Zwo 1600mm and sigma 135mm lens

[graham.hobart (Graham stevens)]

I’m having great difficulty focusing this combo, it seems it needs to go shorter but I can’t find adapters to do it. I have already the canon Eos short adaptor but need someway of shortening distance between this and filter wheel ?
Thanks
Graham

[AdamJL]

would be good to let us know what filter wheel? The Sigma has a backspace of 44mm (more like 44.1mm but anyway).

Your camera takes up 6.5mm. That means your adapter and filter wheel need to be 37.5mm. Your "short adapter" is 17.5mm, so you've got 20mm to play with for your filter wheel and any spacers. What filter wheel do you own?

[JA]

Originally Posted by AdamJL

would be good to let us know what filter wheel? The Sigma has a backspace of 44mm (more like 44.1mm but anyway).

Your camera takes up 6.5mm. That means your adapter and filter wheel need to be 37.5mm. Your "short adapter" is 17.5mm, so you've got 20mm to play with for your filter wheel and any spacers. What filter wheel do you own?

I think all the ZWO filterwheels are 20mm "thick" so they should all be OK to use, refer...
ZWO 7x36mm...
https://astronomy-imaging-camera.com...zwo-efw-7x36mm

ZWO 5x2" & 7x2"...
https://astronomy-imaging-camera.com...7-x-2%e2%80%b3

ZWO 8x1.25"/31mm...
https://astronomy-imaging-camera.com...nch-or-7x-36mm

ZWO EFW Mini 5x1.25"...
https://astronomy-imaging-camera.com/product/efw-mini

ZWO EFW Manual 5x1.25"...
https://astronomy-imaging-camera.com...on-1-25-manual

All specified in their literature as 20mm "thickness". It's must be, as you've suggested Adam, a non ZWO filter-wheel used or there's a wayward spacer in there or perhaps not as shown in the system diagram below.... ???

Graham,
The system diagram for the ZWO 1600mm Pro here


Shows the ZWO1600mm directly screwed on to their various models of 20mm thick filter wheels and then screwed to the "EOS Adapter for EFW +1600" adapter to which the lens is mounted. Check that is what you have setup with no extras, except perhaps for some thin (sub mm) tuning spacers.

Best
JA

[AdamJL]

Cool, thanks JA.

It could also be the thickness of the filter. Say your filter is 3mm (just a random number).. that means your backfocus is impacted by 1mm (usually 1/3 is the rule).

What filters are you using, Graham?