-
Welcome to BirdForum, the internet's largest birding community with thousands of members from all over the world. The forums are dedicated to wild birds, birding, binoculars and equipment and all that goes with it.
Please register for an account to take part in the discussions in the forum, post your pictures in the gallery and more.
. Venus, Uranus and Mars (1 Viewer)
- Thread starter Binastro
- Start date
I wonder if there are any binoculars where something as bright as Venus, at 12 arc seconds across, looks its real size rather than an expanded disc.
Which would be the best binocular regarding this?
I suppose that our eyes also play a part in expanding a very bright object.
Which would be the best binocular regarding this?
I suppose that our eyes also play a part in expanding a very bright object.
An interesting question. I don't know the answer for sure but there are several different factors appear to be important.
If a single photo receptor in the fovea is around 20 arcseconds lets just suppose you would need a diameter of 5 receptor or 100 arcseconds to distinctly 'see' is as a disk rather than a point. 100/12 would be 8.3 x magnification as a minimum.
To see it with optimum acuity the pupil/exit pupil needs to be about 2.5mm so an 8x20 sounds like a starting point, but 10x25, 12x30 or 20x50 would give you increasingly larger discs. Larger EPs reduce acuity and would increase defocus blur.
Venus appears very bright even in the night sky, but it's completely invisible in daylight. So it's not technically too bright even at a 2.5mm EP but I think the extreme contrast means the scatter within the retina is the primary cause of the apparent expansion. You could shrink the EP even further with more magnification but another path is to attenuate the contrast. That probably means neutral density filters.
I already have objectives masks to reduce the EP and I have some sheets of ND filter material which should do for a little experiment. When I get a chance I'll have a go.
David
If a single photo receptor in the fovea is around 20 arcseconds lets just suppose you would need a diameter of 5 receptor or 100 arcseconds to distinctly 'see' is as a disk rather than a point. 100/12 would be 8.3 x magnification as a minimum.
To see it with optimum acuity the pupil/exit pupil needs to be about 2.5mm so an 8x20 sounds like a starting point, but 10x25, 12x30 or 20x50 would give you increasingly larger discs. Larger EPs reduce acuity and would increase defocus blur.
Venus appears very bright even in the night sky, but it's completely invisible in daylight. So it's not technically too bright even at a 2.5mm EP but I think the extreme contrast means the scatter within the retina is the primary cause of the apparent expansion. You could shrink the EP even further with more magnification but another path is to attenuate the contrast. That probably means neutral density filters.
I already have objectives masks to reduce the EP and I have some sheets of ND filter material which should do for a little experiment. When I get a chance I'll have a go.
David
David, thanks for that.
Assuming your estimate for the size of Venus as seen in your 20×50 spotting scope is correct at six arc minutes.
Why does it look so large?
Is it the scope, your eyes are both?
Yes, if you looked at Venus with a 20×50 spotting scope during the day it might look 12 arc seconds across.
Actually, Venus is rather easily seen when high up during the daytime without optical aid. The problem is finding it. Unless it's very near to the moon, or you actually know exactly where it is, it is impossible to find. But if you find it it is surprisingly easy.
The smallest star images that I have seen is probably with the Canon image stabilised binoculars. But this is for faint stars or at least not bright stars. When it comes to really bright objects such as Venus, even though the optics are clearly very well designed, the brilliant Venus is expanded. But here the exit pupil of the 18×50, for instance, is less than 3 mm. Most of the image stabilised binoculars even from other firms generally seem to have small exit pupils. Although the 10x42 Canon and the Bushnell do have larger exit pupils.
Assuming your estimate for the size of Venus as seen in your 20×50 spotting scope is correct at six arc minutes.
Why does it look so large?
Is it the scope, your eyes are both?
Yes, if you looked at Venus with a 20×50 spotting scope during the day it might look 12 arc seconds across.
Actually, Venus is rather easily seen when high up during the daytime without optical aid. The problem is finding it. Unless it's very near to the moon, or you actually know exactly where it is, it is impossible to find. But if you find it it is surprisingly easy.
The smallest star images that I have seen is probably with the Canon image stabilised binoculars. But this is for faint stars or at least not bright stars. When it comes to really bright objects such as Venus, even though the optics are clearly very well designed, the brilliant Venus is expanded. But here the exit pupil of the 18×50, for instance, is less than 3 mm. Most of the image stabilised binoculars even from other firms generally seem to have small exit pupils. Although the 10x42 Canon and the Bushnell do have larger exit pupils.
