Tumbleweed Observatory's

Astronomy Hints



A Review and Tutorial on the Celestron NexImage Astrocamera

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Solar System Photography With The Celestron NexImage Web Cam

If you've been an amateur astronomer for very long, you've probably gotten the itch to try some astrophotography. Maybe you've even tried by holding a camera up to the eyepiece and taking some snapshots of the moon. I've used that hand-held technique a few times myself, with some examples near the end of the 60mm Telescope Project web page.

After taking a few such moon snapshots, you may want to optimize the use of your hand-held digital camera by making a Digital Camera Mount, or perhaps better yet, buy the Celestron Universal Digital Camera Adapter. You can get some excellent moon crater pictures that way, but moving on to planets is a bit difficult with such an arrangement. The difficulty is finding the object in the camera view finder.

That's where something like the Orion StarShoot Solar System Color Imaging Camera astrocamera comes in. The StarShoot is an inexpensive and easy to use system that works as an astrocamera for imaging solar system objects. It works much like the Quickcam Astrocamera that I describe, and its predecessor, the Celestron NexImage reviewed on this page.

The Celestron StarShoot is newer version of my older NexImage unit, and like the NexImage is only capable of short exposures, but can take up to 15 exposures per second. It works very well for lunar and planetary photography, but is not the kind of unit you want for taking time exposures of celestial objects like galaxies, star clusters, and nebulae. To get a good idea of the potential of one of these cameras, check out my ETX 90 Astro-photos page.

The least expensive way to get into star photography is with an SLR (Single Reflex Camera). Yes, I know, this is the digital age. But when you check out the prices of Digital Single Lens Reflex (DSLR) cameras compared to 35mm film SLRs, I think you'll get my point. Even a relatively inexpensive 35mm SLR, like the Vivitar V3800N 35mm SLR Camera w/ 28-70mm Lens, can take star photos. But to get a DSLR that can take long enough time exposures can be a bit expensive. You can find out more about SLR cameras at Choosing A Camera.

The Celestron NexImage Camera

Celestron NexImage Camera

This is what the NexImage camera looks like. The pencil in the image is to help illustrate the size of the unit. As you can see, the size of the camera is quite small. It's basically the web cam electronics in a small container with a 1.25 inch diameter snout on the front. The snout fits nicely into your telescope focuser or Barlow lens.

You may notice that there is a plastic cap inserted into the snout. This comes with the camera, and keeps the dust out of the unit when not being used. Of course, remove the cap before you insert the camera into your telescope focuser.

The Camera is Small and Easy To Use

Celestron NexImage on Meade ETX 90

The above image shows the NexImage mounted on my Meade ETX 90 Maksutov. Doesn't this make for an incredibly compact photographic observatory? Using the device on the ETX 90 is a snap. In this illustration, I've attached my camera adaptor onto the back port of the ETX 90, and inserted the NexImage into the adaptor. With the ETX 90 (and a Questar if I owned one) I can use the flip-mirror to switch back and forth between the camera and the eyepiece. This aids considerably in getting planets to appear on the small CCD array of the camera.

While getting the moon image to land on the CCD is easy, getting a planet to do the same, especially with only a finder scope, is much more difficult. That's why I use my ETX 90 for most of my photography. The flip-mirror system lets me view the targets at higher power, which in turn lets me orient the telescope with much greater precision. Note that in this picture I don't have the ETX 90 in equatorial mode. In actual use, I mount the ETX 90 as shown near the bottom of the Cheap Tripod web page so that the built in clock drive can track my solar system targets.

I have managed to use the NexImage with a simpler telescope. I used my 60mm long-focus refractor to take some nice Jupiter and moon images. You can see the results of my efforts on the 60mm Telescope Astrophotos web page. You may want to check out that page, since those images were taken with the NexImage using a simple altazimuth mounted telescope without a clock drive. The page demonstrates that you can get nice results with the NexImage using any telescope that is on a sturdy mount.

While I have a number of telescopes, my favorite setup is with my ETX 90 and the Celestron NexImage. But I think a comparable setup could easily be had using the popular Celestron NexStar 90SLT Mak Computerized Telescope (Black), which is also a 90mm Maksutov telescope. It's the extraordinary compactness of the Maksutovs that make solar system photography straightforward.

The Celestron NexImage Control Software

Celestron NexImage Software

To collect the pictures from the NexImage camera, you use the camera control software that comes with the unit. The software runs on the Microsoft Windows operating system. The software finds the camera if it's plugged into a USB port. Pull-downs on the software let you select various camera parameters, such as frame rate, image size (up to 620x480), and whether to automatically control exposure or allow manual control.

