Last updated on
4/11/
2006

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Fastar & the SBIG ST-237 CCD Camera

The Celestron Fastar Assembly
Schmidt-cassegrain telescopes have a primary light-gathering mirror that reflects light to a secondary mirror on the back of a glass correcting plate (at the front of the scope). This is shown on the left in the diagram below:

Unlike the Meade LX scopes, in the Celestron NexStar 8” GPS scopes, the Ultima 2000, and some of the older C8 scopes this secondary mirror can be removed and replaced with a lens assembly.  The Celestron Fastar (or Starizona Hyperstar, a similar system) lens assembly can hold a CCD camera in front of the scope and focus the scope’s image into the CCD, as shown on the right in the diagram above.  (The Fastar or Hyperstar assembly includes a lens system because the primary mirror wasn’t designed to focus properly at that point so a corrector lens is needed.  On this page, when the discussion refers to the Fastar lens assembly it is also applicable to the Starizona Hyperstar lens assembly.)  The advantage to this arrangement is that at this location the CCD camera sees a very wide field of view at an optical speed of f/1.95, which is a real advantage for shooting deep-sky objects which usually are much larger than the CCD, and are dim.

The earlier 11” NexStar GPS scopes did have a removable secondary mirror to which the Starizona Hyperstar lens system can be attached. Celestron has discontinued the addition of the removable secondary mirror on the Nexstar 11” and CGE 14” scopes offered for general sales but is providing those scopes, with the removable secondary mirror, to Starizona which makes available their Hyperstar lens assembly for them.

The disadvantages to this system are not major but their cost must be considered.  First, when you remove the secondary hyperbolic mirror you can’t view objects visually - essentially you have created a Schmidt Camera and you need to locate and track objects with a well-aligned finder scope and view them on the computer to which the CCD camera is downloading its images.  Second, you need to be very careful that the CCD camera is not struck or otherwise hit, since it’s mounted to the relatively fragile glass plate that comprises the secondary lens of an SCT.  This danger can be overcome by placing a (solid, not flexible) lens shade/dew shield over the front of the scope, such as the Celestron Lens Shade.  Regardless of whether you really need a dew shield in your climate, it will protect the CCD camera and the glass corrector plate from damage.  (See Christopher Anderson’s Darklight Imagery site for an example of what can happen if you don’t protect the corrector lens. The Celestron Lens Shade (#94017) is about $40, is designed to fit snugly on the U2K or C8 scopes, and will protect the secondary lens when the Fastar option is used.) Third, in this configuration you will need to focus the scope by using the scope’s focus knob to move the primary mirror (rather than using a rear-cell-mounted focuser), which will be an insufficiently precise process unless you add one of the appropriate options described on the Focus Options page.

SBIG ST-237 CCD Camera
To use the Celestron Fastar option you need a CCD camera along with the Fastar lens assembly, to capture deep-sky images. Celestron had in the past sold a CCD camera called the PixCel. It was made by SBIG (Santa Barbara Instruments Group), a company that manufactures some of the most highly respected CCD cameras for amateur astrophotography, and in fact the PixCel was the SBIG ST-237.  Celestron subsequently decided not to market this camera under the Pixcel name, and recommended that Fastar users purchase the ST-237 directly. The SBIG ST-237 is configured (with its pixel size and other parameters) to work reasonably well in the Fastar position as well as at an SCT’s prime focus.  It is physically smaller than most other CCD cameras so it causes the least obstruction of the primary mirror.  The ST-237 can also accept an internal color filter wheel, which allows you to take color photos as well as black-and-white. (The image from almost all CCD cameras is inherently black-and-white. To produce a color image you need to successively shoot the object through each of three color filters, and add the images in your computer using the camera’s software.) By keeping the color filter wheel internal to the camera you don’t lose more effective aperture at the front of the scope.

See the Links page for some Web sites where astrophotographers have posted photographs using the ST-237 with Fastar. Also, see Todd Ellsworth’s e-mail page for detailed information on setting up and using the ST-237 with the U2K - he has a lot of good information there.

SBIG discontinued production of the ST-237 and replaced it with the ST-402.  The ST-402 has a larger CCD and is sold for about the same price as the discontinued ST-237, but it also a larger camera body which appears to be large enough to significantly obstruct the aperture of most SCTs.  The ST-237 is often available in the used astronomy equipment market.  However, several of the Starlight Express CCD cameras are small enough in diameter to work in the Faster configuration without causing excessive obstruction of the primary mirror, and these cameras have becoming very popular with Celestron scope users. See the CCD Camera Comparison page for more information.

How does this all fit together?
With the Fastar and a CCD camera, you will also need a computer to capture the digital images from the camera; the ST-237 and the Starlight Express cameras come with software that is loaded onto a computer to capture the camera’s images. You already have a computer, but in this case a laptop computer will be easier to use so if you don’t have a laptop, purchasing a used laptop is a cost consideration.  The cost of the Fastar assembly and a new ST-237 camera together was about $1,700 and the color filter wheel for the ST-237 adds about $400; Starlight Express cameras are priced in the same vicinity as the ST-237 depending on the model.  Since you presumably already have a computer, this isn’t a bad price for the ability to produce wonderful digital photos.

Tom Shaull has created a Win9X application that calculates exposure times for CCD cameras for different scopes.  It will calculate optimum exposure time for just about any CCD camera, allowing for multiple variables including site darkness, percentage of A/D capacity desired, etc.  It is very helpful and can be found at Tom Shaull’s CCD Exposure.

Since a computer is needed anyway and there is a cable connecting the computer to the scope for autoguiding, many Fastar users doing CCD astrophotography also use sky chart software that shows a view of the night sky and can aim the scope at objects you choose with your mouse, the ultimate in automation.


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