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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!

1: Introduction and unpacking

As you maybe have read I have some vignetting problems with my 2.5" Riccardi Field Flattener in combination with my full frame camera. To reduce that problem I need a bigger field flattener but it's not so easy to find a 3" field flattener without too high cost and it shall of course fit the telescope also.

After asking for advice at Cloudy Nights forum I got this information from Matthew:

Started a new project around an 3" Altair Planostar x1 Field Flattener. I haven't find very much information about this field flattener, but looks like the old Meade field flatteners they used to the big APO refractors, built to handle Pentax 6x7 medium format cameras. This one maybe little bit different.

The earlier Riccardi 2.5" field flattener have very high quality, will this new bigger but simpler field flattener give the quality I need? The Riccardi is built around a three lens system, the new one only have two lenses. However the Riccardi have also a reducer function, x0.75 and the new one is x1. It's much simpler to solve the optical problems with a x1 flattener. So maybe I will have the same quality with the new one but with much bigger image circle.

DHL deliverence

I bought the Altair Planostar 1x field flattener from Altair Astro. The delivery was late but Ian at Altair send me a new one. It came in a big yellow DHL box. The British currency is low now (2016) so I felt it was good time to order one right now.

Small brown box

Inside the big yellow box there was a small brown box. Good because it is well protected inside all this package.

Altair 3" Planostar field flattener and M48 adapter

Three Inch flattener and M48 adapter

My camera is a Canon 6D full frame and I ordered the flattener with an adapter to Canon EOS with M48 flange. I will still get some vignetting because of the camera house narrow inlet and also some from the M48 adapter itself, but I expect it to be much better than my earlier setup. I also reduce the optical system opening from f/5.3 to f/7 with this x1 field flattener, that is easier to handle, but slower.

The lens is really big, it's lens diameter is 68mm, good, because the off-axis guider system need space around the pickup prism to not vignetting the main camera and also have all the light it can get to the guiding camera. My telescope has a 3" focuser.

Mounted M48 adapter on three inch flattener

Here is the M48 adapter connected to the three inch field flattener. Later I will not use this adapter, I will have an off-axis guiding adapter. And in the future maybe a monochrome camera and filter wheel. And if the future will offer big size sensors I think this field flattener will handle a medium format 48x36 mm sensor or maybe even bigger, similar field flatteners are used earlier to the old Pentax 6x7 medium format cameras, but with higher quality demands I think I must use a smaller field of view. The back focus without the M48 adapter is very long, in my system maybe as long as 115 mm (or is it 103 mm in my case? More about that later).

Altair 3" Planostar x1 flattener

By the way, my own telescope is not an Altair 130 mm, my is the TS130 f/7, but I expect it to work with some small modifications.

Now I must I try to take apart my old adapters on the telescope, not any success yet, it's like they are welded together!

I will come back later, but for the moment all to much things to do outside my astronomy hobby, I also have my Raspberry computer project with Kstars and Ekos waiting. This will be a slow progress project!

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