Dark Frames

Very accurate polar alignment is essential for deep sky imaging. Stars will streak across the field of view without precise polar alignment and tracking. Longer exposures of 60 seconds or more also require autoguiding with a separate camera. The Orion StarShoot AutoGuider can be operated with the StarShoot Pro in MaxIm DL Essentials.

To start:

1.Acquire and center the deep sky object into the field of view of your eye- piece. If you are using a mount with an accurate computerized go-to sys- tem, you can keep the camera installed in your telescope’s focuser without using the eyepiece.

2.Remove the eyepiece and replace it with the StarShoot Pro.

3.Set the Mode to Light Color 1x1 and precisely focus the camera by using the telescope’s focus knob and the Focus setting in the Camera Control Window. If necessary, move the telescope to a nearby star to determine the best focus. (Refer to “Focusing”)

4.For best results, set the Mode to Light Raw 1x1 after you have reached focus. Use the Medium setting in the Screen Stretch Window

5.Select Single in the box under the Mode box and take a 10-20 Second exposure of the deep sky object to ensure it is centered well in your cam- era. Adjust the camera orientation if needed, keeping in mind that you may have to refocus the camera after making the adjustment. Reposition the telescope if needed to center the deep sky object.

6.Select AutoSave in the box under the Mode box in the Camera Control Window. Set the number of images you would like the camera to take under Autosave (start with 5 or so), select the file folder in which you would like to save the images with Folder, and enter in a Base file- name for the captured images. Typically the name of the object being imaged, such as “OrionNebula1”, will be entered here. If “OrionNebula1” is the Base filename, and you choose to Autosave five images, then the images will appear in the selected file folder as “OrionNebula1_0001.fit”, “OrionNebula1_0002.fit”, “OrionNebula1_0003.fit”, “OrionNebula1_0004. fit”, and “OrionNebula1_0005.fit”. Try exposures of 30 Seconds to start.

7.Click Expose, and the camera will commence taking the images.

Note: When the camera is taking long exposure images, it is critically important not to touch, shake, or otherwise disturb the telescope, or a blurred image will result. Also, make sure no surrounding light shines into the telescope during the exposure.

Dark frames are images taken with no light coming into the camera. A dark frame is typically taken with the telescope’s objective capped. The only data in the image is the inherent camera noise (Figure 14). The noise contains the

Figure 14. A dark frame contains the background noise. The same background noise appears in your “light” images. Dark frames isolate the noise so it can later be subtracted from your “light” images.

dark current (background noise level), read noise (noise introduced during camera readout and download) and hot pixels (bright dots in the image). All of this noise exists in your raw astro-image too, which distracts from the detail you want to see.

To eliminate most of the camera noise, you can take several dark frames, average them, then subtract them from your astro-images, also called, “light” images.

As the outside temperature changes, so does the camera temperature and noise levels. So the dark frames must be taken close to the same time your light images are taken, so that the camera temperature is nearly the same. So if you take 5 light images, you should take a couple of darks right before or after the light images.

To take dark frames for subtraction from “light” images:

1.Set the Mode in the Camera Control Window to Dark Raw 1x1.

2.Use the same exposure time as the “light” images you have or will take. If your “light” image is 60 seconds, the dark frame must also be 60 sec- onds.

3.Choose Autosave in the box beneath the Mode box.

4.Set the number of dark frames you would like the camera to take under Autosave (3 to 10 will generally suffice, as these will be averaged together)

5.Select the file folder in which you would like to save the dark frames with Folder, and enter in a Basefilename for the captured dark frames. Typically the name of the object being imaged with the word “dark” added, such as “OrionNebula1dark”, will be entered here. If “OrionNebula1dark”