Questions about RGB filter wheel

[Ags]

I have a filter wheel coming soon with some cheap RGB filters - I am just wondering about the right way to use them. Because red will be the sharpest, do I shoot a long sequence of short red frames, then increase the length of frames and shoot shorter sequences of blue and green? My reasoning being red will provide the detail/luminance and the blue and green will only be used for color where sharpness is not so important, and longer individual frames  for blue and green will lower noise.

Or do I just shoot the same number of frames in each color with the same settings?

[vlaiv]

Not sure if using red channel as luminance is such a good idea. I understand that you want sharpness for luminance, but you will get quite unnatural appearance of target if you select red channel to be your luminance.

Human vision is geared towards green being the luminance. For same intensity of source, we perceive green as being the brightest of the three - r, g and b. Here is for example luminance formula when you have sRGB components:

This means that we will perceive green as being about 2 times brighter than same intensity red and even more bright than blue (although above table gives x3.5 and almost x10 - it is still linear and not gamma corrected, after correction this difference is less).

In answer to your question, I would just shoot same number of frames per channel, as sensor is roughly as sensitive as it should be in comparison to luminosity of each part of spectrum. After than just do regular R, G, B compose from these filters and whitebalance image and do wavelet sharpening. That should give you good results.

[JamesF]

I agree.  I'd start with everything the same for each filter.  Once you see the results you can decide if you want to change anything.  I also tend to do one capture with each filter in turn and cycle through them, so if the seeing goes or cloud comes in then I should hopefully have all the data necessary for an image.

James

[Ags]

Thanks, Keeping it all the same and cycling through each channel sounds like the  best approach.

Shooting each channel for one third the time of a mono image sounds like I will get a lot of noise - how to manage that?

Is wavelet sharpening specifically for color images? I don't currently use Registax, only Autostakkert (which doesn't seem to have wavelets).

[Freddie]

No, wavelets can also be used on mono.

[vlaiv]

6 hours ago, Ags said:

Thanks, Keeping it all the same and cycling through each channel sounds like the  best approach.

Shooting each channel for one third the time of a mono image sounds like I will get a lot of noise - how to manage that?

Is wavelet sharpening specifically for color images? I don't currently use Registax, only Autostakkert (which doesn't seem to have wavelets).

I think that @JamesF gave that advice in light of long exposure AP, where you can cycle between filters for each sub (or couple of subs).

With planetary imaging this is of course not feasible since exposures are really short, but you can actually use this approach - shoot 1 minute video in each filter then change and do next filter. When you finish all three (or four if you do LRGB, but not sure if it is worth it - maybe just do RGB to start with) then return to first filter and again do another set. This way you can choose best 6 minutes of video - two for each filter, depending on seeing as you can cover as much as half an hour or more by cycling thru the filters. I'm not sure if you will be able to use all recorded material due to motion blur even if you derotate your videos.

Yes, each channel will have lower SNR compared to lum, but that is simply the way things are if you want to shoot in color - same thing happens if you use OSC sensor since only 1/4 of pixels pick up red and blue and 1/2 green color - again meaning less signal and lower SNR.

This of course should not worry you much as there is no other way to do it that is reasonable (you could in theory use multiple scopes and shoot all filters simultaneously, but like I said, let's keep things reasonable ).

Yes, wavelets work on both mono and color data. In fact wavelets work on mono data "exclusively" - but when you think about it R, G and B when you look at them individually are mono data - they become color only as consequence of "interpretation" by hardware capable of displaying color.

[JamesF]

7 minutes ago, vlaiv said:

I think that @JamesF gave that advice in light of long exposure AP, where you can cycle between filters for each sub (or couple of subs).

With planetary imaging this is of course not feasible since exposures are really short, but you can actually use this approach - shoot 1 minute video in each filter then change and do next filter. When you finish all three (or four if you do LRGB, but not sure if it is worth it - maybe just do RGB to start with) then return to first filter and again do another set. This way you can choose best 6 minutes of video - two for each filter, depending on seeing as you can cover as much as half an hour or more by cycling thru the filters.

That's exactly what I meant.  Obviously I didn't make myself clear enough :)

James

[Ags]

That's what I understood - shoot 1 minute R, 1 minute G, 1 minute B, repeat, repeat, repeat.

[Science562h]

... or just image in "color."   

[Freddie]

A bit tricky with a 178MM cam!!!

[NenoVento]

Mmmm... I tweak the exposure and gain for each channel until I get a more or less similar histogram for them all, otherwise I get almost no signal in the blue one.

NV

Edited January 4, 2020 by NenoVento
Typo due to autocorrecting software set at a different language