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Tutorial News

My astronomy project:
Replacing a 2.5" field flattener with a 3"


  1. Introduction and unpacking
  2. Planning for needed adapters
  3. Disassemble adapters and assemble them again
  4. Replacing off-axis adapter with a temporary guide telescope
  5. First Light and test
  6. Ultra thin M48 adapter
  7. Test with ultra thin adapter
  8. Guide telescope holder distances

I take no responsibility or liability for what are written here, you use the information on your own risk!

3: Disassemble adapters and assemble them again

Today I took out the telescope from the closet and began to test the adapters if they fit together. First problem to solve was how to get them apart, they are like welded together! One must be very careful when both take them apart and mount them together. Any mistake can destroy the threads!

The adapters to my old 2.5" flattener consist of three parts between the field flattener and the telescope.

Old flattener and telescope adapaters

The upper side connects to the field flattener through M68 threads. The middle and upper part connects through M92 treads (I hope) and I haven't solved to get them apart yet. The bottom and middle parts are where I can freely rotate them against each other and bottom connects to the focuser.

After some fights down in the garage I got to solve how to take them apart. I wrapped a rope around them and then pulled them apart without any damage.

Here are the three parts disassembled. I will reuse the two to the left for my 3" flattener. And yes, it was a M92 thread between them, then there is no more need for extra adapters on the telescope side, that's great!

Here you can compare the sizes of the two flatteners. The left one is a 2.5" flattener with M68 threads and already that one is big, the one to the right is the 3" flattener with M92 threads on focuser side. As you can see there is much more opening on the right one. A normal 2" flattener will be very small compared to these. This is the telescope/focuser side.

This is is from camera side. The left one has a tilt adapter and an off-axis adapter mounted, the bayonet is Canon EOS which connects to off-axis guider through M48 threads, they are modified by me to fit, I will say it was complicated. The right flattener has a M80 thread on camera side, I'm not sure how I will proceed here yet.

This is what's needed to fit the 2.5" flattener, from camera, M48 adapter, off-axis adapter, flattener, rotation adapter and then the adapter to the 3" focuser.

This is what's needed to fit the 3" flattener, from camera, M48 adapter, field flattener, rotation adapter and then the 3" focuser. The adapter to the left I maybe replace with a tilt and off-axis adapter and some other adapters.

The 3" field flattener mounted on the telescope and a Canon 6D full frame camera. This system will cause some vignetting from the camera house itself and the M48 adapter, but much less than earlier. I also must test if the telescope has enough back focus, I don't think this will be a problem, but who knows.

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