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- SHARPSTAR 94EDPH APOCHROMATIC REFRACTOR
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- FIELD TEST: THE NOH CT-20 ALT-AZ MOUNT
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A New Look at Yesterday's CCD Imaging Gear
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A NEW LOOK AT YESTERDAY'S CCD IMAGING GEAR
This is intended as a brief recap and overview of the development of the very popular line of astronomical imaging cameras and accessories offered by the Santa Barbara Imaging Group. The primary goal is to examine some of the earlier instruments in this line to see whether the secondhand market in them represents a viable option today for those potential CCD imagers who aren't ready to invest in the powerful but expensive current models.
To provide some context, I'll begin by providing a brief history of the imaging cameras I've owned and used. Then I'll describe the evolution of SBIG's dedicated CCD imagers and try to point out those features which are highly valued in current gear and are also present in those older models, as well as those features which are missing in the earlier units. I'll also briefly compare the older cameras to a few of the current economy model imaging cameras. I'll conclude by describing the decisions I made regarding the suitability for me of some of the early SBIG models. I've also reluctantly attached an appendix explaining some of the numbers which could not be avoided in this discussion.
My first dedicated, cooled CCD imaging camera was a Meade 216XT with 616 color filter wheel. I later owned the SBIG/Celestron ST-5C and a couple of Meade 416XTE cameras. I've also owned and used Meade's DSI, DSI Pro, and LPI cameras as well as Celestron's NexImage and SAC Imaging's SAC-7B. Dedicated astro-imaging software that I own and have used includes Maxim/DL-CCD, Images Plus, CCDSoft, CCDOps, K3CCDTools, Registax, and Astrovideo.
Despite all this, nearly all of my imaging has been done using first a stock Canon 300D DSLR and later an IR-modified 300D. The primary reason is that my first Canon arrived at about the same time as my observatory. I can't resist adding the otherwise irrelevant comment that an observatory is the best imaging accessory there is. My desire to return to monochrome imaging with its improved resolution is what prompted this research.
WHY CONSIDER SBIG?
Fifteen years ago nearly all the great CCD astro-imagers were using either homebrewed cameras or gear by Santa Barbara Instrument Group. SBIG was the major innovator in this field. Their ST-4 was the first standalone CCD autoguider on the market and was eagerly adopted by both film astrophotographers and early CCD imaging enthusiasts. Their next product, the ST-6, was introduced in 1992 and was the first commercially available camera designed for and dedicated to CCD astrophotography. This extremely popular camera featured an imaging chip of 6.5mm X 8.5mm, delivering a resolution of 375 X 241 pixels. The interface to the controlling computer used the computer's parallel printer port. The unit incorporated active cooling and temperature regulation for consistent noise performance. This was to become a feature offered in all of SBIG's products.
THE NEXT GENERATION - SELF-GUIDING IMAGERS
Things got really interesting in 1994, when SBIG introduced the next generation of CCD imaging cameras. These were the ST-7 and ST-8. The ST-7 had a slightly smaller imaging chip than the ST-6 (4.5mm X 7mm) but a higher pixel count (765 X 510 pixels) due to smaller pixels. The big excitement, however, was the addition of a second imaging chip inside the camera body. This permitted autoguiding from the same telescope as the imager - no more off axis guiders or separate guidescopes. The convenience of this plus the avoidance of guiding errors previously caused by guidescope slop or misalignment or by mirror flop in Schmidt-Cassegrain telescopes made SBIG the undisputed leader in the industry.
Even better was the ST-8; this was the same camera as the ST-7 with a different, larger imaging chip installed. The ST-8 had an imaging area of 9mm X 14mm (exactly twice the ST-7's dimensions) in an array of 1530 X 1020 pixels. In those days that was a very large chip, indeed, and imagers snatched them up despite the impressive price tags.
Other SBIG cameras which were basically the same as the ST-7/ST-8 with different imaging chips were the ST-9 and ST-10. The ST-9 has a less dense pixel array of 512 X 512 but the pixels themselves are substantially larger (20 microns). This results in a FOV not much less than that of the ST-8 but at reduced resolution and cost. The ST-10E uses smaller pixels (6.8 microns) than the ST-7/ST-8 but in a larger matrix of 2184 X 1472 pixels, resulting in both a larger FOV and higher resolution than the ST-7/ST-8.
ECONOMY MODELS - A JOINT VENTURE
SBIG also addressed the economy market. A joint venture with Celestron resulted in the creation of the ST-5C, which was also marketed by Celestron as the Pixcel 255. This unit featured a parallel interface and an optional internal filter wheel. The imaging chip was very small (2.4mm X 3.2mm; 320 X 240 pixels) but it was a complete, thermoelectrically cooled imaging system and was bundled with excellent software. The next joint creation was the ST-237, offered by Celestron as the Pixcel 237. The final enhancement of this design was offered in 1998 only by SBIG as the ST-237A. For about $1300 (plus about $400 for the optional internal color filter wheel) one received a very capable, low-noise imager with an imaging area of about 4mm X 5mm and a resolution of 657 X 495 pixels plus very powerful imaging and processing software.
