Although color CCD chips (with Bayer color mask applied directly on chip pixels) are perfectly suitable for still and video cameras, astronomers use color chips only exceptionally. Majority of amateurs, as well as all professionals, use monochrome CCD chips with color filters. Also cameras on space probes and satellites are equipped with monochrome chips and multiple filters. This is because monochrome CCDs are generally more suitable for astronomy (and other scientific application) for number of reasons:

• First of all, monochrome CCD is perfectly capable to take color image of astronomical objects using color filters. But color CCD can create monochrome image only at the price of much lower QE and lower resolution.

• Color CCD chips have one fixed set of filters without the possibility to exchange them or to completely remove them. Number of applications require unfiltered images taken with maximum QE and color information is not important. Also number of applications require images in precisely defined spectral range. Monochrome chip is perfectly capable to take images with narrow-band filters like Hα, OIII, etc. Professionals prefer standard photometric (U)BVRI filter set to (L)RGB filters, aimed at color imaging, for precision photometry etc.

• Color chips have less QE then monochrome ones. Limiting QE from to around 25 % (compared to around 80 % QE of monochrome chips) by color filters only wastes light in number of applications.

• Color CCD chips do not allow reading of binned images. Binning would mix the colors from individual pixels and the color information would be lost.

• Color CCD chips do not allow so-called Time Delay Integration (or Drift-Scan Integration). Image drifts over CCD vertical lines in this kind of integration. But the image drift is synchronized with image vertical shift. This means image is always exposed on the same pixels—when the image moves to another row, accumulated charge in pixels are also shifted into another row. Image is then read line by line in precisely defined time intervals.

All reasons mentioned above contributed to the decision to include filter wheel directly into the G2CCD camera body. The filter wheel has 5 positions for standard 1.25" filters. G2CCD can be sold with pre-installed filters or the user can install filters according to his or her needs.

G2CCD camera filter wheel (left) Changing filters inside the opened camera head (right)

Filters for (L)RGB imaging

Moravian Instruments offers the high-quality Astronomik filters for LRGB imaging. The L-RGB Type 2c filter set is optimized for CCD chips and individual filters offer near 100 % transmission over its spectral range. The L (luminance) filter covers the 380 nm to 670 nm spectral range and blocks UV and IR part of the spectrum for true color imaging.

But it is also possible to combine the RGB filters with C (clear) filter to use all available QE offered by CCD chip for luminance images—handling proper color balance of astronomical images is more or less a question of personal preferences either way. The C filter has the same thickness as color RGB filters so it is not necessary to refocus the telescope when taking luminance image.

It is also possible to use both L and C filters and fill all five positions on the filter wheel with CLRGB filters.

Astronomik L-RGB Type 2c filter set in the box (left) and in the G2CCD filter wheel (right)

BVRI filters for scientific applications

Professional photometry applications (note the professional photometry is often performed by amateur astronomers) require standard photometric filters. The Johnson-Cousins photometric system, used within science community, defines five spectral ranges for U (Ultra-violet), B (Blue), V (Visible), R (Red), and I (Infra-red) light. The U filter is used only exceptionally, because the QE of many CCDs is very low in UV portion of the spectrum and the filter itself has very low transparency. But the remaining BVRI filters are often used. Any scientifically valuable photometric measurement, be it variable star, supernova or asteroid, should be performed using one of these filters to be comparable to other measurements.

Moravian Instruments offers Schüler—Astrodon professional BVRI filter set, made of SCHOTT glasses, for the use in the G2CCD camera.

Narrow-band filters

Great gaseous nebulae, like “North America”, “Pelican” or “California” emit almost monochromatic light at wavelength 656.3 nm (also known as H-alpha—this light is emitted by hydrogen atoms). Planetary nebulae often emit light in different wavelengths, e.g. OIII (495.9 nm and 500.7 nm) or SII (671.9 nm and 673.0 nm).

Using of narrow-band filters ensures elimination of almost all unwanted light, caused by light pollution, and passes the light in which the target object is visible. Moravian instruments offers number of Astronomik narrow-band filters, optimized for CCD imaging:

• H-alpha (656.3 nm) with 13 nm bandwidth

• H-alpha (656.3 nm) with 6 nm bandwidth

• OIII (495.9 nm and 500.7 nm)

• SII (671.9 nm and 673.0 nm)