 |
Stellarium map showing field of view
|
On October 24, 1786, William
Herschel, observing the area of sky around Deneb (the alpha star of the
constellation Cygnus) from Slough,
England, noted
a “faint milky nebulosity scattered over this space, in some places pretty
bright.” The most prominent region was catalogued by his son John
Herschel on August 21, 1829. It was listed in the New General Catalogue as NGC 7000, where it
is described as a "faint, most extremely large, diffuse nebulosity.”
On December 12, 1890, the German astrophotographer Max Wolf noticed
the characteristic shape of the eastern part of the nebula on a long-exposure
photograph, and dubbed it the North America Nebula. In addition, in a paper of
June 10, 1891 he described the region near that nebula as photographed on a 3
hour plate taken on June 1 of that year. However, an accurate position of the Pelican
Nebula had to wait until Sep 7, 1899, where the object was described by
British astronomer Thomas Espin.
In his study of nebulae on the Palomar Sky Survey plates in 1959, American
astronomer Stewart Sharpless realised that the North America
Nebula is part of the same interstellar cloud of ionised hydrogen as the Pelican
Nebula, separated by a dark band of dust, and listed the two nebulae together
in his second list of 313 bright nebulae as Sh2-117. American astronomer Beverly T. Lynds
catalogued the obscuring dust cloud as L935 in her 1962 compilation of dark
nebulae. Dutch radio astronomer Gart
Westerhout also detected the HII region Sh2-117 as a strong radio emitter,
3° across, and it appears as W80 in his 1958 catalogue of radio sources in the
band of the Milky Way.
One of the most famous bright nebulae in the heavens, the
North America Nebula is shaped very much like its namesake. Despite its
relative brightness, its large size and low surface brightness make it
undetectable with the unaided eye except in very dark skies, and even then only
by using special filters to increase the contrast of its line radiation. The North America and Pelican nebulae (IC 5070) are part
of an approximately 100 light year-wide ionised hydrogen region. Their shapes
and apparent separation are due to clouds of obscuring dust lying between us
and them.
What star or stars are responsible for heating the gas has
long been unknown, but recently the 2MASS
infrared telescope, concentrating on the area obscured by dust, has shown that
there is a massive O-type star in the general area of the nebulae, which is the
most likely source of their radiation. Estimates of the distance of the North America and Pelican nebulae vary considerably,
ranging from as little as 1500 light years to as much as 2200 light years.
The image above maps the hydrogen alpha emissions of the
region. This represents “first light” of
my new wide-field imaging system, comprising of a Samyang F2 135mm focal length
lens coupled to a Starlight Xpress SX PRO-694 camera. This gives a 5 x 4 degree field of view.
To carry the lens and camera, I refurbished my old Vixen
GPDX mount, re-greasing it and carefully adjusting the RA and declination worm
drives to try and eliminate the horrendous backlash that had always plagued it.
I also (reluctantly) retired its old and increasingly unreliable Skysensor
control unit, replacing it with a Skysensor EQ5 upgrade kit which allows me to
control the mount via EQASCOM.
Although such a short focal length system probably doesn’t
need auto-guiding, I had a spare guide camera and a Vixen 450mm focal length
guide scope, so I thought I may as well use them.
The set-up is intended to be a portable one, although I will
be setting it up in the same position in my garden. I Araldited three steel washers to the
hard-standing where I would be setting up the tripod, and used the GPDX’s
excellent polar-scope to polar align.
It all seemed to work pretty well first time. PHD reckoned
the polar alignment error was only around 1.5 arc-minutes, with an RMS guiding
accuracy of around 0.6”, way better than needed (and better than my observatory
Avalon mount!), so I think the mount
refurb went pretty well.
At the moment I am manually focussing the lens but at F2, it
is extremely sensitive and I may need some engineered assistance. Spacing between the lens and the CCD camera
is also critical. Fortunately I managed
to find an assembly of various adaptors that connected the lens to the camera
via my old ATIK manual filter wheel that manage to land the Ha focus point
exactly on the “infinity” point of the lens.
Manual focussing required a deft touch but was relatively
easy using the focus indicator on the Astroart camera control module. For
starters, I shot 6 x 300 second exposures in Ha at F2 (via an old 12nm Ha
Astronomik filter) and was very pleased with the sharpness and detail in the
single subs. There was a small amount of flaring around the brighter stars that
I attributed to the filter and some small distortion of the stars in one corner
of the image field, but nothing disastrous.
 |
OIII flares around stars
|
Attempting to use an old Baader filter for OIII imaging was
a complete washout however, as the flaring around all stars made the sub-frames
unusable. That filter will have to be replaced, hence the mono image above.
I used Starnet to remove the stars from the stacked Ha data
as I found the snowstorm of the Milky Way to be distracting. This allowed some
minor selective sharpening and stretching of the nebulosity, although the data
was pretty good even though I only had 30 minutes-worth of subs. I restored the stars by layering a
“de-stretched” and slightly Gaussian-blurred version of the original stack over
the Starnet version in “blend” lighten mode.
References:
https://apod.nasa.gov/apod/ap171201.html
https://en.wikipedia.org/wiki/North_America_Nebula
https://cseligman.com/text/atlas/ngc70.htm
https://cseligman.com/text/atlas/ic50a.htm#ic5070
