ads-medium

High-Resolution Planetary Imaging, Part 2: Designing the Imaging Train

In Part 1, we looked at factors to consider in choosing a telescope and mount for high-resolution planetary imaging. With that major task accomplished, it’s time to design the imaging train.

The imaging train is the series of components that records the image the telescope has provided, including the camera, filters, optics such as a Barlow lens and atmospheric dispersion corrector (ADC), and so on. The camera, telescope optics, and other components must all be chosen to work together.

The author’s imaging train and focusing setup. Credit: Rouzbeh Bidshahri

Camera

The camera is the key to the lucky Imaging technique necessary for getting the highest resolution images. We’re taking a series of so many images that we’re actually shooting a video. I use four different cameras, each with its own advantages. Factors to consider include:

Speed

In lucky imaging, we’re capturing a series of very high-speed frames with shutter speeds of 5 to 50 milliseconds. My typical Jupiter capture is at 120 frames per second. A USB 3.0 camera is a must; USB 2.0 will not transfer data fast enough.

Read Noise

A very important camera characteristic is a low read noise sensor. Because we’re “reading” every time we download a frame, the read noise will really add up with such short exposures. Modern CMOS cameras now have as little noise as 1.0 electron/pixel.

Thermal (dark) noise

Dark noise isn’t a concern with these very short exposures, with dark current not having a chance to accumulate. Dark frame calibration isn’t needed.

Cooling

Unlike with deep sky imaging, cooling isn’t needed with such short exposures. That keeps the cost for these small, simple cameras down to about $300, allowing some imagers to keep a selection of different cameras on hand.

Sensor size

Because the planets are so small, sensor size isn’t an issue unless you’re shooting large lunar landscapes.

Color vs. Mono

Color cameras are easier to use and don’t require a filter wheel. Mono cameras are a lot more work and require a motorized filter wheel and filters, raising the cost. The multiple images will also require de-rotation (more on that later). Mono cameras usually cost a bit more themselves and are more useful for intermediate to advanced users. My best results have been with mono cameras.

Pixel size will be covered in the next part of this series on setting up for an imaging session.

A lunar landscape imaged with the larger sensor of the QHY 5III485 camera. This camera also has good IR response and can be used for IR imaging when seeing conditions are poor. Credit: Rouzbeh Bidshahri

One Shot Color Options

Good candidates are:

  • QHY/ZWO 462MC – New camera, very high IR sensitivity, 2.9-micron pixels
  • ZWO ASI 224MC – Solid performer, larger 3.75-micron pixels
  • QHY 5III 485 – New Camera, large sensor with 2.9-micron pixels, low noise

Monochrome Camera Options

  • ZWO ASI 290MM (not the mini) – One of the best, smaller 2.9-micron pixels, low noise
  • QHY183M – Smallest 2.4-micron pixels, allows imaging without the need for a Barlow. Large chip, very good for lunar imaging, slightly high read noise
  • QHY/ZWO 174MM (not the mini) – Largest pixels at 5.86 microns, large sensor
Popular modern USB 3.0 planetary cameras from ZWO and QHYCCD. Credit: Rouzbeh Bidshahri

Image Amplification

With the exception of long focal length classical Cassegrains and Dall-Kirkhams, most telescope will require further amplification of the image (magnification). This is usually achieved using either a Barlow or a Televue Powermate.

 

(adsbygoogle = window.adsbygoogle || []).push({});

Imagers have had good results with the Televue Powermate. The main difference between a Barlow lens and a a Powermate is that the magnification with the Powermate is almost constant regardless of its distance from the focal plane. This can make things simpler, but it does limit the ability to adjust the final effective f/ratio. Siebert Optics also makes similar lower power devices, e.g., 1.5x, that I’ve used with good results.

Powermates have almost constant magnification as distance from the focal plane changes, with the exception of the 5x model. Credit: Televue.

The other option is a Barlow lens. I would advise against low-cost generic Barlows because of questionable optical quality. I’ve tested the Baader VIP and the Astro-Phyiscs 2-inch Advanced convertible Barlow. I like the Astro-Physics Barlow as it offers more back focus and can be adjusted from 1.8x to 2.5x by altering the distance to the focal plane.

As for determining the best effective focal ratio, we’ll cover that in Part 3 of this series. This can be done by manipulating the Barlow spacing or even switching to an alternative Barlow or Powermate. You can also switch to another camera with a different pixel size.

Astro-Physics 2-inch Advanced Barlow with optional eyepiece holder. Credit: Rouzbeh Bidshahri

Focuser

Perfect focus is critical at very high magnification, so I strongly recommend a good quality focuser.

The internal focuser of an SCT is not ideal. With SCTs, it’s best to add an external Crayford-style focuser.

