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Resolution and image size

The resolution of long-exposure images is always limited by seeing. In our

circumstances, seeing on good nights is just under 2 as. When camera systems are designed,

it is important that the pixel size matches with the expected seeing. It is called the Nyquist

sampling criterion (actually a general theorem in mathematical statistics): the size of a pixel

should be one-half of the smallest detail (i.e. half of seeing, about 1 as). Using larger pixels

is undersampling, which loses detail; smaller pixels mean oversampling, which increases

noise without additional detail. In our setup, the pixel size is adjusted to 0.79 as (remember

this number!), which is the correct match at best seeing conditions.

Given this pixel size and the 2184 × 1472 image size, the field is 20 am × 30 am size;

that means the half-moon just fits on it. Objects that are larger than this (like the

Andromeda Galaxy, or the Pleiades) cannot be imaged with this system. A few objects can

be pictured using a mosaic of a few panels. This technique however is quite complicated,

and it is difficult to achieve even illumination on each panel of a mosaic. Large mosaics

usually involve an impossibly large amount of exposure time.

Narrowband filters

The spectrum of many nebulae is an emission spectrum with only a few bright spectral

lines. For these, it is beneficial to take images with narrowband filters, which filter out all

other colors and so enhance the nebula with all its usually filamentary structure; in galaxies

a narrowband H

filter enhances the red glow of molecular clouds in the spiral arms.

Narrowband imaging requires long exposure times as the filter blocks most of the light.

It is sensitive to dark current noise, so the chip should be cooled as much as possible. Also,

stars colors become impossibly unnatural. To counter that, we take regular broadband

(RGB) images of the same object as well, process them separately, then blend the two

images in Photoshop as two layers. This process follows the same principles as regular

image processing, but requires considerably more care.

Another great benefit from narrowband imaging is that it strongly cuts background light

relative to the nebula.

Three narrowband filters are installed in the camera: a red H

filter, the color matching

the spectral line where most nebulae give out most of their light; a green [OIII] filter and a

red [SII] filter. The latter are centered on spectral lines where many nebulae have emission

lines; however there is usually little light in these, so a very long exposure is required with

these. As a “ballpark number” for some nebula, one might expect to take a ½ hour with the

filter, 2-3 hours with the [OIII], and 6-10 hours with the [SII] filter. Of course, we

cannot do this as an introductory student project.

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