NavList:

Dear Dale,

I do not have the C&P bubble attachment and never saw it closely. All I can suggest as an ИМС-3 owner, where it is possible to see through the bubble chamber at the natural horizon.

If you have big enough open water near by, try viewing the natural horizon overlaid across the bubble. If C&P bubble attachment lets you to see through it - check that the "through-bubble" horizon cuts the center of the bubble and that the double-reflected image of the horizon coinsides with them when you set the index to zero.

The lengths of the fork is irrelevant (I think) the longer fork of the scope tube is to let you to adjust the relative brightnesses of the direct and the double-reflected images (the more you shift the scope to the frame - the brighter the double-reflected image is and vice versa).

Hope that helps.

Warm regards,
Tony
60°N 30°E