With our goal of recording the finest details we can, near the theoretical limit of our telescope, we need to eliminate any factors we can that might degrade our images. A successful imaging session requires planning and proper preparation of both the telescope and the imaging site. It’s also very important to match the optical layout to the camera’s sensor.
The temperature of the telescope and its optics needs to be as close as possible to the ambient temperature. This is usually done by leaving it outside for several hours to cool down to ambient temperature (and avoiding sunlight). If the temperature inside the telescope is warmer than the air outside it, the warm air will rise inside the tube as cooler air enters it, causing internal tube eddies and turbulence that will blur the image.
The telescope needs to be perfectly collimated. Some imagers check regularly, sometimes before every session. Many use a defocused star to get a concentric doughnut for checking collimation. I have had good success using the free software, Metaguide, to fine tune the collimation. Skywave Wavefront Analyzer is a recent release that promises perfect collimation. Whatever method you choose, near-perfect collimation is required to capture high resolution details.
The concentric rings of a defocused star are an indication of good collimation on an SCT.
Another test is a highly magnified, stacked image of an on-axis star, where a symmetrical Airy disc and a lack of coma indicate good collimation.
Because atmosphere motion (seeing) can degrade your image, you should shoot through the least amount of air you can, which is when the is planet highest in the sky. If your shoot for an hour or more on either side of when the planet transits the meridian, you can look at the data later for windows where the atmosphere was the most stable. The atmosphere is also usually calm before sunrise though each observing site can have its own characteristics, which is only learned from experience at the site.
Avoid shooting over a house or chimney where rising heat will blur the image, an effect known as local seeing. Shooting from a balcony where heat is rising off the side of the building will also interfere with the incoming light from the planet.
Similarly, avoid shooting over warm asphalt, concreate, or walls that accumulate heat during the day and radiate it during the night, causing warm air to rise in front of the telescope. This effect, known as ground seeing, can be avoided by over cool grass or other areas that don’t absorb as much heat during the day.
- Atmospheric “seeing” is caused by turbulence and other geothermal phenomena in the upper atmosphere and is the main culprit in blurring the image. Forecasts are often not reliable and the seeing can change from minute to minute. Again, shoot for long periods of time in the hope of finding calm windows.
- The Jetstream, a narrowband of fast-moving wind in the upper atmosphere, can wreak havoc on planetary imaging. In my location in the northern hemisphere, I’ve found the Jetstream more active in winter, so I’ve had better results during summer, but this varies with location. Windy.com can show you the Jetstream location and speed at 10km altitude.
- Very good transparency is often inversely associated with seeing. The air is often more stable when it is warm, humid, and hazy.
- While shorter (bluer) wavelengths have potential for higher resolution, they are also degraded more by seeing effects. Green and red usually yield better resolution as they are less affected by seeing. Near infrared (IR685, for example) can be reserved for nights of poor seeing.
- While location can be very important, that’s not always a factor that can be changed. My advice is to keep trying on as many nights you can, raising the probably of finding a night with better seeing.
- You will have many nights when the seeing is extremely poor. My advice for those nights is save your energy for another night and get some sleep!
Matching the Optics and Camera: Focal Ratio
Great results require having the right focal length and f/ratio. You can change the focal length (and f/ratio) by using Barlow lenses or Powermates. Longer focal lengths will render a larger image and possibly more resolution; however, you can’t simply increase the focal length indefinitely.
Maximum possible resolution is determined by the optic’s aperture. For imaging, we can derive an optimal image recording f/ratio based on the size of your camera pixels.
We won’t get into the math behind the theory here, but as a rule of thumb, most imagers need to aim for 4x to 6x their pixel size for the final effective f/ratio:
Optimal Image Recording f/ratio = 4x to 6x camera pixel size (in microns)
For example, with the ASI224mc with 3.75micron pixels, the f/ratio should be in the range of 4x to 6x times 3.75 microns, which is f/15 to f/22.
With better seeing and more skill, the imager can push the system with higher focal ratios. I’ve been able to push it to 7x at times with my setup, but those nights are rare.
For true high-resolution work, we aim for resolution in the range of 0.10 to 0.15 arcseconds per pixel (yes, you read that right!).
Too low an f/ratio could cause a loss of resolution. Too much amplification (longer f/ratio) and you are oversampling, which means you’re not gaining any extra resolution. This can cause a “soft” image with a reduced signal-to-noise ratio.
With poor seeing, you need to stay in the lower part of the range since fine resolution is not achievable. A smaller, brighter image will yield a higher frame rate, which helps. So don’t consider your focal ratio to be fixed. You have to be able to alter the f/ratio either by changing the Barlow spacing , having a few Barlows and/or Powermates on hand, or having multiple cameras with different pixel sizes to be able to manipulate the system based on atmospheric conditions on that night.
The Atmospheric Dispersion Corrector needs to be tuned for the altitude of the planet.
- The dual prisms should be horizonal with relation to the horizon. A bubble level helps here.
- Place the ADC after the Barlow or Powermate; the ADC will work better at the higher f/ratio.
- Most ADCs require a minimum distance to the focal plane, so it’s best not to have it directly adjacent to the camera.
- Firecapture has a tuner aid that will help with color cameras
- With mono cameras, I use an inexpensive Wratten #47 (violet filter) to tune the ADC by aligning the violet separately from the infrared (IR). This method works well, allowing you to tune the angle of the prisms until the separate UV and IR images overlap. You will need an extra filter wheel slot for this filter.
Unlike with deep sky imaging, you need to be very involved with focusing during data capture when seeking very high resolution. This can take some training to be able to differentiate almost perfect focus from perfect focus. This is easier said than done, as the image is blurred and constantly moving.
- The use of a focus mask is not advisable, nor is slewing to a bright star to focus.
- With a fast computer-controlled motor you can “jog” in and out of focus rapidly to determine the best focus point visually on the fly. This requires practice and skill, but it can be mastered. A fast-moving focuser will make this task easier.
- Don’t “focus and forget”, the focus needs tweaking as often as every 5 or 10 minutes.
- Many filters require an offset for each color. When motorized, you can work these out in advance and apply them automatically with every filter change.
Seen here is Saturn on a relatively good night. Note the blurry image, making it difficult to distinguish the best focus point. Credit: Rouzbeh Bidshahri
With careful session planning and correct setup of the telescope and imaging train, we are better equipped to take advantage of those fleeting moments when the atmosphere happens to be calm.
At times, planetary imaging is a bit like fishing; you never know what you are going to catch that night. On some nights, you will be amazed how clear and calm the image looks on your screen, and that’s when you can bag those prize-winning shots!
In the next section, Part 4 of 4: Planetary Imaging Software and Processing, we will take a brief look at the software required to both capture data and process it.
More from the High-Resolution Planetary Imaging Series
Author’s gallery with more example: https://www.astrobin.com/users/Rouzbeh/
Collimation with Skywave: https://www.innovationsforesight.com/