Using the ST-2000XM CCD Cameras with EquinoX
EquinoX is a pretty spiffy astronomical imaging program for Mac OSX. There is quite a lot it can do – everything from taking the images to changing the filters to auto-guiding and onwards to basic image processing. This tutorial will guide you through the basic steps needed to use the ST-2000XM CCDs (and for that matter, other SBIG cameras) with EquinoX.
- Carefully center and align telescope, finderscope and Telrad
- Make sure that the camera is plugged into the computer; Open EquinoX, connect CCD to computer
- Set temperature regulation on CCD
- Mount CCD on telescope
- 5b. Set up Auto-guide, if using
- Focus and center a sample star using the clear filter (don’t forget to remove three-hole mask when done)
- Move to object to be imaged
- Take dark frame of length of time you wish to image
- Take light frames (AutoL or Light) with clear filter – don’t forget to save images!
- Switch filters and take more images if doing color
- Move to new object if desired, repeat steps 7-9
- When done, turn off temperature regulation and disconnect the computer/CCD link
- Disconnect camera from telescope; pack up
Basic Imaging with EquinoX
- Getting Started: Connecting the CCD to the computer
- Mounting the CCD on the telescope
- Focusing and Centering
- Shutting down for the Evening
- File Names
More Advanced Topics
First, before you open EqX, make sure that the camera is plugged into the computer (via a USB port). If the camera isn’t plugged in when you open EqX, the computer won’t recognize that it’s there and will not provide you with the needed menu. Close EqX, plug in the CCD, and reopen the program if you have problems. You can turn on the camera (with the switch on the black power converter box) either before or after opening the program (though it can’t hurt to go ahead and have it on just to be sure the computer will notice it). The camera has a red LED on the back – it should come on when you turn on the power, and the fan should spin up (you can hear it quite easily).
Now that you’ve got EqX open, connect to the CCD. To do this, go to the SBIG menu (on the far right) and open CCD Control. A control window should open. This window has several tabs, which are quick ways of navigating to most of the features you’ll be using. Under the
tab, click “Connect” to open the CCD-computer link.
Start by turning on the CCD’s temperature regulation. To do this, type in the temperature you want the camera to go to in the “Setpoint” field and then click the “Regulate” box (note that the temperature is in Celsius). If you want to change this value, de-click “Regulate,” change the set point, and then turn “Regulate” back on. Try to keep the temperature control working at about 80% - if it stays near 100% after it has reached the desired temperature, adjust the setpoint so that it doesn’t have to work so hard. There’s no need to burn out the cooling motors.
- Remove the eyepiece after confirming that the main scope, the finder scope and the TELRAD are all as aligned and centered as you can possibly make them. If you had in an angled prism piece, be certain to remove that, too.
- Insert the CCD’s nosepiece into the eyepiece holder and tighten the set screw. As there’s only one screw, and the CCD isn’t exactly light as a feather, it’s a great idea to put the telescope boxes, foam side up, under the CCD in case it drops. For ease of alignment, aim to make the flat side of the CCD parallel to the bottom of the telescope’s support fork.
- Make sure that you’ve got the power and USB cords long enough that they won’t get tight and tug on the scope and CCD while you’re moving between objects and imaging. Keep an eye on this throughout the imaging session, just in case.
When focusing, it is best to select a star that’s not too far from the object you want to image (focusing on a nebula or galaxy is like trying to use a lint roller on a cat – you can’t de-fuzz the naturally fuzzy). A relatively faint star is better than a really bright one, as you won’t have do worry about saturation and therefore can see the star’s shape more easily. If using a 3-hole mask to assist in focusing, carefully place it over the telescope aperture and tighten the screw(s). As you focus with this technique, you will be trying to get the three separate images of your star to merge into a single round star. If not using a mask, just try to make your focusing star as round and compact as you can.
To focus, you will be using the “Focus” button on the Image tab and setting the exposure time to be small. To set the exposure, you can either select a pre-set time from the drop-down menu, or, clicking “Adjust,” have more control over the length using the arrow buttons. For brighter stars you can use as short an exposure as you can; for dimmer ones you might need to go up to multiple seconds. Just be careful not to overexpose. This will show up in the images as long straight lines coming out of the star image, and is caused by the electrons in the CCD “bleeding over” into the neighboring pixel.
The “Focus” feature takes successive images, with a delay in between if you want to set one (a 3-5 second delay is handy for making focusing and centering adjustments). Delay is set using the drop-down menu on the right. Between images, carefully turn the focus knob on the telescope. Remember which way you turned it, and how much! If you go too far or go the wrong direction, you don’t want to have to spend extra time figuring out which way you went and which way you should go.
While you’re at it, use the hand paddle for the telescope to center up your star if you need to. “Focus” has crosshairs – very useful for centering! Set the paddle to the lowest slew speed and nudge the image to where you want it. (On the CCD-telescope setup tested so far, to move the image Right or Left is the customary East and West on the paddle, respectively, but North is Down and South is Up). If your focus star has a Meade number (i.e. Vega is star # 214), center, then match coordinates. This increases the likelihood that your target imaging object will actually be near center (or at least in the picture) when you finally move to it.
To help speed up the focusing process, you can select a smaller area of the image to look at. To do this, ctrl+drag to create a rectangle on the image – the computer will automatically only download data from that section of the CCD chip. Alternately, shift+ctrl+drag will create a square centered on where you initially clicked. Such tricks can be handy especially when readjusting the focus when you are already at your target object. Just select a section of the image with stars that are easy to focus on – no need to move the telescope or worry about the whole image field at once.
To aid further with focusing, you can click on your star in the image window itself to open a smaller focusing window. This automatically zooms in on the star (2x) and takes no-delay images (generally, it’s fast both imaging-wise and upload-wise). It also shows a graph of intensity as taken across the center of the star. Above this graph are a series of numbers. The top left number is the maximum pixel value (0 to 65535). If this maxes out, you’re overexposing. The lower left number is the threshold value. To the right, top to bottom, is the area of the star in pixels, the FWHM (full width at half maximum) in arcsec, and the minimum FWHM achieved so far. On the star profile itself are two lines – the bottom one is the threshold value and the top one is the current FWHM. Aim to get a low FWHM and a sharp peak when focusing. Be careful when adjusting the focus in this set-up – little movements can make the scope shift and the star go out of the field of view. You might need to close the focusing image (which stops focusing), restart Focus and re-click on the star in the main image. You don’t have to re-center the big image until you’re done focusing.
Once you’re satisfied with your focus, carefully remove the three-hole mask (if you used one). Double-check your centering (don’t forget to go back to the full image!), match coordinates if it is a Meade-numbered star, and prepare to move on to your real target object!
Now that you’re all focused, use the hand paddle to move the to the object you want to image. You’ll need to take an initial exposure to check to see if the object is within your field of view – usually 10-30 seconds is reasonable for fainter objects. You can do this with the “Focus” feature (just turn it off once you’ve verified the existence of your target in the image), or you can go ahead and take a real image.
To take images, first take a dark frame of the same length of time as you will be taking the images. Select the image type as “Dark” from the drop-down menu on the top right of the Image tab to do this. EqX’s AutoL feature for imaging automatically subtracts a dark frame of identical time length and temperature set point from your light image if it’s available, so if you change times or temperatures, you’ll want to take new darks (don’t forget to save the darks). If there are multiple darks of the same length and temperature setting, the most recent one will be used for the AutoL subtraction. Don’t let this spoil you, though. The CCD can change slightly over time, i.e. gain new hot pixels, changing the darks. Best results will be achieved by taking fresh darks each time you go out to image. Other than AutoL, you can take Light images and subtract dark frames later in processing. Also, when taking those first images, make sure that the filter type (if you have a filter wheel attached) is set to “clear.” To take an image, click “Expose,” sit back, and wait for it to upload to the computer.
Ding! You’ve got image! Take a peek. Is your object there? Do you need to center it? Check the focusing – do you need to tweak it a bit more? Feel free to use “Focus” to make these little adjustments, as it goes more quickly than “Image.” Once you’re satisfied, take several images using the clear filter. Make sure to save your images! Click “save all” to automatically save all of the non-focusing images to a folder. The default save folders are MPjSBIG/MPjSBIGImages for light images, MPjSBIG/MPjSBIGDark for darks, and MPjSBIG/MPjSBIGFlat for flat frames. You can select a different folder under the
tab at “Set Save Folder.” Images are automatically saved as TIFF files. You can also select “Save FITS files” to save your images as FITS files, but be warned that you can only process (dark subtract, stack, etc.) TIFFs in EqX.
You can set the CCD to repeat as many times as you would like, with some or no delay in between images. Adjust your exposure time as you see fit. Longer exposures capture more light, but keep an eye out for star trails – the telescope doesn’t always track properly with the big heavy CCD on its back. If you start getting streaks instead of dots, reduce your exposure some. (Bad tracking can be helped by using the auto-guide feature, which will be discussed later. You may also need to use a counterweight to balance the telescope better). The images you take now can be stacked together later. The times add up. So, if you take 6 10-second images, when you stack them it will be the equivalent of one 60-second image. This really helps to bring out the fainter details.
