If your talking a 150mm f/8 then I have one. I'll agree the Nager is great - I have a 9, 13 and 20. I also have a Zeiss Jana 10mm orthoscopic. It murders the Nagler for on-axis sharpness and contrast. You just see ~ 1/4 of the area of sky and feel like you are looking down a toilet roll
. I find the two complement each other. For some objects I even use both (eg widish view of eta Carina vs a close up of the Homonculus) but for smallish object like the dumbbell the ortho does a better job. Zeiss orthos are rare but I can also recommend UO othos. I have a 7mm which is great.