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Reflections, ghosting, glare, scatter, EOFB

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#1 Andy-di-Notte

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Posted 11 March 2017 - 05:36 AM

Sorry if this has already been explained, but I'm not sure I understand properly what all these terms mean.

Could someone please explain what the differences are between these terms and possibly also the causes?

Some of these terms seem to be used interchangeably.

 

I've read Don Pensack's excellent explanation of aberrations, but I'm still not clear about the difference f.e. between glare and scatter. Or between these and internal and external reflections.

 

The only one I feel sure of is ghosting, when you see an out of focus duplicate image of Jupiter for example.

But then again, is this due to internal or external (eyeball) reflections?

 

It would be great if someone could post some examples (illustration) of what is actually seen in the eyepiece.


Edited by Andy-di-Notte, 11 March 2017 - 05:37 AM.


#2 Peter Besenbruch

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Posted 11 March 2017 - 03:13 PM

Reflection is a generic term for light bouncing off of something it shouldn't (including the human eye).

 

Ghosting is a type of refection. A bright object has a fainter companion. Especially annoying if the ghost sits on top of the regular image. Kellners ghost a lot, and are sometimes called haunted.

 

Glare is light reflecting off the side of something in the eyepiece and reducing contrast. Often seen more on one side than the other.

 

Scatter is caused by less than perfect optics. All optics suffer from scatter. It is reduced by polishing the optics more, which costs time and money. The effect is a diffuse halo around a bright object. It also reduces contrast, especially on planets. Scatter is less of an issue than people sometimes make it. It's something akin to "soundstage" in the audiophile discussions.

 

EOFB = Edge Of Field Brightening. A brightening of the background toward the edge of the eyepiece field. Like the others, it reduces contrast, especially for deepsky viewing. Another somewhat audiophilish term, as some people see it in a majority of eyepieces, others in almost none. The signature EOFB eyepiece is the Olivon 70° 13mm.


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#3 Starman1

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Posted 11 March 2017 - 03:33 PM

Sorry if this has already been explained, but I'm not sure I understand properly what all these terms mean.

Could someone please explain what the differences are between these terms and possibly also the causes?

Some of these terms seem to be used interchangeably.

 

I've read Don Pensack's excellent explanation of aberrations, but I'm still not clear about the difference f.e. between glare and scatter. Or between these and internal and external reflections.

 

The only one I feel sure of is ghosting, when you see an out of focus duplicate image of Jupiter for example.

But then again, is this due to internal or external (eyeball) reflections?

 

It would be great if someone could post some examples (illustration) of what is actually seen in the eyepiece.

Reflections:

come from light bouncing off something in the system: the edges of the lenses, shiny internal spacers, shiny filter threads, shiny aluminum surfaces immediately above the top lens of the eyepiece, or a lens surface.

Each will have a different result, yet result in the same thing--extra light in the field that shouldn't be there.

Ghosting:

comes from reflection between lenses in the eyepiece (e.g. Kellners and others) or from reflection off the eye and then back to the eye after reflecting from the eyepiece.

Glare: refers to when a spike enters the field of view from a bright object outside the field or when an object in the field is so bright the light spreads all around it.

Scatter refers to light glow being spread by rough surfaces or dust or oils on the optics in the telescope or the eyepiece.  It's usually seen as a glow around the stars.  Dew on optics can mimic bad light scatter.

EOFB: a slight brightening of the edge of the field.  Causes may be multiple but are not really known at this time.



#4 Andy-di-Notte

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Posted 12 March 2017 - 04:35 AM

Fantastic, thanks for the explanation.

Now I have something to refer to when I'm reading about it, or when comparing eyepieces.

 

So, if I understand correctly then glare is a type of reflection.

I assume that these reflections are the only ones that can be reduced by the user, by dealing with any shiny surfaces in the eyepiece. The others cannot be helped, apart from keeping the optics clean.

 

Related to this, I have a 6mm TS/TMB planetary eyepiece, and I've read several posts about people reducing reflections, glare, ghosting, etc, and improving contrast by blackening retaining rings, edges of lenses and/or by flocking the inside of the barrel.

I was thinking of doing this also.


Edited by Andy-di-Notte, 12 March 2017 - 04:40 AM.


#5 Starman1

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Posted 12 March 2017 - 09:55 AM

Eyepieces started receiving coatings to decrease internal reflections and improve transmission.

Ditto for lens edge blackening, blackening of spacers.

