Leveraging Starlight for Sharpening Planets

The field of digital planetary imaging is where art meets science, particularly in post-processing. After using our preferred tools to distill a video of several thousand frames into a stacked and aligned image, there is still much work to do. We need to refine it into a sharpened view of the target before applying some tweaks in a program like Photoshop. To achieve that sharpened image, we have two techniques at our disposal: wavelet sharpening and deconvolution.

Wavelet sharpening is a key feature of the freeware application Registax6, a staple of planetary imagers for many years. The author of the software, Cor Berrevoets, has not issued an update to the venerable program since 2011. He has, however, created a successor in the form of another freeware application called waveSharp. Both of these tools decompose the selected image into layers - from large-scale components of the image to fine scale. By adjusting the sliders, you can selectively enhance these aspects of your image. The finer scale adjustments must be done with a light touch to avoid introducing significant noise to the final result. Fortunately, one can combat the noise by suppressing the finer scale adjustment.

The ability to apply deconvolution is appearing in more tools for the planetary imager. One excellent new addition is the Lucky Stack Worker (LSW) freeware application (a video by the author, Wilco Kasteleijn, is on the ALPO channel). Another is AstroSurface, an application with extensive filters and functions for not only the planetary imager but the deep sky enthusiast as well.

So, what is deconvolution? In brief, it is using a contemporary representation of an Airy disk to help recover detail lost through atmospheric turbulence, soft focus, or optical issues. Under perfect conditions, when examining a moderately bright star, you would expect to see a small disk with diffraction rings emanating from it, known as its Point Spread Function (PSF). The interface on the LSW has two checkboxes - one for "Deconvolve" and one for "Sharpen" (i.e., wavelet sharpening). If we select the Deconvolve and select the "bullseye" adjacent to it, we see that LSW is offering us a synthetic PSF with sliders to allow us to adjust it.

Impacts when adjusting the Seeing Index slider

You could take a few minutes before or after your imaging run to inspect a nearby star and make a note of its Airy disk appearance to emulate it in the synthetic version. But why not record an image to capture the PSF for that particular imaging session? This is exactly the sort of thing that Wilco Kasteleijn advocates in the LSW manual and references a nice article by Marco Lorenzi on how to do this.

A couple of nights ago, Astropheric was displaying that the seeing would be "average" with temperatures in the 40s, so it was an opportunity to try this technique and maybe gain some experience acquiring a PSF image to leverage in my processing. Like a deep sky flat frame, the PSF image should be taken with the same imaging setup and without pointing the telescope too far from the target.

My first attempt using a star near Jupiter did not yield results, as it ended up being too faint and requiring a longer exposure. That, in turn, blurred the Airy disk by its scintillation. For Mars, I targeted a brighter star and had more success with a frames-per-second rate nearing 100. Even so, it is challenging to get it right; in retrospect, I should have lowered the gain further to avoid "blowing out" the Airy disk.

The PSF image taken during the imaging run

So, here are the results! Even with only a fair PSF image processed by Autostakkert, the LSW did a nice job of recovering the details from what turned out to be sub-par seeing. Applying sharpening and denoising to the image resulted in a reasonable, if not admirable, image showing albedo features, the polar cap, and likely cloud features.

To me, this technique of capturing a PSF image as part of the Lucky Imaging session holds great promise. As a final note, here is a Copilot-assisted comparison of how these two techniques help us achieve fantastic planetary images.

Eclipse Prep Notes

Wow - I turn around and suddenly we are down to two weeks to go for the Total Solar Eclipse (TSE)! While I have been trying to read, prepare, and bullet-proof (except for weather - that's the wildcard) for the event there's still things to do on my checklist. Among the things I have checked off my list are the following:

Solar Filter: Glass or Film?

