Hmmm (redux)

[xiphmont]

Flashback: T-2 days to eclipse.

I looked around the workshop and noticed I have boxes of left over optics that didn't work out in various projects over the years, including some nice big achromats I grabbed from Anchor and Surplus Shed a good ten years ago.

And then, later in the day, all the empty shipping tubes from McMaster in the garage caught my eye...

...which brought to mind all that diffuser film I bought for an LCD project and never used...

...and I realized I had every part on hand I needed to make a projection telescope.

Most of the lenses I had no idea of the ratings, so first step was to make a quick mockup and some slap-together mounts out of endcaps to dial in my lens choices.

First thought was to use a pinhole as the field lens (pinholes make the design equations ~ trivial) along with a ginormous objective...

...but I was making a projector, and I wasn't sure I'd be able to get enough light through even if it was pointed at the bloody Sun. Also...

LOL. No.

So I went with a more usual two-lens Keplerian design, especially since I had some really nice surplus microscope photo relay lenses.

How To Mount Precision Optics With Hot Glue

To be fair, hot glue works well enough to hold firmly, but also releases cleanly from the glass when you want to pull apart an assembly.

Getting there: A telescoping (literally) objective assembly with sliding focus. four rings of masonite in the larger tube hold two bands of urethane foam in place to provide the sliding friction and light seal. Less complicated than it sounds, also worked first try.

Field lens in the middle is still movable in order to mark out effective focal lengths before permanent assembly.

Rear cap is a bit of diffuser film to see the focus.

...projected from a nice illuminated target for after-dark focus and distortion testing.

Originally I had planned to use the moon as my after-dark focus test once the sun went down, realizing only too late that right before a solar eclipse, the moon will be New. Duh.

Day zero, T-2 hours. Projection hood all glued up.

Lining up the relay lens baffle and an angle cut for the projection mirror.

Result: JAUNTY

At this point I'd tested the objective focal length and spent a day cutting and gluing without any idea if I'd gotten the subsequent calcs right.

As it turns out, I'd missed by just a bit-- projection mirror was about two inches too close to the relay lens. ONE HOUR TILL ECLIPSE, IT'S GOOD ENOUGH...

Aw yeah, we have a clear image with sunspots! Which the DSLR spectacularly refused to resolve.

Carted it all outside (hot-glued to a Tiltall), AND IT'S STARTING:

End result: COMPLETE SUCCESS.

...which is to say, the kids' verdict was, 'Is that all? This is boring.'

Comments

[sauergeek]

I got to watch the eclipse through an array of pinhole projectors (pinhole in cardboard at the top of a garbage can, white paper at the bottom, hole cut through the cardboard as a viewport, aim behind you), welding glass, and crossed polarizers. Next eclipse, I want something that cool. Sunspots!