MrSpock22 I have exactly the same problem, I can not reach a correct focus while using this adapter, and I removed the 5 mm extender.

That adapter is meant to be used with a camera that has a 17.5mm flange focal distance, or 12.5mm flange focal distance when you include the 5mm spacer.

The flange distance for the ASI071 is 17.5mm, so leave the spacer off, and then fine tune the focus by using the nylon T2 spaces that came with the camera adapter -- these go between the adapter and the camera's tilt plate.

If you still have insufficient spacing, places like Agena sells M42/T2 metal washers/spacers, usually up to 1mm thick and as thin as 0.1mm. You may want to choose the metal ones anyway, instead of the flimsy plastic stuff that ZWO provides.

Just try to focus the lens at non-infinity, measure the extra distance you need with a pair of calipers, and add that much spacer.

With many lenses, where all the lens glass elements all move in unison as you focus, you can just leave the focus at the sharpest non-infinity point and not have to deal with spacers. With lenses that moves only some of the glass elements internally, you will need to use precise spacers so that the stars are sharp when the camera lens is at infinity focus -- if not, stars at the edge of the frame won't be sharp when the stars at the center of the frame is sharp ("field flatness").

Chen

privateer So since the Optolong L-eNhance filter is the same thickness as the ZWO Dual Narrowband filter "1.85mm" would this be the same requirement?

The two filters would require the same backfocus extension if their glass also have the same refractive index. But it is probably "close enough."

If I have 3 ZWO thin plastic spacers available would this be close to .62mm ?

I have no idea how thick the ZWO spacers are. But I suspect that by the time you add enough plastic spacers to make up 0.6mm worth of thickness, you are going to end up with some flexure.

Since the ZWO adapter itself has some unpublished flange tolerance, and the flange to sensor distance of ZWO cameras are also not as precise as higher end cameras, you may need to do some trial and error yourself anyway. But it should be a 0.62mm difference with the filter out and the filter in.

So, I would test first with the plastic spacers, find a suitable spacing by measuring with your caliper or micrometer, and find metal spacers to replace the plastic.

Chen

Thanks Chen for your reply. I will try your suggestions to add more spacers, so far I used all of them, but not good enough... I guess I will have to try again with some more. It is actually quite frustrating... I was hoping it would have been a bit plug and play. I am using some 294mm pro camera with canon lenses and filters and here no problem at all... So could it be the pb with nikon only?
best,
Gael