Taking solar images

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This section is mainly to add notes and information that is useful during planning an observing session and then working on it.

  1. H-alpha surface and prominences
  2. White light for sunspots
  3. Work flow


H-alpha surface and prominences

The scope is fitted with the H-alpha etalon and the blocking filter.  Position the camera so the long axis of the chip is perpendicular to the handle line of the scope. To get the image to fill the frame when capturing flats it is best to use a Barlow lens if possible.

SharpCap is used to record the images. (The words and precise layout of the SharpCap interface depends on the camera. Here the ZWO ASI 120M is used).

1. Image the surface and prominences.

  • Move around the sun to find what it is best to image.
  • Use an exposure that sets the max on the histogram to about 90% with the gain set at about 30 and the minimum brightness.
  • To focus zoom in and get the best focus.
  • Momentarily push the gain right up e.g. 200 and the gamma to 99. Search for the prominence you want and use that for framing. Do not use the histogram but get the best exposure to be able see the prominence without drowning it. Zoom in on the prominence to better adjust the exposure.
  • (a) Capture 500 frames.
  • (b) Revert the settings to the surface ones and capture 500 frames for the surface.
  • The surface and prominence image are combined – see section processing solar images.

2. Capture a flat for flat frame calibration.

  • It is important to do this near in time to the main images as the temperature changes in the scope cause any Newton’s rings to move. The flat can remove the Newton rings.
  • Keep the settings as per the surface (the flat is only used for the surface). Move the scope so that the image fills the frame. Put the scope right out of focus. Capture about 300 frames.
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White light for sunspots

Here the H-alpha etalon is removed and the body of the scope with no filter is fitted instead. It is vital that the Herschel Wedge is used as the diagonal. To reduce the intensity further and to increase detail the Baader Solar Continuum filter is fitted to the nosepiece of the camera.

Focus using zoom and take images using a 90% histogram as for H-alpha.

When using a Barlow to a flat by defocussing and filing the eyepiece with the solar surface. When taking images without a Barlow it is not possible with this set-up to fill the image with solar surface. The best solution for me was to heavily defocus, using a piece of almost transparent plastic over the object to further diffuse the light. Then adjust the exposure so the histogram is at 90%. Generally though a flat was not found to be necessary when no Barlow was used.

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Work flow

After some false starts – particularly not adjusting the angle of the camera so its long axis is E-W, and not taking enough flats to get rid of Newton’s rings – here is what I now do. Maybe it will change as I learn more.

Set the telescope for white light. Do not use a barlow and check for sunspots. Use these to get the camera axis EW as described using the EW arrow keys on the handset. Provided there are some sunspots I take 4 images so they can be put into a mosaic giving a complete solar disc when processed and merger. I use Registax or imPPG to sharpen with subsequent tweaking in photoshop.

Then I install the Barlow and take a flat using the centre of the defocused sun to fill the view and keep the maximum of the histogram to about 90%. Then refocus. Since the camera has been removed it is necessary to redo the EW alignment, although noting the previous position enables it to be roughly correct to start with.

Then image the sunspots magnified with the Barlow.

Change the scope to be h-alpha. With the Lunt this only takes a few moments.

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