Solar Imaging

Equipment

I use a Coronado Personal Solar Telescope (PST) for solar imaging. It's the original affordable solar telescope, but has been eclipsed (no pun intended) by more recent designs. Advanced solar telescopes are quite pricey, so I'm sticking with my PST for now. It does a nice job!

Solar telescopes use an etalon and a blocking filter to reduce the sun's light enough to view and image safely. Most solar telescopes block everything but a very specific wavelength called hydrogen alpha. Viewing the sun in Ha allows you to view prominences, flares, and other solar features that can't be seen through a full-spectrum solar filter.

A solar telescope DOES NOT use the same type of Ha filter one would use for deep-space imaging. Viewing the sun through a deep-space filter (or no filter) WILL result in serious eye or camera damage.

The business end of the PST. This is not the etalon.

Set the telescope mount to solar instead of sidereal rate tracking. I use the handset to find the sun and only switch to computer control when I'm ready to image.

The PST uses a moveable prism to focus. This small knob adjust the prism's position inside the box.

The PST has an integrated solar finder. You know the scope is pointed at the sun when you see it in the center of this small, circular window.

Camera Backfocus

One of the challenges of solar imaging with a PST is obtaining the proper distance between the telescope body and the camera sensor (backfocus). My ZWO 178MM camera can't be placed close enough to the telescope body to find precise focus without a modification.

I found a 3-D printable, low-profile cover to replace the original front cover of the camera, which just screws off. This allows the camera to be positioned almost .5" closer- just enough to obtain critical focus. ZWO used to produce their own low-profile adapter but it has been discontinued.

Tuning The Etalon

My PST has a single etalon that can be adjusted (tuned) to obtain the best contrast possible. To do so I view the sun through an eyepiece, turning the rotator until I've found the best image

While a single etalon provides a great image, adding an additional etalon provides much more contrast. It's on my wish list!

I've taken the outer adjustment ring off to obtain a greater range of movement. I tune the etalon by placing a small screwdriver into one of these holes and using it to turn the rotator

Lucky Imaging

Even through a solar telescope, the sun is so bright that it requires very short exposure times. Also, turbulence in the atmosphere (seeing) blurs the image many times per second, so the chances of any one image being fuzzy are pretty high.

We use a technique called "lucky imaging" to mitigate these problems and obtain the best quality solar image. We shoot a short video with as many frames per second as possible. Most will be blurry but a few will be sharp. Later, we'll use software to pick and stack the best frames, leading to a sharper final image with low noise.

Imaging With Sharpcap

I use Sharpcap to image the sun. It has a number of great features to aid in lucky imaging. Here are the steps I follow:

Find the Sweet Spot

A single-etalon scope like the PST will have a sweet-spot where the image will be brightest and have the best contrast. To start, set a high exposure to find that spot in the field of view.

Center the Sweet Spot

Next, I use the telescope controls to move the the sun so that the sweet-spot appears in the middle. This will give the most natural looking image.

Focus At 100% View

By setting the display to 100%, surface details become more clearly visible. This makes critical focusing easier. Adding contrast with the Display Stretch can also help.

Find The Prominences

Next, I view any prominences on the sun. Proms are dimmer than the surface, so I either brighten the image or increase the exposure to make them visible. I then select an area around the sun that includes the proms (the red box).

Increase Framerate Using ROI

We always want to obtain the highest framerate possible. We can accomplish that by limiting the camera to the Region of Interest we set in the previous step.

Using Display Stretch

The display stretch allows you to change the brighness of the image on the screen without changing the exposure. This lets you brighten the image to find proms or increase contrast when focusing. Changing the screen stretch doesn't change the actual data, just the way it's displayed.

Set Exposure Using The Histogram

The graph along the bottom of this image is the histogram, showing the number of pixels at each brightness level in the image. Here I'm using a "logarithmic" display to get a better look at the highest values. I adjust the gain and exposure until the peak of the histogram is between 85%-90%. A brighter image will lead to blown-out pixels in the final image stack.

Feature Tracking

Feature tracking is great! Here, I've set it to track the sun's center of mass. Sharpcap will send commands to the mount that keep the sun centered in the imaging space.

Capture Video

Now it's finally time to capture some video! I usually capture 1000-2000 frames when imaging the sun. My camera can only achieve 44-46 fps when imaging the full disc, but many more if the ROI is reduced. I'll stack the best 50-100 frames from each video.

Seeing Monitor

Sharpcap's Seeing Monitor can help make the most of your image capture time. Place the red box around a prominent feature and it will measure the contrast in every frame. Frames with higher contrast are generally better. The Seeing Monitor allows you to set a minimum quality level and will only save frames that exceed that threshold.

Livestacking

Use SharpCap's livestacking feature if you don't want to stack and process your captures manually.

The controls let you specify how many frames to stack, adjust the sharpening and brightness, and even colorize the image. It will update the stack continuously and can save the result as an image file.

Here, the stacked and processed image is on the left of the window, while the unprocessed video is on the right. This gives you some idea of what we'll have after manual processing, although we can probably do better.

I have SharpCap save the video captures as .ser video files, which are better quality than MP4s. You can also set your camera to 8 or 16-bit mode. 16-bit mode yields better quality, but file sizes can be huge. I typically stick to 8-bit unless there's a really cool feature to image. My 178MM produces a 1000-frame video, in 8-bits, with a full disc ROI, that is 3.2 gigabytes! Switching to 16-bit mode can fill up a hard drive very quickly!

Now it's time to stack the best frames of our video into a single image. Then we'll process that image for a final, color result that shows the prominences and surface details of the sun.

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