Imaging with custom wide band filter sets

[mmirot]

I reviewed Vicent's work with photo metric filter sets.  I got me to thinking on two points.

1) Current LRGB systems toss out a lot of IR and UV data.
Our idea's have stemmed from our preconceived notions of hypering film and the daylight visual response to our eyes.

There is no good reason this.

2) A large bandwidth overlap of an additive filter set can could replace LRGB by shooting straight RGB analogue filters. 
I believe their is a SN benefit for extracted luminance and chrominance data.

Any thoughts?

Max

[pfile]

well also i think most refractive optics are 'tuned' for visible wavelengths and so IR does not come to focus in the same place as the visible light. so if you let IR thru to the sensor, you get bloated stars due to wrong focus.

of course with a reflector that's not a problem.

rob

[mmirot]

That is consideration for some optical systems

[Andres.Pozo]

of course with a reflector that's not a problem.

Nowadays many (most?) reflectors have focal reducers and/or flatteners attached so the focusing points of IR and UV are also a problem.

[lucchett]

Yes, that is what I am seeing with my Apo+ff. ( ff separates the red wavelengths more, even if within Cfz)
I think that selective focusing by filter can help a lot, no?

I can't figure out if IR is out of the CFZ for the red filter.
Andrea

[NGC7789]

I think that selective focusing by filter can help a lot, no?

Selective focus by filter help ONLY if the bandpass of the filter is sufficiently narrow that all wavelengths fall at the same focus. The question here is if wider bandpass filters, particularly those that admit IR and UV signal, would prevent adequate focus even if you refocus for each filter.

[lucchett]

I agree with you if all the filters were transparent to IR-UV.
If we considered wider band filters, with red extending to Ir, then in my opinion focusing red could get Ir into focus.
For many Apos, Ir is far from green ( which is one common optimization point) but not to far from ha-red. I suppose could be the same for corrected reflectors.
Andrea

[pfile]

of course with a reflector that's not a problem.

Nowadays many (most?) reflectors have focal reducers and/or flatteners attached so the focusing points of IR and UV are also a problem.

i guess that's true (CDK designs, SCTs) but people still use regular newts and RCs, which have no correctors. i guess i should have said "pure reflectors"

rob

[mschuster]

More considerations:

- Detector quantum efficiency in UV and near-IR is typically less than the visual band.
- Spectral gap between green and red filters in some filter sets reduces the effect of light pollution (high pressure sodium).
- Narrower red bandwidth can improve the SNR of the common HII regions by reducing the background.
- Filter sets with ~1:1:1 weighting for the G2V spectral class avoids the need for differing RGB and multiple dark frame exposure times.
- Filter transmissivity across the bandwidth.
- Reflections and star halo effects are issues also.

Mike

[mmirot]

All good points. I am pretty sure an APO would not be my first choice.

Cameras very in their spectral response.     
Is the information contained at wavelengths with QE 10-20% worth imaging? I am guessing yes but I have no experience.

Historically IR and UV Luminance information was rejected because it did match the RGB visual bandwidth.
It was felt to be unsound from a color balance point of view.
Obviously, I am challenging this.  I am thinking more about structures that we are not seeing.