Just as I was heading out on my road trip B&H sent out an eclipse mailer:
https://www.bhphotovideo.com/c/solar-observing/ci/33818/N/3583558376/sba
When I was in Portland I was talking to my rocket scientist friend Phil
about watching the eclipse, he wants to make a camera obscura to project
the image of the sun. It turns out that if you take a long lens
(500mm), and place it back to back with a wide angle lens (samyang 8mm?)
you can then focus a projected image a reasonable distance away.
We did some quick proof of concept with my bigma and my sigma 20/1.8.
Philip just ordered himself an inexpensive Celestron. Here are his
notes from when he was waiting for it:
From the intarwebz:
D = 0.0093P(FLtelescope/FLeyepiece)
D = image diameter, m
P = Projection distance, m
object - sun
And, if your telescope has an all metal and glass eyepiece, you can just
use that for projecting. Even back projection, through a funnel stuck
into the objective tube, to a screen strapped on the end of it.
Reflectors can melt their secondary mirror's mounting if it's plastic.
If so, you can stop it down with a 1-2" aperture on the inlet end.
So 1 1/4" eyepieces may make the best second lens for this. The
Celestron AStroMASTER 114 comes with 10mm and 20mm ones, so it'll
project nice images using those if they don't melt.
I then got this update from him yesterday:
Hey,
The ASStroBlaster 114 arrived. Some things I've learned.
The tripod/mount is acceptable, though the solar image wiggles pretty
easily, that's pretty good mag, so more solid would be better.
Its 20mm eyepiece has just enough FOV to project the sun and maybe the
moon, but the spare on the edges is so slim that you have to constantly
be tracking to keep up with that sucker.
A motor drive is definitely in order. There's one for this beast, I'll
have to track it down.
This situation should improve with wider FOV eyepieces (in theory), but
will get worse with shorter FL ones. The projected image size is nice
but not as wide as I dream of for the Camera Obscura.
So, looking for very wide FOV eyepieces of 10-15mm now.
The 'scope is "A Jones-Bird Newtonian telescope ... is a mirror-lens
(catadioptric) variation on the traditional design sold in the amateur
telescope market. The design uses a spherical primary mirror in place of
a parabolic one, with spherical aberrations corrected by sub-aperture
corrector lens usually mounted inside the focusser tube or in front of
the secondary mirror. This design reduces the size and cost of the
telescope with a shorter overall telescope tube length (with the
corrector extending the focal length in a "telephoto" type layout)
combined with a less costly spherical mirror. Commercially produced
versions of this design have been noted to be optically compromised due
to the difficulty of producing a correctly shaped sub-aperture corrector
in a telescope targeted at the inexpensive end of the telescope market."
(Wikipedia)
So far, the solar image is not sharp like it clearly could be. Have to
see how much of that is the eyepiece. Cool set of big sunspots today,
though.
Projecting through the eyepiece gets you a screen that is naturally at
90 degrees to the sunlight. A small sized screen (1'?) and a simple
up-sun shade could be built onto the 'scope, and you'd have a decent
setup without much trouble.
The cover that comes with it has a hole that makes it a perfect aperture
for not roasting plastic optics. I may have prematurely let some of the
smoke out of it, but it's still alive. Turns out as the sun drifts off
to the side, it wants to melt any black plastic mounting the secondary
optics. Like around that cursed corrector lens mounted at the deep end
of the focuser tube. Or the focuser tube itself. Or the eyepiece.
Motor drive! Maybe a fan for the secondary mirror.
-Phil
=============
In any case, it seems that one critical aspect is some rough
approximation of an equatorial mount, not to keep the image from
blurring but to just keep it relatively centered.
Mark Roberts wrote:
I expect there will be more than a few of us attempting to photograph
the eclipse next month. I've just started a little research into it
and I'm closing in on a decision as to what filter to use. My longest
lens is my FA*300/2.8 and it's happily configured with an in-lens 43mm
filter capability. Here's what I've found that looks most promising:
https://www.optcorp.com/spectrum-threaded-camera-solar-filter-43mm.html
Anyone have any experience or other recommendations?
--
Larry Colen l...@red4est.com (postbox on min4est) http://red4est.com/lrc
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