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The Velvet black background in 4" F15 classic scopes

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#1 starman876

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Posted 22 June 2019 - 11:22 AM

Not sure how many of you have seen it.  That black velvet jet black background when using a 4" F15 scope.  It almost looks like a pattern of some sort.   It looks like it has texture to it.  If you have seen it do you know what it is?


 

#2 dgreyson

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Posted 22 June 2019 - 11:47 AM

That's one of those questions you could easily answer with a million dollar grant and a choice selection of extremely attractive graduate assistants, but finding out wouldn't especially advance the state of the art in any way I guess. I think of those kind of questions a lot too but no one has stepped in to take a look at any. Maybe long focal lengths and larger objectives self baffle a bit and have inherently somewhat better contrast and less light reflections. Or not.

I think I've noticed what you're describing though, so either it's real or I'm just impressionable.
 

#3 rolo

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Posted 22 June 2019 - 12:04 PM

Its just the effect you get from a small aperture and an f/15 focal ratio.


 

#4 Keith Rivich

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Posted 22 June 2019 - 12:08 PM

Its just the effect you get from a small aperture and an f/15 focal ratio.

Kinda what I was thinking. That scope is focusing the light onto a very tiny part of your pupil...the rest of the pupil is bored and trying to find something to do!


 

#5 starman876

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Posted 22 June 2019 - 12:43 PM

Kinda what I was thinking. That scope is focusing the light onto a very tiny part of your pupil...the rest of the pupil is bored and trying to find something to do!

so the answer is a bored pupil lol.gif


 

#6 starman876

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Posted 22 June 2019 - 12:44 PM

Its just the effect you get from a small aperture and an f/15 focal ratio.

and what effect might that be?  In scientific terms?


 

#7 starman876

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Posted 22 June 2019 - 12:45 PM

That's one of those questions you could easily answer with a million dollar grant and a choice selection of extremely attractive graduate assistants, but finding out wouldn't especially advance the state of the art in any way I guess. I think of those kind of questions a lot too but no one has stepped in to take a look at any. Maybe long focal lengths and larger objectives self baffle a bit and have inherently somewhat better contrast and less light reflections. Or not.

I think I've noticed what you're describing though, so either it's real or I'm just impressionable.

I thought for a while that is was background noise.  Do not ask me what kind.


Edited by starman876, 22 June 2019 - 01:50 PM.

 

#8 Bomber Bob

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Posted 22 June 2019 - 12:57 PM

I see it regularly in my Edmund 4" F15, and put it the same category as the Marble Moon Effect seen in some combinations of 40mm / 50mm refractors + certain eyepieces.  Not an illusion, just a function of perception in a limited circumstance.  In fact, the 4" F15 Black Background is palpable to me when the field has a few very faint stars in it.


 

#9 Richard O'Neill

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Posted 22 June 2019 - 01:23 PM

"I thought for a while that is was background noise."

 

 It is 'neuron noise' that originates in the eye's retina while the brain attempts pattern recognition as it slowly adapts to the absence of light. Ask Sheldon's Amy, wink.gif she understands.


Edited by Richard O'Neill, 22 June 2019 - 01:23 PM.

 

#10 kansas skies

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Posted 22 June 2019 - 01:26 PM

Although I'm not sure what is meant by a velvet background, what I find most impressive about my 4" F15 Celestron is the increase in subtle contrast. It's not like details just "pop" out, but with enough patience (along with a considerable amount of practice), details seem to surface that I don't think I would have been able to coax out with scopes of lesser focal ratio. Of course, I'm speaking of achromatic refractors only, since I've spent very little time observing with apochromats and don't have the experience required to form a useful opinion insofar as they're concerned.

 

Bill


 

#11 terraclarke

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Posted 22 June 2019 - 01:29 PM

I also think its a visual artifact akin to signal noise. Probable related to the same kind or neurological response that creates after images. The small exit pupil of a long refractor has focused the light and image onto a minute spot on the fovea where the cone density is highest and you are somehow perceiving the texture of the surrounding cones, with each one is sending a weak signal to the optic nerve and brain? 