There are a few areas where eyepieces still haven't caught up with suppression of scattered light:

1) The bottoms of the eyepieces should be flat black so a bright annular reflection cannot go back to the primary and be refocused in the field, i.e. any surface facing the secondary should be flat black (including the focuser).

2) Internal baffles should be more common to suppress reflection from spacers, which, even though blackened, reflect light.

3) All filter threads should be blackened--without exception.

4) the tops of eyepieces should be flat black to suppress reflections from peripheral light

5) the tops of eyepieces should meet the lens in a knife-edge so that no bright light can reflect into the eye from the aluminum of the eyepiece immediately above the lens.

 

If you take apart an eyepiece to blacken lens edges and spacers, all the above points except #2 can be addressed at that time.

 

Several years ago, I used everything that was known about light scatter to suppress the light scatter in an SCT.  It entailed a lot of flocking and blackening, and it made a profound difference

that was easily noticeable.  SCT manufacturers haven't caught on.  Unfortunately, neither have most eyepiece manufacturers.


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#6 Andy-di-Notte

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Posted 12 March 2017 - 10:51 AM

Don, please help me out here, I'm a little confused, I must have misunderstood something.

I thought that scatter was not due to reflections but to less than perfect optics, rough surfaces, dust or oils, and so could not be helped by blackening?


Edited by Andy-di-Notte, 12 March 2017 - 11:34 AM.


#7 REC

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Posted 12 March 2017 - 02:28 PM

Eyepieces started receiving coatings to decrease internal reflections and improve transmission.

Ditto for lens edge blackening, blackening of spacers.

There are a few areas where eyepieces still haven't caught up with suppression of scattered light:

1) The bottoms of the eyepieces should be flat black so a bright annular reflection cannot go back to the primary and be refocused in the field, i.e. any surface facing the secondary should be flat black (including the focuser).

2) Internal baffles should be more common to suppress reflection from spacers, which, even though blackened, reflect light.

3) All filter threads should be blackened--without exception.

4) the tops of eyepieces should be flat black to suppress reflections from peripheral light

5) the tops of eyepieces should meet the lens in a knife-edge so that no bright light can reflect into the eye from the aluminum of the eyepiece immediately above the lens.

 

If you take apart an eyepiece to blacken lens edges and spacers, all the above points except #2 can be addressed at that time.

 

Several years ago, I used everything that was known about light scatter to suppress the light scatter in an SCT.  It entailed a lot of flocking and blackening, and it made a profound difference

that was easily noticeable.  SCT manufacturers haven't caught on.  Unfortunately, neither have most eyepiece manufacturers.

Good suggestions! I'm not going to take my eyepieces apart, but can blacked the barrel end that has threads for filters. Any suggestions as to what to use to do this with? Does a black magic marker ect. work well enough for this?



#8 Starman1

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Posted 12 March 2017 - 02:53 PM

 

Eyepieces started receiving coatings to decrease internal reflections and improve transmission.

Ditto for lens edge blackening, blackening of spacers.

There are a few areas where eyepieces still haven't caught up with suppression of scattered light:

1) The bottoms of the eyepieces should be flat black so a bright annular reflection cannot go back to the primary and be refocused in the field, i.e. any surface facing the secondary should be flat black (including the focuser).

2) Internal baffles should be more common to suppress reflection from spacers, which, even though blackened, reflect light.

3) All filter threads should be blackened--without exception.

4) the tops of eyepieces should be flat black to suppress reflections from peripheral light

5) the tops of eyepieces should meet the lens in a knife-edge so that no bright light can reflect into the eye from the aluminum of the eyepiece immediately above the lens.

 

If you take apart an eyepiece to blacken lens edges and spacers, all the above points except #2 can be addressed at that time.

 

Several years ago, I used everything that was known about light scatter to suppress the light scatter in an SCT.  It entailed a lot of flocking and blackening, and it made a profound difference

that was easily noticeable.  SCT manufacturers haven't caught on.  Unfortunately, neither have most eyepiece manufacturers.

Good suggestions! I'm not going to take my eyepieces apart, but can blacked the barrel end that has threads for filters. Any suggestions as to what to use to do this with? Does a black magic marker ect. work well enough for this?

 

Flat black paint is best, but a black marker is better than leaving a shiny surface.  At low angles, though, even a black inked surface is quite shiny.



#9 Starman1

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Posted 12 March 2017 - 03:04 PM

Don, please help me out here, I'm a little confused, I must have misunderstood something.

