Using a CFW8a Filter Wheel with a Meade F3.3 Focal Reducer

Before reading this article, please note that this arrangement is not supported by SBIG and using this arrangement takes the CFW8a and the filters provided by SBIG out of its intended design parameters. This worked with my CFW8a and SBIG filters. It may not work with yours. I have not tested this with the CFW8 using the separate IR blocking filter.

My mount has some pretty sudden jumps in Periodic Error which prevents me from imaging using anything more than 700mm focal length. So I use the Meade 3.3 Focal Reducer when I image. I wanted to try RGB imaging. My first attempt at RGB imaging using an Andy Saulietis manual filter wheel yielded some interesting results.

First RGB attempt using the 3.3 reducer and a manual filter wheel.

You can see from this image that the stars at the edges are severely distorted by coma. Here is a close-up of some of the stars that were distorted:

I had an opportunity to borrow a CFW8a to see if I could do better with it than with the filter wheel. I attached the CFW8a directly to my ST7E which is what most do. I got similar results as before using the focal reducer and the standard spacers provided by Meade.

I did some research on the Internet and I quickly discovered that the spacing between the focal reducer to the chip is critical and that being off 1mm forward or backward will cause problems. The Meade 3.3 Focal Reducer comes with a variable length T-Adapter that comes with 15mm and 30mm spacer tubes.

Using the 15mm spacer gives about a 0.5 reduction. Using the 30mm tube gives a 0.33 reduction. Using both (45mm) gives about a 0.3 reduction (this never worked with my system). I was using the 30mm adapter attached to the ST7E to get a 0.33 reduction which was yielding excellent pictures. So 30mm from the top of the camera to the back of the male threads on the T-Adapter is the critical spacing for 0.33 reduction.

The CFW8a is 25mm deep so I needed a way to get an extra 5mm for the critical spacing length. I was talking to a fellow CCD imaging friend and he suggested an Edmund Optics T-Mount Double Male-Threaded Ring. He had a 4.9mm ring which would get me really close to the critical spacing I needed (29.9mm).

I detached the CFW8a from the ST7E (that was attached directly to the ST7E) and placed the t-threads plate (that was removed when attaching the CFW8a) back onto the camera. I then screwed the 4.9mm spacer ring into the t-thread plate. I attached the CFW8a to the 4.9mm spacer by screwing it onto the male t-threads of the spacer ring. I adjusted the alignment of the CFW8a to match the ST7E by loosening the 3 allen set-screws that are part of the t-thread plate.

Once the filter wheel and camera were connected, I removed the spacers from the adjustable T-Adapter so all that was left was the T-Adaptor itself. I attached the T-Adapter to the CFW8a resulting in the arrangement pictured below.

A couple of tips when taking the spacers off of the T-Adaptor:

1. Screw the T-Adapter onto the focal reducer to give yourself some extra grip. There was just not enough to hold onto.

2. Keep the reducer attached to the T-Adapter when screwing it onto the CFW8a. Again, this makes it easier to do this.

The ST7E and CFW8a connected using the 4.9mm ring from Edmund Optics. The T-Adapter from the focal reducer is attached to the front of the CFW8a.

Now that this was completed, it was time to test this arrangement to see if I obtained the critical spacing. On the first cloudless night, I attached the Camera/Filterwheel arrangement to the focal reducer (and my Hutech LPR filter which goes behind the focal reducer). This was attached to the telescope (as shown in the picture below).

Optical train using the CFW8a and Meade 3.3 focal reducer once the critical spacing was obtained.

First RGB attempt using the 3.3 reducer and a manual filter wheel.

Wow! I was pleasantly surprised how well it worked for me. Here is a close-up of the lower left hand corner:

As you can see, the stars are nice and pretty round at the edges of the frame (there are color artifacts because of some field rotation as I eyeballed the polar alignment that night). I did find a peculiar problem with this arrangement, however. The stars looked great when using RGB filters. But when I used the clear filter (No IR blocking), I found some coma on the left side of the image. You can see this in the following examples.



Close-up of some of the distorted stars in the upper-left corner of the green image:

Close-up of the same area in the green filtered image:

The stars on the left side of the image do show some distortion. At first I thought it was the filter not lining up in the filter wheel. Then I theorized that the filter itself was not laying flat in the mounting ring. I finally called SBIG about this and they explained that the IR is probably getting refracted. They explained that the filters are not designed to work at F3.3 and they were not surprised that this would happen. Being that I was using the filters outside of design parameters I accepted this answer (plus, others on the SBIG list thought this was the issue as well). I concluded that for luminance data, I would simply use an empty filter slot and then refocus for the color data. I may purchase an IR Blocked clear filter to see if this prevents the distorted stars.

I am extremely pleased that I am able to use the CFW8a using the 3.3 reducer even though the filters (and the CFW8a) are not designed to work in this arrangement. I find that it works quite well and I will be using this configuration until I can improve my mount (which may be a while!!) There are probably some unnoticable artifacts in my images using this optical train, but I am not as critical as some may be. Your results may vary and you may find these results unacceptable.

I have since purchased a CFW8a :-)

Clear Skies and happy imaging!
Jason Hissong

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