Punching Past Light Pollution, Part 2
Last month I used the Great Orion Nebula, M42, to illustrate that it’s possible to access the brightest Deep Sky objects from the city by using astro-imaging technology. This month I’d like to describe my equipment and approach. There are lots of equipment combinations that can be assembled to achieve the same end; my purpose here isn’t to convince you that I’ve got the best, rather to just describe one system that works for me.
I have been accumulating equipment comprising an “ultraportable” configuration. The simplest definition for this would be a complete imaging system that I can comfortably carry in one backpack, with a weight of 10 kg (22 lbs) or less. It’s a formidable constraint and I have traded off cost, convenience, and image quality to one degree or another. But like a backpacker or serious cyclist I acquire my gear based on its performance to weight ratio. Why?
First, for easy transport. I can carry my full system on Metro North and the subway with no problem. Second, for “grab and go” convenience at home. It is trivial to carry it out back and be up and running within a few minutes. And third, because I can. Advances in cameras, computer power, mounts, and optics give us options that did not exist a few years ago – particularly sensitive CMOS cameras and live stacking software which enable the equivalent of very long exposures without requiring high precision mounts.
So here’s what’s in my backpack for High Line sessions:
Scope: A Borg 55FL astrograph. It’s a fancy modular 200mm focal length telephoto lens with fluorite glass and a dedicated focal reducer that generates pinpoint stars across the field. With the camera attached it weighs just under 3 pounds.
Mount: An iOptron CubePro 8200 mount which I operate in equatorial mode, allowing it to be controlled with the planetarium software on my laptop.
Camera: A ZWO ASI1600MC cooled astro cam.
Battery: A brick-like Tracer LiPo 12V battery. Hanging it under the tripod head adds some helpful additional stability.
Laptop: When I started this hobby in earnest I handpicked the Microsoft Surface Pro 3 as the thinnest and lightest full powered i7 Windows laptop available. Added bonus: it functions in tablet mode with a touch screen that works very well in outreach settings. As you’ll see below, I make full use of all the processing power and memory on this machine.
After assembling the components with the rig pointed north, I start up SharpCap Pro which runs the camera, and Cartes du Ciel, the free planetarium software which can communicate with the mount.
I click on a target in the sky say M31 – and let the GoTo mount swing the scope into the position where it believes M31 to be, knowing that it will only hit the target if I have perfectly aligned the mount with the earth’s axis. But the alignment won’t be perfect and initially the scope will be off. So I take a snapshot of the sky and run it through All Sky Plate Solver, which matches my snapshot against a complete database of sky images from a telescope with the same focal length. To the naked eye it appears that my scope is pointed at empty space, but the sensitive camera records dozens of stars hidden in the light polluted city sky. After 20-60 seconds of hard crunching, the plate solver gets a match and relays the stellar coordinates where my scope is actually pointed to the planetarium software, which can now use this information to redirect the mount to the desired target. Within a few minutes I’ve aligned my rig with a completely invisible target millions of light years away from earth. The mount will compensate for the earth’s rotation and keep the lens pointed right at that target for as long as I want. Sweet!
Now I can adjust my camera settings in SharpCap and take a series of 4 second exposures. The CMOS chip on my astro cam is cooled electronically to -20° C effectively eliminating noise, and it can pick up plenty of faint starlight in 4 seconds. My laptop gets to do some real work when each 24MB image file comes off the camera: SharpCap quickly aligns the stars in the new image against the accumulated stack of images, and then averages its pixel values into the stack, improving the signal to noise ratio of the live image. Within a few minutes a grainy, ghostlike shape resolves into this – a recognizable view of our sister galaxy, Andromeda.
You can view my deep sky images at https://www.flickr.com/ photos/124244349@N07/ or https://www.instagram.com/ mtrastronyc/