ST7E - NABG with CFW-8

I originally had a 'standard' ST7 with anti-blooming gate. While that camera suited my needs for the first 2 years of imaging, I came to want more from the camera. First choice was to upgrade in early 2000 to the new 'E' chip. It has increased sensitivity across the range picked up by the camera, with about 10x sensitivity in the blue - a well-known drawback of the older chips.

At the same time, I opted for the non-anti-blooming version of the chip. Going from the AGB to the NAGB gained me another estimated 30% sensitivity compared to the old setup. This is nice as most of the objects I work on are dimmer deep sky items. The 'blooming' problem with bright stars occurs every now and then, but really isn't an issue as I don't mind much and am not shooting world-class images for the cover of the next Sky & Telescope magazine.

Another option I went for when I had the camera in getting upgraded was to get the more advanced cooling package. A second Peltier cooler drops the camera another 6 degrees Celsius, cutting the dark current at that point in the temperature curve. The water assist feature was also added, taking off another 9 degrees C. This has proven to be fairly effective at helping reduce dark current in my images. Some may say that this is a waste, as I can just take proper darks and subtract them out of my images, and I still do, however it makes the original image just that much cleaner and less digital manipulation of my images means more 'real' data. I have never been a fan of overprocessed images.

Anyway, below are some shots of the camera and accessories for those who may have questions or want a closer look...

Rear View showing new ports for water cooling. The large brass slotted circle is the plug covering the desiccant chamber.

Side view of ST7 with CFW-8 permanently mounted. The CFW-8 adds exactly 1 inch to the optical train. Mounting the CFW8 in it's 'permanent' configuration was very easy and straightforward based on instructions that come with the unit. The nosepiece and adapter on the ST7 are removed, then the CFW8 is opened and the filter wheel removed to get to the screw holes in the back of the case. Tighten a few screw, pop the wheel back in, and close the cover... we are ready to go! Took about 5 minutes.

I opted to use the 'permanent' style mount for a few reasons:

Front View of camera and wheel. Notice red filter (Slot #1) showing through the 2" nosepiece. The trapezoidal bulge 180 degrees from the nose is the raised cover over the wheel motor. The grey cable runs from the filter wheel electronics to the guide port on the camera.

I use the 2" nosepiece for increased stability and an unobstructed light path. Never had problems with the 1-1/4", but you can never be too safe. It also keeps another adapter out of the train because this slides directly into the NGF-S focuser.

Inside side of CFW-8 cover removed. 6 screws on the face easily allow access to the wheel itself.

CFW-8 with cover removed. Note the silver motor and curved electronics board. The wheel is currently mounted with the factory SBIG Red, Green, Blue, and Clear filters (counted as filter slots #1,2,3,& 4 counterclockwise from the red filter). The fifth slot is empty in the image, but holds an H-alpha filter in real life.

Harder to see, but very important are the vertical pins (white dots) near the circumference of the wheel, and also the orange friction ring on the very outside edge of the wheel.

Close up view of the wheel removed from the CFW-8. Removal is necessary to change filters. Note that they screw in from the bottom (ST7 side). Easily visible in this shot are the vertical 'alignment' pins that the wheel uses to determine position. There is one pin near each filter, and two pins ('home' position) by the blue filter. They have a bit of white paint on them (do not scratch off!) for the internal sensor to determine wheel position.

Also seen here is a better view of the orange rubber friction ring that the motor rides against to rotate the wheel around the central pivot pins (1 upper, 1 lower).

Closeup of the wheel motor. Note the small ribbed area on the end of the central motor shaft (between the two support rods, center of image). Perspective makes it look like it is recessed behind the motor mounts, but it is in line with them. This is the piece that comes in contact with the orange rubber ring on the outside of the wheel.

Motor movement is not measured - the motor turns until the sensors pick up the vertical alignment posts.

A view of the 'bottom' side (ST7 side) of the wheel. Note the factory positions of the filters. In MaxIm DLL/CCD, the filters are numbered 1 through 5, starting with the red filter and going clockwise (from this side of the wheel, counterclockwise as viewed through the nosepiece). These are the newer IR-blocking color filters from SBIG that are now the standard on all new filter wheels. You have the new IR-blocking filters if they have the metallic blue retaining ring holding each in its cell (visible in the image below).

These filters are all the same thickness so every shot remains in focus no matter which filter you shoot through after focusing through the clear filter.

NOTE: this 'parfocal' feature is NOT true of the SBIG H-alpha filter that is an optional accessory for the wheel. It is of different thickness and requires separate focusing (I learned the hard way)!! Other filters are also available from SBIG (click here to view SBIG's filter page)