guess what is this and its use?
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wucheeyiun
- Posts: 1758
- Joined: Tue Sep 07, 2004 5:57 pm
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For indoor testing and analysing of light fall-off in scopes and its effects on imaging cameras?
http://www.astro.sg
email: gary[at]astro.sg
twitter: @astrosg
"The importance of a telescope is not how big it is, how well made it is.
It is how many people, less fortunate than you, got to look through it."
-- John Dobson.
email: gary[at]astro.sg
twitter: @astrosg
"The importance of a telescope is not how big it is, how well made it is.
It is how many people, less fortunate than you, got to look through it."
-- John Dobson.
Yes, it is an accessory used in ccd/dslr imaging to facilitate the taking of flat field frames. So those who are new to imaging, whenever we embark on taking an image of a deep-sky object, whether it is taken from a light-polluted area or dark sky, the signal photons coming from the object is actually also "mixed" or "altered" slightly due to the nature of the optical system that we have used to take this object. In short, the object that we are taking may not appear in its "true" form, but slightly altered in a form of either uneven illumination levels or it includes dust motes that have somehow made its way to the optical surfaces.
The way to remove these "altered" anomalies is through the taking of flat field frames. These frames are hence used remove image artifacts due to our optical system. Vignetting and shadows from out-of-focus dust specks (as our systems age, or after several expeditions) are the most common aberrations which these flat field frames eliminate.
The taking of a flat field can be done by a few methods:
1) Taking sky flats - pointing your telescope at a region of the sky away from the sun (before it sets), say around 4-5pm in the east. It is important that you point to any area free of clouds and whereby the sky illumination is as even as possible (with no stars in sight!). You may like to use a white thin shirt to cover the objective if the camera is very sensitive.
2) Taking a whitish wall from a distance with your telescope.
3) Taking flats based on an uniformly-lited "light box" that can be made from simple light source materials (with acrylic, diffusing materials, etc).
4) Taking flats based on an EL Panel (EL short for Electro-Luminence).
What I have bought is based on Option 4, which is an EL Panel with uniform illuminence (from Gerd Neumann).
Below shows the setup taken earlier in the night showing the EL Panel in action.
Once flat field frames are taken with any of the above options, you will notice that they would look like blank, evenly illuminated image which additionally show the variations in brightness due to your optical system. By subtracting these flat field frames (upon stacked) from the original DSO image, these anomalies will be removed.
For tonight, I have taken a picture of Prawn Nebula (in Scorpius, Hydrogen Alpha component) to illustrate the difference a flat-field-corrected image has compared to an uncorrected image.
The first image below shows the uncorrected original image of Prawn Nebula. Observe that it appears that there is uneven illumination with a haze-like cast especially prevalent in the bottom areas. It also difficult to process such an image in terms of contrast levels, since minute contrast details are shrouded or lost.
The second image shows the corrected image. One will notice that the nebula "stands out", and it is more contrasty. The bottom haze is also not present. Overall, the image looks more pleasing to the eye, and this is what we call a flat-field corrected image.
The way to remove these "altered" anomalies is through the taking of flat field frames. These frames are hence used remove image artifacts due to our optical system. Vignetting and shadows from out-of-focus dust specks (as our systems age, or after several expeditions) are the most common aberrations which these flat field frames eliminate.
The taking of a flat field can be done by a few methods:
1) Taking sky flats - pointing your telescope at a region of the sky away from the sun (before it sets), say around 4-5pm in the east. It is important that you point to any area free of clouds and whereby the sky illumination is as even as possible (with no stars in sight!). You may like to use a white thin shirt to cover the objective if the camera is very sensitive.
2) Taking a whitish wall from a distance with your telescope.
3) Taking flats based on an uniformly-lited "light box" that can be made from simple light source materials (with acrylic, diffusing materials, etc).
4) Taking flats based on an EL Panel (EL short for Electro-Luminence).
What I have bought is based on Option 4, which is an EL Panel with uniform illuminence (from Gerd Neumann).
Below shows the setup taken earlier in the night showing the EL Panel in action.
Once flat field frames are taken with any of the above options, you will notice that they would look like blank, evenly illuminated image which additionally show the variations in brightness due to your optical system. By subtracting these flat field frames (upon stacked) from the original DSO image, these anomalies will be removed.
For tonight, I have taken a picture of Prawn Nebula (in Scorpius, Hydrogen Alpha component) to illustrate the difference a flat-field-corrected image has compared to an uncorrected image.
The first image below shows the uncorrected original image of Prawn Nebula. Observe that it appears that there is uneven illumination with a haze-like cast especially prevalent in the bottom areas. It also difficult to process such an image in terms of contrast levels, since minute contrast details are shrouded or lost.
The second image shows the corrected image. One will notice that the nebula "stands out", and it is more contrasty. The bottom haze is also not present. Overall, the image looks more pleasing to the eye, and this is what we call a flat-field corrected image.