The William Optics Flattener dedicated to the Zenithstar 61 optical tube ensures the maintenance of uniform image quality - flatness, because the flattener is a "flattener" - both in the center and on the edge of the frame, eliminating the negative impact of the refractor's optical system, which as you move away from the center of the frame, the sharpness of recorded objects gradually deteriorates.
This product also acts as a reducer, allowing the focal length to be shortened by 0.8x
• M4 / 3
• Sony α
• times: 0.8x
• back focus: 56 mm
• correction distance for SLR and mirrorless cameras: 1 mm
• telescope mount: M54 thread
• camera mount: M48 thread
• possibility of mounting the filter: yes, thread M48
Product page on the manufacturer's website: https://williamoptics.com/adjustable-reducer-flattener-flat61r-for-z61
• flattener for Zenithstar 61 refractor
It should be remembered that this instrument is characterized by a constant focal length value, ie the distance from the lens to the primary focus, eg the matrix of a digital camera. Generally speaking, both the reduction ratio for focal length reducers and its increase in the case of Barlow lenses change with the distance from the main focus, making it easy to choose the desired value.
However, if we deal with flatteners or field correctors, then the focal length value must be strictly defined and it cannot be changed. This means that a sharp image can only be obtained when the user sets the correct focal length, which sometimes requires a precision of 0.1 mm.
Most zoom accessories of this type have the focal length specified by the manufacturer. However, if you cannot find this information in the manual or on the website, make the general assumption that a typical back-focus value in astrophotography is 55 mm. If your camera is not rated to this value, or you use modified equipment, look for the products listed below that will solve your problem: