Techniques - Imaging with the ETX70 and SAC7 CCD Camera  

Background:

The ETX70, SAC7 CCD Camera, AstroVideo, and Picture Window Pro combine to for a very low cost and easy to use set of tools capable of producing some nice astro-images. When I purchased the equipment new, this setup of telescope (OTA, goto mount and tripod), camera, and software, cost me less than $800 total. I do the majority of my imaging from my suburban back yard in central Massachusetts.

Imaging with an ETX70 and SAC7 camera is -really- fun. It's also a very easy and relatively inexpensive way to get started in imaging. I highly recommend the combination.

The key characteristics of the ETX70:

• Large field of view
• Fast focal ratio (F/5)
• Goto mount
• Very light
• Small aperture
• Limited magnification

The Meade Autostar does a very nice job finding and tracking targets. The equipment is very light, telescope, camera and eyepieces can all be brought out in a single trip, on a moments notice. I use the telescope aligned in AltAz mode to make setup quick and easy.

Because of the big field of view and small aperture, the 70 is particularly good for large, relatively bright deep sky objects. The fast focal ratio makes imaging these objects easier by keeping exposure time requirements down. Although capable, the telescope isn't quite as good for Solar System objects and planetary nebulae, as they typically call for more magnification than it can deliver.


The key characteristics of the SAC7 camera:

• It is a color camera
• Supports extended exposures
• Supports .AVI movie format
• Has a small pixel size (5.6 microns)
• Has a small chip area (3.6mm x 2.7mm)

The SAC7 is a nice, rugged, low end color camera. The camera is controlled with the supplied AstroVideo software from COAA. AstroVideo is integration software, enabling the addition of multiple frames to create a final image. It is easy to use and can control all functions of the camera. Planets, Moon and Sun can be imaged by taking .AVI movies and from one to hundreds of long exposure stills can be taken for imaging deep sky targets.


The ETX70 and the SAC7 together:

When used on the ETX70, the field of view of the SAC7 is 26.53 arcminutes high X 35.37 arcminutes wide. This means that the field of view is fairly large (larger in width and only slightly smaller in height than a full moon). The image scale of this combination is about 3.3 arcsec per pixel which is forgiving. The scope and mount can bump/shake/vibrate or otherwise miss by up to 3.3 arseconds without having light cross over onto an adjacent pixel.


Imaging Deep Sky Objects:



Getting good deep sky images takes time both in the field as well as at the computer during image processing. Seeing a faint nebula or other object slowly come into view when adding images together is exciting and rewarding. This is -not- an uncommon experience when summing lots of short exposure images! Success is dependent on finding and centering the object in the camera's field of view, and on taking the following steps:

* Focus carefully

When acquiring the images, one of the most important factors is having good focus. Having an eyepiece that is near par-focal with the camera is a big help. Adding par-focalizing rings to eyepieces works if you don't have any eyepieces that match the focal point of the camera. I have found that one ring on a 12mm eyepiece gets you pretty close. Alternatively, you can count the number and direction of focus rotations going back and forth between the camera and a favorite eyepiece to get you close. In either case, final fine-tuning of the focus is critical to getting a good final image and is worth spending time on up front.

* Get steady exposures

I've tried various exposure lengths, and have had the best luck with 8-12 seconds. Taking 100 exposures of 10 seconds each, I routinely keep over 90 images, with a loss of less than 10% to shakes and trails. The "keeper" rate decreases sharply after about 15 seconds of exposure. Final images are much better if only clear, tight images are used. The drawback however, is that shorter exposures tend to have less detail. In other words, you usually can't "go deep" into image detail with short exposure lengths.

* Gather lot's of data (exposure time)

It's very important to gather as much clear, sharp exposure time as possible. I usually stick to a very limited number of subjects per night, and take lots of images of them for this reason. Good images are summed together, adding their light together into a clear exposure much longer than the mount would ordinarily be capable of. Since only quality images should be used, maximizing their quantity is important.

* Always take dark frames

At least with my camera, extended exposure images have too many hot pixels to be used without dark frames subtracted. I always take a set of dark frames at the end of each session. It's important to keep the camera contrast and gain control settings unchanged for the dark frames. In other words, these settings should be -exactly- the same for both the images and the dark frames. Failure to take a good set of dark frames usually results in a set of very noisy images.

* Apply the dark frame subtraction

Always perform the dark frame subtraction -before- summing the images. I usually sum 5 dark frames together and scale the subtraction at 0.2 in AstroVideo. Occasionally, if background noise is very high, I will use 10 frames scaled at 0.1 but as the number of frames increase, and the scale decreases, the chance for "cold" pixels (black spots) increases. Cold pixels are as bad as hot pixels and need to be avoided.

* Rough sum the images

When you are ready to combine the (already dark subtracted) images from an imaging session, it helps to roughly combine sets of images that can be more finely processed later. When dealing with sometimes hundreds of images, it takes too long to carefully align each one. AstroVideo has a nice manual sum routine that lets you add multiple images together. I usually sum about 90-120 seconds worth of images together (usually 10-20 images). Any longer and the effects of field rotation wreak havoc on your star fields as shown in this example. This star field image was summed using 60 raw 10 second images and registered in AstroVideo using a single point near the top right portion of the image. Note that the field rotation effect worsens as you move further from the registration point.



* Perform a refined sum

After creating the rough sums, convert them from FITS to BMP format for further processing. Assuming an exposure time of 10 seconds, and 200 total frames, there should be about 10 images of 20 frames each to process. I use the Picture Window "two point shift, rotate and scale" option to register and add images together. Using image processing software to carefully register the images eliminates most of the field rotation that is seen when just using a one point registration as with AstroVideo. Here's the same star field, created using exactly the same raw images as in the example above, but registered using the rough sum then refined sum technique just described.



* Process the final image

I usually find that most good images don't require much processing, although it's sometimes hard to keep from over-tweaking. A good rule of thumb is to make small processing changes that can be backed out using undo if necessary. It is also a good idea to save different versions of your work under different names or version numbers so you can always go back to a known good point if you really make a mess of things.


Imaging the Moon and Sun:

When imaging these bright objects, it's good to take .AVI movies rather than single exposures. As noted previously, the Moon almost fits into the field of view. Once the capture of the .AVI starts, you can (slowly) pan the telescope around with the Autostar slew keys. Covering the whole Moon or Sun can be done easily in about 30 seconds.

With both these subjects, brightness is an issue. Of course, always use a solar filter intended for solar viewing when working with the Sun. When imaging the moon, I usually use both red and blue filters at the same time. This keeps the camera from being saturated by all the light and helps to improve contrast and pick up finer detail.

To create a composite image of the moon or sun, I take the following steps:

* Establish "view segments"

Edit the .AVI files into smaller segments that represent a single view of the image. For example, the Moon .AVI can be broken into 2 segments, one representing North, one South. To do this I use MGI VideoWave, an application that I got from another purchase. There are utilities available from multiple sources to perform this function.

* Average lots of frames together

The individual .AVI frames can be averaged together into a final image, greatly improving the signal to noise ratio. I use Robert J. Stekelenburg's fabulous AstroStack freeware on the internet (http://utopia.knoware.nl/users/rjstek/english/software/) to perform this function, but again there are other alternatives available if needed. At this stage one might apply an unsharp mask to bring out additional details in the final images.

* Create the composite

The final step is to merge the image pieces together (2 of them in the case of the full moon) to create a final composite image. If the images are not aligned properly, the seam connecting the images will be visible. Picture Window Pro is particularly good at registering and merging images. Can you see the seam in this full moon composite?




Imaging the planets:



Imaging the planets is similar to imaging the Sun and Moon in that you capture the data with .AVI files and then process them into a final image. I typically don't use filters when imaging the planets, I use barlow lenses to get the magnification as high as seeing and the telescope will permit. With a 3x barlow in place, the SAC7 gets to about 105X which is pushing the little ETX70 to it's limit.

I have never been able to get more than a couple of weather bands visible when using the ETX70. I have tried and failed to pick up the great red spot, and shadows of Jupiter's moons. This telescope really doesn't support the amount of magnification you need when imaging planets. It's the 70's wonderful field of view that makes up for it's magnification limitations. There are always plenty of trade-offs to be considered in astromony!

To create a final image of a planet, I take these steps:

* Identify the best AVI frames

A key to producing a good planetary image is to use only those frames that represent the very best moments of seeing during the capture. I'll usually examine about 100 frames and select from between 10 and 20 frames to use in producing the final image. I use the handy Avi2Bmp utility to select the frames and create .BMP files out of them. This software can be downloaded without charge from http://avi2bmp.free.fr/telechar.htm

* Average lots of frames together

The selected best frames (.BMP files) can be averaged together into a final image, greatly improving the signal to noise ratio. Again, I use AstroStack for this type of work. Unsharp masking can also be applied at this stage to help to bring out details, but if used too heavily, can wreck the image.

* Perform final image processing

Final image tweaking, such as sharpening and adjusting the contrast and brightness curves, has a big impact on the final image. As noted, I use Picture Window Pro for this. The image of Jupiter and moons above was created by making a composite using an extended exposure to capture Jupiter's moons, and an AVI to image the planet itself.


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