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Scattering in Refractive vs. Reflective Optics

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#1 zeehas

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Posted 22 August 2014 - 04:16 PM

In recent discussions about the different contrast levels observed by users of refracting and reflecting telescopes I have noted one factor that hasn't received much attention. That is the simple fact that dust on a primary mirror will forward scatter light toward the focal plane while the same dust on an objective lens will mostly just cast a shadow. Granted, each dust particle on a lens somewhat degrades the image through simple light loss and diffraction effects, but this can not be as detrimental as the forward scatter of light off of dust on a mirror. I suspect that this could account for much of the observed differences in contrast reported by skilled observers comparing refractors and reflectors of known quality under typical field conditions.

 

Does anyone know if this effect has been properly analyzed?

 

 


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#2 Markab

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Posted 22 August 2014 - 06:14 PM

I can't answer your question, but it is a good one. Also worth noting is that different types of lens objectives scatter light very differently as the wavelengths pass through the material. An example: a green laser pointed through FPL-53 and other ED material shows the beam as it passes through the lens (implying scattering), while the same laser pointed through crystalline CaF2 does not.  That is perhaps one reason why CaF2 scopes seems to produce, in many people's eyes, a sharper image.



#3 GJJim

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Posted 23 August 2014 - 09:03 AM

Forward scatter is mainly a factor in solar studies, photometry, and long exposure AP. unless the dust is literally a layer on the optic, it probably has minimal effect for visual observers. The scatter in most observer's aging corneas is far worse than anything produced by dust motes on a lens or mirror.



#4 Dan McConaughy

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Posted 23 August 2014 - 10:17 AM

Scatter is not very important except for bright objects.

I got a Baader 2"  prism star diagonal and it is better than the AP Maxbright - which is a comparison of refractive vs. reflective optics.  However, I've read that 2" prism diagonals do not work well with fast focal ratio apos with regard to chromatic abberation - which is possibly why A-P doesn't sell prism diagonals.

Depending on your eyes, scatter within the eye may be the biggest problem.



#5 BillP

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Posted 23 August 2014 - 10:27 AM

Does anyone know if this effect has been properly analyzed?

 

 

Properly analyze how?  There is certainly a wealth of scientific research documenting the scatter differences between glass and mirrors.  There is also a wealth of studies on scatter related to professional astronomy observatories and scatter behavior and impacts due to dust and mirroring application and overcoat applications. 

 

As far as amateur observers, a lot of anecdotal comments and user reports.  In my Diagonal Comparison it was clearly evident the scatter that mirrors produce compared to prisms and compared to no diagonal.  Readily visible.  On planets I found between the 2 diagonal types the prism showed a much more saturated planetary picture which allowed finest details to portray.  However, can't narrow the reason for that simply due to scatter.  When using my Dob vs my APO the scatter differences are obvious on star fields, but would not characterize it as detail detrimental just an annoying artifact that once you learn to see it, you tend to always then notice it.  When I set up my Dob and APO side by side and level either magnification or exit pupil, scatter around star points to my eye is fairly obvious.  Nothing horrible...but definitely a peculiarity of the design.  But if one can keep their mirror very clean all the time, then the added scatter, while still there, is of course minimized.  It's just another little factor to weight as you personally feel it merits.  Some folks weight it zero...others give it some consideration.  Not a deal breaker in my mind.



#6 GlennLeDrew

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Posted 23 August 2014 - 11:41 AM

Looked at in perspective, in visual observing scatter from some dust is awfully minimal. As noted, the eye (even not so aged) is usually much worse. Scatter from particulates might only be seen where there is very high contrast, such as against the dark sky surrounding a planet. But against the planet surface itself the scattered component is of such low intensity relative to the object that it is quite below detectability.

 

Where dust bits can be injurious is on optics near to the focus where the image-forming beam is quite narrow. For then a particle can have a size significantly large relative to the beam diameter. One such place is the eye lens (or most any eyepiece lens) when the exit pupil is very small.


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#7 BillP

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Posted 23 August 2014 - 12:12 PM

Looked at in perspective, in visual observing scatter from some dust is awfully minimal. As noted, the eye (even not so aged) is usually much worse. Scatter from particulates might only be seen where there is very high contrast, such as against the dark sky surrounding a planet. But against the planet surface itself the scattered component is of such low intensity relative to the object that it is quite below detectability.

 

Where dust bits can be injurious is on optics near to the focus where the image-forming beam is quite narrow. For then a particle can have a size significantly large relative to the beam diameter. One such place is the eye lens (or most any eyepiece lens) when the exit pupil is very small.

