Moon 7-6-2003, prime focusPrime focus photography with the Nikon D100 and Meade 8″ LX-90 telescope, Phoenix, July 2003:

This composite is featured today. Click the image to the left for a 900X1111 50% scale JPEG of the original composite. It’s a small 57K download.

Practice, practice, practice. Last night we used our QuantaRay 2X teleconverter for a 4-part composite of the Moon at about the first quarter. We missed a piece, and there was a big bite out of the moon’s limb that made it look like PacMan.

We had also done a series of two, without the teleconverter. These spliced together easily. Converting to grayscale, it’s hard to see the splice.

The teleconverter is a magnifying whatchamacallit that normally fits between a camera lens and body. If “2x”, it doubles the focal length. A 24-120mm zoom lens becomes a 48-240mm, and so forth. Coupled between the prime focus at the rear of the telescope, and the camera body, you get an image twice the size — and exposure time needs to be increased, all other things being equal.

How does that compare to eyepieces? Going from a 26mm eyepiece to a 13mm eyepiece would be doubling the “magnification”.

  • The rule on magnification is: focal length telescope divided by focal length eyepiece.
  • The Meade is f/10, 2000mm.
  • Another good rule is: don’t try to force more than 50x-60x magnification per inch of aperture.
  • 2000mm / 5mm = 400x; a 5mm eyepiece would give a practical max magnification of 400x for the 8″ aperture Meade LX-90. In an urban area there are few if any nights when we would be able to benefit from a magnification this high.
  • 2000mm / 40mm = 50x for a fairly widefield eyepiece

Unfortunately, I don’t see more detail in the teleconverter shots when the images are scaled to the same apparent size. The teleconverter is fun for daytime shots, but it’s not Nikon optics, and I’ve read many snide comments about less-than-$100 discount teleconverters. How much of that is optics and how much is snobbery?

Hard to say. Before I start knocking QuantaRay, I should concentrate on refining the basics.

  • All images were shot at 1/100, ISO 1000 – just because that’s what I used before. Really, 1/100 is about right with the 2x, meaning 1/200 would be closer to “right” without the teleconverter.
  • Even at 1/100, clicking the picture by hand introduces vibration. Particularly for astrophotography, use of the remote shutter camera attachment would be a good habit to get into.
  • Atmospheric turbulence will always be a limiting factor too. It’s HOT down here!
  • The main telescope focusing knob is OK for casual observing, but dicey for astrophotography. Basically, it is easy to overshoot an “ideal” focus with the finest of twisting movements. Electric focusers are available for micro-control of this all-important feature. It would help to master the focusing tool we presently have.
  • Having a comfortable seating or working position makes it easier to maintain patience and precision adjustments. Elevating the tripod (extend the legs more) to a more comfortable working position would save the back, too.

Tonight: Try to practice these basics. Try to capture the moon in color, before sunset.

Alex 7-7-2003

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