I have two CCDs. My main imaging CCD is the Starlight Xpress SXV M7C and my guiding camera is a Meade DSI-C. I actually bought the DSI-C, which comes with Meade's Autostar Suite and includes an adequate planetarium package and the Envisage CCD control programme.

Envisage and the DSI-C are a well integrated package which serves as a great introduction into CCD astrophotography. Envisage can't control other CCDs though, so when I decided to upgrade to the SXV M7C I needed to get a different CCD Control solution. After a bit of internet research I decided on AstroArt 4.0  as a good CCD control and image pre-processing package. It was around $150 when I bought it, which isn't cheap but is cheaper than other packages like MaximDL and TheSky.

AA4 comes as a standard basic package to which you add a CCD control plugin as well as a host of other free plugins for image processing utilities, etc. In the Imaging section I run through the steps in detail, but I'll cover some of the main points here.

Having dowloaded the right plugin for your camera from the Astroart website, it will appear in the selection box of the CCD Control window (accessed through Plug-in/CCD Camera). You will be presented with a setup window to enter some CCD-specific values - the help files provide plenty of guidance here.

Then you select the binning mode (1x1, 2x2, etc), the exposure length and press Start. Once the exposure has run its course and the image downloaded it will appear on the screen. In the example to the right you can see a raw 60 second exposure of the famous Horsehead Nebula. If you click on the image you'll see a full-size image with a faint outline of the nebula. You may also notice a 'grid' like texture to the image. This is the Bayer matrix, which is a technique used to turn a monochrome CCD into a colour (one-shot) CCD. For one-shot cameras like the SXV-M7C you need to synthesise the colour image from the raw image, which AA4 can do for you. This is covered in the Imaging section.

60 second exposure of B33, the Horsehead Nebula
showing the CCD control dialogue

You may also noticed from the single 60s exposure that the image is rather faint and noisy. To make the images brighter and less noisy you can take longer exposures (which means you need to guide carefully) or you take many shorter exposures and stack them together. I typically take 20-40 5 minute exposures and stack them.  If you look at the CCD Control window again you'll see a tab called Image which is where you can take single images of a defined exposure. There's also a tab called Sequence which is where I spend most of my time in an image run - it allows you to specify a number of exposures to take, which folder to put them in and how to name them. For example, if I called it M42 the exposures would be saved as M42 001.fit, M42 002.fit, etc.

Once you've collected your images, before you can stack them you need to calibrate the individual images. I cover this in some detail in the Imaging section, but calibration (called pre-processing in AA4) involves:

Dark frame removal - every image the CCD takes has a certain amount of inherent noise generated by the electronics in the camera. There are various components to this noise, some of which is constant, some of which increases as the exposure increases. The longer the exposure, the more background (dark) noise is produced. Dark frame removal involves taking a number of 'dark frames' with the telescope cap on so no light is getting to the CCD. This is a record of the CCD's noise for that exposure duration. My camera produces very little dark noise, so I rarely use dark frames.

Flat  field correction - because of the constraints of the optical path along which the light travels to get through the telescope to the CCD, the CCD is rarely evenly illuminated. This is called vignetting. You can also get blemishes on the image because of dust. These are called dust motes and appear as "doughnuts". Flat fields are short exposures taken of an evenly illuminated background (e.g. white wall, twilight sky). They are effectively subtracted from the images by AA4, removing the effects of vignetting and dust motes. I usually collect a few flat fields before starting imaging.

Defect Maps - sometimes a few pixels can get damaged over time, which results in what we call hot or cold pixels. Hot pixels register on the image as very bright and cold pixels very dark. We can create defect maps which are automatically compensated for by AA4. A defect map is semi-permanent and doesn't need to be created each session.

Bias Frames - whilst dark noise increases as a function of exposure length, there is a certain low level of background noise which the CCD electronics introduce which is independent of exposure length. This is called bias. A bias frame is like a dark frame, but only very short duration (e.g. 0.001s). A "master" bias frame can be created by averaging 10 or so individual bias frame and then it can be re-used each session.

Here's a screenshot from AA4 showing an example of each type of file. Note that AA4 scales the images so you can see the full range of pixel values in a file. This means that although a bias frame, for example, is really quite faint, it is 'stretched' by AA4 so it appears brighter.

This all sounds much harder than it really is. You get into the routine of collecting darks and flats and, once you've collected your images, AA4 does the pre-processing for you. In the pre-processing window.....

 ....you simply drag and drop the images, darks, flats, etc into the appropriate panels and click on the Options tab:

 

The Options tab allows you to determine how to combine the images. It also allows you to align the images in case. When you press OK, AA4 will automatically remove the effects of bias, dark noise, defects and flat fields from the individual exposures and combine the exposures into a final combined image.

I usually collect exposures which are between 60s and 5 minutes. The longer the better in terms of getting a good signal-to-noise ratio, but longer exposures place more demands on your mount, how well polar-aligned it is, etc. But if you take many shorter exposures and combine them as described above, you get the effect of a longer exposure.

AA4 also has some facilities for post-processing and digital development, but I use Adobe Photoshop for this as it is so powerful.

There are lots of other features in AA4, such as astrometry (measuring positions of stars, planets, asteroids, etc), photometry (measuring start magnitudes) and scripting (ways of automating the imaging process). There's also a really useful Yahoo! group for tips and advice.


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