 

IMO the bolded part is worded too strongly and a supposition only.  I personally would not discount that it will not have an impact, even if directly not detectable as a scatter component.  But regardless, for me the scatter is aesthetically displeasing after seeing the alternative, so a problem from that perspective. 


Edited by BillP, 23 August 2014 - 12:12 PM.


#8 drollere

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Posted 23 August 2014 - 01:12 PM

one way to parse this is to assume all the "dust" on the mirror represents a surface area within the light column defined by the aperture. say the mirror is filthy, and this amounts to an area that is 10% of the aperture area. this amounts to 30% of the aperture diameter.

 

if the scattering is at the supramicron diameters (mie scattering) of most surface dust particles, then they are essentially reflecting the light, so everything depends on the reflectance index of dust. equivalently, the amount of light reaching the image plane from this dust will be equal to the amount of light reflected from a disk of pure dust that is 30% of your aperture diameter. say the reflectance is a neutral gray, the reflectance is 20%. surfaces scatter light randomly, so only the light that reaches the secondary mirror will be reflected to the image, so this depends on the angular width of the secondary as observed from the mirror distance. say this is an ƒ/8 newtonian with a 25% obstruction, then the secondary will be 0.25/8 radians wide or ~3º in angular area, which is 1/6240 of the forward half space area of about 20,000º into which light will be scattered. some of this scattered light will be reflected outside the area of the field stop, a further reduction i ignore, so the net forward scatter will be 20%/6240 or 1/31,000th the light falling on the equivalent mirror area.

 

if you assume submicron or rayleigh scattering, where photons interact with material atoms or molecules, then you might replace the 20% reflectance of dust with the 60% reflectance of a sky blue lambert disk, which means that you have no more than 1/10,000th of the light reflected into the image.

 

this analysis suggests why many experienced and grizzly astronomers are far more interested to port their telescopes to dark sky sites than to emblazon their objectives or corrector plates with sleeks by cleaning off a bit of dust.

 

it also supports glenn's contention, since we are talking about contrast and no more than a 1:10,000 difference in illuminance. i think that will have a very small leverage to brighten the dark areas of a low contrast target such as jupiter's whorls. small exit pupils acting as pinholes can project dust or motes into the image.

 

in general the problem of scattering arises from material surfaces parallel with the optical axis and from insufficient surface polish in eyepiece lenses or objectives. to the extent refractors have an advantage then i think it comes from the lack of central obstruction, tube baffling, and the antireflection coatings on the objective, which i beleive act as a very weak light pollution filter.



#9 zeehas

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Posted 23 August 2014 - 06:13 PM

Thanks for the insightful comments so far.

 

To those who said that contrast loss due to dust scatter is insignificant, I would argue that nothing is insignificant when one is trying to make out low contrast planetary detail at the threshold of visibility. And that the refractor's relative immunity to dust scatter is an underrated factor that gives it an edge in contrast. 



#10 RodgerHouTex

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Posted 23 August 2014 - 07:18 PM

Thanks for the insightful comments so far.

 

To those who said that contrast loss due to dust scatter is insignificant, I would argue that nothing is insignificant when one is trying to make out low contrast planetary detail at the threshold of visibility. And that the refractor's relative immunity to dust scatter is an underrated factor that gives it an edge in contrast. 

 

Actually, due to the lack of experimental evidence, this is just a personal supposition with no basis in fact.



#11 zeehas

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Posted 23 August 2014 - 08:46 PM

You're right, it is just speculation. The experiment that needs to be done is for someone with a dirty mirror to do some careful planetary observation and then clean the mirror and see how much difference it makes. Has anyone tried this?



#12 zeehas

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Posted 23 August 2014 - 10:33 PM

Allow me to elaborate on why I posted this topic. I have access to both a 6" f/15 achromat and a 12 " f/5 dosbonian. My main interest is planetary. Both of these scopes have good objectives. If anything the reflector star tests a little better, showing an almost textbook star test.  Yet every time I compare them the refractor always shows me more planetary detail against all optical theory. This is after several hours of acclimation and collimated  well enough that the diffraction rings look concentric. This set me wondering why this should be the case. Here in the desert southwest dust is a serious problem.  Nothing stays clean for very long. So I started to suspect that the role of light scattered by dust may play a larger role than is commonly assumed.



