Transit of Venus

Here are some photos over the first 45 minutes or so. They are in sequence, but not evenly spaced in time.

This is from my 8" F/4.5 scope, stopped down to about 3", with photo taken hand-held with my Canon G7 camera off the projection funnel.

Here is the last photo in a larger size.  The sunspots were very nicely visible in the funnel (I counted about 15), and I could even see two without a telescope in the #14 welder's glass.  The photo doesn't do justice to the sunspots, especially the nice bright area that was just barely visible at the bottom of the disc.

Eclipse

Here in central Texas, the sun set before the eclipse hit maximum.  But we had some lovely views of the moon gobbling up sunspots as the sun was setting over the lake.  Photos are taken by a Canon G7 camera, using a solar funnel for projection.  The scope is an 8" F/4.5.  For the first photo it was stopped down to about 3".  The second photo I think used the full aperture.

And for reference, here's the sun in the afternoon, before the eclipse:

More sunspots

I still don't quite have the hang of photographing the projected solar image on my funnel--it's still sharper naked-eye--but the photos are improving. When the clouds aren't in the way.

Sun funnel done

To get the size of sun funnel I wanted, I had to join two different funnels.  But it works!

More sunspots

I was handholding the projection screen material in front of my scope for a 4-5" image.  Wow!  Today's sunspots were really impressive.

I noticed that they were very much in pairs.  Apparently sunspots come in pairs with opposite magnetic polarity.

Sunspots today

A kind soul sent me a 25mm Kellner eyepiece for solar projection purposes for the cost of shipping.  A nice thing about this eyepiece is that the field stop extends quite far from the lens, and will dissipate any sunlight if the sun goes to the edge of the field.

I played with it today to see how far away from the eyepiece I'd need to get to have a 4" image in my 8" F/4.5 (stopped down to two 3" apertures).  Answer: ~26cm.  When I projected the sun on a piece of cardstock, at that size, I was blown away by the richness of the structure of the sunspots today.  And the sun was quite low, as it was shortly before sunset.  Here's today's LMSAL photo.  I saw the two larger groups but somehow missed the smaller group that's down from center.

It's time to build a sun funnel as soon as my projection material comes.

More solar observing experiments

I am continuing to experiment with my solar projector, which uses a hard disc platter as a heliostat mirror, and two Surplus Shed lenses, one positive with 2600mm focal length, and one negative with -800mm.  While I can see sunspots, I can't see almost any structure in them, and there is a lot of chromatic aberration.  A bit of experimenting suggests that most of the chromatic aberration is due to the negative lens, so I've ordered a -1000mm achromatic lens.  We'll see if that improves things.

While ordering from Surplus Shed, I also ordered a 15mm focal-length Hastings triplet.  If it does not make a good planetary eyepiece--I will certainly will try--I will have a nice 15X loupe, replacing my 15X plastic-lens loupe (which is usable, but only in the center of field).

Solar observing mishap

I was experimenting with solar projection with my 8" scope.  I first stopped up the aperture to about 3".  All was well.  Then I went for full aperture.  The images were sharper.

But then the images went foggy.  And I couldn't get them in focus.  And smoke started coming out.

I disconnected the Kellner and noticed the lens was fogged up.  Turns out that while the eyepiece housing was metal, as it should be for solar work, the eyepiece lenses had a plastic spacer.  Which melted.  The spacer is the black thing to the left of the lenses.

The lenses cleaned off fine with acetone.  But I wonder how I can find a metal replacement spacer.

So, that's a warning for solar work: don't assume that just because the eyepiece tube is metal, the inside is metal, too.

Solar projection scope prototype

Warning: With solar observing, be very careful not to look into the optical train, or you are likely to go blind.  Do not leave the optics unattended.  Watch for children!  Don't let any critters walk into the optical path.  Remember that as you experiment with the device, the optical path may be unpredictable.

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)

I cut out holes in the shipping box and mounted the big lens to one side of it.

Next, I cut a hole in a piece of plywood, and mounted the small lens in it, using pieces of an old bike inner tube to hold it in place.  I added two pieces of wood to the bottom of the plywood for a stand.

I had some used hard drive platters.  I used a pair of cheap Harbor Freight helping hands to hold one of the platters as a heliostat mirror (using the alligator clip to hold the platter through a piece of cloth so it wouldn't mar the platter too much). 

I set up the optical train on the driveway.  After some experimenting, I got it working.  Hard drive platter reflects light into the big lens.  About 80 inches further, the small lens stands.  A couple of meters further away (I didn't measure; my calculations suggest 6.3 meters as the correct distance, but it seemed closer to me), in the shade, I have a box with a piece of card stock taped to the side as a projection screen.  