Binastro,
I had a couple of minutes to play with the 20x50 before I went out last night and I my recollection of the apparent size was probably out by a factor of two. 1/50 to 1/60 was closer to the mark but that's still pretty bloated. 14" should be ~1/600th of the FOV.
I only had time to have a quick look at what some ND filter placed between the eye and the eyepiece did to the apparent size. The stuff I used is equal to two photographic stops, a four fold reduction or 25% transmission. One piece didn't do a lot so I tried 3 which should give 6.25% transmission. I wrote down 1/200th of the field diameter, but to be honest I wouldn't trust my estimating skill with an angle that small. 1/200 compared to 1/600 is probably within my margin of error.
It doesn't look like there is much prospect of doing a more careful examination today, but I'll have a go when I can.
The way I understand it the bloating is almost entirely in the eye though optical aberrations and the point and line spread functions will have a minor role. I did discover some fine scratches on the eyepiece which probably explains why Venus appeared more spiky with the scope than the binoculars.
David
I had a couple of minutes to play with the 20x50 before I went out last night and I my recollection of the apparent size was probably out by a factor of two. 1/50 to 1/60 was closer to the mark but that's still pretty bloated. 14" should be ~1/600th of the FOV.
I only had time to have a quick look at what some ND filter placed between the eye and the eyepiece did to the apparent size. The stuff I used is equal to two photographic stops, a four fold reduction or 25% transmission. One piece didn't do a lot so I tried 3 which should give 6.25% transmission. I wrote down 1/200th of the field diameter, but to be honest I wouldn't trust my estimating skill with an angle that small. 1/200 compared to 1/600 is probably within my margin of error.
It doesn't look like there is much prospect of doing a more careful examination today, but I'll have a go when I can.
The way I understand it the bloating is almost entirely in the eye though optical aberrations and the point and line spread functions will have a minor role. I did discover some fine scratches on the eyepiece which probably explains why Venus appeared more spiky with the scope than the binoculars.
David
Last edited:
. Hi David,
. Thanks for trying.
Three neutral density filters would give 1.55% transmission i.e. 1/64.
The problem is that when you reduce the brightness of Venus as viewed by the eye, you are also reducing the brightness as seen by the instrument.
In other words, does a very bright object form a larger visible image than a fainter object?
As to my own eyes, when I look at the bright moon I see a much fainter ghost image in the double glazing. This secondary image is for me much nicer. It is smaller and I see more detail in the secondary image because it is not bright.
I know that many binoculars produce large star images even from average brightness stars, I think that is because of the optical design.
. Thanks for trying.
Three neutral density filters would give 1.55% transmission i.e. 1/64.
The problem is that when you reduce the brightness of Venus as viewed by the eye, you are also reducing the brightness as seen by the instrument.
In other words, does a very bright object form a larger visible image than a fainter object?
As to my own eyes, when I look at the bright moon I see a much fainter ghost image in the double glazing. This secondary image is for me much nicer. It is smaller and I see more detail in the secondary image because it is not bright.
I know that many binoculars produce large star images even from average brightness stars, I think that is because of the optical design.
Oops! you're right on the percent transmission.:t:
Don't forget I put the filters between the eye and the eyepiece. I did try putting them in front of the objective as well, and although my impression was it would probably make little difference, the ND stuff is not optically flat and I was getting a misshaped disc.
If I get another chance I'll try to remember to include a couple of lower resolution binoculars as well in the comparison.
David
Don't forget I put the filters between the eye and the eyepiece. I did try putting them in front of the objective as well, and although my impression was it would probably make little difference, the ND stuff is not optically flat and I was getting a misshaped disc.
If I get another chance I'll try to remember to include a couple of lower resolution binoculars as well in the comparison.
David
As promised, I had another go with stopping down the objectives and using ND filter to see how the relative size of Venus appeared in the view. To be honest I don't feel I learned a great deal more than that first look.
Over the 20 minutes or so I was viewing, the sky got darker and the apparent size generally increased. With the 20x50 it increased from 1/100 FOV to about 1/60FOV. With the use of 3 layer ND filter the apparent size decreased and on both occasions to about 1/600 FOV which is the correct diameter.
To cut an uninteresting story short with both a high resolution and low resolution 10x50 the combination of a 25mm aperture and 3ND filters reduced the diameter to less than 1/1000 of the FOV approximating to the real diameter. The shrinkage with the 8x42 appeared somewhat less than the others and was probably about twice the real size but I didn't try anything extra to shrink it further.
