Lunar Photography

Though the moon exhibits subtle colors, in particular the northern regions, many astro-photographers prefer to use monochrome cameras in conjunction with an IR-pass filter in order to obtain higher resolution and sharpness benefitting from the high near-infrared response of modern astro-cameras.

The Moon is about a quarter the diameter of Earth but exhibits extreme natural and geological dimensions. Some peaks of the rugged mountain chain Montes Apenninus rise as high as 5 kilometers (3 miles) casting long shadows. Crater Copernicus is 3.8 kilometers (2.36 miles) deep as are many more, wide or small. Rupes Altai is an escarpment located south of Mare Nectaris stretching over 420 kilometers (260 miles). Most craters contain a dozen or more craterlets and have central peaks casting shadows. Under the right illumination and timing stunning pictures of such features are possible -- the exciting side of lunar photography.

Thanks to the moon's libration (wobbling effect), 59% of its surface is observable from Earth. The maximum libration angle is: 1.54° (equator to ecliptic inclination) + 5.15° (ecliptic to orbital plane inclination) = 6.69°. Libration occurs in both lunar latitude and longitude and at times brings far side features or parts of them into view, such as the entire Mare Humboldtianum at the northeastern edge above the better known Mare Crisium.

Since the lunar orbit around Earth is of elliptical shape its distance to Earth changes strictly speaking every second (orbital speed = 1.022km/sec). When reaching its closest orbit location the distance is around 362,600 kilometers (225,310 miles) while the farest point is at around 405,400 kilometers (251,903 miles), after all a difference of 42,800 kilometers (26,595 miles), or over 12 times the lunar diameter. Consequently, when nearest, the Moon is apparently larger therefore better positioned for imaging, however, we may be splitting hairs since seeing is the more dominant factor influencing the resolution and quality of our images.

Telescope

In the interest of good resolution, a telescope with at least 5-inches (127mm) aperture, Newtonian or Cassegrain, will be fine. A Newtonian is most affordable and the 'fast' with typically F5 focal ratio sufficing with short exposure times, but is quite heavy therefore requiring a sturdy mount. More costly than Newtonians, Cassegrains come with a focal ratio of F10 and with a long native focal length. A variation, the Maktsutov Cassegrain, has a slower focal ratio of around F12 to F15 and is said to produce sharp images. Cassegrains have a shorter tube and weigh less than comparable Newtonians. Owing to its design, the aperture of reflectors is obstructed by a secondary mirror. Consequently, a 6-inch scope is a little over true 5 inches. If you choose an 8-inch scope you will get true 6.5 to 7 inches depending on the size of the secondary mirror.

Sky-Watcher 150PDS

Ø150mm 6-inch F5 Newtonian

Please be aware that large telescopes do not only collect more light but also way more air turbulences as compared with smaller telescopes. With 10 and more inches aperture you depend a lot on good seeing, but when seeing is really good the images will knock you out. Telescopes from 5 to 8 inches, too, can produce remarkable images. Refractors can as well be used but are quite expensive with growing aperture. So-called "field flatteners" (for refractors) and "coma correctors" (for Newtonians) are not essential because the sensor of a 'planetary' camera is rather small. Besides some budget, choosing a telescope requires a great deal of diplomacy and compromising.

Tracking Mount

The most important gear is an electronic equatorial mount capable of accurate tracking. The heavier the telescope the sturdier a mount is required. High magnifications typically required for lunar imaging call for accurate tracking. The mount should be specified for a payload of, say, 1.3 times the weight of your complete imaging gear. Autoguiding is not necessary.

Camera

ZWO ASI290MM

Monochrome camera with a 5.6 x 3.2mm sensor comprising 1936 x 1096 pixels, same as the ASI462MC color camera which is highly sensitive to infrared.

A "planetary" CMOS astro-camera records uncompressed videos containing thousands of frames from which a certain percentage (typically the best 10% to 20%) are used for stacking. Available in color and monochrome, planetary cameras sport sensors up to full HD resolution (1920 x 1080 pixels), resulting in a narrow field of view which is desirable for imaging the Moon and the planets. Color cameras make it easy to image painlessly within short time. Monochrome cameras offer higher resolution and sharpness. In order to produce a color image with a monochrome camera a set of color filters is required significantly extending imaging and post-processing time. A filter set including wheel costs about the same as a color camera in that ownership of both a color and a monochrome camera is most efficient. Cameras containing the Sony IMX290/462 sensor are ideal for our purpose.

Filters

Infrared Filters

IR-Cut filter (left) for color imaging and IR-Pass filter for near-infrared imaging on a filter wheel.

