Combine Files

The Combine Files command adds the contents of multiple files together to reduce noise, with optional realignment to remove any translation or rotation. One common application of this command is to eliminate the need for accurate guiding. Instead of a single long exposure, a series of shorter exposures are taken. They are then aligned and added together to simulate the effect of a single long exposure.

 

Select Images

Two separate dialogs are used for this function. The first is identical to the File Open dialog. First select the files to be combined. This is done in the standard Windows fashion by holding the Shift key down and selecting a set of files with the mouse, or by holding the CTRL key down and selecting the files individually. All of the files have to be of the same type (file format). You can include all the files in a sequence even if some of them are known to be of poor quality since the second dialog allows you to reject poor images.  

Once you have selected the desired files, click the Combine button. A second Combine Files dialog is presented.

 

Combine Files

When the Combine Files dialog box appears, a special Align image buffer also appears.

The first step is to select the Output mode. This determines how the images are combined. The options are Sum, which adds up the pixels in all the images; Average, which sums all the pixels and divides by the number of images; and Median, which takes the median or middle value from all of the images. The Sum mode provides the best performance in most conditions (e.g. when Gaussian noise is present). The Median mode is useful when some pixels are extremely bright or dark (hot/dark pixels, cosmic ray hits). If Median mode is used, a Normalize option is available. Normalize will remove differences in the image scaling which could interfere with the median processing.

When the images are rotated or resized, MaxDSLR will interpolate the values of the pixels in the input images to produce a more accurate result in the output. Two interpolation modes are available. Bicubic Resample uses a higher-order resample algorithm that produces a crisper result, but requires more processing time. When turned off, a simple bilinear interpolation is used.

The next step is to select the Align Mode. The methods available are:

The auto modes require no further operator input – just click the OK button. Even in the automatic modes you can remove bad images by stepping through using the Next Image and Previous Image buttons, and turn on/off the Reject Image button.

Manual modes allow for image alignment using a variety of techniques. Each alignment method is described below.

None

In this mode, no change is made to the alignment of the images. The images are summed, averaged, or median combined without any other adjustments.

Auto - correlation

A cross-correlation is performed between each image to determine the relative offset. If point sources (stars) are available in the image they are used to refine the alignment to sub-pixel accuracy. The images are only shifted horizontally and vertically; no rotation is performed. This function only works properly if the offset between images is relatively small. To determine if it will work satisfactorily, click the Overlay All Images button to produce a preview.

Auto - star matching

In this mode, the images are matched up by comparing star patterns. If the match is successful, the images are automatically aligned to sub-pixel accuracy, using both shifting and rotation. To determine if the results are satisfactory, click the Overlay All Images button to produce a preview.

Manual 1 star - shift only

This is the fastest manual adjustment method. Each image is presented in turn; the user simply clicks on a particular star or other recognizable feature in each image. The images are aligned using shifting only; no rotation is performed.

In order to speed up the alignment, turn on the Auto Next check box. Every time the mouse is clicked on the image, the next image will automatically be displayed. When all of the images have been registered, the computer will beep.

If Auto Next is not being used, you can move through the images using the Next Image and Previous Image buttons.

If the reference point is a star or similar object, turn on the Use Centroid check box. At each clicked location a centroid will automatically be calculated and used to refine the adjustment to the sub-pixel level.

One of the images can be set as the reference image using the Set As Reference button. The selected image will not be moved, and all other images will be adjusted to align with it.

If any image in the sequence is of poor quality, click the Reject Image button to remove it from the list. You can restore it by clicking on the button again. Also any image that is not registered by clicking on it with the mouse is automatically rejected.

To determine if the results are satisfactory, click the Overlay All Images button to produce a preview.

Manual 2 stars

This method is similar to the Manual 1 star method, but performs both a shift and rotate using two registration points. Each image is presented in turn; the user simply clicks on a particular star or other recognizable feature in each image.

In order to speed up the process, turn on the Auto Next check box. Every time the mouse is clicked on the image, the next image will automatically be displayed. When the first registration point has been set on each image, the computer will beep and the first image will be displayed again. Move to a second star and repeat the process until the computer beeps a second time. The Star 1 and Star 2 buttons will toggle to show you which point is being set.

If Auto Next is not being used, or you want to change a single registration point, use the Star 1 or Star 2 button to control which reference point is being changed. You can move through the images using the Next Image and Previous Image buttons.

One of the images can be set as the reference image using the Set As Reference button. The selected image will not be moved, and all other images will be adjusted to align with it.

If the reference point is a star or similar object, turn on the Use Centroid check box. At each clicked location a centroid will automatically be calculated and used to refine the adjustment to the sub-pixel level.

If any image in the sequence is of poor quality, click the Reject Image button to remove it from the list. You can restore it by clicking on the button again. Also any image that is not registered by clicking on it with the mouse is automatically rejected.

To determine if the results are satisfactory, click the Overlay All Images button to produce a preview.

Planetary

This method is intended for aligning images of planets. It will work only when each image to be aligned contains a single substantial disk, that is, an approximately circular region of bright pixels completely surrounded by background. The images are shifted to bring the centroids of their disks into coincidence. No rotation or scaling is performed. The Reject and Set As Reference buttons are available in planetary mode and work as previously described.

To determine if the results are satisfactory, click the Overlay All Images button to produce a preview.

Auto one star

This method is intended for aligning small images such as subframes or cropped areas. For proper operation, each image must contain one star or other compact source that is significantly brighter than everything else. The images are shifted to bring the centroids of this object into coincidence. No rotation or scaling is performed. The Reject and Set As Reference buttons are available in auto one star mode and work as previously described.

To determine if the results are satisfactory, click the Overlay All Images button to produce a preview.

Overlay

This mode is useful when no clear registration points are visible, such as planetary images with indistinct features.

You must first set one of the images as the reference image. Look through the images using the Next Image and Previous Image buttons. Pick an image and click the Set As Reference button (in many cases the first image can be used).

Next, move to another image using the Next Image or Previous Image button. This image will be overlaid on the reference image, with the reference in magenta and the new image in green. The Overlay Controls can now be used to adjust the position of the image to match the reference.

There are four shift buttons with up, down, left, and right arrows. There are also two rotation buttons with rotate clockwise (right) and rotate counterclockwise (left) arrows. Each time you click a button the image moves. The distance moved is controlled by the Nudge Size (pixels) control. With these controls you can position an overlaid image to a fraction of a pixel.

In order to use the rotation buttons, you must first pick a rotation point. This is the point that the image will pivot about as it is rotated. The best procedure is to align a feature that appears on both of the overlaid images, then click on the feature. Look at another point in the image some distance away and adjust the rotation to line up the second point.

To determine if the results are satisfactory, click the Overlay All Images button to produce a preview.

 

NOTE: The FITS header for the result image will be a copy of that of the image that was selected as the reference, with the following exceptions: