A recent update at Slooh.com included a new FITS viewer that’s built right into Slooh.com, the editor is called JS9.
Update: JS9 is available outside of Slooh.com as a stand-alone application. While this article discusses Slooh.com, the details also apply to this stand-alone version of JS9. Except for how to start JS9, the details of using it are identical regardless of which version you use.
A FITS file contains the raw data the telescope collected during your mission. This type of file is used to extract the most information from an astronomical image and is useful for scientific measurement. FITS files are also used by members to process and produce their own images, often resulting in images better than those the automated algorithms that Slooh.com uses to produce the PNGs in your ‘My Photos’.
Because it uses a special format, viewing a FITS file requires specialized software, like FITS Liberator by ES/ESO/NASA. While FITS Liberator is free, you have to download and install it on your system, plus it requires more adjustments to get a good final image. I discuss FITS Liberator here: Using FITS Liberator To Create An RGB Image. I also discuss SAOImage DS9 here: Creating An RGB Image Using SAOImage DS9.
Slooh.com updated their website and now incorporates a FITS viewer right into the site, so there’s nothing for you to download or install. You just open the FITS viewer, called JS9, and you can immediately view and manipulate your FITS file.
This tutorial demonstrates how you can process your FITS files to produce full-color images that are suitable for your final image or further processing using something like Photoshop, GIMP, or the online image editor Photopea.
In this tutorial, you’ll learn how to:
- Start JS9
- Select the Stretch function
- Measure the pixels in your image
- Adjust your image’s black point
- Convert your image into an RGB image
- Combine a Luminance image with your RGB image for more detail
- Align your images
- Adjust the bias of your image
- Adjust the gamma of your image
- Use the zoom tool
While you’re here, consider downloading my Free book about Slooh.com – Remote Astrophotography Using Slooh.com – A Handbook.
Why Use JS9 In The First Place?
When you take an image using Slooh.com, you’re provided with PNGs of already processed images, so why go to the trouble of creating your own?
Slooh.com (the MicroObservatory and even itelescope.net) generate images automatically based on processing presets that are derived from processing many images. While these presets do a good job in most cases, they are not customized for your particular image.
Using JS9, or FITS Liberator, or SAOImage DS9, you have more control over the process and can develop your images based on what you want to get from them. The process shown in this tutorial is just one way of creating images from FITS – there are many things you can do to emphasize or de-emphasize certain parts of your images.
The Results
Let’s begin with what our result looks like. This is my final version of Helix Nebula, as shown in JS9:
I created this image by downloading the FITS files from the mission, opened the files in JS9, and manipulated the files to produce this output.
The great thing about JS9 is that not only can you combine the RGB FITS files into a final color image, you can also stack your images to include your Luminance data too. Luminance data increases the depth and contrast of your images. I show you how to incorporate Luminance data in this tutorial.
I have made sample files available for you to download to try this tutorial. Click the link below to download a sample observation of the Helix Nebula:
Download sample observation of the Helix Nebula
The download includes four FITS files that you can use with this tutorial.
Starting JS9
Let’s start by opening JS9.
Update: JS9 is available outside of Slooh.com as a stand-alone application. You can start it by visiting this link: https://js9.si.edu/. While this article discusses Slooh.com, the details also apply to this stand-alone version of JS9. If you are using the stand-alone version of JS9 outside of Slooh.com, skip the rest of this section and continue in the following section, “Selecting a Stretch Function”.
Review your missions in ‘My Photos’ under the Missions tab and find a suitable target. The PNGs that Slooh.com produces will give you a good idea about how bright and large your target is before you view the FITS file. Hover your mouse pointer over the mission card and click the arrow as shown:
This brings up a listing of the FITS files that make up your mission:
To the right of each filename are two action buttons: View FITS and a download button. Note the filenames and select ‘View FITS’ on a file that ends in ‘r_cal.fit’, ‘g_cal.fit’, or ‘b_cal.fit’. This opens JS9 in a new window (note that it takes a few seconds to display your FITS file in JS9, so be patient while it loads):
Chances are your image looks much like mine does: washed out with little detail. To fix this, we need to apply a stretch function and then adjust the black point of our image.
