Vario Flattener for apochromats

Long Perng 2-inch flattener with adjustable working distance

• Optimized for f/5-f/6 apochromats (according to LongPerng).
• With a changed working distance, effective flat correction can be achieved from f/4.8 to f/10 (according to our test results).
• Working distance is adjustable from 55mm to 80mm. Camera can remain screwed on.
• 50.8mm plug-in connection with M48x0.75 filter thread on the telescope side.
• Camera side M48x0.75 screw connection.
• Large clear opening (depending on levels up to 52mm).
• Both focal length and focal plane remain unchanged.
• Ohara glass from Japan with partner glass 

Use

• Roughly determine the working distance (see table below)
• Create test photo
• If "coma" is still visible at the edge (undercorrection), increase the distance slightly
• If "banana shape" is visible at the edge (overcorrection), slightly reduce the distance
• Fasten the sliding outer tube at the optimum distance
• If you use the flattener with several telescopes, write down the optimal working distance

Recommended working distance ranges according to LongPerng

• over 1000mm focal length = about 55mm working distance
• between 800 and 1000mm focal length = about 58mm working distance
• between 600 and 800mm focal length = about 60mm working distance
• between 500 and 600mm focal length = about 65mm working distance
• between 400 and 500mm focal length = about 70mm working distance
• The working distance can - due to different optical designs (ED-doublet, triplet, fluorite-APO etc...) - deviate from the values ​​listed above. The table only serves as a starting point before fine adjustment

Flatteners can be used to make the stars sharp even at the edges when taking photos with a refracting telescope without changing the focal length.

Some test images can be seen below with different flatteners and without any flattener.