Logan,
The tight transmission curves at OIII and Ha shows it's much more than an UHC filter.
Yes the peaks are much tighter in bandwidth on the dual narrowband filter compared with the UHC filter. Some time ago I had the idea of using a UHC filter in this way with a DSLR and using the BLUE channel for OIII, and RED channel for Ha. For a dark sky (no light pollution) one could even use the GREEN channel with either the dual narrowband or UHC filter. I had made a comparison of these together with a Nikon D800 sensor response. I will look for the comparison and post it here later.
I had made a comparison of these together with a Nikon D800 sensor response. I will look for the comparison and post it here later.
Best
JA
Here is that comparison of the UHC filter & Dual Narrowband filter against the Nikon D800 spectral response. The curves are aligned by eye at 400 and 700nm.
As is evident the peaks are much tighter in bandwidth on the dual narrowband filter (Bandwidth = ~15nm OIII & ~18nm Ha [even less if the D800 response were to be included) compared with the UHC filter(Bandwidth = ~50nm OIII & ~28nm Ha [with D800 response included - brown line line on top graph]). At a fraction of the cost, or perhaps something that one might already have, I would give it a try in the same way as the dual narrowband filter is used on colour cameras, for "narrowish" - band imaging or of course stick to "normal" colour imaging with it to remove some light pollution.
JA, et al,
I think Glenn was hoping to see some actual AP images with the bi-wavelength filter.....
Hi K,
He didn't mention wanting to see images otherwise I would have pointed him to the sample images in your original link. Image number 5 in particular is an excellent demonstration of a difference, purportedly of the effect with and without the filter.
Processed as an OSC image. Need long subs as the two bands are narrow (10nm). Need 10min subs with the filter -compared to 1.5min with just a uv/ir filter. I'm still capturing lights for these images.
Interesting thread. I was wondering about this filter too as I use the UHC one currently. For me I dont think I can us the bi-wave, though this thread has me thinking, especially seeing the wavelength graph comparisons that I should look at stripping my OSC subs and try processing as Ha Si filtered data and see what i get.
Interesting thread. I was wondering about this filter too as I use the UHC one currently. For me I dont think I can us the bi-wave, though this thread has me thinking, especially seeing the wavelength graph comparisons that I should look at stripping my OSC subs and try processing as Ha Si filtered data and see what i get.
I would definitely try that as the OIII & Ha bands are typically "only" about 2 to 3 times wider with the UHC filter if we also take in to account the camera's Ha response. It would be great to see such.
Before getting the duo filter I used a UHC filter a fair bit. The duo is a very different beast due to the much narrower bandpass as others have mentioned. I'm guessing but think I'd need subs at least 5x longer to get background levels equivalent to those with the UHC filter. Might do a comparison on a dud night.
Chris the STC Duo certainly looks like a mono layered narrowband result. Good investment. I would be tempted to try it with my Nikon (when night's get cooler).