Low Power Wide Angle Finderscope

Rings

I ordered some rings to hold it in a Vixen finder base. Finding rings large enough for a 68mm OD tube was nearly impossible but these seem like they should do:

It turns out the tube, being a PVC pipe fitting is not exactly round; it has some odd bumps I think to get traction with a wrench. The solution was to buy some 58mm 3mm wall aluminum tube and machine this, thus making the finder be completely free of the PVC. 

Those rings are made in the PRC and like most stuff that I am seeing made there, it is beautifully and accurately made. (I bought and returned a camera telescope adapter that looked like it was designed from a photograph: beautiful but inverted in a hard to explain way). 

I have made a similar ring set in the past of white acetal. They made the same correct choices. The base, which uses a 75 degree dovetail for Vixen finder shoe compatibility, has slots across it that the rings fit into very precisely so they will never be not perpendicular to the base. It uses 1/4-20 screws through the base into the bottom of the rings. That means that a variety of ring sizes can use that same part. It also makes machining this into three separate parts, each of which is a simple part to machine.

I really wish there were Americans willing to make products like these. Antares builds smaller rings in Canada; I buy their stuff even though a bit more expensive. These rings cost $30. I am sure that these could be made profitably here and sold for $60 and there would be buyers at that price. If I had the time, I would organize this. I have the capital, the interest, but not the time.

Eyepiece

I bought a SVBONY 25mm Plossl for the finderscope. It gives 6x and 8.33 degrees of field which is fine. It is not a crosshairs reticle. It is a beautiful eyepiece. So I went looking for instructions on adding an illuminated reticle to an existing eyepiece and I found some for an Orion 25mm Plossl, which doubtless came from the same factory.

Okay, maybe it does not need an illuminated reticle. The skyglow here is so much that black crosshairs show just fine. But what if I get a really dark sky in Tennessee? More interestingly, I found this video touting SVBONY's 20mm illuminated reticle eyepiece.

20mm focal length means 7.6x. The 70 degree apparent field of view means 9.2 degree field.

Since I know at least some of you are here to learn about telescopes (no, really!), let me explain Apparent Field of View (AFOV). This is a measure of how much field of view is potentially available. You divide AFOV by magnification (70/7.6 in this case) to get the field of view your eye will see.

This is a big win. Another big win is that as magnification drops, the telescopes exit pupil falls. A dark-adapted young eye can expand to 7mm. If you supply a 10mm exit pupil, only (7/10)^2 of the light will be useable. This is why 7x50mm, 8x50mm, and 6x42mm binoculars are most common; optimizing the available light for young eyes. 

Us old guys are lucky to get 5mm. The 25mm eyepiece gave a 8.2mm exit pupil, so about 62% of the available light is useless. Instead of seeing 100x fainter stars (5 magnitudes), I would get about two magnitudes. At 7.6x, that is a 6.6mm exit pupil about 42% of the light is useless. This should be three to four magnitudes.