I observed the transit from home, despite the less than optimal vantage point, and, as it turned out, less than ideal weather. However, my equipment was there and I thought it best to make do with what I could see rather than traipse all over the countryside trying to get something slightly better. In the end, it was the weather rather than the limited horizon which stopped observations.
I decided
to mount two telescopes with CCDs on the Paramount ME. The ST-8e was attached to the
AP-130 refractor for whole disc images. This combination yields a field of view of
60·7 × 40·4 arcminutes, with pixels 2·38 arcseconds
square. The CCD was windowed down to 1020 pixels square, which helped the readout
times (as it's an old parallel port camera). One frame was to be taken every minute.
A piece of Baader solar filter was over the objective, while the 3-nm H-alpha
filter attenuated the light so that 0·1 second exposures (the shortest
possible with the ST-8's shutter) could be used. It's here that the limitations of
the parallel port interface become painful. Normally, a one minute readout after a
30 minute exposure isn't too onerous, but after a 0·1 second exposure it
really hurts.
I also attached an old 80mm f/15 refractor (a recent acquisition bought with the intention of a dedicated solar telescope fed by a heliostat.) with my trusty Cookbook 245 CCD. This yields a field of view of 18·4 × 13·6 arcminutes, with pixles 2·9 × 3·4 arcseconds. I didn't go for higher resolution as I didn't expect the seeing to be great due to the far northerly declination of the Sun, hence low altitude. A piece of Baader solar filter was over the objective, while a 10-nm H-alpha filter further attenuated the light level. I also needed an ND0·6 filter to further reduce the light so that charge buildup during readout (the Cookbook has no shutter) didn't satuate the image area. 0·25 second exposures were used with this combination. This telescope had to be offset relative to the AP130 as it could only see a portion of the solar disc. (I spent ages agonizing over the alignment and was very pleased when the transit started that I'd got it right!) The intention was to take a frame every 15 seconds with this camera in the hopes of making a small time-lapse movie of the whole event.
For looking at the transit (while the CCDs were busy taking images) I used
another 80mm refractor; this time my f/9 guidescope with a
Zeiss solar eyepiece all mounted on my
"old" mounting. The solar eyepiece is an interesting item. It
consists of a Herschel wedge as the first stage in reducing the Sun's intensity
(with a mirror to divert the heat away from the observer) followed by a right-angle
prism. Normally, the prism would produce a complete reflection from its hypotenuse,
but in this case this surface is in contact with a fluid whose refractive index
is almost - but not perfectly - matched to that of the glass. This destroys the
total internal reflection from the surface and allows most of the light (and heat)
to pass through (the excess being blocked by a filter). The result is perfect
attenuation of all frequencies to a safe level for visual use - the resulting
image is white. It takes 24·5-mm diameter eyepieces so I was limited in
my choice to either the
supplied 18mm Huygens (which didn't reach focus so couldn't be used anyway) and a
25mm Ortho (giving 29×) which is a bit low power but would have to do. (I
don't have a small-size Barlow, unfortunately.) Note the styrofoam light shield
over the telescope to shield the observer from direct sunlight.
The day dawned bright and clear and looked great, but early in the afternoon some clouds started rolling in. I was already set up and aligned on the Sun but we didn't see the Sun after 1.30pm until shortly after second contact. We had actually made the decision to leave the CCDs running and head elsewhere when the sky amazingly cleared enough to see the Sun, with a small bite already taken out the side.
Both CCDs were already taking images, but the alignment was slightly off as the Paramount was running with a pointing model for a different telescope/weight combination; I hadn't bothered updating it for just a few hours observations. So the first few images weren't perfectly centred, but that was quickly fixed. Once the CCDs were aligned I was free to look at the transit through some of the assembled telescopes.
The effect everybody was looking for was the so-called black drop effect; where at second contact the disc of Venus and the limb of the Sun don't part cleanly, byt rather Venus seems to "drip" from the limb. This effect caused significant problems in timing the event in past transits. However, I can't say that I saw the effect, at least to any significant degree. It would momentarily appear when the seeing became bad (I still had some clouds around, plus it was low in the sky anyway) but was never a major problem. I've included many of the frames I took around second contact and in one of them (where the seeing was poor) there is a strong hint of it.
The other observation to be made was whether Venus could be seen with the naked (but safely filtered) eye. The answer to this was a resounding YES. It was easily seen as a notch out of the side of the Sun mid-way between first and second contact (when the clouds parted enough to see the Sun!).
Despite the poor weather (it cleared up after sunset to be a nice, clear night - but who cared then!) we had a great time. Bring on 2012!
Here are a few of the results of my imaging. I've not included the Cookbook movie sequence in the list, just a few selected frames. If anybody is interested in it I can make it available. The edited (clouds removed) sequence is 56MB in size (uncompressed, to get the full resolution), or 4MB compressed.
This is the sequence around second contact from the Cookbook 245 CCD attached to the 80mm f/15 refractor. There is little evidence of any significant black-drop effect. What little there is seems to be due to the seeing.
| 05:25:42 UT |
05:25:57 UT |
05:26:12 UT |
05:26:27 UT |
05:26:42 UT |
05:26:57 UT |
05:27:12 UT |
05:27:27 UT |
Here are a few shots with the ST-8e and AP130 refractor.
| 05:23:12 UT |
05:26:10 UT |
About second contact. | 05:27:10 UT |
Well in now. | 05:30:08 UT |
05:32:07 UT |
05:34:05 UT |
05:34:05 UT |
Same as above, but with the contrast adjusted to show the few meagre sunspots that were visible at the time. Note how changing the contrast changes the appearance of Venus and it's apparent position relative to the limb. | 05:46:01 UT |
06:24:51 UT |
About the last we saw of it! |