In the intervening years the ST- series cameras have been updated with higher-sensitivity imaging chips (ST-E), even higher sensitivity chips (ST-ME), and much faster USB interfaces to replace the original slow parallel port interfaces (ST-?XME). Other, similar cameras have been added to the line, including the ST-402 (a non self-guiding camera using the same imaging chip as the ST-7) and the ST-2000 series (available in both monochrome and one-shot color versions). Accessories include color filter wheels for the ST- and the ST-2000 series (CFW-8 , CFW-9, and CFW-10) and an internal CFW for the ST-402. Another important accessory made possible by SBIG's use of self-guiding cameras is the AO-7 (AO-L for later models). This accessory uses an internal moving mirror to provide guiding corrections at an amazing rate of 10 per second. This permits the use of imaging resolutions far beyond those possible with typical imaging telescope mounts.
During this period, imagers also explored other directions. Many cameras were introduced using VGA webcam chips. These chips were substantially smaller than even the ST-237A and also incorporated Bayer matrix filters to permit one-shot color, significantly reducing their resolution. Canon introduced a series of digital SLR cameras which proved to have strong potential for astro imaging. They had fairly low noise and decent sensitivity plus a huge chip, nearly twice the size of the ST-8 at perhaps one quarter of the cost. They also have that darn Bayer matrix filter, though, and lack the cooling and self-guiding capabilities of the SBIG cameras. Meade introduced their competing imagers, the 416XT and 1616XT. These used the same imaging chips as the ST-7 and ST-8 but lacked the self-guiding feature and used a quirky SCSI interface. They quickly acquired a mixed reputation and were never popular.
ECONOMY MODEL CHALLENGERS
Meade made the next significant challenge with their DSI and DSI Pro series cameras. These have imaging chips nearly the size of the ST-237A and not far below the ST-7. They aren't actively cooled but do have decent cooling characteristics and pretty good sensitivity. They are available either with or without the one-shot color Bayer matrix filter so they can be used at full resolution for RGB imaging (though no automatic color filter wheel is available). They are far less expensive than the current SBIG offerings and have become appropriately popular. The DSI II and DSI Pro II built on the original model with even better passive cooling and larger chips (nearly the size of the ST-7 and with a greater pixel count).
MY PATH DOWN THIS ROAD
Being frugal, I watched the ST-7/8 gang with envy but picked up the much less popular Meade 216XT and 416XT. Once I got the silly SCSI interface running they did work but setup and teardown time made them seem like too much work so I rarely used them. About the time I acquired my observatory I picked up a Canon 300D DSLR. Between the simple camera setup and the fact that everything stayed ready from session to session I finally began to get some productivity and I used that gear for a few years of enjoyable imaging. I'm still an enthusiastic supporter of the use of DSLR cameras for astronomy and I'll keep mine for the foreseeable future.
I did eventually reach the point where I wanted to do some high-resolution imaging of smaller objects and the Bayer filter began to seem like a problem. I picked up a DSI Pro to try some manual RGB shots. I quickly learned two things: The DSI Pro is very sensitive - it makes a terrific guide camera - BUT I found that I didn't want to be running out to the observatory to switch filters all night when using it as an imager. The obvious next step was to dig out the old 416XT with its automatic filter wheel. True to its reputation, the camera died hard during the first night's setup. A brief examination proved that repairing whatever was wrong with it exceeded my level of motivation to do so.
Great - now what? I knew I didn't want one-shot color and although several new monochrome cameras had popped up in the intervening years I didn't want to buy into any more problems. I just want to take pictures. I knew that I wanted the features and quality offered by SBIG but I'm too frugal to invest in their current offerings in case I decide I still like DSLR imaging better. Besides, a big part of my enjoyment in this hobby is seeing just how much I can do with how little investment.
I remembered that back in the day I had decided that when the ST-8's had been around long enough to drop in price I'd grab one. Some shopping around revealed that the availability of the new models with their very desirable USB interface had driven the prices of the parallel models way down. That's what led to the writing of this article. I want to describe what's available now at very attractive prices, and to decode SBIG's confusing model numbers for the uninitiated.
MODELS TO CONSIDER
In my opinion, the first model to consider is the ST-237A. These can be found complete with the internal color filter wheel and plenty of great software for $500-$700. They lack the USB interface of the DSI Pro/DSI Pro II but use active, regulated cooling and a very sensitive chip. Filters can be switched remotely and the camera can also be used as a guider. The lack of USB is a real issue for many laptops but for those of us using desktop computers for camera control they are fine.
