William Optics Adjustable Field Flattener For Zenithstar 73

[astro mick]

Hi.

Anyone using this Flattener.

If so could you let me know the correct back focus for the Flattener,and where exactly do you measure from.

Its double dutch to me.🙄

Cheers.

Mick.

[Annehouw]

Hi Mick,

I normally speak Dutch, so doing it twice is not a big leap...

I do not have this flattener, but I do have the 6A III. Same procedure.

As per the WO website they recommend a spacing of 66.2 mm from the flat end of the flattener to your sensor. (See WO website and then choose "more information").  You do this by using extension tubes between the flattener and your camera and making it precise by using the rotation extension in the flattener itself. 

The numbers are approximate; you will have to star test it to get to the last fractions of a mm. Make short enough exposures of a star field so that tracking does not influence the result. If the star shapes in the edges are comets pointing to the center, you have not enough distance. If the star shapes are little arcs rotating around the center, you are too far out.

Another caveat is that if you use a filter between the flattener and the sensor, this also changes the required distance (most often less than a mm though).  Test with that filter in place...

Hope this makes any sense to you?

[astro mick]

Hi.

Thankyou so much for this detailed answer.It explains everything I wanted to know.

Really appreciated.

Mick.

[symmetal]

The spacing of 66.2mm from the flat rear end of the flattener to the image plane is when the adjustable section is set to zero.

For a DSLR with a standard 55mm spacing between the front of the T adapter and the image plane you would unscrew the adjustable piece shown below until the left edge is at 11.2mm on the scale. So the overall FF spacing is then 55 + 11.2 = 66.2mm, the correct distance. You then use the locking ring (below the blue arrow) to lock the adjustable section.

You have 15mm of adjustment so if you don't use a DSLR you get the physical distance between the rear flat of the FF and the image sensor anywhere between 51.2 and 66.2mm with standard spacers and you can make up the difference with the adjustment.

As Annehouw says using filters requires the 66.2mm to be increased by 1/3 the thickness of the filter. Also the sensor protect window on astro cameras needs to be counted as a filter as well so add 1/3 the thickness of that too.

If you have a 2" filter fitted in the rear of the flattner this stops the adjustable section reaching zero and it can only be adjusted between around 7mm to 15mm.

 

Alan

Edited May 15, 2020 by symmetal

[astro mick]

Thanks for the additional info Alan.It is also very helpful as I do have a DSLR which I don't often use.

Appreciate the time spent in replying to this.

Mick.