I tried a very high-precision FLI Atlas but found it to focus too slowly. I found it’s best to have a fast-moving focusing system, either a motorized focuser or adding a motor to an external focuser.

You don’t need a large focuser since the camera and the imaging train for planetary imaging are lightweight. A 2-inch Crayford-style focuser is great because it’s a friction drive system with virtually no backlash.

The focuser needs to be motorized as constant adjustments by hand will not be as accurate or practical. Micron-level precision is needed and that can’t be achieved by hand. Also, at extreme magnification, touching the telescope will result in a lot of vibration and movement.

I’ve had the best results with the 2-inch Feather Touch focuser. I use the Primaluce Sesto Senso focus motor that directly drives the Feather Touch focuser shaft without gears, eliminating possible sources of backlash. I’ve found this combination to be very responsive and accurate.

The Starlight Instruments 2-inch Feather Touch with the Sesto Senso focus motor. Credit: Rouzbeh Bidshahri

Filters

Color Camera Filters:

Most color cameras will need a UV/IR block filter, i.e., a luminance filter. Blocking both UV and IR will render a sharper image. But if you’re looking at IR imaging, some new color cameras that use the Sony IMX462 sensor are very sensitive in IR and can work with IR pass filters.

Mono Camera Filters:

Mono cameras will require using red, green, and blue interference filters to get a color (RGB) image. These filters are different from the common visual colored glass filters.

While I use Baader filters, many others use Astrodon. Most brands will suffice. You won’t need filters larger than 1.25-inch for planetary imaging.

Infrared filters can be useful too. The IR685nm pass is a good one to invest in. I’ve also found the long bandpass RG610 filter useful for red + IR imaging. IR is useful when seeing is poor, also, IR can penetrate some of the clouds of planets like Jupiter and Saturn and reveal some extra details not seen by visible light.

I recommend a 1.25-inch motorized filter wheel. Since these are small and relatively inexpensive, a 7- or 8-position filter wheel is a better investment than a 5-position wheel since you’re bound to add more filters in the future.

8-position 1.25-inch motorized filter wheel with numerous planetary imaging filters. Credit: Rouzbeh Bidshahri

 Atmospheric Dispersion Corrector

The atmosphere acts as a lens, refracting the light from the planet. The lower the planet’s altitude, the greater the refraction. Because different wavelengths of light are refracted at different angles, the colors are misaligned at the focal plane, just as with uncorrected single-element refracting telescopes (non-achromatic) where the misalignment is referred to as chromatic aberration.

An Atmospheric Dispersion Corrector (ADC), correct for this effect with a pair of prisms, taking into account the altitude of the planet.

I strongly advise using an ADC to sharpen up the image.  With a color camera, this is even more important. ZWO make a cost-effective version for $130. I use the Pierro Astro ADC Mark III ($370).

The author’s Pierro Astro ADC Mark III. Note the compact size and single adjustment knob. Credit: Rouzbeh Bidshahri

 

An example of a complete mono camera planetary imaging train. Credit: Rouzbeh Bidshahri

 In Summary

Optimal imaging train design is very important. The components discussed here should cover the requirements for most planetary work.

The next step is setting these up correctly and adjusting the final focal ratio based on the telescope, camera, target, and seeing, which we’ll cover in the next part of this series.

Beginners will probably want to start with a color camera as it eliminates several components. An ADC becomes more critical with a color camera, though. We’ll also look at ADC tuning in the next article.

 

More from the High-Resolution Planetary Imaging Series

Part 1: Telescope and Mount Selection

Part 3: Imaging Session Preparation

Part 4: Imaging Software and Processing

 

(adsbygoogle = window.adsbygoogle || []).push({});

Useful Links:

Author’s gallery with more examples: https://www.astrobin.com/users/Rouzbeh/

QHY Cameras: https://www.qhyccd.com/

ZWO: https://astronomy-imaging-camera.com/

PrimaLuceLab: https://www.primalucelab.us/

Pierro Astro: http://pierro-astro.co.uk/

Televue: www.televue.com

Astrophyiscs: https://www.astro-physics.com/

Feather Touch Focusers: https://www.starlightinstruments.com/

Siebert Optics: https://www.siebertoptics.com/

About Rouzbeh Bidshahri

Rouzbeh Bidshahri is a mechanical engineer with a lifelong passion for astrophotography. He has tested dozens of telescopes ranging from 3 to 20 inches in aperture and has spent several years optimizing systems for very high-resolution planetary imaging in the sub 0.1 arcsecond/pixel range. He has contributed to several institutions such as ALPO (The Association of Lunar and Planetary Observers). His main area of interest has been designing and operating larger setups, and he is currently focusing on high resolution, long exposure photography for both broadband and narrowband deep sky imaging.

Related posts