200827 - DSLR Modification
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click image to enlarge
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EQUIPMENT: camera1 (left)=Nikon D90 DSLR - stock optics=Nikon 70-300mm zoom, f/4-5.6 (fits both cameras; good for larger DSO's) camera2 (right)=Nikon D90 DSLR - modified optics=T-ring adapter w 1.0x flattener (ready for 2" slip-fit connection to a telescope; good for smaller DSO's) filter=Radian Ultra Quad NB free standing lens (far left)=Nikon 18-105mm zoom, f/3.5-5.6 (fits both cameras; good for wide-angle starscapes and Milky Way) |
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Astrophotography is the perfect blend of Science, Technology and Art. But because each of those endeavors is a full hobby unto itself, a progression of learning is required for anyone choosing to embark "down the rabbit hole" (as it is so often described) of AP. Several aspects of the AP learning curve revolve around the camera, with the overriding theme being a 'deep-dive' study into long-exposure photography.
Many imagers can trace their entry into the hobby through ownership of a DSLR camera; often borne out of a love of daytime photography. But the techniques required by nighttime long-exposure photography are very different from those of daytime sub-second exposures. One of the first aspects to be learned is how to control the camera. The priority of modern DSLR camera design is almost totally centered on daytime short exposures. As a result, it is imperative that the amateur learn how to turn ALL automatic functionality OFF; including the now venerable 'Vibration-Reduction' feature, and automatic white-balance. ...complete manual control of basic functions like, focus, aperture, ISO, exposure time, and remote shutter release must be learned. Typical "one-shot" digital cameras (there are as many variations on that description as there are manufacturers) need not apply; they typically do not have this prerequisite ability for manual control; and, the lens on many of them cannot be removed - another prerequisite for mechanical connection to a telescope.
Another important aspect is to gain a basic understanding of light propagation and wavelength as it relates to color. Modern DSLR cameras use an electronic sensor to capture light. The physics behind that process dictates that the sensor's sensitivity is biased heavily toward the red end of the visible light spectrum. This is very different from the daytime reactivity of the human eye which, is biased toward green. To adjust for this daytime mismatch, camera manufacturers incorporate two (2) adjustments into the designs of their cameras:
1) An infrared-cut (IR-cut) filter is permanently mounted in front of the sensor to tone-down the reds. This filter is not the same as an IR-cut filter used for purposes of nighttime AP; rather, this 'daytime photographic version' has a transmission characteristic designed to adjust the daytime color reception characteristic of the sensor. ...It is very different from what is needed at night.
2) A color filter array ("CFA") is permanently affixed on the face of the sensor consisting of individual pixel-sized Red, Green and Blue broad-band filters. The way in which the resulting RGB colors are re-combined (along with an aspect of luminance - or Blacks) is what enables us to see all the colors in the resulting images. But because of the human eye's propensity toward daytime Green, the Mfr's put twice as many Green filters on the sensor than what they do for Red or Blue.
The most common atomic element present in the universe is Hydrogen. It is also the element most prevalent in deep space nebulae. When a Hydrogen atom is struck by a very energetic photon of light, the atom transforms into its ionic form where its one and only electron is stripped away. While in this very excited, energetic state, it seeks to return to its preferred, non-ionic state by recapturing an electron. In order to 'relax', the atom must release its energy previously gained, in the form of a photon of light. The characteristic wavelength of this released photon is that of a deep red color, known as the Hydrogen-Alpha (Hα) wavelength (656nm). All stock DSLR cameras with the manufacturer's original daytime IR-cut filter are challenged to "see" Hα light.
As soon as most beginning imagers understand how to control their camera, they almost immediately begin to see that their camera is not recording the nighttime light the way they want it to. In particular, the reds that are so prevalent in DSO nebulae just don't seem to want to come-thru. As a natural consequence, one begins to look for ways to improve the light-capturing capability of their camera. ...hence, the reason for astro-modifying it. This modification, in its simplest form (there are as many variations to this modification as there are companies who offer to perform them), consists of removing the stock IR-cut filter from in front of the sensor, thus opening it up to much more red light. ...a very desirable capability for AP. Unfortunately, this modification tends to be permanent. ...but with that said, there are strategies that can be used to maintain one's ability to continue participating in daytime photography, while still also engaging in nighttime AP...
1) Acquisition of a daytime photographic IR-cut filter that can be temporarily mounted to the front of the camera lens will quickly 're-enable' a modified camera for daytime use.
2) Some DSLR cameras (not all) are capable of adjusting their internal white balance settings to fully compensate for a missing IR-cut filter.
3) In the image presented here, two (2) Nikon model D90 DSLR cameras are shown. One (left) is un-modified (stock), while the other (right) was sent to a specialty camera shop and modified for AP. These are both vintage (2009) cameras which today can be acquired very $inexpensively online. The cost of a used D90 body-only together with the cost of shipping and modification compares very favorably against the cost of a dedicated astronomical camera. Both cameras accept all the same Nikon type "F" lenses.
