Planetary Imaging Guide
Planetary imaging can be one of the most satisfying areas of astrophotography, partly because the planetary targets are usual visible to the naked eye. So they can be viewed through a telescope in real time and it can take as little as two minutes to image a target.
But this vastly understates the challenge. Planetary imaging takes a lot of practice, requires subtle processing and a great deal of luck. Imaging at such a high magnification means planetary images are incredibly sensitive to atmospheric interference and any temperature fluctuations in the optics.
Fortunately advances in imaging processing, in particular so called "Lucky Imaging" means that it is possible to take images of Jupiter and Saturn from your back garden at the level of detail that would be difficult with even professional telescopes 40 years ago.
Taking a detailed planetary image involves 4 steps:
1) Telescope setup - acclimating your high focal length scope to the ambient temperature and using the right equipment
2) Image capture - taking a short video to collect as many frames as possible of your target planet before it rotates significantly
3) Lucky Imaging - stacking as many high quality "lucky" frames as possible (while ignoring the low quality frames that were impacted by poor seeing conditions) to develop a high resolution image
4) Imaging processing and enhancement - using wavelets to enhance details and derotation algorithms to stack longer periods of data without blurring due to the planet's rotation
In this guide we will discuss each stage of this process in more detail.
1. Telescope Setup
The most essential piece of equipment for planetary imaging is a long focal length telescope.
Because we are trying to obtain fine detail, of course aperture makes a difference and a large reflector has the potential to resolve much more detail than a smaller refractor. But in general a telescope with an aperture over 100mm and focal length of 1500mm can produce very good results.
I think the simplest setup is a manually tracked Dobsonian and a video camera. This is my 300mm Flextube with a ZWO ASI 290MC camera. Admittedly this is probably an excessive telescope for many people. But a 6-8" Dobsonian and a colour camera for under £400 could achieve great images under the right conditions.
Unlike Deep Sky Astrophotography the mount is relatively insignificant here. A simple ALT-AZ mount or even a manually hand-tracked Dobsonian mount is sufficient for planetary imaging where the high frame rate videos are less detrimentally impacted by drift of the telescope.
Because you are imaging at such a high focal length, you need to ensure the telescope has been taken outside at least 1 hour before imaging to acclimate your optics to the ambient temperature. It is also important that your optics are in good colimation and focus.
For more information about setting up a Dobsonian telescope click on the following link
There are two additional pieces of equipment that you will find useful:
1. A barlow lense to magnify the target; and
2. An Atmospheric Dispersion Corrector to reduce the light dispersion effect of the atmosphere
A Barlow Lens is simply a magnifying lens that increases the focal length of a telescope by a magnification factor. For example, this 2.5x Powermate increases the focal length of my 300p Flextube from a native 1,500mm to 3,750mm. This is extremely useful for picking up additional detail without having to store a 4 meter long telescope.
However, since the aperture of your telescope remains unchanged, Barlows also increase the focal ratio of your telescope which makes the optics slower and the image fainter so will reach a maximum level at which magnification continues to be beneficial.
As a general rule of thumb when selecting a Barlow lens, try to keep the focal ratio of your telescope below 20-25 (you may be able to go higher under better seeing conditions).
As the name suggests, an Atmospheric Dispersion Corrector (ADC) corrects for the dispersion effects of the atmosphere.
Particles in the atmosphere act almost like prisms spreading red and blue light apart and distorting higher magnification images.
An ADC contains a pair of small prisms that re-focuses the extreme wavelengths of light to correct for this phenomenon.
They can be relatively fiddly devices to use and require a fair amount of trial and error. But there is no question they can greatly improve image quality - especially when imaging planets at lower than 45 degree altitude where the atmosphere is thicker and the dispersion effect is greater.
The ADC should be added to your image train like this. It can be added either in front of your barlow lens to maintain the barlow's magnification or can be added behind the barlow lens to add additional back focus and increase the magnification if your seeing conditions support higher magnification imaging.
Ensure the spirit level is centered (this will need to be adjusted throughout the imaging sessions as the planet rotates throughout the night relative to the earth's atmosphere).
