2" filter adapter

[Mr Flibble]

I have a 1.25" UHC filter which I've had for years which I now use with this adapter and I place the filter in the 2" adapter of the focuser body. This saves me having to swap the filter between eyepieces during viewing. I use mostly 1.25" eyepieces with the longest f/l eyepiece having a field stop of 21mm, and I also use a 28mm 2" nirvana which has a field stop of 43mm.

I'm now looking to buy an oiii filter and would like to know if there are any disadvantages to buying a 1.25" version and using it with the filter adapter compared to buying a more expensive 2" version. In particular I'm wondering if the narrower 1.25" filter will partly block the light when using the wider 2" nirvana?

[John]

Yes, it will block the light partially. The full field of your 2" 28mm 82 degree eyepiece will not be fully illuminated - it will be as if the field stop has been reduced to about 30 mm which is about the equivalent of approx a 60 degree 28mm eyepiece I reckon.

[umadog]

You could look on the second hand market for a used 2" filter. Most good 2" to 1.25" adaptors will allow you to thread in a filter.

[Mr Flibble]

OK thanks John and umadog.