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.

Result: Eight inch image of the sun.  Acceptable chromatic aberration along edges.  Moderately fuzzy.  One clear sunspot.  (Today's 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.

Solar projection telescope

I'm ordering lenses from Surplus Shed for my big solar projection scope.  The plan is a 150mm PNM objective lens with a 2600mm focal length, followed by a -800mm MNM lens spaced at 1890mm from it.  This should result in 202mm image of the sun approximately six meters away, for an effective focal length 23.1 meters!

I still don't know how I'll direct sunlight into the scope.  My first attempts will use a hard drive platter as a super-cheap mirror.

Solar projection

I made a little cardboard solar projection box for my 8" Coulter, and a mask to cover all but 2.9" of aperture.  I read a hint to put two holes in to get the resolution advantages of the aperture--I may try that one day.  Or I might open up more of the aperture and use IR-rejection film, though that may be risky for the eyepiece.

It's easy to point the scope at the sun--you just move the scope until the shadow of the tube is as small as possible.  For safety, I make sure all the observers are on the side of the telescope away from the eyepiece.

With a 27mm Kellner eyepiece, after I got things into focus, the box did show sunspots.  (Note that the big yellowish smudge on the right is glue on the inside of the box, rather than a solar feature.  I will have to stick in a clean piece of card paper there.)  The image was actually a little too bright to view comfortable.  The photo of the projected image worked, though, once I set the camera to minimum aperture and exposure time.  I need to make some registration marks on the projection screen, so that I can take two photos in the future, one with exposure for the bright disc and the other with exposure for the registration marks, and use the registration marks to undo the skewing you see in the photo.

The sunspot positions match those here.

The aperture mask was quite warm, but the eyepiece didn't seem to get hot.

Solar observing safety precautions: Do not look through the eyepiece.  Do not look at the sun naked-eye, either.  Stop down the aperture to prevent the eyepiece from shattering from the heat (which is a waste of an eyepiece, and might cause harm from flying shards), and push telescope away from the sun every couple of minutes to cool off.  Don't use an expensive eyepiece.  Never use the cheap solar filters that screw into the eyepiece.  They can shatter while you're viewing and the sunlight is then likely to blind you (and I had one as a kid that faded with time--it got less and less dark! fortunately, I never used it in the telescope).

Sunspots

I've tried to see sunspots by using solar projection with a 68mm refractor, with no luck.  Last night, close to sunset, I pointed my 8" Coulter towards the sun, with a 30mm eyepiece and a card in front of it.  I got a very sharp image with a number of distinct sunspots.  Couldn't see any detail in the spots themselves--they were just dots--but they were there.

Normal solar safety rules apply here: Don't ever look through a telescope or eyepiece pointed in the vicinity of the sun.  (I took care of that by making sure that all the observers were on the side of the scope opposite to the eyepiece.)  Cover up any magnifying finder scope (a red dot finder is fine--just don't use it!).  Don't look at the sun while pointing the telescope (my best method for pointing was to move the scope until the size of the shadow red-dot finder was minimized, and then to wiggle the scope until I saw solar glare on the card).  Don't keep the scope pointed at the sun for too long--give it some cool-off time between observations.

On reflection, I should have stopped down the aperture to prevent eyepiece melting.  I didn't have the eyepiece melt, but maybe that was because it was close to sunset.  I am going to try to make a more comfortable projection setup today.  This is all a sort of preparation for the transit of Venus in June 2012.

My solar viewing was before the Waco star party.  Later that night, we were showing Jupiter to girl scouts, waiting for clearings between clouds.  I think everyone who stuck around got a view.  Some of the time the seeing was horrid, but at one time I did see a fuzzy barge atop the northern belt.  After the girl scouts left, the sky mostly cleared up.

Paper craft sundial

Here are instructions for a paper craft sundial I built last night.  The hard part was doing the trigonometry for getting the gnomon to be at the right angle after folding--I hadn't done much trigonometry for a while, especially in regard to laying out a three dimensional object (the gnomon is wider at bottom than at the top).  With the script complete, construction takes about fifteen minutes.

Sundial completed

It's done now, and I made an Instructable with detailed instructions.

Now, can I persuade my kids' school to have it painted somewhere?

Working on sundial

I'm planning to draw a large analemmatic sundial with the kids, with chalk on the driveway. But that's not as much fun as writing a perl script to generate a pdf file with all of the dimensions. The beta version of the script is now running here. Source is also available, of course.
Update: Just executed it in sidewalk chalk. That's the best I can do in terms of permanence, since we rent our house from the University.