The final piece of equipment needed is a camera. Compared to deep space astrophotography, cameras suited to planetary imaging tend to be relatively inexpensive. Cooling is unnecessary for such short exposure times. A camera with a small sensor and high framerate is useful to maximise the amount of images you can take in a short space of time.
2. Image Capture
Once your telescope is assembled, aclimated, colimated and focused you can begin imaging. For planetary imaging this means taking a video of your planet.
There are a number of free applications that you can use to control your video camera. The most popular is undoubtedly FireCapture. Although it includes a lot of features that beginners may not need. I find ZWO's own ASI Studio to be elegantly simple - especially when using one of ZWO's own popular CMOS cameras.
Locating your target planet shouldn't be too difficult because planets tend to be so bright they can be easily located in a finder scope. But at high magnification this may still require a bit of manual searching the surrounding area.
You will need to adjust the exposure settings to get the optimal image. I generally use 15ms exposures for Jupiter. 20ms for Saturn and then adjust the gain settings so that details are just visible on the planet. Aim to have the histogram peaking around 75% saturation.
The total length of your video is also very important. Jupiter and Saturn spin with a surprisingly fast 10 earth-hour day length. So over the course of just 15 minutes, 5% of the visible hemisphere of the planet will have rotated. So if your video is too long the planet will rotate and this will smudge details when the individual images within the video are stacked.
In general there is deminimis rotation of Jupiter and Saturn during a 3-4 minute video. But anything longer could result a blurred image. (If you intend to use a monochrome camera with different filters, this means taking a 1-1.5 minute video for each RGB channel)
De-rotation algorithms can be used to combine videos of longer duration (see processing section).
Your goal here should be to take as many 4 minute videos as you can, saving them as .ser files. Over the course of just 20 minutes I regularly see considerable differences in seeing conditions that result in vastly different image quality.
3. "Lucky" Imaging
Throughout the course of your imaging session and even within a single 3 minute video, the seeing conditions are likely to change. This means some frames will be very clear due to momentarily calm atmospherics while other frames may be much more blurry due to atmospheric turbulance.
Lucky Imaging uses software to identify the optimal frames from your video and then stacking only the best data to produce the sharpest image. There are a number of pieces of software that will do this but AutoStakkert is probably the most popular due to its speed and simplicity.
All you need to do is open your video file in AutoStakkert (click on the top left box titled "1 Open") and it will automatically align your frames with the planet centered.
Select "Planet (COG)" and then go to the window showing the image of your planet.
Now you need to place Alignment Points on the planet so that Autostakkert can sample, align and compare the image quality of each frame. Choose an AP size that results in the planet surface being covered with about 25-30 overlapping squares.
On the other window click "2) Analyse"
This provides a graph analysing the overall quality of your individual image frames. You can use this graph to assess how many higher quality images you were able to obtain.
The green curved line on the quality graph shows the frames ordered from high quality to low. This example shows that just under 25% of the frames have a quality estimate of over 50%. If the curve drops in a more shallow way, you might be able to select more frames to stack. If the curve is steeper, you may need to select a lower % of frames.
Based on the number of frames above your quality threshold, adjust the "Frame percentage to stack" box.
I usually also select "Sharpened" which adds a deconvolution algorithm to the final image. Other than that, I would recommend using the default settings.
Once you are happy with the settings click "Stack". This may take a bit of time based on your computer speed and the size of your video frame.
Less is More
This image shows a comparison of the same image with the same processing, using the best 15%, 45% and 75% of the frames.
As you can see from this comparison, adding more frames often results in a lower quality image. To repeat, the goal of lucky imaging is not to obtain the most amount of data; it is to select the most amount of the best data.
You will need to look at the quality graph and possibly experiment with different sample rates to obtain the best image.
The output image is a stack of the best or "lucky" images you obtained from your video.
4. Image Processing
By now you should have a pretty good image that should be significantly better than the individual frames in your initial video.
Occasionally at this point people can be quite disappointing with their image. But dont fear, even for what appears to be a blurry image, there could be good quality data hidden below the surface that just needs to be enhanced.
The next step of the image processing is to apply "Wavelets" to enhance the various details on the planet. Again there a number of applications available on the internet to assist with this. But Registax offers a superb tool that is free, simple and extremely powerful.