I thought that scatter was not due to reflections but to less than perfect optics, rough surfaces, dust or oils, and so could not be helped by blackening?

Scatter is technically when any light in the system goes anywhere it's not supposed to be.

Dispersion, diffusion, ghosting, glare, and reflections are all sources of light scatter, though they may have different causes and require different remedies to suppress.

It can certainly be from rough optical surfaces, dust, oils and even intrusion into the field (in fact, light from outside the exit pupil can come through the eyepiece and get into the eye if the exit pupil is smaller than your pupil).

So light scatter can come from reflection (lens surfaces, spacers, barrel interiors, eyepiece tops, etc.) and it can be from sources extraneous to the eyepiece like surfaces elsewhere in the telescope.

A completely optimized eyepiece needs to be in a completely optimized telescope to truly suppress all sources of light scatter.

 

So if internal reflections cause the light of a planet, for example, to appear anywhere in the field other than where the planet is, whether a glow around the planet, or a ghost image elsewhere in the field, one would have to consider that light scatter.  One of the hardest to suppress is reflection from the cornea.  Ironically, the shape of the cornea and the lens surfaces below it is almost never considered when designing an eyepiece.

 

One of the big ones that seems to be ignored is having light from the sky enter the bottom of the eyepiece directly over the top of the tube opposite the focuser.  There are commercially-available light shields to block this, but I see few in use in the field.

 

When sources of light scatter are removed, blocked, or even reduced, the effect is improved contrast, and contrast is one of the major criteria for object and detail visibility.


Edited by Starman1, 12 March 2017 - 03:05 PM.


#10 John O'Hara

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Posted 12 March 2017 - 03:47 PM

A frustration of mine is a glow around bright planets, like Jupiter, when using my 7 inch Starmaster Oak Classic at high power. l've not isolated the cause. The Zambuto mirror star tests almost perfectly and the image of the planet is sharp and highly detailed.

 

I've flocked the entire length of the tube, and even use a flocked extension on the upper tube.  One thought is the shiny base of my TV 2.5x Powermate that I normally use with T-6 Naglers for high power.

 

Have others experienced this?



#11 Starman1

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Posted 12 March 2017 - 03:59 PM

A glow, which is a very small amount of scatter and almost coincident with the target, can have many causes:

1) Dust.  The most common.  Add a "haze" on the optics (micro-dust or organics).

2) water vapor deposition.  Even before dew is visible to the eye on optics, a small amount of water droplets condensed on the optics will cause this scatter.

3) rough optical surfaces on either the scope optics or the eyepieces.  Scope optics are usually more likely to blame than the eyepieces, though eyepieces are not immune.

4) light scatter in the eye--the cornea (roughness), the lens (incipient or forming cataracts), floaters (though out of focus, can cause scattering), and the retina (light intensity activating

the firing of adjacent rod cells not in the actual image.

5) old mirror coatings.

 

Some questions concerning the above:

1) Have the mirrors been recently cleaned, like in the last month?  Did it make a noticeable difference?  I clean my mirrors quarterly and ALWAYS see a difference in light scatter.

Dust is not your friend.  And organic haze is even worse.

2) How do you keep the secondary and eyepieces free from moisture?  In the winter, removing the eyecups may allow more circulation between warm, moist, eye and the cold lens.

Moisture won't condense on surfaces that are warm or under a breeze.  One of the side advantages of a fan pushing up.

3) Do you see this with all eyepieces?  Do you see it with the same eyepiece in another scope?  If it's different in another scope, the fault isn't the eyepiece.  If it's different with another

eyepiece, it isn't your scope.  Experiment to see.

4) Can't do much here.  An eye exam may give you a clue.  If you try the other eye is the issue exactly the same?  That doesn't prove it's not in your eye, but if there is a difference between

your eyes, it points to the eye as a possible problem.

5) I've seen many a mirror's figure magically improve with a new coating, and light scatter virtually disappear.  Coating more than 8-10 years old?  Get the mirror re-coated.


Edited by Starman1, 12 March 2017 - 04:02 PM.

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#12 areyoukiddingme

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Posted 12 March 2017 - 04:06 PM

A frustration of mine is a glow around bright planets, like Jupiter, when using my 7 inch Starmaster Oak Classic at high power. l've not isolated the cause. The Zambuto mirror star tests almost perfectly and the image of the planet is sharp and highly detailed.

 

I've flocked the entire length of the tube, and even use a flocked extension on the upper tube.  One thought is the shiny base of my TV 2.5x Powermate that I normally use with T-6 Naglers for high power.

