Flocking the baffle in a GSO RC

I bought the GSO 8" Ritchie Chrieten telescope in April 2009. It is a great little scope except in one particular area of imaging. The finish of the mirror baffle is such that when bright stars are just outside of the field of view, very bad reflections appear in images. This most often occurs when imaging galaxies. I have set about to correct this problem while awaiting GSO's response to change the interior surface of the baffle. To that end I wanted to flock the baffle or at the very least re-paint the baffle with flat black chalk board paint.

Disassembly

As with any instrument I consider it necessary to place registration marks on the side of the telescope. This aids in reassembly of the telescope and ensures that you have the telescope back as it was before disassembly. In this regard I used temporary tape and placed a couple of registration marks around the OTA at the junction of the tube and the mirror back. Take a bit of time with this and be as accurate as possible.

The first thing you need to do after installing the registration marks is to undo all the screws at the mirror back connection of the tube. Be very careful as the mirror back will just slide out after all the screws are released. Pull the tube up and place it with the secondary mirror facing down. Then as quickly as you can grab the mirror baffle and unscrew it in an anticlockwise direction. It is not hard to remove this.

Once you have removed the baffle place the mirror back (which contains the primary mirror) into a dust proof environment. The mirror is over coated but it is best to protect the mirror. I used what we Australians call an esky (beer cooler).

Now you can get on with the job of working with flocking material or painting the baffle tube. I initially tried using chalk board paint, which on second coat became a bit shiny. Obviously not the desired effect. I then found some left over flocking paper from a previous project and gently installed this throughout the inner surface of the baffle tube. Not knowing the exact trace diagram of the light rays I am assuming that this will not affect the light cone or cause much vignetting. It does not look pretty but as can be seen below (sorry about the out of focus image) the level of light reflection is now nil.

Once you have installed the flocking material, give its a bit of a clean with a cigarette light to take away the dags of fluff and then screw the baffle back into position. Then just reassemble the OTA in exactly the reverse of the way you disassembled.

Once everything is reassembled then all you need to do is collimate the instrument. I have found that collimation is easy enough when using live view in my 40D and centering the secondary while seeing the diffraction rings live. Pay particular attention to collimation as it will mean the difference between good images and bad ones.

Final Analysis

The images below show the effect of the flocking of the baffle.The image directly below shows NGC4945 before the flocking. You can see three rings which are reflections. Obviously this is not an acceptable situation.

The image below shows the effect of the flocking. There are no reflections visible. I am very happy that this was an easy and pain free modification. I will be notifying GSO of the modification and asking for a baffle that performs in a similar manner.

Best of luck if you do this and remember you do this at you own risk.