#13 GlennLeDrew

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Posted 24 August 2014 - 10:56 AM

If low contrast features differing in surface brightness by, say, 10% were to be sensibly impacted by contrast-lowering veiling glare, said glare might re

quire to be somewhere in the neighborhood of 10% as bright as the features under observation. But let's be stricter, and require glare only 1% as bright in order to cause harm. If the object is Jupiter, and glare is 1/100 as bright as its surface, said glare is 5 magnitudes lower in surface brightness, or a bit fainter than 10 magnitudes per square arcsecond (Jupiter is 5.3 MPSAS). That's equivalent to bright twilight. And are not good observations made in twilight, by Jove? ;)

 

A telescope which suffers veiling glare this badly could do so only with a flashlight shining down the thing.

 

P.S. This editor does some weird stuff.


Edited by GlennLeDrew, 24 August 2014 - 10:58 AM.


#14 wh48gs

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Posted 24 August 2014 - 11:59 AM

In recent discussions about the different contrast levels observed by users of refracting and reflecting telescopes I have noted one factor that hasn't received much attention. That is the simple fact that dust on a primary mirror will forward scatter light toward the focal plane while the same dust on an objective lens will mostly just cast a shadow.

 

That is not so. It is exactly "casting the shadow" what is the primary mechanism through which such small obstructions in the wavefront cause light scatter, and it is identical in both, lens and mirror objective. Remember that any pupil obstruction acts like inverse aperture projecting its (inverse) diffraction pattern onto the main pattern. Since these obstructions are so tiny, their central maxima will be enormously large, and that is the main factor causing the transfer of energy resulting in light scatter from this cause.

 

Where lens objective do have advantage is in the coating effect. Their coatings are refractive, hence transferring much less (roughly by a factor of 4) of their irregular structure onto the wavefront. An improperly applied, or deteriorated reflective coating can scatter up to 10% of light, or even more, per surface. That's a lot.

 

Vla



#15 drollere

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Posted 24 August 2014 - 12:34 PM

Allow me to elaborate on why I posted this topic. I have access to both a 6" f/15 achromat and a 12 " f/5 dosbonian. My main interest is planetary. Both of these scopes have good objectives. If anything the reflector star tests a little better, showing an almost textbook star test.  Yet every time I compare them the refractor always shows me more planetary detail against all optical theory. This is after several hours of acclimation and collimated  well enough that the diffraction rings look concentric. This set me wondering why this should be the case. Here in the desert southwest dust is a serious problem.  Nothing stays clean for very long. So I started to suspect that the role of light scattered by dust may play a larger role than is commonly assumed.

 

the missing piece here is magnification. your objective magnification is 9.1x with the achromat and 6.1x with the dob. assuming you're observing jupiter with an exit pupil of around 2 then you're using a 30mm eyepiece yielding 76x with the achromat and a 10mm eyepiece yielding 153x with the dob.

 

if you equate magnification and work at 153x with the achromat, then your exit pupil is 1.0 and you are probably getting diffraction blurring of the planetary detail. if you equate the other direction and use 76x with the dob then your exit pupil is 4 and you are likely losing detail the objective can pull out.

 

separate from all that, rings that look concentric may not be sufficient collimation criterion in an ƒ/5 -- separate from how well a mirror that steep has been figured -- and the larger aperture is going to be much more sensitive to atmospheric turbulence.

 

two issues: telescopes are each unique tools and there are many factors that can affect a side by side comparison. and, looking to forward reflecting mirror dust is a pretty long reach considering the more basic factors that are more likely to affect your image evaluation.

 

but i agree: seeing is believing. so clean the mirrors and refractor objective and report back your observational results.



#16 zeehas

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Posted 24 August 2014 - 04:53 PM

I'll try giving the reflector a better collimation next time. I would also like to install a boundary layer fan. I'd really like to see what 12 inches of quality aperture can do. When I get a chance I'll try the mirror cleaning experiment. Unfortunately we are now experiencing a cloudy season, so it may be a while.

 

In the mean time, here's an experiment anyone can do to illustrate the effect I am talking about. Your computer monitor probably has some dust on it right now but you don't really notice it since it's illuminated from behind. Now turn off the monitor and shine a light on it. The dust is now readily visible.



#17 GlennLeDrew

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Posted 24 August 2014 - 05:25 PM

But shine a light on the screen while it's on. The light must be fairly intense in order to render the dust visible, and the screen surface itself might be visibly scattering as well. The lesson to be derived is that scatter must be rather intense if it's to impact contrast.



