AstroStack: First Light

AstroStack: First Light
A Getting Started Guide for Digital Camera Astrophotographers
Introduction
This guide was written to help digital camera astrophotographers learn to use the freeware
program AstroStack to improve the detail in their images by stacking. This tutorial uses 11
mediocre pictures or frames of Jupiter but stacking can help most all planetary, moon, and sun
images. Once you are familiar with stacking it can also be used to provide off-chip integration or
simulate longer exposures for deep space objects. After reading this document and following along
with your own images you should have enough information to do any type of stacking in
AstroStack.
Quantity is Quality
The key to stacking for detail is to capture as many pictures of the subject as you can using the
exact same camera settings and eyepiece, and to fire these pictures off as quickly as possible to
reduce the effects of seeing and rotation. Many digital camera astrophotographers take hundreds
of frames and then sort them and stack only the better images. AstroStack does have a limit to
the number pictures that can be stacked and this limit is based on the width of the input images.
Preparing to Stack
AstroStack was originally written for Video Astrophotography and “still camera” image support was
an afterthought. Unfortunately AstroStack will not accept the images as they come out of your
camera and therefore another program must be used to prepare the images. Each image must be
the exact same size and saved as a Window’s Bitmap or BMP file. For this example we will use
Paint Shop Pro 7.0 (PSP) to prepare our images but you can use any standard Photo editing
software.
To prepare our image we must to crop them to less than 768 pixels wide. For this example we will
crop them to 320 x 240 with the subject in the center of each image. Open the raw image(s) from
your camera in PSP. Click the “Crop” tool. Find the Tool Options window by either expanding the
Tool Options bar or opening the tool options window under View Toolbars Tool Options Palette.
The Tool Options should look like
the screen to the left. Now click
the “Crop Settings…” button and
a dialog box pops up. Set the
dialog box to look like the screen
on the right and then click ok. A
white outline of the crop box
should be displayed on your
image in the top left corner.
Drag the box so that the subject
is in the middle of the box. Now
click the “Crop Image” button on
the Tool Options tool window. The
image should be cropped. Using
these settings crop all of the images. It is essential that every
image be the same size.
After the image is cropped you must save it as a Windows Bitmap. Choose File | Save As.. from
the menu in PSP or use the F12 function key and then change the type pull-down to Windows or
OS/2 Bitmap (*.bmp) and, under most circumstances, leave the name intact but save the image to
a new location such as a directory called cropped. AstroStack is very picky about the name and a
name like Alpha0001.bmp to Alpha0043.bmp will work well. If your camera uses filenames that
are all numbers adding a “letter” to the beginning of each file name will ensure that AstroStack will
accept and process your images. Click Save.
Repeat this process for each of your images saving all images in the same directory. You can
probably imagine, and will be well aware after a few frames, that the preparation is the most
tedious and disliked aspect to using AstroStack.
Once you are finished you should have many images all 320 wide by 240 high and saved as BMP.
The image below is an example of the way your images should look (hopefully better) before
importing them to AstroStack.
AstroStack at Last
By now you are probably tired of drawing little boxes all the same size. But the results are worth
the effort. Open AstroStack and notice that it has two image windows at the top. The leftmost
image window is the current image and the rightmost window is the result.
Adjust your screen to look like the image below and make sure that the “width” is set to 640, the
“full color” option is selected, and the Full Image checkbox is checked all under the result image in
the “Show Result” section. We will not discuss these options in detail as this is just a guide to get
you started. After you have learned how to stack you can go back and investigate all of these and
other options. For now we are going to stack those images that you have labored over for the last
half an hour or more.
Now click the BMP button at the top left of the AstroStack screen. AstroStack will prompt you for
the “The name of the FIRST bitmap in the sequence.” Select the lowest numbered image for
example DSCN2103.bmp and click open. AstroStack will now ask you for “The name of the LAST
bitmap in the sequence.” Select the highest numbered image for example DSCN2113.bmp and
click open.
Observe that the current image window and the status bar will change indicating that it is loading
the images. AstroStack will let you click around but, instead, wait until all of the images are
loaded.
Garbage In, Garbage Out
Before we stack the images they must be aligned. Notice the “Align
Pictures” section below the BMP button. Now adjust the “Align with
Picture” box by clicking the “up arrow” button until you have selected
the middle image. For our example, we will use image 6. Make sure
that you clicked the Auto option and then click the “Go!” button.
Watch as AstroStack attempts to align your pictures. Depending on
your cropping accuracy during the preparation of images the current image window will go Black
and White and dance while AstroStack aligns your images.
First Stack
With your images aligned you are ready to stack them. Click the [->Calculate<-] button in the
center of the screen. Watch as AstroStack stacks your images and notice that much of the noise
has been reduced while the detail has been enhanced. This is why we did all the work!
Pulling your Weight
The first Stack should have shown definite improvement over a single raw image but there is even
more that you can do to improve your result. AstroStack has the ability to perform a weighted
stacking of your images based on your selection of good, bad, average images as well as images
that should not even be included. Just below the current image window is the Set Quality Factor
section. Using the “up arrow” and “down arrow” buttons on the “Nr. of input picture” area notice
that the current image changes to the next or previous image in the sequence. Move through the
images one time and noticing the good and bad images. If any image looks distorted or
extraordinarily fuzzy note this as well. Now start back at input image 1 and click the various option
buttons below “Don’t” (ignore), “Bad”, “Average”, and “Good” corresponding to your estimation of
the raw input images. Go back through again and check that the bad are marked bad, and good
are marked good. Now click the “Calculate” button again. You should notice that the weighted and
newly stacked result looks even better than the standard stacked image.
Unmasking the Detail
AstroStack can improve your image even further by performing an “un-sharp” high pass filter on it
during the calculation. Notice the “(De)convolution” section at the bottom left. Click the checkbox
next to “Unsharp Mask” and press the “Calculate” button. Does it look better? Try adjusting the
radius of the Unsharp mask until you have the best looking image. Too much and it will turn to
mush. Too little and you can not notice a difference from just stacking.
A Color Boost
AstroStack, I hope, has shown you what a
difference it can make to your digital camera
astrophotography but there is still more that you
can do. Use the “Save As”, at the bottom center,
and save your image to your pictures folder or
some other location you can remember. Using
the Save Result button will hide your result in the
results folder below the AstroStack program
directory.
Now open the image in PSP again and click the
“Colors\Histogram Functions\Histogram
Adjustment” menu option and adjust the colors of
your image. You might be able to use the exact
settings below.
Finally resize your image to 240x180 (this will also sharpen the image) and then Save As JPEG. Be
sure to adjust the compression in the Options menu. With your completed image saved you can
now post it the digital_astro group!
Conclusion
Using our example of 11 shots was just an example. Ideally you should take hundreds of images,
sort out the good ones and stack 40 or adjust the width even smaller and stack in excess of 163
images. The example images were taken under terrible seeing and low in the horizon but the
stacked images still show dramatic improvement over the raw images and were used to
demonstrate AstroStack and the benefits of stacking.
This guide was written by Gregory A. Pruden with help from Ken Florentino for the Digital Camera
Astrophotography List
http://yahoogroups.com/group/digital_astro
Corrections or suggestions should be sent to [email protected]
AstroStack is a freeware program and Copyright (c) 1999, 2000 Robert Stekelenburg, Netherlands
http://utopia.ision.nl/users/rjstek/english/software/index.htm