 

#12 Joe1950

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Posted 22 June 2019 - 01:50 PM

No objective, regardless of size or f-ratio forms a point image. On a star it forms the classic Airy disk and diffraction pattern. A refractor at 4” diameter and at f/15 has a rather apparent Airy disk and thin, lower lever diffraction rings.

 

Extended objects are countless multiples of that Airy disk that overlap. Even dark backgrounds, unless completely devoid of any light which would probably not exist, are a formation of these overlapping Airy disks.

 

So my guess, and that’s all it is, would be that with dark adapted vision and the larger Airy disks of your instrument, you are seeing a kind of diffraction pattern caused by the interfering and overlapping Airy disks, in some cases being additive and in some cases canceling the trace light, causing a textured effect.

 

A 4” f/15 refractor probably hits the ‘sweet spot’ having sufficient light gathering ability and that large Airy disk. In short, I think it is an interference pattern of the dark, but not completely dark, background causing that velvety look.

 

Again, just a thought. I’m not a physicist or optical engineer. But I did stay at a Holiday Comfort inn!

 

joe


 

#13 starman876

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Posted 22 June 2019 - 01:55 PM

No objective, regardless of size or f-ratio forms a point image. On a star it forms the classic Airy disk and diffraction pattern. A refractor at 4” diameter and at f/15 has a rather apparent Airy disk and thin, lower lever diffraction rings.

 

Extended objects are countless multiples of that Airy disk that overlap. Even dark backgrounds, unless completely devoid of any light which would probably not exist, are a formation of these overlapping Airy disks.

 

So my guess, and that’s all it is, would be that with dark adapted vision and the larger Airy disks of your instrument, you are seeing a kind of diffraction pattern caused by the interfering and overlapping Airy disks, in some cases being additive and in some cases canceling the trace light, causing a textured effect.

 

A 4” f/15 refractor probably hits the ‘sweet spot’ having sufficient light gathering ability and that large Airy disk. In short, I think it is an interference pattern of the dark, but not completely dark, background causing that velvety look.

 

Again, just a thought. I’m not a physicist or optical engineer. But I did stay at a Holiday Comfort inn!

 

joe

so you are saying this only happens after staying at a Holiday Inn?

 

I think you could be right of overlapping airy disks.   There are times I just stare it.   I do notice it changes as I look at different parts of the sky.   I also, thought about the stars that we do not see, but are really still there in the background.  I doubt I am seeing dark matter. 


 

#14 rolo

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Posted 22 June 2019 - 02:08 PM

This velvet background stuff means nothing. From a dark site every scope shows a velvet background, even with 40% obstruction.


 

#15 Chuck Hards

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Posted 22 June 2019 - 02:22 PM

Not sure how many of you have seen it.  That black velvet jet black background when using a 4" F15 scope.  It almost looks like a pattern of some sort.   It looks like it has texture to it.  If you have seen it do you know what it is?

 

You're not truly dark-adapted, or maybe macular degeneration?  (Just kidding!)

 

 

Good baffling and other diffraction management is the best guarantee of excellent contrast, assuming a good polish on the objective itself, a lack of surface irregularities such as scratches also.  F-ratio is secondary.  Rolo has a point, sky-glow is going to reduce contrast from less than optimal sites and all scopes will do better under a dark sky.


 

#16 Joe1950

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Posted 22 June 2019 - 02:23 PM

so you are saying this only happens after staying at a Holiday Inn?

 

I think you could be right of overlapping airy disks.   There are times I just stare it.   I do notice it changes as I look at different parts of the sky.   I also, thought about the stars that we do not see, but are really still there in the background.  I doubt I am seeing dark matter. 

No, according to the commercial, several years back, you perform much better, in this case, my thinking of a reason, if you stayed at the Inn and got a good nights sleep.

