Photographing the Night Sky

Prime Focus Imaging using a DSLR through a Scope

Imaging through the telescope we will be replacing the camera lens with the scope optics. To do this we need to connect the camera to the scope. Clearly the camera should have lens removed and the scope should have eyepiece removed (the camera replaces the eyepiece). For a DSLR a T ring adaptor connects to the camera and gives a generic threaded connection that can be used to connect to many external devices. You also need a specific adaptor for your scope which might be to fit into a 1.25 eyepiece tube or the rear cell of a Schmidt-Cassegrain scope. Note that for long exposures a telescope with an accurate equatorial drive is needed. The better the telescope drive the longer the exposure you can take. A good telescope drive will permit exposures of maybe 1-5 minutes. But even if your drive permits images of just 20 seconds before you get star trails, you can still get excellent final images by adding images together as described in an earlier section.

  T Ring adaptor

  Connector to join T Ring to a Schmidt-Cassegrain telescope.

Photographing the moon through the telescope is the easiest to do. Exposures will be short, typically a 100th second or so, thus the telescope drive is not important. Indeed a telescope drive is note really necessary for this short an exposure. Bright planets like Jupiter can also be imaged through the scope without the need for a drive.

Moon (phase 59%), Meade 80mm APO Refractor, Canon 550D DSLR. 1/320th second exposure at iso200.

Moon, (lunar phase 58%), Meade  LX200 SCT  f6.3, Canon 450D DSLR at prime focus. Average of 9 images x 1/200th second exposures at iso400

Longer exposures will require your telescope to be equatorially mounted with a drive.

M42 - The Orion Nebula - 10" Meade LX200 SCT f6.3, Canon 450D EOS camera. Average of 8 one minute exposures.

  M42 - The Orion Nebula - 10" Meade LX200 SCT f6.3, Canon 450D EOS camera. Single 10 second exposure.
 

HDR Processing

Notice that the M42 images above show that a single exposure cannot capture both the trapezium stars and the faint details of the nebula. Fortunately modern processing techniques can come to the rescue ! High Dynamic Range (HDR) processing is becoming popular as a way of increasing the dynamic range of an image. In land camera use the technique involves taking a series of bracketed images of the same subject, with different exposures. Most DSLRs will capture bracketed images automatically.
Software is available that combines images to produce a single image, where all parts are correctly exposed. Major image processing programmes include plug-ins or routines that permit HDR processing. Free software is available on net, eg FDR tools. The image below shows a final image of M42 that is a combination of short and long exposures using HDR processing. Notice that it shows both the trapezium stars and the faint outer nebula.

  M42 image at f6.3 using a Modified Canon 1100D DSLR. HDR processing of 1 minute and 10 minute exposures.


 
M78, Bright Nebula, Orion, 10" Meade LX200 SCT f6.3, Canon 550D DSLR. 26 minute exposure.

  M8, Lagoon nebula in Sagittarius. Large emission nebula with embedded cluster and dark lanes. 10" Meade LX200 SCT at f6.3, Modified Canon 1100D DSLR, exposure 15 minutes.

  M17, Omega nebula in Sagittarius. Large emission nebula with embedded cluster and dark lanes. 10" Meade LX200 SCT at f6.3, Modified Canon 1100D DSLR, exposure 12 minutes.

  M1, Crab nebula, Taurus, 10" Meade LX200 SCT f6.3, Modified Canon 1100D DSLR. 30 minute exposure.

  M57 The Ring Nebula in Lyra, 10" Meade LX200 SCT f10, Canon 450D EOS camera. Exposure time 14 minutes.

  M27, 10" Meade LX200 SCT f10, Canon 450D EOS camera. Multiple exposures : Effective integration time 20 minutes.
 

 

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