#18 zeehas

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Posted 24 August 2014 - 05:28 PM

Regarding magnification, I generally use between 150-300X. The refractor shows more detail more clearly at any power in that range. Although the larger reflector brings out the colors much better. I'm not involved in any serious research programs. I just like to look at planets and compare them with maps, seeing what's the finest detail I can make out.



#19 zeehas

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Posted 24 August 2014 - 05:33 PM

Glenn, you may have a valid point. Only experiment will tell for sure.



#20 BillP

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Posted 25 August 2014 - 03:57 PM

Allow me to elaborate on why I posted this topic. I have access to both a 6" f/15 achromat and a 12 " f/5 dosbonian. My main interest is planetary. Both of these scopes have good objectives. If anything the reflector star tests a little better, showing an almost textbook star test.  Yet every time I compare them the refractor always shows me more planetary detail against all optical theory. This is after several hours of acclimation and collimated  well enough that the diffraction rings look concentric. This set me wondering why this should be the case. Here in the desert southwest dust is a serious problem.  Nothing stays clean for very long. So I started to suspect that the role of light scattered by dust may play a larger role than is commonly assumed.

 

 

Can be a host of reasons...

 

1. Dust causing scatter on mirror (as you presently feel is a candidate)

2. While you may think the mirror is acclimated, boundary layers of warmer air can be stubborn and stay on the mirror all evening

3. Body heat from you may be passing over the entrance of the Dob affecting your view without you realizing

4. While the Dob appears collimated, perhaps not as well as you suspect as collimation for Dob is way more complex than for refractor

 

Something to try if you are able...during day set up both scopes and point them at a picture you place down field.  Set both scopes at 500x and then evaluate the views.  Is the image just as sharp in both...if not then either collimation is off or figure of optics not as good as you think.  Is anyone showing a more washed out view, even though it is sharp (dust, scatter, or poor baffling could be the culprit).   I make sure my new instruments pass this test for me...usually using 100x to 150x/inch magnification.  I want to make sure I have them collimated precisely enough and their optics are good enough to produce a pleasingly sharp view in a controlled setting.  Once this is done, then any time the night time view is not as sharp, I know it is either seeing related or thermal management related.



#21 GlennLeDrew

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Posted 25 August 2014 - 04:20 PM

Bill,

When comparing, bes

t to not use the same magnification when aperture differs significantly. Use the same exit pupil. At high powers, the tinier exit pupil on the smaller scope (at same mag.) will likely appear to be the worse view.  [Added in edit: This editor is bizarre; sometimes what I format here appears differently when posted, and some actions like entering a hard return are not accepted. Or backspaces are not applied. And the autosave function is suspect.]

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


Edited by GlennLeDrew, 26 August 2014 - 09:27 AM.

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#22 zeehas

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Posted 26 August 2014 - 01:47 AM

Thanks for the good advice BillP. I'll give that a try. Although it seems that thermal issues could be as much of a problem during daytime testing. How do you cope with that?



#23 drollere

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Posted 31 August 2014 - 03:14 AM

In the mean time, here's an experiment anyone can do to illustrate the effect I am talking about. Your computer monitor probably has some dust on it right now but you don't really notice it since it's illuminated from behind. Now turn off the monitor and shine a light on it. The dust is now readily visible.

 

yes, but that's exactly the point. the visual effect of the dust with the computer on shows you how very little dust affects the image quality. 



#24 drollere

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Posted 31 August 2014 - 03:16 AM

Bill,

When comparing, bes

t to not use the same magnification when aperture differs significantly. Use the same exit pupil. At high powers, the tinier exit pupil on the smaller scope (at same mag.) will likely appear to be the worse view.  [Added in edit: This editor is bizarre; sometimes what I format here appears differently when posted, and some actions like entering a hard return are not accepted. Or backspaces are not applied. And the autosave function is suspect.]

 

glenn, a few of us got together and made a special request that the new CN interface has a special editor, just for you.

 

enjoy.



#25 zeehas

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Posted 31 August 2014 - 06:41 PM

 

In the mean time, here's an experiment anyone can do to illustrate the effect I am talking about. Your computer monitor probably has some dust on it right now but you don't really notice it since it's illuminated from behind. Now turn off the monitor and shine a light on it. The dust is now readily visible.

 

yes, but that's exactly the point. the visual effect of the dust with the computer on shows you how very little dust affects the image quality. 

 

 

Actually, the point was that dust is more obvious under reflected light than transmitted light. The question which remains to be answered is how much effect this has under actual field conditions.   I won't argue the issue any more until I've had a chance to do the experiment.








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