This page was created for those using the Nikon D70, D50, and D40 Digital SLR cameras for astrophotography. However, the ideas equally apply to those conditions of manual focusing using telescopes, microscopes, and manual focus lenses. Besides the D70 viewfinder view, which is covered in the owner's manual, this page will compare the merits of using the following viewing and focusing aids; each of which works with D70. The DG-2 finder has a very small eyelens, making it impossible to see the complete Field of View (FOV) with the naked eye; requiring scanning the field by moving the eyeball around. Also, the small number of lens elements degrades the image with significant Chromatic Abberration (CA). The DR-6 finder shows the complete field and all exposure data; including the manual focus light at 1x. At 2x, the DR-6 vignettes the lower right/left, eclipsing the view of the manual focus LED. At 1.25x, the Canon Finder C only shows the shutter speed and f/stop in the FOV. At 2.5x, the Finder C vignettes most of the exposure window. All views with the Nikon and Canon finders are bright and free of CA.
|Description||Nikon DG-2||Nikon DR-6||Canon Finder C|
|Diopter Range||-5 to +3||-8 to +6||-5 to +3|
|Adapter Required?||Nikon 2370||None||Ed-c (included)|
|Weight (w/adapter)||25 gm||90 gm||110 gm|
|Eye Lens Diameter||9mm||17mm||19mm|
|Carrying Case Included?||No||Yes||Yes|
The Digital Single Lens Reflex (DSLR) camera (so far), does not allow real-time through the lens viewing of the image, as does a fixed lens digital camera. The view is obscured by the mirror blocking light from reacing the CCD sensor. But after the photo is taken, the resulting image is immediately displayed on the D70 LCD during the period of recording to the CF storage card. However, because of the hi-speed operation of the D70, the image is displayed only a fraction from real-time. The drawback of the D70 LCD, is the small 1.8 inch diagonal screen. This drawback can be overcome by using a larger higher resolution external monitor. When using an external monitor, all D70 LCD display functions are shunted to the external monitor. In astronomy, this feature is nice to have, as the telescope and camera can assume very awkward positions when pointed at high elevation objects.
Any monitor with an RCA phono input plug can be used. A TV is not suitable because the dot pitch is too coarse for accurate focus and display of stars. Don't be confused by terminology like horizontal lines, number of pixels, etc. Only the "Dot Pitch" parameter is of interest, and this number should be under .20 mm for accurate focusing. The black/white security monitors are ideal if one has access to the mains and an enclosure (observatory). Another monitor choice are those included with the better DVD players (e.g. Toshiba SD-P1850 is very good). My choice for a monitor was the small professional grade Marshall VLCD4-Pro. The key features of this monitor was portablility, 12V operation, .17mm dot pitch, dual video inputs, and multiple image adustments; including the option of a B/W display. This image shows a setup with the monitor attached to the D70 (note: shown are crosshairs of sewing thread taped across the screen for purposes of Video Guiding. See Thomas Dobbins, "Video Astronomy").Monitor Sources
Plug monitor into video out jack of D70. Switch camera to manual focus (MF) mode. Initial focus is performed visually, or using one on the magnifier's above. Take photo of object, then evaluate result on monitor screen. The displayed image can be magnified many times by pressing the Enter button on the lower left, then adjusting the zoom factor by pressing the ISO button, while rotating the main command dial. This allows a detailed view of focus accuracy. Repeat the process by re-focusing and image evaluation. Normally, accurate focus can be achieved in 2 or 3 iterations.
Since my experience is with the Nikon's D70 and D50 DSLR camera's, this outline will use ImagesPlus software as an example. For the Nikon DSLR, ImagesPlus seems
to be the most feature packed software for the Nikon's mentioned above. ImagesPlus versions 2.50 and above will operate the D70 remotely using a laptop computer, and a modified cable controlled Nikon
ML-L3 remote control. In this case, the parallel port version made by Hap Griffin was used. The Photo shows a Dell Inspirion 600m laptop (one of the few laptops today with parallel and serial ports), and Nikon D70
on a table-top tripod. The D70 is connected to the 600m with a USB cable ( USB camera setting is PTP ), and the IR remote is attached to the DB-25 socket on the 600m; with the remote overlooking the IR sensor of the camera using Velcro buttons to anchor it to the side of the flash housing (Note: The IR sensor is located on the left of the camera body on the D70, and on the right for the D50). This is the only hookup needed to remotely control the shutter for automated imaging in any exposure (or mixed exposure) setting, and to focus the camera. Note that focusing is still a manual operation; similar to the procedure of using an external monitor above. The advantage of using the software, is that much of the guesswork focusing is eliminated by using the brightness of the luminosity to judge focusing. At maximum luminosity, the star will be in focus. On the ImagesPlus 2.5 CD, there is a video tutorial that explains how to do remote camera control. The video example used a Canon 350XT DSLR. There are a few slight differences with the Nikon. Here are the basic steps for hooking up the Nikon D70 and D50 DSLR.
There are some slight differences in using the Nikon D40/D40X with ImagesPlus. First one has to use ImagesPlus version 2.82 or later. The major diference is the use of several different options for determining focus accuracy. This is not suprising, as focusing seems the major issue among a plethora of many for astro-imagers. I like the HFD (Half Flux Diameter) method. Unlike the previous IP versions, where one had to write down or memorize luminanace settings, HFD tracks focus accuracy statistics on a graph as illustrated. For details, see mlunsold.com.
We are all familiar with a helical focuser. This is the same type of mechanism used on Single Lens Reflex (SLR) camera lenses; both film and digital types.
Most astronomical telescopes use a strict linear focusing mechanism (e.g. Rack & Pinion (R&P) or variation (Crayford Type) ). Linear devices consists of a spur gear (pinion) and a toothed rack.
The rack is basically a spur gear with the circumference laid out in a straight line. Thus the radial motion of the pinion is translated into linear motion of the rack. Since the focus knob is not much
larger in diameter of the pinion gear pitch diameter, small rack movement is difficult using manual means.
The helical focuser provides greater accuracy. Here the focuser is a larger diameter compared to the R&P focus knob, and the helical thread pitch is smaller than the pitch of the gear. Thus if the thread pitch is .75mm, it takes a full turn of the focus ring to move the focuser in linear (lead) .75mm. The Borg 7758 Helical focuser DX for SLR's is a very precise focuser. It has graduations covering slightly less than half its circumference. If the focuser is moved one graduation, this translates into a linear movement of .14mm or .0055 in. The photo shows the 7758 attached to a Nikon D70 DSLR and a Vixen 80SS refractor telescope. The Borg parts required for this hookup ( for scopes with 2 inch drawtubes) are, part no. 7425, 7522, and a T-adapter for your camera.
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