Archive for the ‘Sun’ Category

Two views of the Sun

Posted: June 28, 2012 in Imaging, Sun

What a great lightning storm in Newcastle today! And a really eventful drive home which at one point involved navigating through a small lake of drifting traffic cones, in much the same way as I used to navigate a spaceship to avoid hitting asteroids in my youth.

The last thing you’d expect on a day like this is to be setting up the telescope to grab some images.  By 8pm the Sun was beginning to peek through the clouds.  I abandoned the Italy-Germany match and got set up in the bedroom because the Sun was too low to see from the garden.

Here is the view in Hydrogen Alpha:


The Sun has been almost blank in white-light in recent days – barely a sunspot to shake a stick at.  That’s all changed now as the next image shows:


Now it’s getting dark and the sky is completely clear.  Now what….bed or noctilucent cloud hunting?  The night is still young and the camera battery is charged.


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.

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.

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 🙂

Processing my solar images

Posted: May 28, 2012 in Imaging, Sun

My preparations for the Venus transit involve making sure I can take good pictures of the Sun through two different telescopes.  My main telescope for this purpose will be the 8 inch Meade LX10, but yesterday I was trying out a smaller 4 inch refractor: the Celestron NexStar 102SLT.  Here it is on top of the HEQ5 Pro mount in the garden yesterday.

The stylish looking cap on the end of the telescope is a solar filter containing a piece of Baader solar film.  It basically dims the light of the Sun by a factor 100,000 so that it’s comfortable (and safe) to look through the eyepiece.  This type of filter makes visible surface features such as sunspots and faculae.

Here’s an unprocessed picture of the solar disk taken with a camera plugged into prime focus.   The size of the image is 3872×2592.

The Sun doesn’t occupy a lot of the image so the first step in the processing is to crop the unwanted parts of the image.  This will reduce the processing time of the next steps.  The cropped image is now 1276×1231.

There are some small sunspot groups in this image but they’re not that easy to see at the moment.  The next steps are about enhancing the contrast on the solar disk to make the groups more prominent.

I’m using the free software called GIMP to do the image processing. If you have Photoshop you’ll probably have equivalent methods for the following.  We’ll begin by adjusting the brightness levels in the picture.  In GIMP you can do this from Levels on the Colors menu.  For the above image we get this:

The Input Levels show a histogram – basically a count of the pixels in the image and how bright they are.  The Sun image basically contains dark pixels (the background sky) and bright pixels (the Sun).  You can see those two peaks in the histogram.

Now, the detail we’re interested in is contained in the brighter pixels of the Sun.  That means we want to stretch out the brightness differences among the Sun’s pixels to make them more visible.  In other words, it’s the second peak that we need to focus on.  We’ll move the left and right sliders on the Input Levels to bracket the second peak (the middle slider will move to preserve the overall intensity automatically).  It’s subjective where you place them but I went for this:

It means that any pixel brightness below a certain level (154) is treated as black in the new picture.  Anything above a certain level (230) is treated as full intensity.  The image now looks like this:

Those sunspot groups are now much more prominent in the image! Also, notice that the edge of the Sun is dark than the middle of the disk.  This is a genuine physical effect called limb darkening.  At the edge of the Sun we are looking through the cooler, less dense layers of the Sun.

We can enhance the surface features further by applying an unsharp mask.  This will sharpen the finest features in the picture.  You’ll find it in GIMP on the Filters menu in the Enhance sub-menu.  Pick a region of the Sun where there is some detail visible.

Play around with the sliders to get the best effect (very subjective!)  The radius and amount determine the degree of sharpening applied.  The threshold is useful for confining the sharpening to particular features.  It’s easy to get carried away with this kind of enhancement and end up with an image that looks too….fake?  Over processed?

This is where we are at after sharpening.

The picture contains a bit too many pinks and purples for your taste.  This originates with the Baader filter itself.  Sunlight is white – a mixture of all colours in roughly equal intensities.  You can make it appear more natural in the next step.  Or give it a yellow or orange tinge.  It’s up to you.  Or leave it alone!  Look for the Colorize option on the Colors menu.

 The Hue setting controls the overall colour of the image.  The setting shown here gives the Sun a pale orange colour.  Use Saturation to make the colour more or less intense.  Here’s the final image:

 You don’t have to follow these steps precisely to improve the image from your camera.  You might also want to experiment with some of the more advanced settings – particularly with GIMP.  For example, there’s a plugin called wavelet sharpen which is a great alternative to the unsharp mask.

Have fun!