Taking Narrowband Images
The most obvious difference between taking narrowband images and taking
regular RGB images is that the required exposure times are much longer. In
practice, exposure times can increase by as much as 10 times over standard RGB
exposures. So, if you are used to taking 3 minute exposures, get ready for
half hour ones now! In the end, the amount of detail that can be achieved
is well worth the extra time invested.
The methods employed are basically the same as normal RGB imaging, with a few
The filters used in narrowband imaging typically have bandpasses in the
range of 3-5nm. The most popular filters are made by Astrodon and Custom Scientific.
Older Custom Scientific narrowband filters had a bandpass of 3nm. Current
filters typically have a bandpass of 4.5nm, while the popular Astrodon H-alpha
filter has a 6nm bandpass. The bandpass was expanded to
allow the use of the popular faster-focal-ratio scopes such as the Takahashi and
TeleVue refractors, etc. At fast focal ratios, narrowband filters shift
off band, meaning they shift away from the wavelength they are designed to
capture. The effect is a significant decrease in sensitivity. A
wider bandpass allows the required emission line to remain within the filter's
highest transmission zone even if it has shifted slightly. For very fast
systems, an even wider bandpass is required. For telescopes faster than
f/4, such as the Takahashi Epsilon astrographs (f/3 to f/3.8) and Celestron
HyperStar-equipped SCTs (f/1.9), a 10nm bandpass filter is required.
Very long exposure times are often used with narrowband filters.
Several hours spent imaging each color is not uncommon. Typical individual
exposures might be in the range of 10-20 minutes, with 5 or 6 exposures of that
duration taken in each filter. Spending a whole night imaging one or two
objects is typical.
Usually exposures are kept the same for each filter, but as with RGB imaging
it is possible to determine the camera's sensitivity at each wavelength being
imaged and adjusting the exposure times accordingly. In general, 1:1:1
exposure times are used.
Finding a guide star can be tricky with narrowband imaging. Many
imagers forego self-guiding CCDs when it comes to narrowband imaging and choose
to use a separate camera on a guidescope instead.
This has several advantages. The biggest is that since the guide CCD is
not looking through the narrowband filter, the stars appear considerably
brighter and finding a suitable guide star is easy. Also, the guidescope
does not need to be aimed at the exact target the primary scope is on, so the
available area from which to choose a guide star is expanded.
The potential drawback to a guidescope is flexure, which causes the main
telescope to not be perfectly aligned with the guidescope during the course of
the exposure. This is potentially a problem when an SCT is used as the
main scope and a small refractor is used as the guidescope. However, a
good number of imagers are using this setup with quite a bit of success, but
caution is advised. If using this setup, be sure to use sturdy hardware to
lock down the guidescope, and try to avoid crossing the meridian while imaging
as this can cause the primary mirror of the SCT to shift.
In practice, it is usually possible to find a suitable guide star using a
self-guiding CCD through a narrowband filter. This is good news for those
who prefer not to shell out extra money for a second CCD and guidescope setup.
However, patience is a virtue when it comes to finding a guide star this way. Some
searching may be required, and a guide exposure of 10 seconds or more is not
unheard of. Fortunately, most of the targets for narrowband imaging are
located in the plane of the Milky Way along with thousands of potential guide
Focusing is done in the same way for narrowband imaging as for regular
imaging, but again the exposures are longer. To get an accurate focus, be
sure the exposure is long enough that the star is very distinct from the
background noise. This is easy to distinguish when looking at the star
profile, for example in the Inspect tab in MaxIm DL. The star profile
should be clearly visible above the background noise.
There tends to be a large difference between the thickness of the standard
clear and RGB filters and typical narrowband filters. This means if the
telescope is focused with the clear filter it will be fairly far out of focus
when a narrowband filter is put in place. There is also some difference
from filter to filter within the narrowband filters, so refocusing for each
filter is often necessary. Once the difference between filters is known it
is easy to input this information into a computerized focuser, another reason
autofocusers are handy to have.
Some RGB filter sets, such as the Astrodon filters, are designed to be parfocal
with H-alpha filters, but there still may be some slight difference between,
say, the H-alpha and the OIII filters.
The HyperStar imaging system offers a revolutionary way to take narrowband
images. The advantage of HyperStar for regular RGB imaging is primarily in
its speed -- faster than f/2! Applying this speed increase to narrowband
images can take the excessively long exposures down to very manageable times.
Instead of spending an entire night capturing just one or two objects, a dozen
targets can be imaged in a single night.
Another huge advantage is that self-guided CCDs can easily detect a guide
star. In four full nights of testing the HyperStar 14 system with
narrowband filters, every target imaged had a suitable guide star visible
without losing ideal placement of the primary target or even having to search
for a guide star. One was on the guide chip every time.
An important note concerning bandpass with the HyperStar and other fast
systems: At fast focal ratios (faster than about f/4), the bandpass of
a filter shifts. So, an H-alpha filter is no longer centered at 656.3nm.
The easiest way to correct for this (and still have a filter that is usable on
multiple systems) is to use a filter with a wider bandpass. This allows
the filter to shift bandpass slightly and still capture the proper wavelength of
light. For systems faster than f/4, including the HyperStar, a bandpass of
10nm is recommended.
See HyperStar 14
Narrowband Images Here
Next, processing narrowband images...