When one begins to gain adequate skills for manual control of their camera, it makes perfect sense to start looking for other ways to improve... Optics, motorized tracking, auto-guiding, specialty filters, computer control and development of advanced processing skills, and even the camera itself (stepping-up to a dedicated astro-cam); ...each involves significant learning and investment of time, effort and $resources. But the outcome is always amazing and rewarding. Revealing God's universe through science and technology is a wonderful endeavor!
Many imagers can trace their entry into the hobby through ownership of a DSLR camera; often borne out of a love of daytime photography. But the techniques required by nighttime long-exposure photography are very different from those of daytime sub-second exposures. One of the first aspects to be learned is how to control the camera. The priority of modern DSLR camera design is almost totally centered on daytime short exposures. As a result, it is imperative that the amateur learn how to turn ALL automatic functionality OFF; including the now venerable 'Vibration-Reduction' feature, and automatic white-balance. ...complete manual control of basic functions like, focus, aperture, ISO, exposure time, and remote shutter release must be learned. Typical "one-shot" digital cameras (there are as many variations on that description as there are manufacturers) need not apply; they typically do not have this prerequisite ability for manual control; and, the lens on many of them cannot be removed - another prerequisite for mechanical connection to a telescope.
Another important aspect is to gain a basic understanding of light propagation and wavelength as it relates to color. Modern DSLR cameras use an electronic sensor to capture light. The physics behind that process dictates that the sensor's sensitivity is biased heavily toward the red end of the visible light spectrum. This is very different from the daytime reactivity of the human eye which, is biased toward green. To adjust for this daytime mismatch, camera manufacturers incorporate two (2) adjustments into the designs of their cameras:
1) An infrared-cut (IR-cut) filter is permanently mounted in front of the sensor to tone-down the reds. This filter is not the same as an IR-cut filter used for purposes of nighttime AP; rather, this 'daytime photographic version' has a transmission characteristic designed to adjust the daytime color reception characteristic of the sensor. ...It is very different from what is needed at night.
2) A color filter array ("CFA") is permanently affixed on the face of the sensor consisting of individual pixel-sized Red, Green and Blue broad-band filters. The way in which the resulting RGB colors are re-combined (along with an aspect of luminance - or Blacks) is what enables us to see all the colors in the resulting images. But because of the human eye's propensity toward daytime Green, the Mfr's put twice as many Green filters on the sensor than what they do for Red or Blue.
The most common atomic element present in the universe is Hydrogen. It is also the element most prevalent in deep space nebulae. When a Hydrogen atom is struck by a very energetic photon of light, the atom transforms into its ionic form where its one and only electron is stripped away. While in this very excited, energetic state, it seeks to return to its preferred, non-ionic state by recapturing an electron. In order to 'relax', the atom must release its energy previously gained, in the form of a photon of light. The characteristic wavelength of this released photon is that of a deep red color, known as the Hydrogen-Alpha (Hα) wavelength (656nm). All stock DSLR cameras with the manufacturer's original daytime IR-cut filter are challenged to "see" Hα light.
As soon as most beginning imagers understand how to control their camera, they almost immediately begin to see that their camera is not recording the nighttime light the way they want it to. In particular, the reds that are so prevalent in DSO nebulae just don't seem to want to come-thru. As a natural consequence, one begins to look for ways to improve the light-capturing capability of their camera. ...hence, the reason for astro-modifying it. This modification, in its simplest form (there are as many variations to this modification as there are companies who offer to perform them), consists of removing the stock IR-cut filter from in front of the sensor, thus opening it up to much more red light. ...a very desirable capability for AP. Unfortunately, this modification tends to be permanent. ...but with that said, there are strategies that can be used to maintain one's ability to continue participating in daytime photography, while still also engaging in nighttime AP...
1) Acquisition of a daytime photographic IR-cut filter that can be temporarily mounted to the front of the camera lens will quickly 're-enable' a modified camera for daytime use.
2) Some DSLR cameras (not all) are capable of adjusting their internal white balance settings to fully compensate for a missing IR-cut filter.
3) In the image presented here, two (2) Nikon model D90 DSLR cameras are shown. One (left) is un-modified (stock), while the other (right) was sent to a specialty camera shop and modified for AP. These are both vintage (2009) cameras which today can be acquired very $inexpensively online. The cost of a used D90 body-only together with the cost of shipping and modification compares very favorably against the cost of a dedicated astronomical camera. Both cameras accept all the same Nikon type "F" lenses.
When one begins to gain adequate skills for manual control of their camera, it makes perfect sense to start looking for other ways to improve... Optics, motorized tracking, auto-guiding, specialty filters, computer control and development of advanced processing skills, and even the camera itself (stepping-up to a dedicated astro-cam); ...each involves significant learning and investment of time, effort and $resources. But the outcome is always amazing and rewarding. Revealing God's universe through science and technology is a wonderful endeavor!