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Hi Jeelan,
In your situation screwing a 2" filter onto the nosepiece of your diagonal should be a good option. In a small number of cases this isn't a great option as filter performance can suffer, but in your case it should work well, without any negatives other than cost and a few positives.
Filters generally work best when the light cone passes through the centre of the filter and the light cone itself is narrow. Some filter manufacturers publish recommended F-ratios of telescope for use with their filters. This might sometimes be in the range of F4 to F15. The reason for this is that the light cone widens as you get further away from the field lens of the eyepiece and the rate the light cone widens increases with a decreasing F-ratio.
Consequently, the light cone will be at its narrowest when the filter is used on the bottom of the eyepiece. The bottom of the star diagonal is a couple of inches further away and the light cone will be wider than it would be if the filter was used on the bottom of the eyepiece. However, with your scope being an F10 SCT it shouldn't make too much difference. I have used my filters on the bottom of my focuser drawtube on both my F4.5 newtonians for over 10 years and I haven't seen any drop in filter performance. The filter in my situation is closer to the bottom of the eyepiece than it would be on the bottom of a star diagonal but the scope is much faster. I can only imagine this being an issue if you used the filter on the end of a star diagonal with an F4-F5 refractor and even then it may still work ok, depending on the filter.
I would recommend the DGM Optics NPB filter as being the best choice for all round Deep Sky use. This is a UHC filter (narrowband) with a very tight band pass and it works very well on most targets which benefit from a UHC filter or an OIII filter. That one filter would cover 98% of your deep sky observing needs.
The big advantage of using a 2" filter on your diagonal is that it will work equally well with 1.25" or 2" eyepieces and you can change eyepieces without having to unscrew and rescrew the filter.
Cheers,
John B
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