Two types of filters are essentially required because a CMOS camera is sensitive to both visual and near infrared wavelengths. If used without filters images will look notably smeared with pastel colors. An infrared cut filter passes visual light only. It is indispensible for color photography. An infrared pass filter blocks visual light and can be used with both color and monochrome cameras while images are usually converted to gray scale because infrared is colorless. When the atmosphere is turbulent, an IR-pass filter can help reduce the effect of blurring. IR-pass filters are available with kick-off wavelengths between about 600nm and 850nm. With increasing wavelength the camera's response drops in that filters with long wavelengths require longer exposure times. CMOS cameras containing the Sony IMX462 sensor are highly sensitive to infrared, therefore hardly compromising exposure time when using IR-pass filters up to 750nm. When frequently imaging in both wavelength domains, a (mechanical) filter wheel can help reduce adrenaline flow.

Computer

Consider a laptop with SSD disk and powered USB-3.x port since mechanical hard disks are often too slow resulting in intermediate buffering of frames in memory, then saving the buffered frames to hard disk, a process which can significantly extend the time needed for writing a video file. Since you will be saving several Gigabytes per video file (ex.: 6000 frames, 8-bit depth = 12GB) the capacity of the hard disk should be generous, at least, say, 512 Gigabytes.

Software

• Capture: Please look for SharpCap or FireCapture or applications offered by camera manufacturers.
• Stacking: Please look for Autostakkert!3 which is the optimal choice though many prefer Registax6.
• Post-processing: GIMP, Photoshop or similar image processing applications.
• Mosaics: For image stitching, among other choices, MS-ICE is an accurate, simple-to-use application.

Other

Do not forget to prepare a table and a chair plus a power supply for both the laptop (and the tracking mount). An LED light will help avoid hitting the wrong keyboard keys. Outside there is no roof to hit or go through.

If you cannot rule out giving up the hobby sooner or later, a low-cost entry equipment may be a reasonable start. A lot of people use a Sky-Watcher Mak127 (5-inch) on a AZ-GTi altaz mount (sold bundled) with a ZWO ASI224MC or QHY224C color camera. It is a low end camera but its sensitivity and image quality are everything else but low. The Mak127 is also great for visual observations in case your kids take over.

Provided you intend to enjoy the hobby for a long time you may consider an 8-inch Cassegrain on a solid mount with a ZWO ASI290MM/MC or QHY290M/C camera. Assuming affordability and endless motivation, go for a 11 or 14-inch Cassegrain telescope (plus a 6-inch Newtonian for times when seeing is bad and the night still long).

Affordable equatorial mounts are offered by manufacturers such as Sky-Watcher, Celestron and Orion. Make sure the mount can handle the weight of the telescope plus imaging gear.

Whether you choose a Cassegrain or a Newtonian, you may wish to increase the focal length (magnification) using a 2x or 3x barlow lens of high quality since the barlow is often the weakest element in an optical train. Too much magnification is not recommended.

8-inch Cassegrain

The Celestron C8 sports 8 inches aperture with a native focal length of 2000mm.

Since the camera's infrared response is usually lower than its response to visual light (vice-versa for the IMX462 sensor) the IR-pass filter cut-off wavelength should not be too long. Small aperture telescopes are less prone to air turbulences in that about 640nm are sufficient. For larger telescopes, say, 10 inches and more, 740nm will be more suitable. Filters are usually not parfocal, meaning that the telescope needs to be refocused when swapping filters.

Lunar imaging does not require a high-end laptop. With its fast SSD and USB, the Thinkpad E590 laptop, for instance, is ideal for video capture, but not water-proof, so, place your double-star drink away from the laptop.

The aim is to produce videos at shortest possible exposure time and fast frame rate in particular when seeing is poor and persistently rolling in clouds are disturbing. Equally important is low noise. Noise can be reduced by choosing the lowest feasable gain and a large number of frames to stack. Naturally, imaging the Moon at good seeing is the best guarantor of success.

Infrared pass filters compensate a lot for poor seeing but need longer exposure times as the camera's infrared response is lower than its color response. In case of the ASI290MM, the exposure time with an IR640nm pass filter is about half that of an IR740nm filter -- a significant benefit when seeing is not too bad.

Quality IR-cut filters pass more than 95% of the light hence hardly sacrificing exposure time. It is a dance on a rope. Either you record in visual light resulting in the shortest possible exposure times and fastest frame rates, or in infrared with longer exposures and slower frame rates, but less prone to poor seeing. When your time and clouds allow, image in both domains and select the keepers after post-processing.

