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Let me see if I can answer a few of the questions:
Nightcal-
1-You can place the mask over the dew shield. It will work just fine.
2-It is not critical to make a mask that is optimized for focal length. The calculation for the cutout is to minimize the distance to the first order spectrum. In reality most use the third order anyway.
3- Great question! According to Pavel Bahtinov The first order is the most accurate to focus by because it is closer to the center. However, the third order works just as good if you over expose the image like you would do using live view anyway. I have an image that shows the first and third order on what you would see using live view.
4- The shift in the first order and third order move equally with the focus. So no real advantage on either.
Andrew-
Yes the spikes do move as the focal point moves, therefore as seeing changes the spikes move. This is an excellent point and a method to judge seeing.
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