Adventures in Optics

One of my great frustrations from moving out of the mountains of Horseshoe Bend was that I had magnitude 6 skies. (The Milky Way was so bright it washed out the constellations.)  Where i am now, magnitude 2 stars are about all i can see. Diffuse magnitude 4 objects like Orion Nebula are visible but the loss of so many fainter stars means finding obscure objects like Ring Nebula is not practical.   It seems about 40% of the distance between beta and delta Lyrae.  These are magnitude 3 stars. 

Optical aids for telescopes are in two major categories: 0x red dot devices that most shooters know, and finderscopes that are a small refractor with crosshairs.  

The red dot devices are very natural to use and have full field of view of the naked eye. If you can see an object unaided you can move your telescope so the object is in the center of the illuminated red circle.  Because they have glass concentrating light, you het whatever your natural limiting magnitude is.

Finderscooes are little tefrwctors with a crosshair eyepiece.  These range from 5x24mm to 9x60mm.  The magnification is useful for picking out details on the Moon or other large objects.  The light gathering lets you see fainter stars.  A 50mm objective gives 10x the diameter of the average older adult pupil so 100x the light.  

However, the exit pupil of the finder is objective diameter divided by magnification.   If you get a 10mm exit pupil, half the light will be useless for your eyes.

What is need is a very wide angle finder that gathers a lot of light.   Ideally, a 0x50mm finderscope.  These do not exist.  I have found a source for 50mm achromatic lens focal length 153mm.  I can buy cheap 40mm Plossl type eyepieces.  This would give me 3.82x.  Apparent FOV would be 13 degrees.   Exit pupil 10.4mm. More than half the light wasted but still 40x brighter than baked eye or four magnitudes deeper than naked eye. 

I would need to machine a housing for the lens and an adapter to interface one of my existing helical focuses to the tube.  I can do it for about $40 in parts.