Deep-sky astrophotography is said to be easier than taking
photographs of the Moon or the planets because there is an excellent
chance that a good image will be captured on the first attempt. However,
this does require the acquisition of extra equipment and skills.
The telescope needs to be mounted equatorially for a start, either with an equatorial mount or a wedge. Added to this, an eyepiece with crosshairs will be needed for guiding the telescope. The reason for having to guide the telescope is because telescope motor drives are hardly ever perfect, which means that manual corrections have to be made in the east-to-west direction in order to smooth out any erratic motion. Also, any tiny errors in polar alignment will need to be counteracted in the north-to-south direction.
If shooting the sky in piggyback mode, with a camera mounted on top of the telescope, guiding is easy; you just watch a guide star through the telescope, keeping it on or close to the crosshairs. When piggybacking with a medium-power telephoto lens attached to the camera, precision guidance is not essential - just keep the guide star somewhere close to the crosshairs.
The secret to good guiding is to ensure that only the equatorial motor drive is running, set the slew rate to a low value and turn off any backlash compensation to make sure that there is no unexpected jerking. Guiding corrections are best delayed rather than for them to be made suddenly or irregularly.
When taking shots through the telescope, guiding can be more challenging because there is little room for error, with the guide star having to be kept right on the crosshairs. But how is the telescope guided when it is being used to take a photograph through?
One method is to employ a separate guidescope that has a high enough magnification up to x 500, which is really far too high for proper observing. However, guidescopes only work with refracting and reflecting telescopes but not with Schmidt-Cassegrains and Maksutov Cassesgrains. This is because the mirror in catadioptric telescopes tends to move slowly as the telescope tilts while following a star. This creates image movement that the guidescope does not detect.
One way to get around this problem is to use an off-axis guider. This intercepts part of the main image that would not fall on the camera's image sensor, which means that the same image that is being photographed can be guided on. The difficulty with using an off-axis guider is locating a suitable guide star - more often than not there is no nearby star brighter than magnitude 12 in the appropriate place. But photographing open clusters and deep-sky objects close to the Milky Way makes it easier to find suitably bright guide stars compared to taking photographs of galaxies.
Because guiding is a tedious chore, most CCD cameras can actually do it for you. Such CCD cameras are called autoguiders, which can report on their accuracy as they go. Autoguiders replace the crosshair eyepiece. But there again how do you guide when taking a CCD shot? The same CCD cannot make guiding corrections whilst exposing an image simultaneously, can it? So, a second CCD is used in an off-axis guider. Or even by putting a single CCD to double use: to track and record at the same time. Such a set-up can take a short exposure, check for image shift, make another exposure, and shift as needed to match. Then the two images are combined, and so on over and over again.
The telescope needs to be mounted equatorially for a start, either with an equatorial mount or a wedge. Added to this, an eyepiece with crosshairs will be needed for guiding the telescope. The reason for having to guide the telescope is because telescope motor drives are hardly ever perfect, which means that manual corrections have to be made in the east-to-west direction in order to smooth out any erratic motion. Also, any tiny errors in polar alignment will need to be counteracted in the north-to-south direction.
If shooting the sky in piggyback mode, with a camera mounted on top of the telescope, guiding is easy; you just watch a guide star through the telescope, keeping it on or close to the crosshairs. When piggybacking with a medium-power telephoto lens attached to the camera, precision guidance is not essential - just keep the guide star somewhere close to the crosshairs.
The secret to good guiding is to ensure that only the equatorial motor drive is running, set the slew rate to a low value and turn off any backlash compensation to make sure that there is no unexpected jerking. Guiding corrections are best delayed rather than for them to be made suddenly or irregularly.
When taking shots through the telescope, guiding can be more challenging because there is little room for error, with the guide star having to be kept right on the crosshairs. But how is the telescope guided when it is being used to take a photograph through?
One method is to employ a separate guidescope that has a high enough magnification up to x 500, which is really far too high for proper observing. However, guidescopes only work with refracting and reflecting telescopes but not with Schmidt-Cassegrains and Maksutov Cassesgrains. This is because the mirror in catadioptric telescopes tends to move slowly as the telescope tilts while following a star. This creates image movement that the guidescope does not detect.
One way to get around this problem is to use an off-axis guider. This intercepts part of the main image that would not fall on the camera's image sensor, which means that the same image that is being photographed can be guided on. The difficulty with using an off-axis guider is locating a suitable guide star - more often than not there is no nearby star brighter than magnitude 12 in the appropriate place. But photographing open clusters and deep-sky objects close to the Milky Way makes it easier to find suitably bright guide stars compared to taking photographs of galaxies.
Because guiding is a tedious chore, most CCD cameras can actually do it for you. Such CCD cameras are called autoguiders, which can report on their accuracy as they go. Autoguiders replace the crosshair eyepiece. But there again how do you guide when taking a CCD shot? The same CCD cannot make guiding corrections whilst exposing an image simultaneously, can it? So, a second CCD is used in an off-axis guider. Or even by putting a single CCD to double use: to track and record at the same time. Such a set-up can take a short exposure, check for image shift, make another exposure, and shift as needed to match. Then the two images are combined, and so on over and over again.
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