David
Over the 20 minutes or so I was viewing, the sky got darker and the apparent size generally increased. With the 20x50 it increased from 1/100 FOV to about 1/60FOV. With the use of 3 layer ND filter the apparent size decreased and on both occasions to about 1/600 FOV which is the correct diameter.
To cut an uninteresting story short with both a high resolution and low resolution 10x50 the combination of a 25mm aperture and 3ND filters reduced the diameter to less than 1/1000 of the FOV approximating to the real diameter. The shrinkage with the 8x42 appeared somewhat less than the others and was probably about twice the real size but I didn't try anything extra to shrink it further.
David
. Thanks David.
With one of your neutral density filters the brightness of Venus is reduced to the brightness of Jupiter, so I wouldn't expect much change.
With three neutral density filters, i.e. 1/64 the brightness, Venus's brightness is reduced to the brightness of a first magnitude star, approximately the brightness of the four main stars in Orion. Rigel is brighter, but on average they are about right. That is a quite bright, but not very bright stars.
So your findings are about what I would expect.
. But I still don't know whether the binocular contributes to the enlarged image of Venus or whether it is only due to our eyes.
Can you think of a test that does not involve our eyes, to determine whether the binocular itself increases the image size with a very bright point source or nearly point source object?
Do spot diagrams take this matter into account?
With one of your neutral density filters the brightness of Venus is reduced to the brightness of Jupiter, so I wouldn't expect much change.
With three neutral density filters, i.e. 1/64 the brightness, Venus's brightness is reduced to the brightness of a first magnitude star, approximately the brightness of the four main stars in Orion. Rigel is brighter, but on average they are about right. That is a quite bright, but not very bright stars.
So your findings are about what I would expect.
. But I still don't know whether the binocular contributes to the enlarged image of Venus or whether it is only due to our eyes.
Can you think of a test that does not involve our eyes, to determine whether the binocular itself increases the image size with a very bright point source or nearly point source object?
Do spot diagrams take this matter into account?
I think the the comparison of placing the ND filters in front of the objective and behind the ocular convinces me that the eye is the source of the expansion.
I suspect it's possible to do the same experiment with a camera. With the 2.5mm exit pupil I suspect you will need a phone camera with a suitable app to control the settings but it's not something I know much about or could help you with.
David
I suspect it's possible to do the same experiment with a camera. With the 2.5mm exit pupil I suspect you will need a phone camera with a suitable app to control the settings but it's not something I know much about or could help you with.
David
. Hi David,
I don't think that the position of putting three neutral density filters in front of the objectives or behind the eyepieces is of any significance in determining the source of the expanded disc for a very bright object.
Either way, you are reducing the intensity to 1/64 of the original.
So this in itself I don't think proves one way or the other the source of the expanded disc.
However, it may be that the sensor, either film, digital sensors, or our eyes may be the cause of the expansion.
Using a camera would not help with a very bright object as the sensor or film would be overloaded and may produce an expanded disc, either smaller or larger or the same size as seen by our eyes.
And it may be that different people's eyes produce different sized discs. Even if our eyes focus properly at Infinity.
I think that it was may be the Univex Corporation, possibly of New York?, who produced the unusual half frame, sector shutter, Mercury camera, who had up their sleeve a device that could look at the Sun without any glare. My memory is poor, perhaps it used phosphorus?? Anyhow, I don't think they ever found an application for this device.
I will have to ask a lens designer friend whether he can advise me on this topic.
I don't think that the position of putting three neutral density filters in front of the objectives or behind the eyepieces is of any significance in determining the source of the expanded disc for a very bright object.
Either way, you are reducing the intensity to 1/64 of the original.
So this in itself I don't think proves one way or the other the source of the expanded disc.
However, it may be that the sensor, either film, digital sensors, or our eyes may be the cause of the expansion.
Using a camera would not help with a very bright object as the sensor or film would be overloaded and may produce an expanded disc, either smaller or larger or the same size as seen by our eyes.
And it may be that different people's eyes produce different sized discs. Even if our eyes focus properly at Infinity.
I think that it was may be the Univex Corporation, possibly of New York?, who produced the unusual half frame, sector shutter, Mercury camera, who had up their sleeve a device that could look at the Sun without any glare. My memory is poor, perhaps it used phosphorus?? Anyhow, I don't think they ever found an application for this device.
I will have to ask a lens designer friend whether he can advise me on this topic.
. Regarding post 31, the lens designer could not help. He says that chromatic aberration could produce a white image depending on the position along the axis. I think the same for spherochromatism.
