Things That Can Go Wrong

Camera movement and poor camera alignment over the eyepiece cause blurred images. As your exposure time increases, hand holding the camera steadily and accurately over the eyepiece becomes more difficult. To solve these problems the camera can be mounted on a tripod next to the telescope eyepiece, mounted on the telescope or mounted on the eyepiece. There are several options for mounting the camera: (1) use an adapter that mounts the camera on the telescope to position it over the eyepiece, (2) put an eyepiece in a universal camera adapter that is attached to the camera (Figure 2.4 and 2.5), (3) mount the camera onto an

Figure 2.4.

ScopeTronix 28 mm camera to T-thread adapter, Orion Telescopes & Binoculars 20mm Sirius Plossl eyepiece and camera adapter before assembly.

Figure 2.4.

ScopeTronix 28 mm camera to T-thread adapter, Orion Telescopes & Binoculars 20mm Sirius Plossl eyepiece and camera adapter before assembly.

Figure 2.5.

Assembled ScopeTronix 28mm camera to T-thread adapter, Orion Telescopes & Binoculars 20mm Sirius Plossl eyepiece and camera adapter.

Figure 2.5.

Assembled ScopeTronix 28mm camera to T-thread adapter, Orion Telescopes & Binoculars 20mm Sirius Plossl eyepiece and camera adapter.

Figure 2.6. Tele Vue 10mm Radian™ and 35mm Panoptic™ eyepieces with Tele Vue 28mm camera adapters.

Figure 2.7. The older Nikon Coolpix 990/995/(4500 not shown) digital cameras are excellent afocal photography cameras when used with eyepieces having 28mm camera threads. Eyepieces from left to right: ScopeTronix 14mm & 18mm, William Optics 24mm and ScopeTronix 40mm.

eyepiece using an adapter (Figure 2.6) or (4) mount the camera onto an eyepiece with a built-in adapter (Figures 2.7 and 2.8).

Another common problem is that some images may have dark corners or a dark circle frame (Figure 2.9). This darkening is called vignetting and occurs when the camera's imaging chip is not fully illuminated. Also, your images may have a dim fuzzy spot or circle in the center of the image. Both of these problems are the result of a poor telescope, eyepiece and camera optics configuration. You

Figure 2.8. The

ScopeTronix 18mm eyepiece with 28mm camera threads attaches directly to the Nikon Coolpix 990 camera.

Figure 2.6. Tele Vue 10mm Radian™ and 35mm Panoptic™ eyepieces with Tele Vue 28mm camera adapters.

Coolpix 990 Afocal Telescope

Figure 2.7. The older Nikon Coolpix 990/995/(4500 not shown) digital cameras are excellent afocal photography cameras when used with eyepieces having 28mm camera threads. Eyepieces from left to right: ScopeTronix 14mm & 18mm, William Optics 24mm and ScopeTronix 40mm.

Figure 2.8. The

ScopeTronix 18mm eyepiece with 28mm camera threads attaches directly to the Nikon Coolpix 990 camera.

Figure 2.9. Daytime moon images showing vignetting at minimum zoom in the left image and removing it by increasing the camera zoom in the right image can reduce or eliminate these undesirable effects by using compatible telescope, eyepiece and camera configurations.

Your telescope and eyepiece have an exit pupil and eye relief. Your camera has an entrance pupil and a field stop. Typically, for best results, the telescope and eyepiece exit pupil should be equal to the camera entrance pupil, and the eye relief should be equal to the distance from the top eyepiece lens to the camera lens field stop. The camera lens field stop is where the camera iris diaphragm is located.

When the telescope and eyepiece exit pupil are larger than the camera entrance pupil, the camera field stop blocks some of the light and this light is not recorded as part of the image. The result is equivalent to using a smaller aperture telescope. Also, you might see a dim fuzzy spot or circle in the center of your image if you are using a telescope with a central obstruction like a Newtonian, Maksutov-Cassegrain or Schmidt-Cassegrain telescope.

The telescope exit pupil is the eyepiece focal length divided by the telescope focal ratio. For example, the exit pupil of an 18mm eyepiece used with a 150mm aperture Maksutov-Cassegrain telescope (f/12, 1800mm focal length) is 18mm/12 or 1.5mm.

The camera entrance pupil is the camera focal length divided by the camera focal ratio. A camera with a zoom lens has a range of entrance pupils. For example, the Nikon 990 zooms from 8.2mm to 23.4mm with focal ratios of f/2.5 to f/4, and its entrance pupil ranges from 3.3mm to 5.9mm (Table 2.4).

When using an 80mm aperture refractor telescope (f/5, 400mm focal length) with the 18mm eyepiece, some light is not imaged when the camera is set at its minimum 8.2mm zoom. At minimum zoom the camera entrance pupil is 3.3mm and will block some of the light because the telescope eyepiece exit pupil is 18mm/5 (3.6mm), which is 0.3mm larger than the camera entrance pupil. On the other hand, a 150mm f/12 Maksutov-Cassegrain telescope with the same 18mm eyepiece does not suffer from this problem because its 18mm/12 (1.5mm) exit pupil is smaller than the 3.3mm camera entrance pupil (Table 2.5).

Table 2.4. Nikon Coolpix

990 Entrance Pupils

Focal Length

Fastest Focal Ratio

Entrance Pupil

8.2mm

f/2.5

3.3mm

13.0mm

f/3.0

4.3mm

18.0mm

f/3.5

5.1mm

23.4mm

f/4.0

5.9mm

Table 2.5. Telescope and Eyepiece

Exit Pupils Less Than the Nikon 990 Camera

Entrance

Pupil at Minimum

Zoom

Eyepiece

Telescope Focal Ratio

Focal Length

f/5

f/10

f/12

4mm

0.8mm

0.4mm

0.3mm

10mm

2.0mm

1.0mm

0.8mm

14mm

2.8mm

1.4mm

1.2mm

18mm

3.6mm

1.8mm

1.5mm

20mm

4.0mm

2.0mm

1.7mm

24mm

4.8mm

2.4mm

2.0mm

35mm

7.0mm

3.5mm

2.9mm

40mm

8.0mm

4.0mm

3.3mm

55mm

11.0mm

5.5mm

4.6mm

Note: Exit pupil =

eyepiece focal length/telescope focal ratio. Shaded area shows

a

Nikon 990 camera is at minimum zoom and its 3.3mm entrance pupil is greater than the

telescope eyepiece exit pupil.

When a 150mm Maksutov-Cassegrain telescope is used with a 55mm eyepiece, a large dim fuzzy circle appears in the image. It is caused by the telescope's central obstruction and the large 55mm/12 (4.58mm) exit pupil compared to the smaller 3.3mm camera entrance pupil at minimum zoom. In an extreme case, a camera entrance pupil could be so small that the camera field stop blocks all the telescope light and the camera entrance pupil sees only the central obstruction shadow. To fix this problem the camera zoom is increased until the camera entrance pupil is greater than the telescope's 4.58mm exit pupil. Vignetting changes with different zoom settings and it is usually reduced at maximum camera zoom.

Eye relief is the distance from the eyepiece lens to the point where your eye can best see clearly the full field of view. Your eyepiece eye relief should match the distance of the eyepiece outer lens to the camera lens field stop. Matching the eye relief to the camera field stop is difficult because the field stop distance is not specified by camera manufacturers, and in some cameras, like the Nikon Coolpix 990, it can change position with different zoom settings.

Cameras like the Nikon 990/995/4500 series with small lenses have a better chance of being optimally positioned with respect to the eyepiece eye relief when using an eyepiece with a long eye relief and a lens at the top of the eyepiece. See Afocal Photography Trouble-shooting (Table 2.6) for other problems, causes and solutions.

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