48mm Thread Filters for WO Flat 61 ?

[Newforestgimp]

Ok, so, Ive taken delivery of my WO FLAT61 and i notice it has a 48mm internal thread for use of a filter only I cant seem to find any 48mm Filters ? LOTS of 49mm or any other size you care to mention.

Anybody using a particular light pollution fileter in conjunction with the WO Flat61A ?

Many Thanks.

[Davey-T]

They're usually listed as 2"

Dave

[symmetal]

As Dave says 2" filters are actually 48mm thread. My Flat 61A has a 2" filter in it and it fits perfectly. 🙂

Note also that the range of 'back-focus' adjustment on the FF is reduced by the filter thickness so the 0 to 15mm adjustment without a filter, is about 6 - 15mm if a filter is inserted in the FF.

Alan

Edited January 19, 2021 by symmetal

[johninderby]

 

[Newforestgimp]

Thanks guys, I did think it might be 2” even though that isn’t 48mm.

with regards back focus I’ve set it to the 12.9mm as recommended on the WO website, I’m using a dslr currently so I’m hoping that’s correct.

[symmetal]

7 minutes ago, Newforestgimp said:

Thanks guys, I did think it might be 2” even though that isn’t 48mm.

with regards back focus I’ve set it to the 12.9mm as recommended on the WO website, I’m using a dslr currently so I’m hoping that’s correct.

Yes that's the recommended setting for a DSLR. The total back-focus distance is 67.7mm, (with the adjustable section set to zero). The DSLR with T-adapter is nominally 55mm so adding the 12.9mm on the FF adjustment gives close to the 67.7mm required. If you put a filter in the rear of the FF you need to add a bit to the total back-focus distance, 1/3 of the filter glass thickness is usually used, so increasing your FF adjustment setting to around 13.5mm will probably be closer to optimum.

Alan

[Newforestgimp]

17 minutes ago, symmetal said:

Yes that's the recommended setting for a DSLR. The total back-focus distance is 67.7mm, (with the adjustable section set to zero). The DSLR with T-adapter is nominally 55mm so adding the 12.9mm on the FF adjustment gives close to the 67.7mm required. If you put a filter in the rear of the FF you need to add a bit to the total back-focus distance, 1/3 of the filter glass thickness is usually used, so increasing your FF adjustment setting to around 13.5mm will probably be closer to optimum.

Alan

Perfect thank for the advice 👍