Mewlon 180 & Feathertouch Focuser - Please Advise

Several months ago I purchased a Feathertouch focuser (Model
FTF2008BCR) and M-1 Adapter to use with my Mewlon 180. I also
purchased an AP Maxbright 2" diagonal to use with this combo. Due to
taking a new job and business constraints last night was the first
time I have had a chance to check everything out.

With the FT focuser, adapter and AP diagonal on the Mewlon 180 the
only eyepiece that will focus is my TV 35mm Panoptic. None of my
other eyepieces (Pentax XW 5mm - 20mm & Tak 5 &7.5 LE) will focus no
matter where I place the primary mirror focus knob. With the mirror
racked all the way "OUT" and the focuser racked all the way "IN" the
35 Panoptic will just come into focus from one direction.

If I remove the AP diagonal and view straight-thru using the FT
focuser and adapter all the eyepieces will focus just fine. In this
configuration I am able to place the mirror focuser ~ mid-range and
the FT focuser has plenty of adjustment to approach focus from both
direction with all eyepieces.

Has anyone else experienced this issue? If so, please advise what you
did in order to use the FT focuser/adapter configuration w/ a
diagonal? I don't enjoy viewing straight-thru with bi-focals thus the
desire to use a diagonal.

Thanks in advance for your help!

Best regards,

Keith

Hi Keith

I had this problem with my Mewlon 210, exacerbated by using binoviewers (in a diagonal) as well as a Moonlite focuser (which in contrast to another poster I am very happy with). My solution was to create more backfocus by moving the secondary mirror closer to the primary. Loosen the locknut holding the central set screw on which the secondary holder/mirror pivots, and then turn the set screw clockwise. (You'll need to loosen the 3 collimation screws first.) Turning the set screw clockwise will push the secondary holder/mirror toward the primary. If you need even more backfocus, you can purchase a longer M5 set screw (say 15mm or 16mm) to push the secondary even closer to the primary. That is what I did. I use Bob's Knobs which are long enough to still grip the secondary holder when backed out a few mm. I don't know about the stock collimation screws (I don't have them to hand). You can also move the primary a little closer to the secondary (same backfocus effect as moving the secondary toward the primary) but that is a little more difficult to explain and you mightn't need to do this if moving the secondary is enough. I haven't noticed any vignetting or other optical disadvantages by doing any of this, but I have plenty of back focus available now.

Regards
Ian