Selecting a Stretch Function
A stretch function distributes the pixels that make up your image in such a way that it makes it easier to view and improves contrast. There are many stretch functions, so let’s explore a couple of them.
The ‘Scale’ menu contains stretch functions as well as other capabilities that affect your image.
From the menu, select ‘Scale’ and then select ‘linear’ – your image will darken quite a bit, similar to the following:
Try another Scale function, like ‘histeq’, or ‘asinh’ to see the effect. As you make your selections, you are only manipulating the FITS file in memory – you are not affecting the FITS file stored in your ‘My Photo Hub’ area or on your system.
The ‘log’ scale function is the most useful for most images you open in JS9, so select it to return to where your image was when you opened it (the log function is the default for JS9).
The image is washed out because the black point is set too low at the moment. You can adjust the black point by first measuring the darkest part of your image and then making the adjustment.
Measuring the Pixel Values In Your Image
You can measure the pixels in your image in a couple of different ways, however, the easiest is to use something called a Pixel Table. A Pixel Table simply shows you the value of the pixel under your mouse pointer in a large and easy to read table format.
Open the Pixel Table by selecting ‘View’, ‘Pixel Table’ from the menu. This brings up the pixel table as shown:
You can move the pixel table out of the way of the image by clicking and dragging at the top of the table.
Use your mouse to hover over different parts of your image and note the value displayed at the center of your pixel table:
You want to find an area of your image that’s dark and doesn’t have any stars or nebula in it. Your goal is to find the lowest value in your image. In my image, I found that the lowest value is around 930. You don’t have to be completely precise – you can always change the value later.
Setting the Black Point of Your Image
Once you find a low value, close the Pixel Table by clicking the X at the top right. Now that we have measured the black point of our image, let’s set that value for our image. From the menu, select Scale, enter the value you measured into the box marked ‘low’, and press Enter to make the change to your image. You’ll notice your image becomes darker and you can now see more detail:
Click anywhere on the image to dismiss the menu.
Opening Multiple Images
So far, we have just one image open and we need to open more images to combine them to produce a full-color image. JS9 only downloads your first image from Slooh.com directly – you’ll have to download and open images from your computer to open all of the images from your mission.
Download the remaining images from your mission to your computer (you can switch back to Slooh.com in the previous tab).
You can open multiple files in JS9 – the only limitation is the amount of available memory on your system.
There are a couple of ways of opening a file in JS9: you can select ‘File’, ‘Open’ from the menu and browse to find the file you downloaded, or you can simply drag the file from Windows File Explorer onto the JS9 window.
For now, open the two remaining R, G, or B files that make up your mission (you previously opened one of the R, G, or B files above so now open the remaining two).
Understanding the Active File
From the File menu, you can see which file is the one in your view because it is marked with a yellow and green dot next to the name. We’ll be switching between files during this tutorial.
Let’s make the Red file the active one, so select File, find the filename ending in ‘r_cal.fit’ and select it.
Chances are that your image is zoomed in, so from the menu select ‘Zoom’, ‘zoom to fit’.
Adjusting the Black Point of Your Other Images
If your image is washed out, you need to measure and adjust the black point of your image, so go ahead and do that for each image, following the directions above to measure and adjust the black point.
Assigning Color To Your Images
Now it’s time to assign color and combine the image into a full-color image.
You’ll have to know which image you’re working with to create a color image. You can tell which image you’re working with based on the filename, but there is a more reliable way: view the FITS header. The FITS header has a field called ‘Filter’ which tells you which filter was in use to capture the image. Select ‘File’, ‘display’, ‘FITS header’ to view the header and look for the field ‘FILTER’.
Go back to the Red file, the filename ending in ‘r_cal.fit’, and select it from the File menu. Now let’s assign Red to this image:
- Select ‘Color’ from the menu
- Select ‘Color controls’ from the menu
- Select ‘red’ from the dropdown menu in the window that pops-up
When you select ‘Color controls’ in step two, another window pops-up. You can leave this window open and switch to another image.
Assign the corresponding color to the active image (your image will change to the color you select).
Combining Images to Create an RGB Image
Now that you have assigned a color to each image, you can combine them into an RGB image.
From the menu, select ‘Color’, ‘rgb mode’ and your display will change to show the full-color image:
Note the name of the file that’s active when you do this since we’ll need it in a few moments (check the File menu to see which of your images is the active one).
