Converting a finderscope to finderguider

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Converting a finderscope to finderguider

Postby worcspaul » Fri Feb 10, 2012 11:03 am

This is for the straight version of the SkyWatcher finder rather than the right-angled one...

Equipment needed:
Webcam adapter from Modern Astronomy.
SPC900NC webcam or similar (I've found the adapter also fits a logitech quickcam e1000 lens thread too)
SkyWatcher 9x50 finderscope
PTFE tape

2 A4 sheets of black paper (or 4 x A5 sheets)
bubble wrap
Glue stick
Double-sided tape

Preparing the finderscope:

  • Align the finderscope with your main scope by first aiming your main scope at a distant object and centering it in the eyepiece then adjusting the finder in its bracket until the target is centred.
  • Remove the finder from the main scope but leave it in its bracket. (This means that when the finder is re-attached later it should me more or less aligned with the main scope).
  • Completely remove the objective lens from the finder.
  • Remove the focus lock ring. This is to allow sufficient inward travel of the objective lens when focusing. Some have reported being able to achieve focus with the ring in place, but turned around.
  • Wrap PTFE tape (as used in plumbing) around the thread. One complete turn should do as we're looking to remove play from the thread, not lock it completely.
  • Screw the objective lens back on about half way.
  • Unscrew the eyepiece from the barrel of the finderscope.
  • Screw in the adapter til it's finger tight.

Preparing the camera:
  • Unclip the focus ring from the camera (Dion's PolarCam tute show's how to do this).
  • Unscrew the lens from the camera.
  • Take the finderscope and offer it up to the camera. Carefully screw the adapter into the camera making sure the thread's properly engaged before screwing in further. No need to do this too tightly.

Setting up:

This can be done in daylight using a distant object such as the top of a transmitter mast, or at night using a bright star such as Sirius or Vega

  • Re-attach the finderscope bracket to your main telescope
  • Aim your main scope at the target and centre it in your eyepiece
  • Connect webcam to computer and run your preferred Webcam software. One that can display a reticule (such as Sharpcap) will do.
  • You should hopefully have the target appear on screen (though probably way out of focus).
  • Carefully and slowly screw the objective lens further onto the finder. I found that I achieved focus with about 3mm of thread visible. If you're doing this at night a bahtinov mask will help.

Optional: (*)

I found when first using my finder that it would fog up depending on atmosphere/weather at the time so I made a dew shield:

  • Fold each A4 sheet of paper in half and cut along the fold (not necessary if you have A5 paper)
  • Remove the lens cap from the objective
  • Wrap a sheet of paper tightly around the objective lens holder, lining the longer side up with the edge of the lens assembly then mark where the paper overlaps
  • Turn the paper around and repeat. This should give you a mark on each edge of the paper.
  • Using a glue stick, apply glue between the end of the paper and the marks (shaded area in pic):
  • With the glued side outermost, wrap the paper round the lens assembly again, lining up the un-glued edge with the marks and pressing down along the edge.
  • You should now have a cylinder of black paper.
  • Leave for a few minutes for the glue to take hold
  • Take a second sheet and apply glue to the whole of one side
  • Wrap this second sheet around the first, gluing it to it
  • Cut a sheet of bubble-wrap to the same size as the paper (I used some of the wrap that the finder came wrapped in)
  • Apply double sided tape around each end of the paper cylinder. To make this easier, remove the paper cylinder from the finder and slip it over something of a smaller diameter such as a piece of piping or a broom handle.
  • Peel the backing off the double-sided tape and carefully line up the bubble-wrap, wrapping it around the cylinder. Use more tape to stick down the overlapping edge
  • Apply glue to the back of a third sheet of paper and wrap it around the bubble-wrap
  • Repeat with the 4th sheet
  • You should now have a lightweight dewshield which can be slipped over the objective lens and stop the lens from misting up

*Instead of fiddling with paper, glue and bubble-wrap you could use neoprene or the kind of foam that camping mats are made from! :)

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