I have a William Optics (WO) Z61 with a WO Flat 61A field flattener and a (38 mm thick) 48 mm T mount adaptor for my Sony camera. In a moment of Black Friday madness I purchased a ZWO Off Axis Guider which is 16.5 mm thick. The documentation on the WO site is primarily for Canon or Nikon cameras and it doesn't mention the Sony cameras. The Flat 61A says it has to be set at 12.9 mm (purportedly camera non specific). After some measuring, that 12.9 mm is not for a Sony camera like mine. It should be 11.7 mm. Back focus for the Flat 61A HAS to be 55 mm. This means that given the T mount is 38 mm, the OAG is 16.6 mm, I understand that the distance to set on the Flat 61A is 0.5 mm to give that 55 mm back focus. I haven't tested it yet due to the never ending wind recently but does that sound ok? This does limit my options to put in a Clear Sky Filter as there is no room for it in the 55 mm limit. A clip in for my camera seems the only option. You could buy a thinner T mount for the Sony but it has disappeared. Does anyone have one and are my calculations about back focus correct when using my equipment. Thanks for taking the time to read this.
Stephen
Last edited by stephen2615; 07-12-2019 at 02:41 PM.
I have the previous Flat61 without the built in adjuster, but yeah your numbers sound like it should be worth a go. The test will be how the stars look in the corners.
Does the new flattener have a M48 thread (or larger) for threading on a filter? If so, give it a go...the extra glass will throw the back focus back further (by 1/3 x thickness of glass), so don’t forget to extend the adjuster out further if that’s the case.
I believe that with only my 0.5 mm to spare, I can't put in the filter as I can't reduce it any further. However, there does seem to be somewhere inside the Flat 61A where a filter might go. See attachment.
As per normal with most things, there is no label on the inside diameter. I think it is a 42 mm thread but I can't be certain. Also, I can't see an easy way of threading it in if it fit.
Firstly, I decided to get a clip in CLS filter for my Sony. Saves any hassles with back focus settings. I can't see the point of other specialised filters for an unmodded colour camera but see below
Secondly, my back focus is a mess. Last night I ventured out into my back yard in the moonlight with lots of smoke and some wind to test the 55 mm back focus with my setup. I tried to focus on a star with a Bahtinov mask and I couldn't do it as it was always out of focus.
I extended the Flat 61A from 0.5 mm back to 11.7 mm (for Sony?) and was able to focus on the star. I then did a couple of quick shots of the Orion Nebula and it was not in focus. Crappy conditions made me call it a night. The forecast looks dismal for the next few nights so I have to wait and see. I will have another go at taking a few subs of the Orion Nebula with the 0.5 mm setting on my Flat 61A and see what happens. I am concerned about not being able to focus properly without the 11.7 mm setting on the Flat 61A.
In the mean time, I am trying to stop myself from buying a ZWO ASI 1600GT. I have to stop my drooling or my wife will think I am getting old timers disease and put me in a nursing home. I will still need to get back focus even if I do get the 1600GT.
That link you sent makes a lot more sense about how to install a filter inside the Flat 61A housing. Thanks for that. I misread it and thought it went on the camera end of the housing.
I think I need to take a break from this. I have just confused myself with various distances.
The 55 mm back focus cannot be achieved with the WO 44 mm T mount adaptor as it is 38 mm thick. A normal imaging train is:
18 mm (sensor flange) + 38 mm deep (44 mm T mount) = 56 mm with the Flat 61A doing a "minus 1 mm adjustment" from 12.9 to 11.7 for a Sony. A magical number of 67.7 mm is needed for the Flat 61A.
With the OAG, 18 + 38 + 16.5 (OAG width) we have 72.5 mm which is longer than anything the Flat 61A can do. I have to lose 4.8 mm. The old 28 mm deep T Mount would do it for me but they seem to have moved to another planet. Does anyone have one they don't need?
Indeed I do. I found one on Amazon that is 27.6 mm in depth. The OAG has a 42 mm thread like most optics and I kept looking for a 48 mm thread when I didn't need to.
The measurement is now:
18 mm (flange depth) + 27.6 mm (T Mount) + 16.5 mm (OAG) = 62.1 mm. As the Flat 61A has that 12.9 mm that can be reduced, I will have to change that adjustable Flat 61A measurement to give that magical 67.7 mm that the Flat 61A wants to use.
William Optics don't do themselves any favours with the documentation. They INSIST on the 55 mm back focus AND the 12.9 mm set distance on the Flat 61A. Simple addition means that 55 and 12.9 does not equal 67.7. Perhaps 0.2 of a mm is not worth the effort to adjust.
All I can say is that with a fixed depth Flat 61, getting exact distance would be a pain. It helps to discuss these issues as it clears your head.
Now for some nights without this damn smoke and/or wind.
Indeed I do. I found one on Amazon that is 27.6 mm in depth. The OAG has a 42 mm thread like most optics and I kept looking for a 48 mm thread when I didn't need to.
The measurement is now:
18 mm (flange depth) + 27.6 mm (T Mount) + 16.5 mm (OAG) = 62.1 mm. As the Flat 61A has that 12.9 mm that can be reduced, I will have to change that adjustable Flat 61A measurement to give that magical 67.7 mm that the Flat 61A wants to use.
William Optics don't do themselves any favours with the documentation. They INSIST on the 55 mm back focus AND the 12.9 mm set distance on the Flat 61A. Simple addition means that 55 and 12.9 does not equal 67.7. Perhaps 0.2 of a mm is not worth the effort to adjust.
All I can say is that with a fixed depth Flat 61, getting exact distance would be a pain. It helps to discuss these issues as it clears your head.
Now for some nights without this damn smoke and/or wind.
Thanks again,
Stephen
Dont stress too much on the measurement, use it as a guide. I've had a WO flatenner and a WO flatenner/reducer both fitted to a WO scope and the measurements stated by WO for that specific scope weren't correct, both required further adjustment. The Flat IV was out 5mm!
Get it close and then use a CCD inspector to make the final adjustment.
Do you have some device (e.g, OAG or filter wheel) in between your flattener and your camera? The main problem I have is finding a T Mount adapter for a Sony that can replace the WO T Mount that gives "perfect 55 mm back focus".
Finding T Mount adapters with a M42 x 0.75 mm thread is as rare as hen's teeth. Why astro gear has to be using a "non standard" M42 thread is beyond me.
I am at a disadvantage as I don't have enough hair to pull out anymore.