This is a pretty good image, considering we have done just a few minutes of work. Let’s try to improve the image by bringing in our Luminance FITS to improve the contrast and bring out some more detail.
Adding a Luminance Image To Your RGB Image
Open one of the files ending in ‘l_cal.fit’ in JS9. Measure the black point and adjust it as you did with the other images, and zoom to fit.
To bring in your Luminance image, you blend it with your color image and JS9 makes the process easy.
Make sure your color image is the active one (select it from the File menu) and from the menu, select ‘View’, ‘blending’. This will bring up a new window as shown:
The active image is shown with a green box around it. There are checkmarks next to each filename, indicating that the image will be blended when we enable blending. For now, clear all of the checkmarks, except for those of the active image, and of the image ending in ‘l_cal.fit’.
Click the ‘Image Blending’ option at the top of the list of files, as shown, to blend the images:
If your image looks blurry, chances are that the telescope moved during your mission while taking the image, so we’ll have to align the image.
Make the luminance image the active one by clicking inside the box with the filename ending in ‘l_cal.fit’, as shown:
Aligning Your Images
We’ll have to zoom in to be able to align the image more accurately, however, JS9 treats your images separately when you zoom. You’ll have to remember to zoom in or out of each of your images separately.
From the menu, select ‘Zoom’, ‘zoom 2’.
Make the combined RGB image the active one (you noted the filename above) by clicking in the box with its name in it. From the menu, select ‘Zoom’, ‘zoom 2’.
Now we’ll make the Luminance image the active one to align it. Make the luminance image the active one by clicking inside the box with the filename ending in ‘l_cal.fit’
Using your mouse, right-click, hold the mouse button down, and drag to align the images. This will take some time to get it right, but try to align on bright stars.
Once you have the images aligned, zoom out to view your progress (remember to zoom out on your combined RGB image as well as the luminance image).
Adjusting the Bias Of Your Image
In my case, the image is a little washed out because the Luminance has brought in quite a bit of light color:
We can make a simple adjustment to fix this.
Make the Luminance image active (select it from the File menu). Select ‘Color’ from the menu, and adjust the ‘bias’ value in increments or decrements of 0.1, up or down, until your image looks better. Make small adjustments and press Enter to make the change on your image.
Adjusting the Gamma
Another adjustment you can make is the gamma. Gamma adjusts the luminance of your image and can sometimes bring out a lot more detail. To adjust the gamma, from the menu, select ‘View’, ‘Image Filters’. This brings up an image filters dialog that you can move out of the way of your image. Adjust the gamma by clicking the ‘gamma’ button, and adjust the slider left and right to manipulate your image. If you don’t like the effect, slide the slider all the way to the left to zero, or click ‘undo gamma’ at the bottom of the dialog box.
Note that sometimes the Gamma adjustment and other adjustments in the Image Filters menu don’t do anything. This seems to be a bug in JS9 and there’s no way to make it work once it stops working, except to reload the page (and lose your image) and start over again.
Saving Your Final Image
Once you have your image the way you like it, select File, Save, PNG and your file is downloaded by your browser using the name of the active file with a PNG extension.
This is the final version, as downloaded from JS9:
Removing A Color Gradient
If your image has a color gradient on it (a color cast), try my tutorial here where I show you how to remove the gradient: Removing Color Gradient From Your Images. If you find that process removes too much color, there’s a less harsh version here: Remove A Soft Color Gradient From Your Images.
Stacking Multiple Exposures
If you have multiple exposures and need to stack them, I have another article that provides you with the details of how to do that, building on the skills you learned in this article. Here’s the link to the article about stacking: Creating an LRGB Image Using DeepSkyStacker, JS9, and Photopea.com
Conclusion
You learned how to start and use JS9 in this tutorial. Using the steps in this tutorial, you can produce a good quality image without having to learn complicated steps or install anything on your computer.
You can improve this image by editing it further in Photopea. In this tutorial, I show you how to remove the color gradient from your images – a great way to recover images that are otherwise good, except for a color gradient.
Looking For More About Using Slooh.com?
I wrote a free book about using Slooh.com – you could call it the missing manual for Slooh.com. Download the free book now!
You must be logged in to post a comment.