The Gemini telescope consists of a 51 cm (20 in) diameter f/6.8 Cassegrain optical tube assembly on an equatorial fork mount. The imaging camera is a high-QE back-illuminated sensor cooled to -30 C. The sensor has 2048 x 2048 (4M) 13.5-micron pixels. The field of view is 28 arcmin x 28 arcmin, with a plate scale 0.82" per pixel.
There is a 12-position filter wheel with 50mm diameter glass filters, each 3 mm thick. The filters are: N = none , B = blue, V = visible, R = red, Sloan g’ and r’, a W (650+ nm) long pass filter, narrowband (5 nm) filters centered at H alpha, OIII, and SII, and two grisms (filter codes 3 6). All filters are Astrodon research grade filters whose bandpass characteristics are given on the Astrodon website..
Choosing exposure time.
The choice of exposure time depends on whether the object is point-like (planets, stars,asteroids) or extended (comets, nebulae, galaxies). For point-like objects, the online exposure calculator should be used. For most extended objects, it is unlikely that the image will be overexposed even for long exposure times, so the appropriate exposure time is determined more by the science objectives. For relatively bright extended objects, like M51 and M27, a single 60 sec exposure is usually adequate. For fainter objects, multiple 60-sec images can be taken and median-averaged later in Maxim. Alternatively, longer single exposures can be used, but this is risky, since the telescope may not track accurately over long exposures (a few minutes or longer).
The exposure time also depends on filter choice. Here's an example of a single 60 sec exposure of the bright galaxy M51 using a luminance [clear] filter.
The color filters (B, V, R) only allow ⅓ the light that the N filter does, so a single 60-sec exposure may not be adequate. Here's an image of M51 made by combining NBVR filters to form a color image in Maxim/DL.
An example of an overexposed image is shown below: this is Jupiter and its Galilean moons at 10 sec exposure. The features on Jupiter's surface cannot be seen because it is quite overexposed. The Galilean moons are also overexposed, but are clearly seen.
If one wants to see the surface features of planets, a very short exposure time is needed. The image below is a 0.05 sec exposure of Jupiter using an oxygen narrowband filter.
and a 0.05 sec blue filter exposure of Saturn.
Astronomical objects can look quite different in different colors. A good example is the Crab Nebula (Messier 1), a supernova remnant whose progenitor star explored 900 years ago. Here’s a 3-color image:
Here’s the same object in the H-alpha filter, which isolates the hot hydrogen filaments:
Here's another example, the spiral galaxy NGC2903. Here's an image in visual (G) filter.
This is the same galaxy using a narrowband (H alpha) filter.
The bright dots on the spiral arms are star-formation regions which emit most strongly in the hydrogen line (H alpha) line.
A tri-color image of the spiral galaxy Messier 63 using the H alpha filter as the R filter is shown below. Using an H-alpha filter emphasizes emission at the H alpha line (656.3 nm), and shows the giant HII regions in the spiral arms, that are most luminous in H alpha.
Color imaging is done by combining several monochrome images taken with different filters, typically R (red), G (green), B (blue) and L ('luminance'). The latter is a clear filter, and brings out fine detail in the images (because it is more sensitive than the bandpass RGB filters. Below is an example showing the separate RGB images of the planetary nebula M27 and the composite color image. Image combining can by easily done in the program Maxim/DL.