I got the following lenses from Surplus Shed:
- 150mm diameter PMN, 2600mm focal length (stock L3855D, $25)
- 68.7mm diameter NMN, -800mm focal length (stock L4380, $4)
Aiming hard drive platter to reflect the sun was easier than I expected. (At this point, the safety stuff becomes crucial--make sure the sunlight doesn't get focused in anybody's eyes. Likewise, make sure no animal walks into the optical train.) I could see a bright spot reflected from the platter. I moved the spot onto the big lens. I could then see an unfocused spot on the ground between the big lens and the small lens. I centered that spot on the small lens, and put the screen box to catch the image. I then moved the small lens to focus the image.
solar photo online showed a smaller sunspot near the larger one, and I did not see that.)
It should work for the transit of Venus. I hope it will get a bit sharper when I find a good way to collimate the setup, maybe with a laser. Right now the collimation was all eyeballed (on the other hand, this is very long focal length work, since I am only using a small portion of the 150mm lens since the platter doesn't give much illumination, so it's less crucial.) I also hope things will improve if align the optical train in such a way as to make the sun be closer to being at right angles to the mirror, which will increase the amount of the primary lens illuminated by the mirror.
At some point, I will try with a 3.5" diagonal instead of the platter, and if it makes a significant difference to quality, I may need to buy a mirror.
While one can no doubt be harmed if fairly well focused sunlight within the optical train hits the eye, it is worth noting that if the big lens is correctly pointed at the diverging lens, at no point in the optical train is there an image of the sun smaller than about 40mm, so the amount of heat concentration is not so great.