ST-7 AND VARIANTS
The next model to examine is the ST-7. Here's the model number breakdown: The original, parallel interface, dual chip self guiding camera is the ST-7. An economy model was briefly offered as the ST-7I without the self-guiding chip. A model with enhanced blue sensitivity was offered as the ST-7E. A model with microlenses to focus all the light onto the pixels and away from the spaces between to increase sensitivity yet further was called the ST-7ME. Adding an AX@ to any of the later models means it has the very desirable USB interface. New price on an ST-7XME is about $2200. The CFW-8a filter wheel accessory adds about $900 to that. You can find used parallel interface, self-guiding ST-7's and ST-7E's for about $800-$1000. With the color filter wheel figure $1300-$1500.
ST-8 AND VARIANTS
Next is the ST-8, arguably the sweet spot in performance v. price. The model nomenclature follows the ST-7 format. A new ST-8XME sells for about $4500. A self-guiding, parallel ST-8 or ST-8E will go for $1300-$1500, an ST-8I for less. For a self-guiding, parallel ST-8(E) with color filter wheel (these cameras use the same CFW-8(a) as the ST-7 series) figure on about $1800-$2000. Yes, the chip is about half the size of a DSLR but it's cooled for less noise, has terrific sensitivity, no Bayer matrix filters, and comes with great software. It was designed solely for the purpose of astro-imaging.
CAMERA PIXELS PIXEL SIZE CHIP SIZE FOV IMAGE SCALE (MICRONS) (mm) arcminutes arcseconds/pixel
DSI PRO 510 x 492 9.1 x 7.5 3.7 x 4.6 13 x 16 1.71
DSI PRO II 752 x 582 8.3 x 8.6 5.0 x 6.2 17 x 21 1.74
ST-237A 657 x 495 7.4 x 7.4 3.7 x 4.9 13 x 17 1.52
ST-7 765 x 510 9.0 x 9.0 4.6 x 6.9 16 x 24 1.85
ST-8 1530 x 1020 9.0 x 9.0 9.2 x 13.8 32 x 48 1.85
MY RESULTING DECISION(S)
My problem is solved. A few quick swaps of unneeded gear (thanks to www.astromart.com and www.cloudynights.com ) has yielded an ST-237A with internal filter wheel, an ST-7E, a CFW-8, and an ST-8 to play with. A trip to CompUsa and $60 netted me two extra PCI parallel ports so I can run all three cameras at once while I fiddle with them. For general RGB imaging, I plan to use the ST-8 and CFW in self-guiding mode. For narrowband Ha or OIII imaging I'll use the ST-8/CFW-8 with either the ST-7E or the ST-237A as a guider (one problem with self-guiding cameras is that the guide chip is looking through your filters so with narrowband filters there's an issue with getting sufficient light). I'll have a spare available in case Murphy strikes again.
All these were at the top of the industry not so many years ago and in my opinion they represent terrific values on the used market. A quick Google search will reveal the quality of the images folks obtained using these old standards. SBIG is still very much in the business so there's no worry about service availability. These cameras aren't likely to depreciate any further so you should be able to count on recovering your money if you change your mind or decide to trade up. It's entirely possible that, if you do the same analysis, your conclusion will be similar to mine. You may decide that an early SBIG camera is the right choice for you at this time. On the other hand, if CCD downloads and microlens technology are essential to you, then these dinosaurs won't hold much interest for you. That's astronomy - there's something for everyone!
What do all these numbers mean? The pixel matrix array is pretty simple to understand; it defines the overall size of the image created by the camera. The chip dimensions determine the field of view the camera can see with a telescope of a given focal length. This can be calculated by the formula, 57.3/Focal Length of Objective (mm) * chip size in mm. The pixel size determines the resolution attained for a given focal length; smaller pixels result in higher resolution. Oversampling occurs when the pixels are so small relative to the focal length that the resulting resolution exceeds the limitations of seeing or of the optical system. This wastes camera performance but does take advantage of all the resolution the optical system can deliver. Undersampling results when the pixels are too large, failing to take advantage of the available resolution of the instrument. Imagers usually try to select a camera and telescope which will deliver on the order of 1 to 2 arcseconds of resolution. For a 1000mm telescope, this would require pixels of about 3.5 to 7 microns. Hint - rather than do the math, just download CCDCalc, a free CCD calculator from ANew Astronomy Press@ at http://www.newastro.com/newastro/default.asp .
Disclaimer: I have no commercial relationship with Meade or with SBIG beyond that of being a purchaser and user of their products. I also have no commercial relationship with the author or publisher of any of the books offered on the website to which I have supplied the URL.
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