This evening the planet Uranus is 0.3° is south of Mars at 20:00 hours. Of course if you want to have a look, it would be a bit earlier, and Mars is not very bright. But at least you could find Uranus again. You might see a disc with a spotting scope, perhaps at high magnification.
This evening the planet Uranus is 0.3° is south of Mars at 20:00 hours. Of course if you want to have a look, it would be a bit earlier, and Mars is not very bright. But at least you could find Uranus again. You might see a disc with a spotting scope, perhaps at high magnification.
Binastro,
You certainly don't need any optics to demonstrate disc expansion in the eye. All you need is a powerful but dimmable light source and a pinhole in something like aluminium foil and you can see for yourself in a minute or two. A very well documented effect. You can throw binoculars into the mix if you like, but last time I tried it is made no difference.
You do need manual control over exposure but Venus is not very bright in the scheme of things. I once built a million lux light source that would melt plastic in seconds and got very good images at a few millionths of a second.
I suggested a phone camera as the very short back focus means you still should have decent f-numbers even on a 2.5mm EP. Some apps appear to give you manual control over exposure but I don't have anything suitable at the moment to test it out.
David
P.S Very cloudy here this afternoon!
. Thanks for your explanation, David.
It surprised me that the lens designer does not take bright images into account.
At least you measurements of the disc size of Venus gives empirical data with different instruments.
When photographs are taken of star fields, different star brightnesses gives different star image sizes.
What camera did you use that takes exposures of millionths of a second, or was the shutter open for longer?
I did see a camera that went down to 1/15,000,000 of a second.
It surprised me that the lens designer does not take bright images into account.
At least you measurements of the disc size of Venus gives empirical data with different instruments.
When photographs are taken of star fields, different star brightnesses gives different star image sizes.
What camera did you use that takes exposures of millionths of a second, or was the shutter open for longer?
I did see a camera that went down to 1/15,000,000 of a second.
The camera was a Basler machine vision camera, which sounds exotic but isn't. The sensor is a standard Kodak CCD, which is mainly used in CCTV cameras and technically no different from the type used in consumer cameras, just built to be driven by a computer. It's a while since we did it but this was a basic version and the shortest exposure was 8 microsecond and synchronised with a 20 microsecond light pulse so we didn't have complete melt down if I remember rightly. As you say, high speed cameras are much faster.
I don't know much about star cameras, but a point and shoot with a 2micron pixel size will probably only have an 8 bit (256 level) dynamic range so the level of over exposure will be enormous on the brighter stars. Last time I looked 16 and 18 bit monochrome cooled scientific cameras were available but I imagine higher specifications still are available.
David
Last edited:
I realised after that last post I could have explained the camera thing better, and while I'm at it there is probably a lot about our conversation that could do with spelling out.
Binastro is the astronomer and I'm a birder and I'm sure to make mistakes, but I hope he will forgive an amateur and correct me tactfully.
I'm going to skip planets and talk about stars for the moment. I'm sure we've all looked at the sky and checked out the big ones and then looked for patterns in the small ones. It rather surprised me to realise in fact they are all absolutely minuscule. With the biggest telescope any of us are likely to afford they would still make a smaller angle to the eye than a single receptor on the retina. What we've been discussing is why we don't see this and why the stars and the planets like Venus can look hugely bigger than they really are. They look bigger the brighter they are for a number of distinct reasons, but I'll only mention a few that seem from my reading to be the most important.
As soon as light passes through the atmosphere and glass there is a spreading of the profile of the light due to diffraction and to some extent scatter. Most of the light stays in a peak but there is a boundary to the spot distribution that spreads out from the centre. This is progressively more obvious in a photo with over exposure as that will boost the intensity of this boundary and that alone will make brighter stars look bigger. A diffraction spread also occurs in the lens of the eye. Any additional optical aberrations in the eye or the optics will again increase the spread further due to blur. Unfortunately the rest of the eye has a bunch of imperfections as well that increase the scatter further, and those imperfections accumulate as we get older.
In fact the eye has similar problems with overexposure as a camera. The technicalities are different, but the eye has comparable metering properties to a camera and tends to average a scene leading to over exposure very bright spots.
There is one more important factor to consider that can be particularly significant to the astronomer dealing with very high contrast light sources. Once the light hits the camera sensor or more particularly the retina of the eye it isn't all absorbed. Some of it is scattered across adjacent pixels or retinal photo receptors and it is this what I understand is often more important than the other spreading functions in making bright stars look bigger to the eye.
