Manual Image Calibration. QSI 520c

Q S I 5 0 0 S E R I E S U S E R G U I D E taken with the CCD at the same temperature as your light frames. If you’re taking LRGB or other filtered images, ideally you should take a set of flat fields through each filter.

Setup your telescope and possibly your light box to take your preferred style of flats. Adjust the exposure so the average pixel is filled to approximately half its full well depth. With QSI

500 Series cameras you should strive for pixel values of roughly 30,000 ADUs. As with dark frames, you should take at least as many flat frames as the light frames you took through each filter. Taking additional flat fields will produce a master flat with less noise, yielding higher quality images. Do not change your focus. To get correct flat fields, it is important that the focus be identical to the focus used for the light frames.

Flat Darks

As with any exposure using a CCD camera, dark current builds up while taking a flat field image therefore the dark current must be subtracted with a dark frame taken at the same exposure as the flat field. Taking flat-darks is easier and less time consuming than regular dark frames because the exposure time of flat field images is typically much shorter than your light frames. If you take flat field images, it is critical that you also take a series of flatdarks. They’re easy to take with the shorter exposure, so take at least 15 flat-darks.

Manual Image Calibration

MaxIm LE supports both automatic and manual image calibration. This section describes basic manual image calibration in MaxIm LE. For complete instructions on calibrating images, please see the online help in MaxIm LE.

Subtract Dark Frames

Begin by creating a master Dark Frame for each exposure length. A master dark frame is created by combining all the individual dark frames using an average or median combine.

See the section below Combine Frames in MaxIm LE for detailed instructions on combining multiple images.

Tip: In general, an average combine of multiple images will yield a combined image with slightly less noise than a median combine, but this requires that none of the frames being combined have any anomalous groups of light or dark pixels caused by, for instance, cosmic rays.

Other programs, such as MaxIm DL, include additional combine methods that attempt to provide the low noise of an average combine with the anomalous pixel rejection of a median combine. You will need to experiment to achieve the best results.

49

Q S I 5 0 0 S E R I E S U S E R G U I D E

Dark frame showing two cosmic ray hits in lower right hand quadrant

Inspect your dark frames for cosmic ray strikes or other anomalous pixel groupings. The dark frame above shows two cosmic ray hits in the lower right quadrant of the image.

Cosmic rays are energetic particles originating from somewhere in space. They can come from any direction and depending on the angle that they strike the CCD can be very long if they are nearly parallel with the CCD or just a bright circle if they strike perpendicular to the

CCD.

If multiple dark frames in your set each have cosmic ray hits, doing a median combine of all the images may yield a better combined image than averaging just the clean frames.

Experiment to get the best results.

After you’ve created your master dark frame for a given exposure length, subtract it from each of the light frames with that same exposure. Do this by selecting “Pixel Math…” from the Process menu.

50

Q S I 5 0 0 S E R I E S U S E R G U I D E

Screenshot showing Pixel Math command ready to subtract a master dark frame.

Select your light frame for Image A at the top and your master dark for Image B. Select

“Subtract” for the operation. Click OK. You should see many of the bright pixels scattered around the light frame, disappear or get visibly darker. Do this with each of your light frames. After dark subtracting the individual light frames, the light frames can be combined into a master light frame. If you took multiple images through different filters you will need to create a master light frame for each color.

Scale by Flat Fields

Scaling dark subtracted images by flat fields follows a similar process to subtracting dark frames.

Note: Taking proper flat fields can be tricky. If the flat fields aren’t exposed and processed properly, scaling an image by an improper flat field can produce results worse than not calibrating with flat fields at all. Experiment and practice for the best results.

51