Partial Phase - Aug 21 2017

For the 2017 TSE I purchased a Seymour glass solar filter for my 80mm Vixen. That worked out pretty well, allowing me to capture photos with my Canon t6i with good clarity (click the image at left to enlarge). However, this time around in an effort to capture even more of what promises to be a stunning corona near sunspot maximum, I have purchased a used full-frame Canon body. The solar disk is smaller so I'll need sharp focus to enable enlargement without degradation.

One question in my mind was whether a good quality mylar solar film type  might yield sharper images. I invested in a Baader filter from Aegna Astro and did a side-by side comparison a couple weeks ago. The glass filter gives a slightly better color (yellow-orange) but of course our star is actually closer to white in appearance - which is how it looks with the Baader. While not a ton of difference between them, the Baader may be yielding a slightly crisper rendering of the sunspots. But on the other hand the Seymour is easier to slip on & off the scope. 

Comparing Baader film (white) to Seymour (red)

Filtered Smart Phone

At the HAL meeting the other week I mentioned that a simple, low-tech hack for getting images of the partial phases might be trimming one of the eclipse viewing glasses into a makeshift filter taped to your smart phone. The phone I used for this was an older Motorola (Android) that I recently replaced. I cut around the filter of one of the eyeglasses and taped it to cover the camera lens, very simple and quick. Unfortunately, all my attempts to capture the Sun were wildly overexposed. The phone's camera interface supposedly did allow me to adjust the ISO and camera shutter in "Pro" mode, but I was hard pressed to see any difference. While it didn't work for me, it is still an inexpensive and easy modification to your phone, and some better phones (I am quite frugal in what I'll spend for a phone), like an Apple iPhone or Google Pixel, might allow shutter speeds that will render a proper looking Sun. I will say that it is hard to see the Sun on the camera's screen when pointing it at the Sun, so rigging up some sort of shield might pay dividends if you find your camera can snap solar images.

SETnC Trial

If you've been doing some reading about photographing an eclipse you have probably come across the advice of "don't do it, just enjoy those precious moments!" And there is truth in that statement. At the 2017 event I spent a lot of time looking at the eclipse via my pad through which I was controlling the camera. I still took time during totality to soak in the corona and twilight horizon, but not as much as I might have liked. Part of the issue was that a week before the eclipse I decided that the software I was hoping to automate taking the photos to free me up was a little to unpredictable, so I opted to go the manual route. And to be sure, I treasure those shots I took that day.

This TSE will hopefully be different. I came across the Solar Eclipse Timer and Camera controller application (SETnC) and, although the interface is a little "busy", it seemed quite straightforward and nicely programable for firing off shots during the eclipse. You specify your location and the software identifies the C1 - C4 times and provides an Excel-style grid where you define what exposures to take at specific times during the eclipse.

True to forecasts, Sunday was a cloudless day that was perfect for a full dry run - all the equipment set up as if it were the real thing. The only difference is not taking the filter off during "totality"! 😉I began the session about 12:25 pm, roughly when it will start in Texas the day of the eclipse. The software worked flawlessly, so I feel very comfortable turning over the picture taking to it during totality. I did find that while the camera battery and Celestron Power Tank for the mount lasted throughout the simulated eclipse, my laptop would come close to running out of juice near C2. That was solved by my small Jackery 300 portable battery generator.

I also discovered that the HEM27 mount gradually began to struggle tracking the Sun once it had crossed the meridian. I found to my surprise that doing the meridian flip was not as horrific as I had imagined. The main thing was picking up the laptop and walking it to its new position once the slew was finished, plus re-centering the Sun. If you haven't practiced the meridian flip and are planning to take photos, I'd strongly suggest you practice this at least once in case you find you need to do it.

Tip: Sun Centering & Solar Projection

One of the challenges in Solar observing with a telescope (or binoculars mounted on a tripod) is centering the Sun in the field. One great technique that I learned as a teenager is that by watching the shadow of your telescope/binoculars on the ground and adjusting your instrument to create the smallest possible shadow, the Sun will pop into view. Here is a video I created demonstrating the technique. 