The eye is reported to produce an image over a trillion fold range of light intensity, but it only does so with a very small range at a time. We can only discriminate proximately 250 shades of grey (I'm sure there must be a joke there!). You're computer screen or your photographic printer usually works on a 256 scale as does an very basic camera. The metering the ISO, f-number and exposure move that 256 range up and down some of that trillion fold range to hopefully produce an image to match the eye. A high quality camera will capture a luminance range way beyond anything the eye can manage. At 16 bits it will produces digitised information 256 times the range the eye can deal with in one go and the very best cameras will do more than that. If that information is compressed into a single 256 scale image the brightest stars will look disproportionally larger due to over exposure and the feint ones will look smaller, due to under exposure, not unlike the eye, but in fact the captured data can reveal that all the stars have in fact the same apparent diameter; if not the true one. The eye doesn't have this scaling capability and as a result differences in brightness means they always means appearing to be different sizes.
I hope I got most of that right.
David
Binastro is the astronomer and I'm a birder and I'm sure to make mistakes, but I hope he will forgive an amateur and correct me tactfully.
I'm going to skip planets and talk about stars for the moment. I'm sure we've all looked at the sky and checked out the big ones and then looked for patterns in the small ones. It rather surprised me to realise in fact they are all absolutely minuscule. With the biggest telescope any of us are likely to afford they would still make a smaller angle to the eye than a single receptor on the retina. What we've been discussing is why we don't see this and why the stars and the planets like Venus can look hugely bigger than they really are. They look bigger the brighter they are for a number of distinct reasons, but I'll only mention a few that seem from my reading to be the most important.
As soon as light passes through the atmosphere and glass there is a spreading of the profile of the light due to diffraction and to some extent scatter. Most of the light stays in a peak but there is a boundary to the spot distribution that spreads out from the centre. This is progressively more obvious in a photo with over exposure as that will boost the intensity of this boundary and that alone will make brighter stars look bigger. A diffraction spread also occurs in the lens of the eye. Any additional optical aberrations in the eye or the optics will again increase the spread further due to blur. Unfortunately the rest of the eye has a bunch of imperfections as well that increase the scatter further, and those imperfections accumulate as we get older.
In fact the eye has similar problems with overexposure as a camera. The technicalities are different, but the eye has comparable metering properties to a camera and tends to average a scene leading to over exposure very bright spots.
There is one more important factor to consider that can be particularly significant to the astronomer dealing with very high contrast light sources. Once the light hits the camera sensor or more particularly the retina of the eye it isn't all absorbed. Some of it is scattered across adjacent pixels or retinal photo receptors and it is this what I understand is often more important than the other spreading functions in making bright stars look bigger to the eye.
The eye is reported to produce an image over a trillion fold range of light intensity, but it only does so with a very small range at a time. We can only discriminate proximately 250 shades of grey (I'm sure there must be a joke there!). You're computer screen or your photographic printer usually works on a 256 scale as does an very basic camera. The metering the ISO, f-number and exposure move that 256 range up and down some of that trillion fold range to hopefully produce an image to match the eye. A high quality camera will capture a luminance range way beyond anything the eye can manage. At 16 bits it will produces digitised information 256 times the range the eye can deal with in one go and the very best cameras will do more than that. If that information is compressed into a single 256 scale image the brightest stars will look disproportionally larger due to over exposure and the feint ones will look smaller, due to under exposure, not unlike the eye, but in fact the captured data can reveal that all the stars have in fact the same apparent diameter; if not the true one. The eye doesn't have this scaling capability and as a result differences in brightness means they always means appearing to be different sizes.
I hope I got most of that right.
David
. Thanks David for the excellent post 36.
I intend asking my imaging friends how the size of stars and their images relates to the magnitude or brightness of the stars. I don't know if this is a linear relationship or logarithmic or something else. Also I don't know whether the light leaks to neighbouring pixels with overexposure.
I will also ask my friend who still uses film.
I have though noticed that some top quality, say 10×42, binoculars produce smaller star images than do other binoculars. I still don't know whether this is of any significance regarding the size of bright star images, or in the case of Venus. a very brilliant, slightly nonpoint source, object.
I intend asking my imaging friends how the size of stars and their images relates to the magnitude or brightness of the stars. I don't know if this is a linear relationship or logarithmic or something else. Also I don't know whether the light leaks to neighbouring pixels with overexposure.
I will also ask my friend who still uses film.
I have though noticed that some top quality, say 10×42, binoculars produce smaller star images than do other binoculars. I still don't know whether this is of any significance regarding the size of bright star images, or in the case of Venus. a very brilliant, slightly nonpoint source, object.