 

Have others experienced this?

 

I used to get this effect every once in a while. Recently I'm getting it a lot using my Leica zoom in my 12.5" Portaball on Jupiter.

 

The effect comes and goes, so it may have something to do with sky conditions. I will check for the formation of condensation on the eyepiece next time it happens (but never saw evidence of that when it happened in the past). Other than that, I'm wondering if my eye sight is the cause (got glasses for the first time about 8 months ago).



#13 Starman1

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Posted 12 March 2017 - 04:50 PM

Transparency of the air varies, often, from minute to minute at many observing sites.

And in a visibly clear sky, too.

That variability can't but have an impact on glow around bright objects.

I wouldn't rule out something else in the system, but if you EVER see an image with no glow around the 

bright target, the issue is not in the optical components.

 

I've had occasion to use my telescope at high altitude in superlatively dry and clear air and the images I saw were simply an order of magnitude

better than the average view.  It's not that I mind the average view, but, having experienced what the sky CAN look like, I want to experience it again.

Incredible contrast, fainter details visible than normal on everything, and eyepieces that haven't really impressed me showing image quality I thought impossible.

 

Why, oh why, do such nights have to come only once every year or two?  Argh!



#14 John O'Hara

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Posted 12 March 2017 - 05:22 PM

Don,

Thanks for the detailed reply.  I'll have to do some work to eliminate some variables.  My mirror coatings are fresh, only 1.5 years old, done at Majestic Optical in NJ. This is the same company that did the original coatings on my 12.5" Teeter, and I've been very pleased with that scope's performance, so I was comfortable using Majestic for my 7" Oak Classic.  Now, I don't recall this glow around Jupiter prior to that coating.  However, I'm 51 years old and have not had an eye exam in about 3 years.  I do have, and always have had, floaters, but perhaps there's something more going on.  Perhaps I should set up my Teeter, or 6' f/8 refractor side-by-side with the Oak Classic and compare the views with the same eyepiece/barlow combos.  Another option, as you suggested, is to use different eyepieces, and perhaps I could use a standard 2x barlow to get the Powermate out of the formula.  One thing I did try was using my 12 mm UO HD Ortho in the Powermate, but the effect was still present.

 

Dust is another possibility, as I've not cleaned the mirrors in the Oak Classic since their recoating.  Generally, it stays pretty clean in that closed tube, but it bears checking.  While I clean mirrors only when I have to, I'm not afraid to do it and know the proper procedures.

 

As I'm sure you know, the 2.5 Powermate has a bright shiny surface surrounding the lens cell at the base of the unit.  I'd thought that the design, with the metallic surface angled away from the lens would prevent such a problem, but it could be reflecting back from the inner surface of the focuser tube (a Moonlite CR-2).

 

The secondary has never dewed up, in the enclosed tube, especially with the extension I use on the upper end, and the glow is there immediately, before the secondary could have cooled to ambient temperature.  I use dew heater straps on the eyepieces, and even have a warmer box I constructed to keep them in when not in use, so that's not the issue. 

 

I'll experiment as you've suggested, and will try to rule out my eyes.  I'm planning to schedule an exam also.  One thing I do know, I'm beginning to be effected by oncoming car headlights at night, especially bright blue HID and LED lights, so we'll see....

 

Thanks,

John



#15 MartinPond

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Posted 12 March 2017 - 07:14 PM

Don, please help me out here, I'm a little confused, I must have misunderstood something.

I thought that scatter was not due to reflections but to less than perfect optics, rough surfaces, dust or oils, and so could not be helped by blackening?

In a perfect world in a dust-free clean-room, the glass might explain a lot,

but glare reflections and diffusions from even a little dust can diminish contrast all the way down the path.

 

You can put your eyeball where the eyepiece would be and learn a lot about the sources of scattering.

It can be quite surprising, and as you cut them down the difference can be great. 

Even space telescopes have rather elaborate stray light treatment and 'light traps', despite all

their perfection and coatings.


Edited by MartinPond, 12 March 2017 - 07:15 PM.


#16 Peter Besenbruch

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Posted 12 March 2017 - 08:35 PM

Related to this, I have a 6mm TS/TMB planetary eyepiece, and I've read several posts about people reducing reflections, glare, ghosting, etc, and improving contrast by blackening retaining rings, edges of lenses and/or by flocking the inside of the barrel.

I was thinking of doing this also.

