General Thoughts on Imaging
CCDs are sensitive on visual,
ultra-violet and infra-red wavelengths. Lets forget about UV by now!
IR, quiet interesting, has a very different focal plane with
refractive optics than visible light (it's all about the
Snellius-stuff, e.g. chromatic aberration), thereby opening two
possibilities:
- Block visual completely (a waste of light since IR is pretty well attenuated through our atmosphere)
- Use pure reflective optics (i.e. Newtonians).
I am still playing with the thought of
the second option for the future. The problem here: The available
telescopes of this kind having a reasonable size and a not too
long focal length (about 70mm aperture and not more than about
500mm focal length) are usually of extremely cheap quality. To get
some decent images the mount of the telescope (and it's tracking) has
to be rather stable, usually then the telescope on a quality mount is
again much bigger (i.e. focal lengths about 2m), making the field of
view rather small, also the f-number ususally shifts to „darker“
(aka slower) values. All these things are supposed to be teleSCOPEs,
optimised for visual applications usually.
Still one option to go for, a „cheap“
scope on a good mount. But remember, focal-reducers are no option
here, these would include „chromatic“ aberrations again.
Thus, a pure reflective telescope (i.e. Newtonian design) with
absolutely no refractive element, the greatest possible aperture and
the shortest possible focal length would be the intrument to go for,
preferrably with a parabolic mirror (most of the cheapoes have
spherical primary mirrors).
Presently I am using two basic setups
for imaging, both including an IR-cut filter. The first setup
is the relatively cheap, computerised refracting telescope ETX-70 by
Meade, having an aperture of 70mm and a focal length of 350mm (making
it f/5, a rather fast setup). The ETX-70, meant to be a beginners
level scope, has quiet inaccurate tracking, thus exposure times are
limited to about half a minute (still recording stars fainter than
14th magnitude!), field of view (FOV) is less than about 1°. The
second setup consists of a webcam and a photographic objective (have
a look the Bellow-Cam MK-II page for details). The setup is tracked
by standart "cheap" hobby material, namely an EQ-2 mount
(usually provided with very very simple telescopes) and the
appropriate right ascension (RA) motor. Compared to the focal length
(mostly 50mm) of the system this mount tracks well enough to expose
for quiet some time. Drawback on this system: the camera fitting best
mechnically (QC4000pro) is not as good as the one used together with
the ETX-70. Advantage though: even faster optics, the 50mm lens, for
example, is f/1.8, an IR-cut filter (not the best, better than
nothing) present in the base of the CCD... Even faster lenses are
available (e.g. on ebay) and, besides the webcam and the motor,
everything in this setup was obtained via ebay for a real bargain
total amount of money.
The alternative setup to the ETX-70 is
a SK8035 (SkyWatcher 80mm 350mm f/4.4 achromatic refractor). The
newest addition to the family is a SK15075 (SkyWatcher 150mm 750mm
f/5 achromatic refractor), which performs really nice; more starry
nights needed!
More on filters? Yes, there still is
something to mention, I would recommend filters of all kinds cutting
out Na- (Sodium) and Hg- (Quicksilver) lines. The visual impression
might be disturbed, the photographic will be fine.
