Canon 6D modified with Baader filter + Astronomik HA 12nm + Samyang 135 - Out of focus once filter is installed - Help!

[Oliver2278]

So I have just had my Canon 6D modified to make it more sensitive to HA. I also thought I would buy the Astronomik 12 nm filter to take advantage of the mod and do some HA imaging.  I knew it would be hard to get good focus with a Bahtinov Mask while the HA was in place so I decided to focus the Samyang 135 lens without the filter installed in the camera and then tap off the lens so focus did not move, once that was done I would and then place the HA filter back into the camera. My issue is that looking at the images taken they all seen to be out of focus. Perfect focus on my Samyang 135 with the Bahtinov mask is right at the point where it stop turning. Is there something g I am doing wrong?  The two images I have attached are one with the HA filter in and with out, both images were taken with the lens focused in the same place to infinity right where you are unable to turn the lens anymore.  I was a little scared of placing the filter in the camera, its a tight fit and the mirror lockup I was convinced I was going to break the camera!  Any tips or advise would be most welcome. Thanks Oliver

[900SL]

A filter changes the focal point of the image due to the additional refraction. If your filter is mounted behind the lens it (I think) focuses behind the sensor. The rule of thumb is filter thickness/3 for the shift. You might not be able to reach infinity focus with both the Ha mod and filter.

With an astrocamera it's fairly easy to deal with, you simply add a shim between the camera and corrector to compensate for the increase in backfocus 

Edited July 11, 2022 by 900SL

[newbie alert]

The focus for Ha will be different than it is for broadband.. you need to focus for Ha with the Ha filter in place... Up the iso and exposure time for focus and then back it off once done..

[david_taurus83]

Who done the modification?