| You already noticed the technical card [1], but I can describe some of the details that go into this for those unfamiliar with the items on it. 1. The scope they used is roughly equivalent to shooting with an 800mm telephoto lens. But the fact that it's 8" wide means it can let in a lot of light. 2. The camera [2] is a cooled monochrome camera. Sensor heat is a major source of noise, so the idea is to cool the sensor to -10deg (C) to reduce that noise. Shooting in mono allows you shoot each color channel separately, with filters that correspond to the precise wavelengths of light that are dominant in the object you're shooting and ideally minimize wavelengths present in light pollution or the moon. Monochrome also allows you to make use of the full sensor rather than splitting the light up between each channel. These cameras also have other favorable low-light noise properties, like large pixels and deep wells. 3. The mount is an EQ6-R pro (same mount I use!) and this is effectively a tripod that rotates counter to the Earth's spin. Without this, stars would look like curved streaks across the image. Combined with other aspects of the setup, the mount can also point the camera to a specific spot in the sky and keep the object in frame very precisely. 4. The set of filters they used are interesting! Typically, people shoot with RGB (for things like galaxies that use the full spectrum of visible light) or HSO (very narrow slices of the red, yellow, and blue parts of the visible spectrum, better for nebulas composed of gas emitting and reflecting light at specific wavelengths). The image was shot with a combination: a 3nm H-Alpha filter captures that red dusty nebulosity in the image and, for a target like the horsehead nebula, has a really high signal-to-noise ratio. The RGB filters were presumably for the star colors and to incorporate the blue from Alnitak into the image. The processing here was really tasteful in my opinion. It says this was shot from a Bortle-7 location, so that ultra narrow 3nm filter is cutting out a significant amount of light pollution. These are impressive results for such a bright location. 5. They most likely used a secondary camera whose sole purpose is to guide the mount and keep it pointed at the target object. The basic idea is try to put the center of some small star into some pixel. If during a frame that star moves a pixel to the right, it'll send an instruction to the mount to compensate and put it back to its original pixel. The guide camera isn't on the technical card, but they're using PHD2 software for guiding which basically necessitates that. The guide camera could have its own scope, or be integrated into the main scope by stealing a little bit of the light using a prism. 6. Lastly, it looks like most of the editing was done using Pixinsight. This allows each filter to be assigned to various color channels, alignment and averaging of the 93 exposures shot over 10 hours across 3 nights, subtraction of the sensor noise pattern using dark frames, removal of dust/scratches/imperfections from flat frames, and whatever other edits to reduce gradients/noise and color calibration that went into creating the final image. [1] https://www.astrobin.com/w4tjwt/0/ [2] https://astronomy-imaging-camera.com/product/asi294mm-pro/ |
> They most likely used a secondary camera whose sole purpose is to guide the mount and keep it pointed at the target object.
I did use a guide camera with an off-axis guider, I'm not sure why it wasn't in the equipment list. I've added it.
> The RGB filters were presumably for the star colors and to incorporate the blue from Alnitak into the image.
This is primarily an RGB image, so the RGB filters were used for more than the star colors. This is a proper true color image. I could get away with doing that from my location because this target is so bright. The HA filter was used as a luminance/detail layer. That gave me a bunch of detail that my local light pollution would hide, and let me pick up on that really wispy stuff in the upper right :)
> The processing here was really tasteful in my opinion.
Awe shucks, thanks :blush: