Gallery XXXVII - Deep Space - 10/5/04

The Camera comparison.

        I have acquired over time three B&W video chip CCD cameras.  The Stellacam EX, a Stellacam II and a SAC 8. I've used the first two more then the second and the third least of all. The Stellacam's get more use, because they are easier to use. The ability to see the object to be imaged in "real time" is a big plus. It helps with positioning and centering issues. The idea behind purchasing these was to have a simple way to image some of the brighter deep space objects. The video cameras seemed to offer ease of focusing as well as overall ease of use. As an added benefit the video cameras can provide "real time" or "near real time" views of objects on a TV or B&W video monitor or a computer screen. I got the SAC 8 as when it was introduced, it was the closest thing that there was to a Peltier cooled Stellacm EX. (The new SAC 9 may actually be closer to this goal).

            The SAC 8 has a smaller FOV and slightly smaller pixel size but it is a cooled camera (thermoelectic) and can image for longer periods of time. it has "real time" video capibilities, but these are rather anemic compared to the Stellacam's and is useful for focusing on bright stars but will not let you see DSO's in "real time". You can't see the DSO to be imaged until you take a short exposure of it. It has a Peltier cooler for the chip.

            The Stellacam EX has a very sensitive chip and can integrate up to 128 video frames (it calls this "sense up"). But it does more then just integrate. It takes longer then 2 seconds to maximize image brightness. It sometimes takes a minute or more (I've never really timed it) to "warm up" (I say this for lack of more precise understanding of what it is doing during this period of time) and come to maximum image brightness after being told to "sense up". This suffers from AMP glow on the left hand side of the image or video screen so DSO's are best positioned to the right side of the frame. Even so, when applying image stretching with the "curves" tool this amp glow can produce a graded shading across the image which is brighter to the left.

            The Stellacam II has a less sensitive chip then the EX, but it can integrate for up to 8.533 seconds worth of video frames (vs. 2 seconds and the "sense up" magic if the EX) and does not have the pronounced AMP glow. It is more of a WYSIWYG camera in that if it is told to integrate for 2 seconds it takes 2 seconds to generate the image, if set to 8 it takes 8 seconds. What ever the last image the camera generated was, that image is displayed as a constant video output until the camera updates the image the next time. This gives 'pseudo real time integrated video' or serial display of integrated images. The camera controller is also easier to use then the one for the Stellacam EX. This and the lack of amp glow are a big plus. And I believe that it's maximum sensitivity mode is more sensitive then the Stellacam EX's.

            I went up to the DCMO to the planetary nebulae NGC6781 with the C14 with all three cameras. I had a Lumicon "deep sky" filter on the Optec f3 reducer. The Stellacams were attached to the F3 reducer with the Optec "C" thread adaptor and the SAC 8 required the use of the "T" thread adaptor. The Stellacams were nearly parfocal, the SAC was not (this may relate to the different sizes of the "C" and "T" adaptors). I tried to set the Stellacams to their maximum settings.

            The Stellacam II was set to: maximum gain, gamma none, integrations 256. 

            The Stellacam EX was set to sense up 128, manual gain -> maximum, ELC mode.

            The Sac 8 was set to long exposure mode, twain driver, 10 second image.

            The Stellacam II imaged for 80 frames (682.64 seconds), the Stellacam EX for 341 Frames (682 seconds), and the Sac8 for 68 frames (680 seconds). This was done to replicate what I'd done the other night. All images were dark frame subtracted. All were acquired "on the fly" with Astrovideo stacking the frames during the exposute to produce one ".fit" file as output.

            For the table below the raw frames were cropped in GIMP. No processing was done.

The Raw Frames

Stellacam II, raw frame cropped , baseline camera settings

Stellacam II, cropped raw frame, brightness and contrast settings were manipulated in Astrovideo prior to acquisition.

Stellacam EX, raw frame cropped

 slightly cropped SAC8 output, notice the larger image magnification because of the smaller pixel size.

          

The processed frames

These were processed in GIMP using the "curves tool" and the "levels tool"

Stellacam II - processed baseline settings

Stellacam II - processed "tweaked" settings - raw frame was brighter but final image is no better

Stellacam EX - processed - looks noiser then the Stellacam II,  I think that this is the difference in SNR between the 2 second and 8.5 second exposures, but it may just be more hot pixels trails appearing black (DFS image) against the amp glow

SAC 8 -  a little cropped - processed

 

SAC 8 with and without dark frame subtraction

For the SAC 8 images the Contrast was about 90% and the brightness 80% and the gain was about 1/3 up.

 

Raw Frames

No dark frame subtraction. I have no clue why it looks like this.

 

Dark frame subtracted - a little cropped

 

Processed SAC 8 images

No dark frame subtraction, cropped, processed.

Notice the Stellacam EX-like AMP GLOW?

 

Dark frame subtracted, cropped and processed.

 

I thought that this night be an interesting example of what these cameras can do in short exposure mode on one of my favorite PN's in Aquilla. These are basically 11 and a half minute exposures.

 

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