ASI462MC/QHY5III462C

Response of the ASI462MC/QHY5III462C color cameras. The response in near infrared is higher than in color, a unique characteristic which helps reduce exposure times for infrared imaging.

ASI290MM/QHY5III290M

Response of the ASI290MM/QHY5III290M monochrome cameras. The response in near infrared drops past 600nm and is around 50% at 850nm. However, the response is still high with typical IR-pass filters of 640 to 740nm.

The curves should be taken with a grain of salt as for most cameras the accurate relative response, also specified as quantum efficiency, QE, is unconfirmed.

News just in: In November 2020, QHYCCD has launched its QHY5III485C, an 8.3 megapixels 4K color camera with 3840×2160 pixels coring the 7.2 x 4.5mm dimensioned Sony IMX485 sensor. It is basically QHY's QHY5III290C but with four times the chip area resulting in twice the field of view. In 2x2 binning mode the cell size doubles to 5.8µm while still providing HD resolution.

With Baader UV/IR-Cut Filter

With Sightron IR640nm Pass Filter

With Astronomik IR742nm Pass Filter

Color Spaces

The keys to brilliant images next to good seeing are short exposure time and a consequently fast frame rate (fps). The shorter the exposure the faster the achievable frame rate. The frame rate depends on various factors.

• Monochrome cameras provide 8-bit or 16-bit Mono 'color spaces' where 8-bit yields the faster frame rate, about twice as fast as 16-bit. Color cameras provide '8-bit and 16-bit Raw', 'RGB24', and '8-bit Mono'. 8-bit Raw yields twice the frame rate of 8-bit Mono.
• The camera can use either a 10-bit ADC (high speed mode ON) or 12-bit ADC (high speed mode OFF). The 10-bit ADC provides 1024 brightness levels (faster frame rate) while the 12-bit ADC provides 4096 levels hence the better contrast and detail but at about half that of the 10-bit frame rate. There is hardly any difference in frame rate when the exposure time exceeds 10ms (as found with an ASI290MM). For the moon and planets 8-bit depth with 10-bit ADC is basically sufficient, but note that the 12-bit ADC can help reduce read noise and increase contrast.
• Reduction of the image size (ROI = Region of Interest) too results in faster frame rates. While for the Moon the maximum size is desirable, planets do not require that much space. Often an ROI of 320 x 240 pixels is quite sufficient for planets. IMX290 sensor based cameras sport a frame rate of 170fps at maximum ROI and can exceed 300fps with 640 x 480 pixels or 700fps with 320 x 240 pixels through the camera's 10-bit ADC.

NOTE: 16-bit mono and 16-bit Raw provide 65,536 brightness values, however, the camera's 12-bit ADC outputs 4,096 maximum. Therefore, 16-bit color spaces can safely be ignored.

File Formats

Select the most robust...

All cameras can save videos as *.AVI or *.SER. While AVI can be played on numerous applications, SER is more robust and less error-prone as specifically concepted for astronomy image capture. The choice of SER is recommended and fully compatible with the Autostakkert!3 stacking software.

Post Processing

In Photoshop...

There are certainly more professional procedures while the quality of unprocessed images vary. The following table contains the basic steps employed by the author. Please remember, "less is more".

Hint: Purposely under-expose the SER video a little bit to avoid saturation during processing. Then, in Photoshop,

 

Menu Item	Image	Description
Image/Adjustment/Curves		Raise the Curve, crop away stacking artefacts and duplicate the layer.
Filter/Other/High Pass		Apply high pass filter (100%) and blend the layers with Soft Light. This adds sharpness, contrast and lets the image look deeper. You can adjust the opacity of the layer if need be. Next, select Layer/Flatten Image.
Filter/Blur/Gaussian Blur		Apply a Gaussian Blur (Radius 0.5 pixels) to smooth noise.
Filter/Sharpen/Unsharp Mask		If not sharp enough apply a light Unsharp Mask (radius 2-5 pixels), and another Gaussian Blur (do not overdo sharpen).
Filter/Noise/Reduce Noise		In case of a color image (not for monochrome), apply Noise Reduction and disable all adjustments except Color Noise Reduction.
Image/Adjustments/'Hue/Saturation'		If you wish to create a mineral moon, increase Saturation in several small steps of say 20 or 30 each until colors emerge to your liking.
Image/Mode		Autostakkert!3 saves file in RGB color 16-bit format. Reduce to 8-Bits/Channel when the SER video is in 8-bit. Select Grayscale if you are processing monochrome or infrared images. This will significantly reduce file size.