Digital Cameras

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The Nikon Coolpix 990/995/4500 are great cameras for afocal photography. Their small lens provides a good optical match to eyepieces. They have automatic operation and full manual control of focus, aperture and shutter speed. Also, they

Figure 2.10. Adjusting the Nikon 990 camera field stop distance at minimum zoom to match the ScopeTronix 40mm eyepiece eye relief. From left to right: camera too close to the eyepiece lens, camera at best position and camera too far from the eyepiece lens. Images of the Moon were taken during daylight to show vignetting. With the ScopeTronix 40mm eyepiece you can adjust the distance of the camera from the top lens of the eyepiece.

Figure 2.10. Adjusting the Nikon 990 camera field stop distance at minimum zoom to match the ScopeTronix 40mm eyepiece eye relief. From left to right: camera too close to the eyepiece lens, camera at best position and camera too far from the eyepiece lens. Images of the Moon were taken during daylight to show vignetting. With the ScopeTronix 40mm eyepiece you can adjust the distance of the camera from the top lens of the eyepiece.

have 28mm threads on the camera body so they can be mounted directly to eyepieces and eyepiece adapters. Other features include digital zoom for easier focusing, image histogram for accurate exposure adjustment, manual color balance, manual camera image processing, a remote shutter cable and a swivel camera body to position the camera monitor so that it is easily used when the camera is mounted on the telescope (see Figure 2.10).

I use the Coolpix 990 in manual exposure mode with the aperture set wide open and the shutter speed adjusted for correct exposure. Manual focus is set to infinity, and the flash is disabled. Camera sensitivity is best set to ISO 100 based on my experience with test images at ISO 100, 200 and 400. The higher ISO speed allows faster shutter speed but the images become very noisy. White balance is set to direct sunlight for all images. Contrast and brightness are set to normal, causing the same contrast and brightness adjustments on all images. Image sharpening is set to high and image size is set to the maximum of 2048 x 1536 pixels. Metering is not used.

Finally, the image quality is set to the highest-quality JPEG format. The camera can save images as TIFF, which gives better image quality than JPEG. However, the TIFF file size is approximately 9 times larger than the JPEG file so it takes much longer before the camera is ready to take the next image. I have found it is better to take many high-quality JPEG images and catch a moment of good seeing rather than take fewer TIFF images. Also, the quality difference between the JPEG and TIFF formats becomes less of an issue if you are reducing the images for use on a Web site.

Table 2.6. Afocal Photography Troubleshooting Chart

Problem

Possible Causes

Possible Solutions

Vignetting

Camera is not centered and perpendicular to the eyepiece optical axis.

Telescope exit pupil is larger than the camera entrance pupil.

Adjust the mounting of the camera above the eyepiece.

Decrease the telescope exit pupil by decreasing eyepiece focal length or increase the telescope focal ratio. Increase the camera entrance pupil by changing the camera

Move the camera closer or farther from the eyepiece so that the eye relief equals camera field stop distance. Use an eyepiece with longer eye relief.

Use an eyepiece with a lens at the very top of the eyepiece. Use a camera with a small lens that has a shorter camera field stop distance.

Focus the camera at infinity or use the camera macro mode.

Round dim spot in the center of images

Central obstruction is caused by the telescope exit pupil being larger than the camera entrance pupil.

Central obstruction is caused by mismatch of eyepiece eye relief and camera field stop distance from the eyepiece.

Increase the camera entrance pupil by changing the camera

Decrease the telescope exit pupil by decreasing eyepiece focal length or increase the telescope focal ratio. Move the camera closer or farther from the eyepiece so that the eye relief equals camera field stop distance.

Blurry images

Camera movement or telescope vibration is caused by pushing the shutter button.

High magnification and a telescope that does not track the sky.

Mount camera on telescope or eyepiece, use remote camera shutter control or camera self timer.

Decrease camera zoom, decrease exposure time or use a telescope mount that tracks the sky movement.

Table 2.6. Continued

Problem

Possible Causes

Possible Solutions

Camera auto focus cannot

Turn off auto focus, set the

focus.

camera focus at infinity and focus with the telescope focuser. Image the Moon terminator that has bright and high contrast areas.

Camera infinity focus setting

Try other manual focus settings.

is not correct.

Uneven focus across image.

Align the camera to be centered and perpendicular to the eyepiece optical axis.

Poor focus.

Use maximum optical zoom and digital zoom in when focusing. Use focusing mask to find best focus.

Use electric focuser to prevent telescope vibrations when adjusting the focus.

Focus on high contrast object (Moon terminator, bright star, planet's moon, etc.). Use a magnifying glass to look at the camera monitor or large external monitor when focusing.

Poor seeing (atmospheric

Take image when object is

distortions).

higher in the sky and do not take images over houses, hot pavement, etc.

Decrease magnification by zooming out the camera or use an eyepiece with a longer focal length.

Take many images to catch a good seeing moment. Decrease exposure time and increase camera ISO.

Telescope, eyepiece or

Wait until optics cool down.

camera optics are hot.

Telescope vibrates because

Use vibration suppression pads

of the wind or breeze.

under the telescope mount and shelter the telescope from the wind.

Small camera monitor is

Use external monitor with

sharper than image.

camera.

Table 2.6. Continued

Problem

Possible Causes

Possible Solutions

Camera monitor shows a better quality image because it is a real-time stacked image.

Take an image and view single image with magnification on camera monitor.

Noisy images

Image is too dim.

ISO setting is too high.

Camera noise for long exposure times.

Camera is hot.

Camera is sharpening the noise.

Increase exposure time or stack images.

Decrease ISO setting and increase exposure time.

Turn on camera noise reduction or take dark images and dark subtract the image.

Turn off camera for several minutes between images to let it cool down and keep the camera monitor off as much as possible between images because it generates heat in the camera.

Decrease or turn off camera sharpening.

Dim images

Exposure time is too short or at night the image on the camera LCD monitor looks brighter than it is.

Too long an effective focal length.

Increase exposure time and/or stack multiple images.

Reduce effective focal ratio.

Images with bright streaks coming from the very bright white areas.

Image is overexposed and blooming occurs.

Decrease exposure time or decrease ISO setting.

Increase the effective focal ratio by increasing camera zoom.

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