While using the Meade #777 Off-Axis Guider with my D70, I noticed how difficult it was to achieve a proper image focus. I decided to purchase a Nikon DG-2 Eyepiece Magnifier (2X) from B&H Photo. Here you can see some views of the DG-2 attached to my D70, which is attached to the Off-Axis Guider on my LXD75-8"SC telescope.
As you can see in the first photograph, the magnifier is hinged and can be flipped up to allow direct viewing through the eyepiece. To attach the DG-2 you remove the rubber eye-guard from the viewfinder and slip the magnifier on in its place. I did notice that flipping the DG-2 up and down takes some force so it is possible to move the telescope during this operation, resulting in changing the image position. Fortunately, this was not a significant problem with my LXD75 mount.
In actual use I found that the DG-2 is an improvement over trying to focus using the viewfinder alone. However, it is still not a perfect solution. You first have to center the object in the viewfinder as that is the portion that receives the 2X magnification. Faint stars and extended objects (nebulae, galaxies) are almost impossible to use for focusing, meaning that you will most likely want to use a bright star for the initial focusing and then slew to the target object. Using a bright star still may not get you a good focus due to the lower than desirable image quality when using the DG-2. The DG-2 has a diopter adjustment by turning the eyepiece but it is easy to change this adjustment inadvertently which can make focusing more difficult.
Here are a couple of photographs taken during my 15 June 2007 visit to Oracle Observatory:
These 1 minute, ISO 1600, prime focus exposures look pretty good but if you zoom in you notice that the focus is still not precise:
I suspect that as I gain more experience focusing with the DG-2 I will be able to improve the images. Certainly focusing with the DG-2 is easier than without it but it is still challenging.
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