out of focus.

But I think I will stick to my 4" doublet ;)

Colour: http://www.astrobin.com/full/141089/D/

Mono: http://www.astrobin.com/full/148715/D/

[gregbradley]

That's impressive Glen. I would not have though you would get round stars to the corners with a 19% obstructed scope.

As Roland pointed out in the original post I posted you need the large 40-50% central obstruction to cover a large sensor.

Its amazing how light will bend around the obstruction.

Greg.

[glend]

Well Greg it does have a thick front corrector plate, and while I am not sure exactly how that bends the light ( ala a refractor objective shape) it obviously is doing a good job.;)

[Shiraz]

Looks really good Glen - terrific results

Greg, the SW MN gets by with a small obstruction, because the designer did not insist on full illumination away from the field centre. This will produce some vignetting, but it looks to be quite manageable. This design will still produce round stars up to the edge of the field, because coma is controlled - which means that the atmosphere determines the star shape, not the scope.

The light does not bend around the secondary at all (apart from a little diffraction at the edges). The secondary obstruction produces a light cone that has a hollow core, but it still focuses to a geometrical point. The actual point spread function of an obstructed scope will have significant energy outside of the central peak, mainly due to the diffraction of the secondary - but for DSO imaging with the scopes we are considering, that does not matter, because the spot size due to the atmosphere is maybe 10x bigger than that due to the scope (in area). Provided aberrations are controlled, what the scope does is almost immaterial. That is why a scope with large central obstruction (eg RHA) can still produce high contrast images even though the design has very low theoretical contrast - it is simply that the contrast (and resolution for that matter) is all down to the atmosphere and not the scope. In any event, contrast can be adjusted in processing - it really is not a major issue for imaging (although it is for visual).

When we compare larger scopes by looking at their images, we are actually comparing atmospheres - above about 6 inches aperture, scope performance (assuming reasonable quality) has only a modest effect on the outcome, except in exceptional seeing. In typical Australian conditions, Glen's scope should do almost as well as anything, including the AAT (although not as quickly).

[gregbradley]

Yes that what I meant Ray the diffraction of the secondary mirror.

When out of focus you can see donuts but when it comes to focus its a solid dot of light. Amazing. One of the confusing aspects of light and the is it a wave or is it a particle question?

Greg.

[Slawomir]

Light is neither wave nor particle. It only behaves as a wave or a particle :-)

And since matter is also neither waves nor particles, we only use abstract concepts in an attempt to explain reality.