AO-7 Quick Release Adapter

Why a Quick Release Adapter?

I primarily image with two different scopes, a Takahashi FSQ-106 and a Takahashi Mewlon-250, and a single SBIG ST-7 camera.  The FSQ has very little back focus (especially with the JMI NGF-S focuser I use for electronic focusing), so I cannot use the AO-7 when imaging with the FSQ.  When imaging with the Mewlon, the AO-7 allows me to be a bit lazy about mount calibration, so I almost always use it with the Mewlon.  As all AO-7 owners should know, attaching and detaching the AO-7 is a real pain since you must remove the camera adapter ring from the camera (this adapter is shown in the pictures below), take apart the AO-7 (yes, remove the four socket cap screws on the back and pull the AO-7 apart), bolt the AO-7 to the camera face, and put the AO-7 back together.   For a one-time installation, it is not a big deal.  To do this every time I want to image with a different scope is a nightmare, and certainly cannot be done "in the field".  That said, I devised a quick-release adapter for the AO-7 that adds minimal back focus and allows the AO-7 to be quickly and easily attached and detached from the AO-7.

The Adapter

Here it is!  I machined this on my Grizzly mini-lathe out of T6061 aluminum.  Since I was pretty much learning machining as I went along, the cuts are not as pretty as I would like, but it should work great.   The narrower part at the top has an angled cut on the side to mate with the camera adapter as shown below.  The big hole in the middle is threaded to reduce internal reflections.  The final step is to paint the inside of the hole with flat black paint to further reduce reflections.   I would also like to bead blast it (prior to painting of course) at some point and make the whole thing thinner by machining down the side sitting on the table, but first I just want to try it out and make sure it works as expected and doesn't have any strange effects on the AO-7 performance due to changing the mirror-to-chip distance of the setup.   Currently the adapter adds just over 1/4" of backfocus, though this could be cut down to about 1/8".  My Mewlon has plenty of backfocus, so I'm not going to worry about that right now.

adapter_only.jpg (14818 bytes)

adapter_on_ao-7_small.jpg (43154 bytes)

Here is the adapter attached to the AO-7.  The AO-7 bolts onto the adapter just like the AO-7 normally bolts onto the face of the camera using the same bolts.  The barrel with the silver cap sticking out to the top left is the f/9 reducer for my Mewlon-250.   Believe it or not, the regular SCT adapter on the AO-7 is removeable!  There is a tiny hole with a tiny set screw that holds it on.  Just unscrew the set screw (the set screw jams into the threads of the SCT adapter tube that screws into the AO-7 body) and unscrew the SCT adapter.  I had a heck of a time unscrewing the adapter- it was stuck solid.  The magic that got it off was a flat rubber jar opener that you can get for about $.50 at the grocery store.

Here you can see how the adapter will attach to the camera.  The squarish black part bolted to the camera (which I'll refer to as the "camera adapter") above the label (yes, the label is not in its usual place, but that's another story) has a "socket" (for lack of a better word) with three set screws that hold in a 2" nosepiece or other accessory, such as my AO-7 adapter.  On some cameras this socket arrangement is replaced with a t-thread, which has its own advantages.  Of course the t-thread style camera adapter will not work with my adapter, but it is easy enough to machine a camera adapter with a socket as shown at right.

camera_adapter_small.jpg (53180 bytes)

connected_small.jpg (26755 bytes)

Here is the whole combo.  The AO-7 adapter fits into the camera adapter's socket.   When I attached the AO-7 I tried to make it as square to the camera as possible since I am guessing the AO-7 algorithms expect this which makes a movement of the mirror in the X direction move the star only in the X direction, and vice versa for the Y direction.   If the alignment is off, a mirror movement in the X direction will move the star in both the X and Y directions depending on how far off the alignment is.  So, I need to test it.  If it turns out to be an issue I will have to modify the adapter (and possibly make my own camera adapter part) that has some sort of key slot to ensure the AO-7 is perfectly aligned with the camera.

Anyway, the next step is to paint the inside of the adapter and see how it works.   Wish me luck!

Update April 8, 2002
I painted the internal surfaces flat black and have since taken a few images with the new setup.  It works great, and taking the AO-7 off my camera is now much simpler.

Last Updated: April 8, 2002
Copyright (c) 2002.  All rights reserved.