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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!

2: Planning for needed adapters

Now after have done some measurement on the field flattener I have found out what the treads on the telescope / focuser side is, M92, and on the camera side M80.

Focuser side:
My 3" focuser shall already have M92 so it should connect directly on that side, I have to verify this.

Camera side:
I must check this very carefully, it's very easy to misunderstood something and get wrong expensive parts. The back focus from the flattener is about 115 mm in my system so I have plenty of space to work with. I have at least three choices for this:

  • 1. With use of an off-axis adapter

TS has one big off-axis adapter with M68 threads and 62 mm free opening, if I choice this one I can built it like this, from field flattener M80 to camera in order:

After an email to TS and asking what adapter is needed they say this one is the correct one. Little hard to understand the info data about this one. This adapter connect the M80 field flattener to off-axis M68 threads.

TS big off-axis adapter with M68 threads, maybe this will be a good choice. But I already see one problem with this off-axis adapter, the tube that connect the prism to the guide camera is very narrow and will reduce the light to the guide camera, same problem that I have with my off-axis guider system today. This is normal for all off-axis adapters off "thin" construction. There is no space to have a wider hole up to the guide camera.

Then I also need the distance between the field flattener and sensor to be correct, one or more of these extension tubes:

My telescope in this setup is only a slow f/7 optical system and then any tilt of the sensor is not very sensitive. But if there is problem It could be wise to have some tilt adjustment after these extension tubes, this is what they offer from TS:

Maybe it could be wiser to have the tilt adapter before the extension tubes.

And then an adapter from M68 to M48 threads.

And finally an adapter from M48 to Canon EOS bayonet.

All these parts add up to more than 600 Euro, much more than I paid for the flattener!

  • 2. In future with some medium format camera

If this future camera is a color camera, then about the same solution as in point 1. but different adapters against the camera.

If the camera is monochrome I must fit in a big filter wheel after the flattener.

This is a longtime goal.

  • 3. With use of an external guide telescope

Then I can use it as is. Instead of spending money on adapters I have to spend them on guide telescope and tube holders. Something that I maybe already have and then save a lot of money.

But this solution give some problems. Takes a lot of more space and weight, it could be some flex between the telescopes. I had this solution earlier and didn't like it.

Even better solution will be no guiding at all, a high precision direct driven mount could solve that problem. Maybe right way to go in future and just have a temporarily solution with a guide telescope for the moment.

Almost all solutions with an adapter with M48 thread will give a slight vignetting with a full frame DSLR camera and big telescopes. I can built an adapter with a wider opening from M68 to connect to my Canon full frame camera. But no meaning with that, the camera house itself give vignetting in a full frame DSLR camera body with bigger telescopes. If I do some modification to the camera house it will be better. The camera mirror cause heavy vignetting problems in a DSLR full frame camera when connected to a telescope like this, I maybe remove the mirror in future.

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