Taking images through different filters can really add depth and beauty to your final picture. Most of the CCDs are equipped with a CFW-8 filter wheel having a set of red, green and blue filters as well as a standard clear filter (one has a set of BVR filters). These filters are easily controlled by EquinoX.
To take color images, first make sure that CFW-8 is selected under the “Accessories” option in the EqX Setup tab. Then go back to imaging as normal, but instead of using the “clear” filter, select a different color instead. You may need to extend the total exposure time for different filters. The STX-2000XM CCDs are least sensitive in the red range, so take more or longer images with red than you do with blue or green (remember to take new darks when you change exposure lengths). Keep an eye on your focusing, too, as different filters can change the focus somewhat. Take a break and re-focus if you need to.
Once you are done with your imaging for the night, don’t forget to turn off the CCD. First, turn off the temperature regulation. If you forget to do this, the camera can continue to attempt to regulate its temperature. We don’t want that. Once you have turned off the temperature control, click “Disconnect” to break the computer to CCD link. At this point, you may now go ahead and turn off the CCD’s power supply and carefully disconnect it from the telescope and put everything away.
EquinoX has an automatic naming system that seems a little complicated at first, but is actually quite helpful when it comes to processing your images. The basic format can be demonstrated with a sample file name:
The first part refers to the type of image and can be Light_ (the clear filter), LightR_ (the red filter), LightG_ (green), LightB_ (blue), LightS_ (the spare filter – we don’t use this), Dark_, or Flat_. Next comes a series of four letters. The first letter refers to whether or not a dark frame has been subtracted. If yes, it will look like _DXXX_, if not, _NXXX_. The next letter refers to the flat field. There will be an F if you’ve used a flat field on your image, otherwise an X. Thirdly, alignment of images is noted. If you have gone through and aligned images, an A will replace the X. The last letter is reserved for stacked images. If this file is a combination of images that have been stacked together to form one, the X will change to S (there are four types of stack you can do – discussed later – but all will save with the S in the name, so keep track of the stack type you use).
The next information relayed in the file name is the type of binning and temperature setpoint used. The example above used 2x2 binning and a setpoint of -20C. Finally, the last section of the name tells how long the image was taken—here, 300 seconds (or 5 minutes), and that this was the third image with these settings. Again, note that when you stack images, it will put the length of one of the images taken, and not add them all up for you. Keep track of that yourself.
If you want to just set up the CCD to take lots of images in a row, automatically switching filters, use the “Sequence” option (found in the SBIG menu). Set the length, number and binning for exposures using each filter. If not using Auto-guide, keep an eye on the images to make sure that your object stays in view! Also, make certain you’re saving all the images as they are taken!
Solar system objects can be trickier to image than more distant celestial ones. It is especially important to get good focus if you want to have good detail for your planets. Focus carefully on a star before moving to your planet. For the Moon, you might also need to focus on a crater or some other easily sharpened surface feature.
For the Moon and the closer planets, you’ll need to take very short images to avoid overexposure (the shortest the ST-2000XM can take is 0.01 sec). You might also want to add in a moon filter or keep on the 3-hole mask (maybe even covering a hole or two) to reduce the light intake.
If you’re still just getting bright ovals or circles even with all these precautions, don’t panic. Much of the time you can adjust the levels of the image and discover you really had a lot more there than you ever expected. A bright white oval can turn into a crisp image of Saturn, complete with a visible Cassini division in the rings, with just a little processing. However, when imaging planets, take
of images. Since the images are short, seeing can cause random blurring quite easily. Even after adjusting levels, you might find that only one out of ten images in any color is worth using. So, take a lot of images – since the exposure length is so brief, it won’t be a hassle.
Also note that if you are trying to capture details like moons or surface features, try to keep the entire LRGB imaging process to as short a period of time as possible (for example, I've read 10 minutes for Jupiter imaging from start to finish). Otherwise fast-moving features will cause the colors to line up improperly.
If you find the computer screen is too bright and harming your night vision, you can either adjust the settings for the monitor itself or turn on the night vision function of EquinoX. From the View menu at the top of the screen, select “Night Vision” to adjust the color of the screen to the more eye-friendly red. As long as you have EquinoX open, all programs on the computer will display like this. Just remember that closing EquinoX (or turning off night vision) will bring the monitor back to its normal bright levels.
We're working on it.
EquinoX has the built-in ability to do some basic image processing, specifically with images using the EqX file-naming format. Processing is where those cumbersome file names really come in handy.
Image processing is started from the Files tab. From here, you can “Open TIFF/FITS File” and look at your images – EqX assumes that all image files are 16 bit grey scale. You can try to open a non-EqX file by holding down the option key before clicking this button, but such a file might not display correctly or even crash. Also remember that EqX can only process TIFFs – you can view FITS, but not change them.