If you have an actual TMB Planetary 2, you don't need to go through that. If you have the later clones, it would be a good idea. The eyepiece benefits from 6mm and shorter.



#17 AxelB

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Posted 12 March 2017 - 09:56 PM

A frustration of mine is a glow around bright planets, like Jupiter, when using my 7 inch Starmaster Oak Classic at high power. l've not isolated the cause. The Zambuto mirror star tests almost perfectly and the image of the planet is sharp and highly detailed.

I've flocked the entire length of the tube, and even use a flocked extension on the upper tube. One thought is the shiny base of my TV 2.5x Powermate that I normally use with T-6 Naglers for high power.

Have others experienced this?

This happens to me all the time in cold weather. the cause is dew on the eyepiece. I have the very annoying tendency to have very wet eyes. When the eyepiece is cold, this moisture from my eyes condenses on the lense.

I should get heat bands for eyepiece and while I'm at it, for the objective as well.

Edited by AxelB, 12 March 2017 - 09:57 PM.


#18 Andy-di-Notte

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Posted 13 March 2017 - 12:19 PM

 

Don, please help me out here, I'm a little confused, I must have misunderstood something.

I thought that scatter was not due to reflections but to less than perfect optics, rough surfaces, dust or oils, and so could not be helped by blackening?

Scatter is technically when any light in the system goes anywhere it's not supposed to be.

Dispersion, diffusion, ghosting, glare, and reflections are all sources of light scatter, though they may have different causes and require different remedies to suppress.

It can certainly be from rough optical surfaces, dust, oils and even intrusion into the field (in fact, light from outside the exit pupil can come through the eyepiece and get into the eye if the exit pupil is smaller than your pupil).

So light scatter can come from reflection (lens surfaces, spacers, barrel interiors, eyepiece tops, etc.) and it can be from sources extraneous to the eyepiece like surfaces elsewhere in the telescope.

A completely optimized eyepiece needs to be in a completely optimized telescope to truly suppress all sources of light scatter.

 

So if internal reflections cause the light of a planet, for example, to appear anywhere in the field other than where the planet is, whether a glow around the planet, or a ghost image elsewhere in the field, one would have to consider that light scatter.  One of the hardest to suppress is reflection from the cornea.  Ironically, the shape of the cornea and the lens surfaces below it is almost never considered when designing an eyepiece.

 

One of the big ones that seems to be ignored is having light from the sky enter the bottom of the eyepiece directly over the top of the tube opposite the focuser.  There are commercially-available light shields to block this, but I see few in use in the field.

 

When sources of light scatter are removed, blocked, or even reduced, the effect is improved contrast, and contrast is one of the major criteria for object and detail visibility.

 

Thanks Don, I think I get it now.

So scatter can be used as a kind of umbrella term for all the others...


Edited by Andy-di-Notte, 13 March 2017 - 12:30 PM.


#19 Andy-di-Notte

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Posted 13 March 2017 - 12:29 PM

 

Related to this, I have a 6mm TS/TMB planetary eyepiece, and I've read several posts about people reducing reflections, glare, ghosting, etc, and improving contrast by blackening retaining rings, edges of lenses and/or by flocking the inside of the barrel.

I was thinking of doing this also.

If you have an actual TMB Planetary 2, you don't need to go through that. If you have the later clones, it would be a good idea. The eyepiece benefits from 6mm and shorter.

 

I have a TS planetary which is practically identical to the original first series Burgess/TMB planetary.

 

There is a diy fix for a retaining ring that creates reflections in these eyepieces.

Instructions are here:

https://www.cloudyni.../Burgesstmb.pdf

 

Additionally some recommend also flocking the barrel. 

If l hold the eyepiece up towards the light, and look through the field lens I can see plenty of light reflecting off the black interior of the barrel.


Edited by Andy-di-Notte, 13 March 2017 - 12:29 PM.

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#20 Starman1

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Posted 13 March 2017 - 12:31 PM

Yes, but we have different names for some of the sources because they have specific causes.

Some cannot be suppressed without making your own optics or buying optics from a company with a reputation for producing

superbly smooth optics.

Few can afford that, however, so the best we can do is to suppress all the sources of light scatter we can so that, at least, we will see no more scatter than

the minimal scatter from the optical surfaces themselves.

A significant percentage of all the posts on this CN forum relate to one or more of the issues discussed in this thread, some directly, some indirectly.