 

Example: In an operating room a surgeon was barking orders in medical talk and then said, “Okay, that’s it, close it up.” A nurse next to him said, “That was a brilliant operation, doctor!”  He answered, “Oh, I’m not a doctor. But I did stay at a Holiday Comfort Inn last night.”

 

They did one of a pilot making a difficult landing of a commercial jet and others. Very clever commercials among most that are just annoying and foolish. I used to work in the media and how some of these ad agencies get paid for what they do is beyond me. But these were excellent, IMHO.


 

#17 rolo

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Posted 22 June 2019 - 03:12 PM

Just remember that all have here are a bunch opinions with no real scientific fact. Just chatter to keep us entertained...


Edited by rolo, 22 June 2019 - 03:13 PM.

 

#18 starman876

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Posted 22 June 2019 - 03:57 PM

Just remember that all have here are a bunch opinions with no real scientific fact. Just chatter to keep us entertained...

and some catch bigger fish than anyone else in the same pond lol.gif


Edited by starman876, 22 June 2019 - 03:57 PM.

 

#19 Bomber Bob

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Posted 22 June 2019 - 04:56 PM

Background skies are as black in my 60mm F20 as in my 100mm F15, but I don't see the same textured effect.

 

I wouldn't be surprised that the "lumps" are due to stars too dim for my retina, but within the capabilities (lowest limits) of the refractor -- and there would be more of those in the 100mm than in the 60mm.


 

#20 terraclarke

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Posted 22 June 2019 - 06:39 PM

so you are saying this only happens after staying at a Holiday Inn?

 

I think you could be right of overlapping airy disks.   There are times I just stare it.   I do notice it changes as I look at different parts of the sky.   I also, thought about the stars that we do not see, but are really still there in the background.  I doubt I am seeing dark matter. 

Maybe it’s kind of an Obler’s paradox thing? ;)


 

#21 Joe1950

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Posted 22 June 2019 - 06:43 PM

All I know is that around these parts, the only way I can see a velvety black sky is to leave the lens cap on the scope! crazy.gif


 

#22 starman876

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Posted 22 June 2019 - 07:05 PM

All I know is that around these parts, the only way I can see a velvety black sky is to leave the lens cap on the scope! crazy.gif

I live outside Washington DC and I can see the black velvet with the lens cap off.


 

#23 luxo II

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Posted 22 June 2019 - 08:02 PM

It is indeed noise - the neuronal noise floor, between the retina and brain. You can see this phenomenon in a photographic darkroom, or a cave with no light at all.

 

http://math.ucr.edu/...e_a_photon.html


Edited by luxo II, 22 June 2019 - 08:41 PM.

 

#24 starman876

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Posted 22 June 2019 - 08:07 PM

It is indeed noise - the neuronal noise floor, between the retina and brain. You can see this phenomenon in a photographic darkroom, or a cave with no light at all.

So do some see this neuronal noise more than others?  and why do I only see it when I look through my scope?  I should be able to see this neuronal noise when I close my eyes in a dark room.   I actually see something besides noise when I close my eyes in a dark room. It can be really interesting.


Edited by starman876, 22 June 2019 - 08:10 PM.

 

#25 T1R2

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Posted 22 June 2019 - 08:07 PM

Not sure how many of you have seen it.  That black velvet jet black background when using a 4" F15 scope.  It almost looks like a pattern of some sort.   It looks like it has texture to it.  If you have seen it do you know what it is?

The only texture I've seen is when viewing along the Galactic Plane through Cyg. down through Sag. If you look closely you can see the red-ish dust contrasted by other dark clouds and darker areas, its shapes being lit up by the myriad tiny stars within its neighborhood thats only faintly visible to the eye even with a telescope and better seen with long exposure imaging, some stars you can even see embedded within the dust lanes barely peeking out obscured by the dust.

 

Edit: I've also seen the "noise" that others are referring to, but I try to avoid it, usually more apparent in my weaker eye.


Edited by T1R2, 22 June 2019 - 08:11 PM.

 


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