Saturday, January 4, 2014

Legacy Series, General Thoughs on Imaging

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:

  1. Block visual completely (a waste of light since IR is pretty well attenuated through our atmosphere)
  2. 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.

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