As soon as the setup parameters are selected, the image of whatever is available to the camera is displayed. In the example, the Plato region of the moon is the target.

When you first setup for an evening's photography session, I suggest that you start by pointing the telescope and camera at a distant streetlight or some other easy to locate target. Focus the unit on this target so that you have a rough focus for any new target. This is particularly important for photographing planets. If you are way of out focus when you go planet hunting, you can actually pass right over a planet and not even know it.

I usually start with the unit in an auto-exposure mode. This will always show the target when I run across it, as long as focus isn't way off. When the moon is the target, I've found that I can usually leave the unit in auto-exposure mode for my exposures.

Planets will totally wash out in auto-exposure mode. That's fine for finding them, but you'll then need to switch the camera control to manual mode so you can adjust the exposure for best planetary detail.

Taking Exposures

Celestron NexImage Image Capture

Once you have your target in view and the camera parameters set to you liking, it's time to take a series of exposures. You begin by clicking on the File pull-down and entering a file name for your image. Don't forget to do this for each new exposure, or else you'll end up writing over previous exposures. Each of these files will be stored in a movie .avi file. Typically you'll likely take between 5 to 10 exposures per second. So a few seconds of image capture is all you'll need.

When the file name has been selected, click on the Capture pull-down. It offers a number of options for setting the type of exposure, but when all that's set, just click Start Capture. That click will bring up another window confirming that you're ready to expose. Click that and the exposure begins. It will either expose for a specific number of seconds, if you chose that option, or collect until you again click the Capture pull-down and select Stop Capture.

My procedure is to create 2 or 3 movies of each target, refocusing in between. This way I increase my chances of having at least one file of images being in sharp focus, and some frames taken during good seeing.

How To Process Your NexImage Photographs

The advantage of using this kind of camera for lunar and planetary images is that each collected file can have dozens or even more image frames. Now that you've collected some files, you need to turn them into the best images possible.

RegiStax image processor

To help you in this endeavor, Celestron packaged a version of the RegiStax image stacking program, commonly used for working with astronomical images. The RegiStax program can directly read .avi files (and some other formats), so processing is pretty easy.

You begin with the Select Input button in RegiStax, which lets you browse your image directories and pick the file you want. It then loads all the frames into memory, and is ready for processing. There are a number of options on the initial screen, and I admit that I've not tried them all. I'll just describe the simple procedures I use to get images processed.

RegiStax Software

After loading an avi file, you'll notice that when you mouse over the image, a square selection box follows the mouse. You use this box to select the area of most interest for the stacking algorithm. You can just see the selection box over the Clavius crater in this illustration.

The software will use this area to align all the frames. Note that the camera can take color images, and once a file is loaded you can select to either process with color or not. With moon images, you might as will decline color processing, as the moon is pretty much shades of gray anyway.

At the bottom left of the RegiStax screen, you'll see a check-box labeled Show frame list. You can click on this if you like, and step through each frame, selecting those to use for processing, and those to ignore. If you don't do this, all frames in the file will be processed.

Toward the lower left of the RegiStax screen you'll see an area labeled Alignment box (pixels). Notice that by clicking on the assortment of size options, you control the size of the selection box. Pick an appropriate size, move to an area of interest, and left click.

The software will change to the Align screen and show any data graphs selected. You can use these graphs to determine optimal alignment settings. The right area of the align window presents some tuning options for the alignment and stacking operation. I usually use the defaults.

Clavius crater with Celestron NexImage
Clavius Crater
NexImage camera and ETX 90

When done tuning, click the Align and Stack button. The software will step through the frames, optimizing the images and stacking them. It may do this for several passes if the optimizing logic dictates. Shortly it will finish and display the final image. Note that you get the total common area image, not just the area you selected for alignment purposes.

This phase of processing will bring up a new screen that gives you options for re-stacking with different options, or saving your final image. Clicking save will allow you to enter a file name and file type for saving.

The image shown is an output from stacking about 50 frames of Clavius crater. You've probably seen sharper Clavius images, but this one shows the amazing details one can achieve with the NexImage and stacking software, even with a modest telescope.

How Does Stacking Images Work?

Sinx/x with noise
Stacked Sinx/x
Raw Frame
10 Stacked
Stacked Sinx/x
Stacked Sinx/x
30 Stacked
100 Stacked

Stacking images that are meticulously aligned greatly reduces electronic noise that exists in digital images, appearing as pixelization. It can also help reduce the effects of atmospheric distortions caused by scintillation. The technique tends to work very well because the noise and atmospheric distortions, frame to frame, are different. But somewhat buried in each frame is the true data, which repeats frame after frame.

The above images illustrate the point. I added random, normal noise to an image of Tycho crater for illustration. I created 100 frames of this image, each with independent random noise added. The image on the left shows how a typical frame looked. As you can see, the crater image is barely discernable.

The next image is the result of processing 10 frames by averaging the images. You can see that the pixelization is greatly reduced, and details are emerging.

Image 3 is the result of processing 30 frames by averaging. Now the details of the crater are becoming clear, and the noise is almost gone.

The last image (right-most) is the result of processing 100 frames of the noisy data. Now the crate image looks clean of noise, with features showing clearly.

That's the amazing result of aligning and stacking images. It's really quite surprising the level of detail that can be retrieved using this process. Of course it can't reveal detail beyond the limit of your telescope, but can help retrieve nearly all the resolution available through your optics.

So What if You're a Linux User?

If you, like me, are primarily a Linux user, the bad news is that you must still use a copy of Microsoft Windows to run the camera control program supplied with the Celestron NexImage camera. However, since the camera is basically a Phillips web cam, you might find a Linux package that can collect images from it. I used camstream to collect images from my Quickcam Express homemade astrocamera. That worked, but only let me collect about 1 frame per second. I've not yet located a Linux package that will control the NexImage camera.

NexImage Frame List

But the good news is that after capturing your images, you can use the supplied RegiStax program with the wine utility that's available for Linux. Wine can run many Windows programs quite well, and for most of its functionality, the RegiStax program is one of them.

So far, the only RegiStax feature I've found that doesn't work in wine is the loading of the avi files. That may sound rather tragic, but it's really not a big deal. RegiStax will load a sequence of jpeg image files. Just click the Select input button, then click on the first jpeg file you desire, and shift click on the last file you desire. RegiStax will load the entire sequence and operate on them just as with the avi files.

I did find one little hitch with this procedure. When selecting a range of jpeg files, the RegiStax software builds a frame list that wraps the first and last images. You can see that from this illustration. The image shown is the frame list from a jpeg load. I loaded files 00000010.jpg through 00000050.jpg. But the frame list shows file 50 as the first frame, followed by files 11 through 49 in sequence, with file 10 as the last frame.

If you're taking images through a non-tracking telescope, this anomaly will goof up the alignment phase of processing because the target will be drifting through the sequence of frames. Getting the frames out of sequence causes jumps in target position that confuse the alignment algorithm.

It's easy to fix. Just bring up the frame list and turn off the first and last frames of the list. Then click on the first used frame to set the display. Now you can use the selection box to pick an area of interest and go through the steps previously outlined.

Fine, But How do You Get Jpegs From Avi Files?

If you want to do your work in Linux, you need to convert the frames in each avi file to a sequence of jpeg images. It's actually pretty simple to do if you have the mplayer program installed. Mplayer can play the avi files created by the Celestron image capture software. It can also be instructed to break the individual frames out into a sequence of jpeg files. The command is:

mplayer -vo jpeg fname.avi

Just substitute the file name of your specific avi file for the fname.avi of the example. Now you can use wine to run the RegiStax program and process as described.

To make things simple, I made a script file named avi2jpg that does the process for me. My script is a tcsh script, but you can adjust it to use any scripting language you prefer.

#Script to convert avi frames to jpeg files
#Just type avi2jpg fname
mplayer -vo jpeg $1

Summary and Personal Notes

Is the Celestron NexImage camera for you? It is if you want the following features:

  • Inexpensive - around $100
  • Uses computer to operate camera (get out your laptop)
  • Runs on Microsoft Windows, but RegiStax will run in Linux wine
  • Easy to use
  • Can make images up to 620x480 resolution, b/w or color
  • Works for solar system objects: moon and planets
  • Makes movie sequences of objects
  • Requires stacking software, comes with RegiStax
  • I've used this web cam imaging process before with my Quickcam Express conversion camera, and already knew what the potential was. The Celestron NexImage camera was certainly no disappointment. It works well, is easy to operate, and has the potential to let me take some fantastic solar system object photos. I would definitely recommend it to anyone whose needs aren't outside the features listed.