Another image of the Sun taken with the PST yesterday afternoon:

My best yet!  This was made possible by an interesting discovery about my Nikon camera.  The pictures I’ve been taking prior to this have been careful not to saturate the chip – typically using exposures of about 1/100s at ISO200.

Yesterday I tried to do some longer exposures to try and extract more signal from those prominences around the limb at the cost of overexposing the disk.  The PST only allows a specific colour of red light through to the camera so I was expecting to see a washed out red disk with more obvious prominences.  I began taking pictures with exposure times ranging from 1/20 to 1/5 of a second.

What I actually recorded was a glowing orange ball similar to the final image above!  How can you get orange when the only colour available is red?

The camera chip is an array of red, green and blue sensors.  At fast exposures those photons of red light were activating the red sensors on the chip.  At slower shutter speeds the red sensors become saturated and the signal leaked into the green sensors.  A lot of red plus a dash of green equals orange in this situation.

Splitting the raw image into its RGB components gives this:

 The “B” component was a black image – nothing had leaked into that channel.  The green channel contained a nicely exposed image of the disk, with all the detail around the edge contained in the red channel.  Presumably with an even longer exposure the signal may have leaked into the blue channel as well!  From what I can tell, the green channel contains a higher quality image than the equivalent red channel – it certainly seemed to stand up to the contrast enhancement and sharpening process that followed.


Happy days

Posted: June 18, 2012 in Personal

The summer solstice (for us northerners) approaches and I’m happy for two reasons.  Firstly, the solstice marks the start of a period when days begin to shorten and I can get back to doing some astronomy at a reasonable hour of the night.  My second reason to be cheerful is that I’ve got a solar telescope to play with until that happens.  Never again will you spoil my fun Earth’s axial tilt!

Ten minutes of Sun!

Posted: June 17, 2012 in Imaging, Sun

But that was just about enough to grab some more pictures through the solar telescope.  Here’s the result:

I found a great page on the Sky & Telescope website with tips for processing these images and I think it paid off with this one.  In particular I corrected the non-uniform illumination which seems to afflict these early images.  Also, I think I’ll try and blend the disk image and ‘edge’ image a bit more smoothly next time.

The Sun broke through the clouds late this afternoon and I was able to snap about a dozen images of the Sun through the PST before it disappeared.  Without further ado this is what I ended up with after processing several of the best images:

Here are some of my initial thoughts about the PST after one day of use. Well, about 30 minutes of use in total:

  • Focussing is a bit difficult – small image with the supplied eyepiece.  Trying to make the edge of the Sun as sharp as possible but small disk size makes it hard to tell when you’re there! Also, looking at a purely monochrome image takes getting used to.
  • First view through the eyepiece was startling: prominences all around the edge of the disk with one particularly large one.  Sunspots, prominences, filaments and plage visible on the disk.  A wealth of detail – more than is seen in the picture and very sharp when focus is perfect.
  • Disappointed that the PST couldn’t bring an image to focus in the Nikon D80 using the same adaptor/setup I have on my other telescopes.  In the end I dismantled a 2x barlow and plugged it into the T-adaptor.  That moved the focus far enough back and made the image bigger on the chip.
  • The pictures I took showed non-uniform illumination across the solar disk – depending on where the Sun was in the field of view.  Not so noticeable through the eyepiece but it means that several images (at least) with the Sun at different places in the FOV must be stacked to reduce the effect.

Again, I’m hoping for better weather tomorrow…

First light for the PST

Posted: June 16, 2012 in Observing, Sun

There was enough Sun shining through gaps in the cloud to get glimpses of it through my new Coronado Personal Solar Telescope (PST).  Despite the wobbly camera tripod and the interfering clouds, the view was fantastic!  So much detail was visible on the surface of the Sun and there was an incredible prominence visible on the edge of the disk. It sort of resembled an anteater walking along the edge of Sun!

I’d hoped to get some pictures but I quickly realised that my normal method for attaching camera to telescope wasn’t going to work – the focus range of the telescope wasn’t enough to bring the image to focus in the camera.  Think I’ve got a better system sorted now….but it’s raining again.  Maybe tomorrow.

I wrote recently that the season of noctilucent clouds had started.  Last night, as twilight faded, I saw some appearing in the northern sky.  It was about 11.30pm – just as I was getting ready to go to bed that the first hints of them appeared.  I took a wide field shot with the camera from the road just outside the garden:

It’s a very subtle display at this point! Another five minutes or so confirmed to me that the sky was getting darker and those clouds appeared to be getting brighter.  Here’s a narrow field shot of some of the brighter parts of that cloud:

In this picture you can see the electric blue of the noctilucent clouds and some characteristic ripples in the structure.  Whether this display got better or subsided – I’ve no idea – really had to get to bed for an early morning start.

There’s some evidence that noctilucent cloud displays are inversely correlated to the sunspot cycle.  But strong displays can also happen during the solar maximum so the relationship is probably more complicated than that.  See here for an accessible discussion of this link.  The frequency of noctilucent cloud appearances is variable and it may be that we are in for a couple of lean years.  I hope we get at least a few more displays this year.

Summer astronomy

Posted: June 13, 2012 in Imaging, Planets, Sun

The summer solstice approaches and dark nights have all but vanished from Northumberland.  It seemed like I was just getting into deep sky imaging and now I’m waiting for the autumn to come so I can get back to it.  Realistically, I’ve got a couple of months before I’ll be doing that kind of imaging again.  With that in mind I’ve deferred getting an autoguider (and new finderscope) in order to get a solar telescope.  While I’m waiting for that to be delivered I took the chance to get some ‘white light’ images this afternoon.

Here is a composite of two images taken through the NexStar 102SLT (and 2x Barlow) filtered with Baader film.

Some very prominent groups of sunspots today.  I hope they’re still there when I get my PST in the next day or two!

After getting some solar images I had an attempt at finding Venus.  With the HEQ5 mount not accurately polar aligned it’s not that easy.  My method was to find the difference in altitude and azimuth from the Sun then slew the telescope to the approximate place.  Skymap Pro told me to move the telescope 9 degrees lower and 10 degrees right of the Sun.

Here’s a star chart showing the location of Venus this afternoon.  With an elongation nearly 12 degrees west of the Sun the planet was far enough to safely observe without taking steps to block out sunlight falling into the telescope tube.  Here’s a Stellarium rendition of the afternoon sky.

The real sky was much cloudier – lots of slow-moving patchy cloud.  After a gap appeared in the clouds Venus was easily visible in the camera field of view.  Venus is currently very narrow crescent – shimmering through what was obviously very unsteady air.  There followed a frustrating hour in which switching between eyepiece and camera was thwarted by the ever shifting clouds.  Eventually I had Venus in the field of view with the camera and 2x Barlow lens.  I began clicking away on the remote…

This is a stack of 33 images taken with the Nikon D80 through the NexStar 102SLT (and 2x Barlow) at ISO100 and 1/250s exposure.  Stacked in Registax 5 and post processed with GIMP.

Now that Venus is west of the Sun it will soon be a prominent morning sky object before sunrise.  As it recedes from Earth the angular size will grow smaller and the phase will increase.

It seems that you can do astronomy in the summer – you just have to change your targets, that’s all 🙂