Detailed procedure with MaxDSLR

Let's take the plunge. In what follows, I'll tell you exactly how processing is done with MaxDSLR and MaxIm DL, using the versions that were current at the time this book was written. (They are alike except that MaxIm DL offers more features.) The overall workflow in Nebulosity is similar, although the details are different. Regardless of what software you're using, reading this procedure will help you understand what is to be done. I assume you are starting with:

• one or more (identical) exposures of a deep-sky object, taken with the camera set to raw mode with long-exposure noise reduction turned off;

• one or more (identical) dark frames, which are exposures taken with the lenscap on, matching the original exposures in duration and ISO setting, with the camera at the same or a slightly lower temperature (lower because it is better to undercorrect than overcorrect).

You can also include flat fields at the same step as dark frames.

Raw images of celestial object

Dark frames

Combine by averaging

Raw images of celestial object

Dark frames

Combine by averaging

Calibrated ~U raw images

Calibrated ~U raw images

Decode color (de-Bayerize)

Astronomical image processing software

Linear color images

Align and stack

Align and stack

The unadjusted image may look very dark until the next step.

Gamma correction (Lighten midtones)

Other adjustments as desired

The unadjusted image may look very dark until the next step.

Gamma correction (Lighten midtones)

Other adjustments as desired

Photoshop or other general-purpose photo editor

Finished picture

Figure 12.1. The path to a high-quality digital image from raw image files and dark frames. Good software automates much of the process.

12.3.1 Screen stretch

MaxDSLR (and Nebulosity) will mystify you if you don't know about screen stretch. The key idea is that you view the image with much more contrast than the image actually has. That is, the contrast on the screen is much higher than in the image file, and you see only a portion of the image's brightness range (hence the term "stretch"). Think of it like magnification in the dimension of contrast rather than height or width.

Screen Stretch

? X

1 1

A

L.

Mi ni murr

Maximum

W

1347G±j

515.69^

1ri

Figure 12.2. Screen stretch window allows you to view the image with more contrast than it actually has. The actual image is not affected, only the visible display.

Figure 12.2. Screen stretch window allows you to view the image with more contrast than it actually has. The actual image is not affected, only the visible display.

Screen stretch only affects how you view the image, not the image itself. Changes to screen stretch do not affect the image and will not be saved when you save it to disk.

Screen stretch is controlled by a small window shown in Figure 12.2. If it's not visible, go to the main menu and choose View, Screen Stretch Window.

What you are looking at is a histogram of the image. It usually takes the form of a big pileup at the left (consisting of very dark background pixels) and a few much brighter pixels trailing off to the right. Below the histogram are two triangular sliders that control how much of this brightness range you are looking at. MaxDSLR tries to set them automatically to help you see the detail in your image (not to make it look good - it often looks awful!). You can pull the sliders to the left or right manually. You can also set the amount of automatic stretching; I usually set it to "Low."

12.3.2 Subtracting dark frames

Telling MaxDSLR about your dark frames

MaxDSLR will automatically combine your dark frames and subtract the average of them from your images. For this to happen, you must tell MaxDSLR where to find the dark frames and how you want them handled. The easiest way to do this is to choose Process, Calibration Wizard. This will step you through the process of filling out the Set Calibration menu, shown in Figure 12.3.

The key idea is to define one or more "groups" of calibration frames (darks, flats, etc.). The files in each group are combined before being used. In this case, there is just one group, containing the dark frames, and they are combined by averaging without scaling.

If the dark frames did not match the exposure time, ISO setting, or temperature of the original images, MaxDSLR could scale them in an attempt to make them match. In my experience, scaling is more useful with small astronomical CCDs than with DSLRs. But it's a technique to keep in mind if you have a valuable image and no dark frame to match it.

Set Calibration

17 Calibrate Bias 17 Calibrate Daik [7 Calibrate Flat [7 Dark Subtract Flats 7 Bias Subtract Fldts

j Mame

Type Filter

Duration

Image Sire

| Binning

Setpoint

Count

PI Dark 1

DARK

0.00s

JOSS X 2056

N/A

N/A

3

Learn Photoshop Now

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This first volume will guide you through the basics of Photoshop. Well start at the beginning and slowly be working our way through to the more advanced stuff but dont worry its all aimed at the total newbie.

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