Narrowband Imaging II: Components

Last time I wrote about reasons for why you might want to give narrowband imaging a try.  To summarize, try it out if you enjoy astrophotography, live under significant light pollution, and enjoy a challenge to your learning skills and budget.

Here's what you might need in addition to what you already have:

• A monchrome CCD camera (includes adapter for prime focus imaging, power supply, software for camera control)
• A laptop computer
• A mount that is capable of carrying the weight of your telescope and camera
• Narrowband filters for Hydrogen-alpha, OIII and SII wavelengths
• A filter wheel

Let's consider each in turn, in each case looking.

CCD cameras come in a bewildering variety.  If you've been using a DSLR with a nice big APS-C multi-megapixel sensor, don't automatically assume bigger is better. A one megapixel CCD is really all you need for imaging.  Smaller sensors don't extend as far off axis where distortions such as field flatness and coma can become apparent. The also make smaller images that are less demanding in terms of memory, download time, and processing time. Smaller CCDs also allow you to use 1.25" filters, which are much less expensive than 36 mm or 2" filters. The downsides of smaller sensors are that their field of view is smaller and target acquisition and composing can be more difficult..

If you can afford it, get a CCD camera with temperature regulation. An example is the ATIK 314L+. An unregulated CCD is less expensive, but you'll probably want to shoot dark frames before and after each imaging session. (Some CCDs are said to generate so little thermal noise that dark frames are unnecessary. My opinion is that dark frames are always worth using.)

Options: Most CCD cameras come with an adapter, either 2" or 1.25" for use in a telescope focuser. This allows you to do prime focus imaging. Most manufaturers either include an AC power supply or sell one separately. Unless you plan on running off a battery, be sure to get a power supply.   Software to control your camera during imaging is usually included with the camera, but ease of use may vary.  If you're not happy with the included software you can opt for a commercial product or choose freeware for this task.

A laptop computer is the central nervous system of imaging.  Your camera control software will run on it, passing instructions to the camera via USB cable. The acquired images will be sent to the laptop for storage by the same cable. After gathering images you may use the laptop for processing.  Processing images can be time comsuming, so if you need to buy a laptop make sure it has enough speed and resources.  Software for Windows laptops is a little more common than for Macs. Avoid any version of Windows released prior to XP. If you want to use a Mac, check first for software supports it, and confirm that software will support the CCD you intend to purchase.

The mount is the most important component of imaging. Add up the weights of your telescope, camera, filter wheel, and autoguiding telescope (if any) to see how heavy a mount you need. It's typical for the weight-carrying ability of mounts to be described for visual observing. Imaging is more demanding.  Generally speaking—and this is very general—any mount that sells for less than $1,000 can carry imaging gear equal to about half of its stated carrying ability. More expensive mounts have carrying weights that are closer to those suited for imaging. If in doubt, consult the Cloudy Nights forums for the opinions of actual users. Even if you don't intend to use autoguiding, I would strongly suggest obtaining a mount that accepts autoguiding commands.

Narrowband filters are a source of continual debate. Are inexpensive ones as good as those that are priced dearly?  Again, Cloudy Nights forums can be a useful guide.

Filter wheels are more than a convenience because they help keep filters clean while in use. If you already have a filter wheel loaded with LRGB filters, you may wish to obtain a new carousel for swapping your NI filters in and out. Consider an 8-filter wheel, so that you can house both narrowband and broadband filters in it, in the event you go to a dark sky location for imaging. Some filter wheels can be controlled by commands passed by USB.  This is a very convenient feature well worth considering.

Imaging Telescope. This isn't all that important. All you need is an achromat with a good focuser. NI does not require an apo, as chromatic abberation is largely eliminated by using narrowband filters.  (Remember to refocus when changing filters!) You may want to upgrade your focuser if it's not two-speed, it slips when carrying the weight of your camera, or it won't keep the camera squared up to the optical axis. You can also use a camera lens as a telescope, but that can get into messy issues of adapters and spacing rings. (I plan on doing this for wide field imaging in 2012.)

My personal NI setup is what might be considered an example of an high entry-level system: If it seems a little pricey, it's because I was sure I would take to NI and be doing it for years.

Camera: SBIG ST-8300M (yes, I went for the megapixels and big sensor size; if you are more frugal than I am, do as I say, not as I do)
Mount: Celestron CGEM (my total imaging weight is about 20 pounds) There are a number of mounts in this price range, and all get good reviews
Narrowband filters: Baader 7nm (not the cheapest, but certainly not the most expensive.  They have worked well for me)
Filter Wheel: SBIG FW8-8300 (USB controlled)

If you really get into NI, you can easily spend many thousands of dollars on premium equipment. You have been warned!


Next Time: Guiding during long exposures.