Field Flatteners, Back Focus

Suggestions, corrections or comments about astronomy / astrophotography books written by Allan Hall
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Field Flatteners, Back Focus

Post by karl.w.lewis » Fri Aug 08, 2014 12:15 pm

My new HOTECH field flattener came with an instruction sheet that indicates that the sensor should be 55 mm back from the flattener. My Nikon SLR claims to have a flange distance of 46.5 mm, which leaves me 8.5 mm to fill.

I haven't even tried hanging the camera on the back of this, yet, but I already know, I don't have an 8.5mm extension tube; and it isn't clear that I can get one. Do I have to head to a machine shop?



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Re: Field Flatteners, Back Focus

Post by AllanHall » Mon Aug 11, 2014 6:19 am

Generally the field flattener attaches to the camera with a T-Ring, this ring can increase the backfocus by up to 10mm. In addition, they sell T-Thread spacers that go between the field flattener and the T-Ring in increments of 10mm, 20mm and 30mm (separately or in a kit with all three sizes). These are available from many places including from Orion.


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