If you haven’t been using AutoL for imaging, the first thing you will want to do is to subtract dark frames from your images. Select “Subtract Dark.” You will be prompted to select the file(s) you want to process (these are the light images). Select as many files as you want at once, but remember that you’ll be subtracting darks of the same length and temperature setting as your lights (recall that time and temperature are shown in the file names of your images). Do one set of images at a time if you changed either of these values during imaging. Next you will be prompted to select your dark frame. With this process, you can choose a dark of a different temperature setting, but this isn’t recommended for best results. When darks have been subtraced, new files will be created with a ‘D’ in the file name (_DXXX_).
If you have taken flat frames, the same process can be followed to process images with these (time and temperature not important here). Use the “Process Flat Field” button. Files will be created with an ‘F’ in the name (_XFXX_).
If you have several flats or like-timed and -temperatured darks, you can make a master file of them. The “Make Master File” function will average or median any number of darks or flats together, adding an ‘M’ into the new file name (_MXXX_ or _XMXX_). A master file will be the first searched for when using AutoL to image.
Next, you might want to stack images to increase your effective exposure length. You will first want to align the images in order to correct for any drifting that the objects might have done across the imaging field during the period you imaged. The “Align Images” button accomplishes this automatically. You will be prompted to select an image to align the others to, and then select the images you want aligned. Equinox will automatically select several references in the base image to use to align the others (it’s kind of a fun process to watch, if quick). New files will be created with an ‘A’ in their name (_XXAX_). You might run into images that can’t be aligned to the others. This means that the program couldn’t match all the references (usually stars) it was looking for. Check to see that you didn’t try to align an image of another object. You might have to just align images and complete the processing process in another program if you have a particularly hard time aligning. (We haven’t tried using EqX for planetary imaging yet – don’t know how aligning goes for that).
Now that the images are aligned they are ready to be stacked (this can only be done to aligned images). When selecting “Stack Images,” you can also select the type of stacking you wish to do with the check boxes below the button. Sum simply adds all the pixel values to create a file with higher pixel values. Log increases the pixel values without over saturating brighter areas. Avg averages the pixel values, while Med takes the median values of the stack. Both increase picture quality, but not pixel values. You will be prompted to select an image to act as the base of the stack, and then which images you desire to be stacked. With any method of stacking, a new file will be created with with an ‘S’ in the name (_XXXS_), so keep track of which method you used, especially if you try several. The file name of the stacked image will be based on the base image’s name, and will have that image’s exposure length. The lengths of the images will not automatically be summed for the new file name. Again, keep track of this yourself. Also, you’ll only want to stack groups of one filter type at once. Multi-color stacking comes later.
Display tab allows you to make basic adjustments of the levels of your images. You can select to view your image at ¼, ½, ¾ or full size, flip it east/west or north/south (left/right or top/bottom) or view the negative. To adjust the low, mid, and high level settings yourself, unclick the “Auto” box (which automatically sets the levels to bring out faint detail). Click and drag the appropriate arrows on the histogram to adjust these settings as you would like. Also, you can move the cursor over the image to view the pixel value (0 to 65535) to get an idea of how you need to make adjustments.
Note that all of these adjustments are made to the viewed image as you make them, but do
change the saved file.
The Color tab is where color images are created. Load your images to the luminance, red, green and blue channels (usually stacked images) by clicking “LRGB Files” and selecting the desired images as prompted (be careful to note which color it is asking for each time!). The histograms for all four colors are displayed to the left. To view all four colors at once, click the “LRGB” checkbox. To view each color individually, de-select this box and select the color you wish to view. Histogram adjustments will be made only to the selected color when not on “LRGB” mode, and to the entire image when “LRGB” is selected. Again, you also have the option to flip the image N/S or E/W. Any changes that are made
affect the final image. When you are satisfied with your color adjustments, click “Save JPEG” to save your final image (with all its adjustments) as an uncompressed color JPEG file.
You can also modify and save grey scale images by selecting the “B&W” checkbox. With this option, only the luminance adjustments will be available. When you save these images, they will be saved as 8 bit grey scale JPEGs.
The EquinoX image processing options are obviously not Photoshop, but they are a good basic option for creating final images. Feel free to use other programs such as Photoshop to get more control over your adjustments or use other processing techniques not available with EquinoX.
For more information or to acquire EquinoX, go to the Microprojects homepage. All images of EquinoX are credited to Microprojects and from version 5.1.1. Also, this tutorial could not have been written without the aid of their instruction mannual. -BEB, 4/2005