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#21 Andy-di-Notte

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Posted 13 March 2017 - 12:40 PM

A glow, which is a very small amount of scatter and almost coincident with the target, can have many causes:

1) Dust.  The most common.  Add a "haze" on the optics (micro-dust or organics).

2) water vapor deposition.  Even before dew is visible to the eye on optics, a small amount of water droplets condensed on the optics will cause this scatter.

3) rough optical surfaces on either the scope optics or the eyepieces.  Scope optics are usually more likely to blame than the eyepieces, though eyepieces are not immune.

4) light scatter in the eye--the cornea (roughness), the lens (incipient or forming cataracts), floaters (though out of focus, can cause scattering), and the retina (light intensity activating

the firing of adjacent rod cells not in the actual image.

5) old mirror coatings.

 

Some questions concerning the above:

1) Have the mirrors been recently cleaned, like in the last month?  Did it make a noticeable difference?  I clean my mirrors quarterly and ALWAYS see a difference in light scatter.

Dust is not your friend.  And organic haze is even worse.

2) How do you keep the secondary and eyepieces free from moisture?  In the winter, removing the eyecups may allow more circulation between warm, moist, eye and the cold lens.

Moisture won't condense on surfaces that are warm or under a breeze.  One of the side advantages of a fan pushing up.

3) Do you see this with all eyepieces?  Do you see it with the same eyepiece in another scope?  If it's different in another scope, the fault isn't the eyepiece.  If it's different with another

eyepiece, it isn't your scope.  Experiment to see.

4) Can't do much here.  An eye exam may give you a clue.  If you try the other eye is the issue exactly the same?  That doesn't prove it's not in your eye, but if there is a difference between

your eyes, it points to the eye as a possible problem.

5) I've seen many a mirror's figure magically improve with a new coating, and light scatter virtually disappear.  Coating more than 8-10 years old?  Get the mirror re-coated.

I've always thought that even small amounts of dust can effect contrast, and I try to keep my optics clean, including the corrector and the primary mirror on my C8. 

However, there are many people, including experienced observers, who state that dust on the primary mirror or corrector has little or no effect on the image, and that you should rarely need to clean them, and only if there is really a thick layer of dust or grime.


Edited by Andy-di-Notte, 13 March 2017 - 12:41 PM.


#22 Jeff Morgan

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Posted 13 March 2017 - 01:12 PM

Ghosting is a type of refection. A bright object has a fainter companion. Especially annoying if the ghost sits on top of the regular image. Kellners ghost a lot, and are sometimes called haunted.

 

Ghosting to some extent can be suppressed by the eyepiece designer by insuring that the ghost does not come to focus near the image plane (at which point the light just become scatter).

 

A good discussion of the mathematics and issues of ghosts and coatings can be found here, starting about page 11:

 

http://www.brayebroo...ofEYEPIECES.pdf


  • Peter Besenbruch likes this

#23 Andy-di-Notte

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Posted 15 March 2017 - 01:50 PM

Thanks for that link Jeff.   Lots of interesting stuff .

Such as: "Single internal reflections cause light to be scattered across the field, only double reflections can lead to ghost images."

 

I've learned a lot  from this thread, thanks to everyone for that.

For example (if I understood correctly), ghosting is one of the problems that cannot be suppressed with DIY blackening or flocking.

In fact, although some people complain about ghosting in the TMB planetary eyepieces, the issue that is addressed in this eyepiece by flocking the inside of the field lens retaining ring, is glare (or scatter of some other type, but not ghosting).



#24 Starman1

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Posted 15 March 2017 - 02:03 PM

Correct.  Ghosting, though it may appear similar to back-and-forth reflections from the cornea and top lens surface, occurs by reflection between the elements within the eyepiece.

The very best broadband anti-reflection coatings can mitigate it, but some eyepieces are just prone to it by design (e.g. Kellners).

Sometimes, merely spacing the elements differently (which may require different curves, glass, or more elements) will eliminate the ghosts.

 

In modern multi-element designs, though, it is either an example of where design doesn't match the intended purpose (for example, a planetary-specific eyepiece that has ghosting

as part of its design) or simply a cost-cutting aspect of manufacturing (i.e. lesser coatings on the glass).

 

As for cornea reflections on bright targets, this can sometimes be mitigated by having better coatings on the glass, but I think the shape of the top surface of the eye lens 

in combination with the shape of the observer's cornea as a cause for the reflection is an aspect of eyepiece design that has not, heretofore, been accommodated for in

eyepiece design.  Like EOFB (edge of field brightening), it has been inadequately researched.




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