Clement Focuser and Astrodon TAKometer Upgrade
In January of 2007, I
completed an upgrade to the focuser on
my Takahashi FRC-300 that I thought others might like to see (of course BRC-250
owners could do exactly the same thing). I have always had a love/hate
relationship with the big Takahashi Helical focuser that comes with these
scopes. The Takahashi helical focuser was very precise, strong, and well made,
but it didn't lend itself at all to automated focusing. The standard fix or path
to automated focusing with these scopes was to install a secondary focusing
device like a FLI PDF focuser, and I did that myself for the past several years.
But there were a few problems I saw with that approach.
The secondary focusers are inserted into the light path on the camera side of the FRC's corrector lens. Focusing adjustments therefore change the metal back distance (MBD) between the corrector lens and the CCD. While it can be argued that the MBD change is so slight that it is unimportant, there have been several tests posted that show changes to the image quality with just a few mm's change in the MBD. I've also been a bit concerned about the ability of some of these secondary focusing devices to handle the increasing loads created by all the extra stuff (large filters wheels, AO type devices, etc) and the larger cameras we are all hanging off them now. The final issue for me was that the secondary focuser consumes a lot of the MBD, and I wanted to free up enough room to be able to fit a remotely operated Camera Angle Adjuster.
My imaging partner and I found the solution with a Clement Focuser. We had Don at Clement make a custom 4.25" Bellerophon II focuser. With the large 4.25" aperture we could remove the helical focuser all together, replace it with the Clement, and mount the corrector lens on the camera side of the focuser. With this arrangement we can maintain a consistent (and exact) MBD, and free up room to install one of Astrodon's TAKometers to handle the remote CAA function. It sounded too good to be true – but it wasn't! For about the same cost as a PDF, Don made up one of his complete automated focusers.
I completed the installation a few weeks ago and couldn't be happier. The Clement Focuser is very precise, on par with the original Takahashi Helical Design and with similar resolution to the PDF, and fits nicely in the original focuser envelop. The appearance is a bit usual, but it grows on you - especially when you see how well it works! The automation interface is a Robofocus, so there is really nothing new to learn there. As a final plus, the nature of the Clements focuser design gives it a very nice load capacity. Since there are no guide pins or bushings with clearances used with this focuser, there is little of no chance of flexure being introduced by the focuser. I'm very pleased, and find that the v-curves produced in FocusMax are some of the best quality I have ever created with the FRC. Now if I just had that new imaging camera.............
Here are a few images of the set-up: