Thread starter signalpath Start date Nov 9, Tags digital filter. Nov 9, Joined Aug 1, Messages 88 Likes I'm thinking "Brickwall" but have no clue.
And we really don't have the time or bandwidth to do comprehensive listening trials on this. Thank you in advance. Last edited: Nov 10, My humble opinion is that I can't hear any differences between ESS filters but I'm too old to be able to hear above 15kHz.
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Thread starter ChordElectronics Start date May 10, Tags chord electronics chord hugo hugo tt hugo tt2. Prev 1 … Go to page. Go to page. First Prev of Go to page. This is a mask that you have to fasten to the front end of your OTA O ptical T ube A ssembly and is primarily used by planet and binary observers.
I've come across many different opiniont about this mask: some say it's brilliant, some say it does not perform any better than your regular colour or neutral filter. I've even forgot about it until recently the Jupiter was just in such a great position on the sky that I had the idea that I should not be manipulated by anyone's opinion and give it a try and see what this mask adds to the observation. So this article will show you how to create your own apodizig filter and I'll also describe and illustrate my own experience with the mask I've built.
The apodizing mask often referred as apodizing filter is something that was invented to enhance contrast, but paradoxically it achieves this goal while it also corrupts the details of the image.
This is done by a special diaphragm to block some parts of the aperture, or so to say we theoretically corrupt the image of the scope while we also enhance the image. I know this sounds weird, so I have to emphasize that it's only the outer parts of the image that become pretty much useless, while the central part in about a diameter of two Jupiters becomes a lot better. As it is the diffraction that ruins the contrast the most, it is clear that this mask enhances the view of optical systems where there's something that blocks part of the aperture - such as Newtonian reflectors, catadioptric systems, etc.
Some optical basics To understand how this mask works, we must get a little bit into some optical concepts like the Fresnel diffraction, Airy-disk, resolving power, diffraction rings and so on.
I know these might sound exotic to some of you, but don't worry, I'll try to explain the bare basics as much as I can, but bear in mind that I'm not that much experienced in optics so I cannot guarantee that I'll be always prefectly right, but anyway it's not necessary to get into the depths of optics to understand why the concept of apodization might work.
Anyway, feel free to correct me if you find something that's wrong. First of all, it's good to know that the resolution of our telescope - or with other words the level of details we can glimpse with it - is primarily affected by its aperture plus all the things that block the way of the light till it ends up on our retina.
One of these things is actually the open end of the OTA, or the ring at the open end of the telescope as this is itself a baffle, or something that blocks our field of view. The diameter aperture of the telescope changes resolution in a positive direction: the bigger the scope the better the resolution. The next figure shows you how the intensity of a beam of light changes in the focused image: The hill in the middle of the intensity diagram draws the actual star or a dot-like detail on the surface of a planet.
This, as you can see, is not a point-like feature, but it's actually a little disk, therefore the image of the star will never be a pinpoint but instead it will always be shown as a really really small disk that gradually but quickly fade near its edge - this is the so called Airy-disk. With a larger aperture this central hill is narrower the Airy-disk is smaller compared to a small aperture telescope, as the perimeter of the larger OTA's end ring that causes a diffraction - something when the light is detoured from its ideal path is growing in a smaller rate compared to its surface area.
So while with a small telescope the perimeter of the OTA's diffracting end ring is relatively large compared to its surface, this rate gradually decreases when moving forward larger diameters. This can be verified easily if you calculate the perimeter and area of various optics.
Now we see that larger aperture means narrower central hill, smaller Airy-disk, and if you relate the construction of a planet's image to a computer monitor's image constructed of pixels, you can easily understand that the smallest detail of a telescope's image will also get smaller with a smaller Airy-disk, just like a monitor is capable of displaying a higher resolution if its pixel size is smaller.
As by a small OTA the Airy-disks are relatively large, they are blurred on a larger area, making the image of the scope softer, less contrasting, showing less details. However, if you have larger optics, the Airy-disks are smaller, they blur in a smaller area, resulting in an image that features more details, or so to say an image with higher resolution. Let's take a look again on the above figure.
You may notice, that not only is a central hill is resulted from diffraction, but there are also many smaller waves that are higher near the center of the light cone and gradually losing their amplitudes toward the edge until they are quiesced out. In the concept of diffraction we speak of n-th order images, which means that the central, original image is the image of the object itself is called the 0th order diffraction image, while on its sides you will see the 1st, 2nd, 3rd, etc.
These are diffracted images, that are in case of white natural light actually decomposed into its components, just like the colours of the rainbow.
These are in fact spectral images of the source of light, that are getting gradually fainter. I will not get into details about this now, but you can find many DIY CD-spectrograph websites around the web.
Back to the waveform: these weavelets with their decreasing amplitudes are the reason for the so called diffraction rings, the phenomenon of the gradually fading concentric circles of light around focused stars. Using smaller optics these waves are wider, therefore the diffraction rings are more pronounced, while larger optics this quiescing of waves is happening at a much faster rate, so these optics sometime barely display any diffraction ring around star images.
Although the details of an image is mostly specified by the size of the Airy-disk, merging diffraction rings are also strongly corrupting contrast. It's worth to mention that of course not only is the edge of the OTA that creates diffraction, but virtually anything else that's inside the path of the light: the secondary mirror is the most guilty with its relatively large perimeter. But it is not very common to have an intensity profile like a gaussian curve, or something with the borders going to zero.
This bokeh is great to be true neutral - the background blur is not stressed. Here only a short simple explanation how the bokeh is formed.
What we see as the "bokeh" borders, are the iris blades, or their surrounding perfect circle shaped housing while the iris is set full open. When we close the iris, we see the iris blades. Most people seem to prefer circle shaped defocus spots - so there is the need for circle shaped iris blades, and many iris blades. This is the reason, why some claim that lenses with many iris blades have a good bokeh.
But these blade number is only interessting with most lenses as soon as one close the iris! Sony has made their apodisation element out of two glas elements. One would be out of a neutral density filter glas, the other out of clear glas.
The two parts could probably be a kind of parallel plate, if the glas is the same for the two glued together lenses. But they have to take this into their optical design. One could buy apodizing filters - but still the optical design should be changed for those.
For us tinkerers this is not a good way to go! But there is a way to get the apodisation effect with other lenses than the Sony STF lens with its long mm focal lenght - not as perfect as that lens, but way cheaper and you could test it with different lenses:. You need access to the iris. With some lenses this is pretty simple. But at this lens the iris surrounding is not perfect for this task, the iris will probably not close after the modification.
You need to measure the diameter of the open iris. Search a lab who could do film recording for you at a cheap rate for a small number of slides.
Check what resolution they need. I got my slides here in Germany from Eye.
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