When you open Registax, you just need to click "Select" and open your image output from AutoStakkert. Select "Stretch Intensity-levels"
The beauty of Registax is the simple design of its Wavelet sliders located on the left.
To many people (me included) Wavelets appear to be be some sort of magical wizardry. Essentially they attempt to identify patterns in your data and by increasing the slider it enhances those features over others. If you right click on the "preview" button next to each slider it will show you the features each wavelet will attempt to enhance.
If you are interested in understanding more about Wavelets I would recommend watching this superb presentation Warren Keller produced for the Astro Imaging Channel:
https://www.youtube.com/watch?v=gDJKnRVxhXw
I generally find most planetary detail is located in the 2nd and 3rd layer by increasing the slider upto 50%. I find other layers tend to add as much noise as they do detail which can be detrimental as we shall see,
These wavelets are very powerful. But remember: with great power comes great responsibility...
It is very tempting to push the sliders too far and overly process planetary images. This is most commonly seen when an image starts to look like a cartoon drawing of a planet as you can see here.
In this example I have intentionally pushed the wavelet settings too far.
The definition of the banding on the planet is definitely sharper. However, the wavelets have added considerable noise to the image and more importantly, this is not what Jupiter actually looks like!
Take a look at the images of Jupiter taken from Juno and you will see the bands on the planet are not solid blocks but rather subtle circles and swirls of marbled colours.
https://www.nasa.gov/mission_pages/juno/images/index.html
Obviously this level of detail is not remotely realistic from a ground based telescope but its worth noting what the target should be before processing.
I believe the goal of astrophotography should always be to create a realistic record of an astronomical object. Anything beyond this is art (which has it's place too, but is a different field). So I would only use processing the improves real detail and increases the signal to noise ratio to the image: but not artificial clarity.
The best advice I have heard is sharpen the image until you think the details have become slightly too "blocky". Then dial it back a bit.
Then dial it back a bit more because you've probably still gone too far.
I would also suggest using the RGB Balance function to adjust the colour levels.
This is an example of an image I would consider to have the right level of processing. Some of the important details have been highlighted but it has not become a blocky cartoon drawing of Jupiter. The image has retained subtle marbling between the layers.
The final step of planetary image processing is to combine multiple stacked images together.
As discussed earlier, because the gas giants rotate so fast, its not possible to simply combine 10 or 20 images together because the details will have moved as the planet rotates. So we need to somehow derotate the image of the planet.
Fortunately there is free software available that can do this, WinJupos.
Once all of your 4 minute videos have been stacked down to a single image, open them up in Winjupos. Open the application, select the planet you are processing, and then click on "Recording", "Open Image".
Go to the "Adj." tab. Press F11 to let WinJupos automatically align the planet. Then manually adjust the settings until your planet is well aligned vs the planet outline and equator of the WinJupos simulation.
Once all of your frames have been recorded, you are now ready to derotate the individual images and combine them.
Click on "Tools" and then "Derotate".
You are presented with a number of options including the relative weightings of each frame. It makes sense here to more heavily weight the higher quality frames you have.
You may also want to adjust the quadratic pixel size to increase the amount of black space surrounding the planet.
Once you are satisfied with the setting, click "Compile image"
The output you receive is a derotated stack of the individual images you have produced. From here you can export the image to your favorite post-processing application to finalise.
To recap, there are 4 key steps to creating a planetary image.
First you take a video of the planet to obtain several thousand images but for a short enough duration to avoid any rotation of the planet.
Next use AutoStakkert to perform lucky imaging to stack only the best frames from your video.
Use Registax to balance the colour levels and apply wavelets being careful not to overly process the image.
Repeat this process for each video of the planet and then finally, derotate the individual images to produce a final image of the planet that can go into post processing.
This really is an extreme form of astrophotography where the goal is regularly to combine large amounts of data into 1 single image. If you derotate 10 images of a planet you could be looking at converting over 100,000 initial frames into 1 final image!
It is worth the effort when you finally produce a stunning image of a planet. Just make sure you have enough hard drive space for the process.