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My astronomy project:
Three Inch field flattener system

Replacing a 2.5" field flattener with a 3"

  1. Part 1: Introduction and unpacking
  2. Part 2: Altair 3" Planostar filed flattener and M48 adapter
  3. Part 3: Planning for needed adapters
  4. Part 4: Disassemble adapters and assemble them again
  5. Part 5: Replacing off-axis adapter with a temporary guide telescope

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.

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

Cloudy Nights forum, answer by 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. But 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 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.

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2: Altair 3" Planostar field flattener and M48 adapter

Thrre 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.

Altair Planostar 3" flattener

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 filed flatteners are used earlier to the old Pentax 6x7 medium format cameras, but with higher quality demands I think it must be much smaller. The back focus without the M48 adapter is very long, in my system maybe as long as 115mm.

Altair 3" Planostar x1 flattener

By the way, my own telescope is not an Altair 130mm, my own 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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3: 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 too 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:

TS M80 to M68 adapter

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 M68 off-axis adapter

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 to 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:

TS extension tubes M68

My telescope in this setup is only a slow f/7 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:

TS tilt adjuster M68

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

And then an adapter from M68 to M48 adapter.

And finally an adapter from M48 to Canon EOS adapter.

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 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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4: 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 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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5: Replacing off-axis adapter with a temporary guide telescope

I decided to mount a guide telescope as a temporary or maybe permanent solution to have guiding opportunities, I don't want to stress to find a off-axis guider solution. I have an old telephoto lens 400 mm f/6.3 laying around. I have used it long time ago for guiding and it worked. My main telescope has 910 mm focal length in this configuration, the guide camera has a little bit smaller pixels so it match fairly good.

The telephoto lens has Pentax M42x1.0 mm thread and the guide camera has T2 which has M42x0.75 mm thread, the pitch are different. I have to place an adapter between them, I already have one, I also mounted an extension tube to get the camera in focus.

My tube rings for the main refractor are already prepared to take extra gears. Perfect, here I can mount an extra pair of small tube rings. My telephoto lens diameter is about 65 mm, in my desk I found a pair of 90 mm tube rings. I can use them and fill the gap with something. I mounted the guide telescope as close to RA axis as possible, then I need less counter weight and it will be more compact also.

One drawback of having two telescopes is that I need two heater bands, I took the one I have for my wide field lens and it fit almost perfect.

It's hard to see here with all black painted parts how I mounted the tube rings, but you can see how I filled the gap to hold the lens/camera. I have also rearranged the focuser so it points towards the RA axis to save some balance weight. It's important to check that nothing on the telescope hit the mount when it moves.

A pair of 65 mm tube rings should be perfect, but the smallest tube rings I have found is 3" or 75 mm. If this guiding work as it should I maybe keep this solution for a while and replace the tube rings with smaller ones as a more permanent solution.


Now I got my new 75 mm tube ring holders, the earlier ones with 95 mm was to big.

75 mm tube ring holders

The front holder, I cut a new thermo insulation for the heating band that fit the guide telescope better.

Heaterband with insulation

Detail of tube ring holder and heating band. The main telescope tube rings have recesses where you can mount accessories, eg guide telescope tube rings.

The tube ring holder at the camera end. I have to mount the tube ring holder upside down to fit it to the main telescope's tube ring holders. The guide camera is an old QHY5, not very sensitive but should work in this configuration.

Connection side of camera. I don't use the ST4 guide port, I guide through software and ASCOM.

Overview, the two "boxes" you see behind the guide telescope is the 5 volt and 8 volt voltage reducers from 12 volt. If I don't get too big problems with flex with this guide telescope I maybe keep it. Looks very good and it's stable. Don't take very much space either. I don't have any IR filter in this configuration, get more photons to the guide camera but less sharpness. It's not that important to have the guide star sharp in a guide camera, better with more light I think.

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