Back focus adjustment for a typical 2mm filter thickness?

[fwm891]

I'm guessing add circa 1/3rd the filter thickness to the back focal distance so 55mm becomes: 55.66mm ?

 

 

[symmetal]

That's right, though I doubt you can measure it to 0.01 mm though.  

Alan

[fwm891]

@symmetal thanks Alan, I’ve just got an Askar 120 APO and want to use my 2600 duo with it and that means using the 54mm mountings. That also means taking the 48mm fittings off the scope which adds 18mm to the 55mm back focal distance (73mm) now.
With my current extension tubes and filter holder I need to turn 1.4mm off the length of one extension tube that will then incorporate the extra back focus needed. 73.6mm.

  Camera bf 17.5mm + filter drawer 20mm + 21 + 16.5 extension tubes = 75 I need 73.6 = 1.4 mm.

I’ve ordered an extra 16.5mm extension tube £19 rather than pay £190 for the variable bf adjuster Askar are selling…

[Stuart1971]

2 hours ago, fwm891 said:

@symmetal thanks Alan, I’ve just got an Askar 120 APO and want to use my 2600 duo with it and that means using the 54mm mountings. That also means taking the 48mm fittings off the scope which adds 18mm to the 55mm back focal distance (73mm) now.
With my current extension tubes and filter holder I need to turn 1.4mm off the length of one extension tube that will then incorporate the extra back focus needed. 73.6mm.

  Camera bf 17.5mm + filter drawer 20mm + 21 + 16.5 extension tubes = 75 I need 73.6 = 1.4 mm.

I’ve ordered an extra 16.5mm extension tube £19 rather than pay £190 for the variable bf adjuster Askar are selling…

Instead of the 16.5mm extension tube could you not get a 15mm one to replace, then you are there…? 15mm tubes are available in M42, M48 and M54…

Edited February 11 by Stuart1971

[symmetal]

If you opt for a 15mm m54 extension I'd be wary of the Baader ones as they have a short thread depth and I've found many male threads will reach the end of the available Baader female thread before they're fully mated. I've tend to use AliExpress for extensions and stainless steel spacer ring sets etc. as they are normally better made, with deep thread depths and knurled outers on the extensions, and are a lot cheaper. 🙂 They take Paypal too.

M54 15mm extension and spacer rings from a company I've used and that's also been recommended on here.

Alan

Edited February 11 by symmetal

[Stuart1971]

21 minutes ago, symmetal said:

If you opt for a 15mm m54 extension I'd be wary of the Baader ones as they have a short thread depth and I've found many male threads will reach the end of the available Baader female thread before they're fully mated. I've tend to use AliExpress for extensions and stainless steel spacer ring sets etc. as they are normally better made, with deep thread depths and knurled outers on the extensions, and are a lot cheaper. 🙂 They take Paypal too.

M54 15mm extension and spacer rings from a company I've used and that's also been recommended on here.

Alan

Yes +1 for Aliexpress,

i have had a boat load of stuff off there including all sorts or adapters, extensions and such like, that are just  not available here in the U.K., and as you said always well made too. 👍🏻

[fwm891]

Thanks all, I’d already ordered a direct replacement from FLO, as I’ve doctored my ZWO one that came with the duo which seems to be working well tonight (between clouds)!