At the March HAL meeting I also mentioned that using Solar Projection was a time-honored technique for sharing a view of the Sun with a group of people. In short, you pop in an eyepiece and direct the sunlight emerging from the eyepiece onto a white surface. You focus until you have a nice, crisp view of the Sun for all to see. This technique also works for binoculars as well, especially suitable if they can be mounted on a tripod. The main caveat is to occasionally give the telescope's optics a break and move it off the Sun for a few minutes. You can see a video of the technique here.

Activities for Maryland

Even if you are not able to get into the totality line, there are some interesting observations you can make with a deep partial eclipse such as what Maryland will experience. First, as you hit maximum eclipse (about 3:21 pm in central Maryland), can you detect any Purkinje Effect? In brief, due to the diminished sunlight, the cones in your eye are used less and the rods begin to come into play. Because the rods are more blue-light sensitive, you may notice that red and green colors lose some of their saturation. See the video below for a fuller explanation:

The second phenomenon to observe is how a thin crescent Sun will impact shadows on the ground. Shadows are normally a bit fuzzy on their edges because the Sun appears as a significant disk in our sky, not a point source. That’s in part why the shadow of one of Jupiter’s moons on its clouds is much sharper than our Moon during an eclipse since the Sun is much smaller as seen from Jupiter. If you have a rod/stick oriented tangential to the crescent, and another perpendicular to it, then the tangential one should have a sharper shadow. Check out this video for more information on this effect.

Whether you are heading out to catch totality or staying put to enjoy the deep partial eclipse, my hope is for clear skies and my #1 tip is to be sure you do your observing safely. With any luck we'll all have some wonderful stories and photos to share at HAL in upcoming meetings!

Registax Heir

 A critical part of the planetary computer assisted video imaging (CAVI) - aka "lucky imaging" - is the sharpening of the stacked and aligned image with wavelets. It can seem like a black art as you push and pull levers to apply various wavelet changes to the image to make it clearer. There is also a huge amount of art here, where one balances between a heavily processed and artificial looking output vs. one that has left details on the table that should be brought out.

Registax has been the de facto freeware for the application of wavelet sharpening for quite some time. However, the last release was in 2010 - a virtual lifetime when it comes to a software product. As a result there have been alternatives emerging that the amateur imager may want to evaluate.

One of the new kids is waveSharp, and part of the attraction is that this is an open source project by Cor Berrevoets, one of the primary forces in the development of Registax. As stated in his announcement on CloudyNights of the availability of waveSharp at the end of last January,  

(Registax) was developed in 2011 only for windows 32bit computers, waveSharp is developed for 64bit computers and multiple operating systems (WIN64, LINUX, MACOS). Therefore this application is only meant to sharpen/enhance images that have been created using other software (alignment/stacking).

You can download the compiled executable for placement on your computer at the project's GitHub repository.

The interface is pretty clean, allowing you to open your image and then use sliders to apply the strength of your wavelets. Unlike Registax we have only 3 sliders, with the first one affecting the small-level detail, the third increasing the contrast on larger scale features. The "Denoise" (smoothing) sliders are not activated by default but must have their checkbox selected. 

The user also can select one of three filters to apply. Gaussian appears to be very similar to Registax, with the changes slow and gradual to create a sharpened image. ZeroGauss is quite strong, with minor adjustments having significant effects. The third choice, Bilateral, is supposed to help avoid the "rind" effect that we often see in planets like Venus and Mars with bright limbs. 

I've been using waveSharp for about two months now and have been very impressed with it. The changes are applied quickly (possibly a reflection of its 64-bit architecture) and the various filters are a nice touch. You can also save your settings and recall them for application to new images. The one thing that is probably not as easy as Registax is correcting RGB alignment (atmospheric distortion) to your image - it appears to be more automated in Registax. 

Change is inevitable but not always positive. While waveSharp is still in its early development it seems to be a suitable heir to the venerable Registax - give it a try!