Articles & FAQ

Articles & FAQ

LRGB vs CRGB

LRGB vs. CRGB Blocked Luminance Discussion Using NGC2403

Don Goldman

STATEMENT OF THE PROBLEM

As indicated in the Why Astrodon? section, the use of a NIR-Blocked Luminance filter or an unblocked Clear filter for the high resolution "Luminance" portion of an image has been and is still being hotly debated in the imaging community.  There are many opinions, conjectures and beliefs posited, but very little data.  The work that follows is a systematic attempt to collect data in an attempt to find out what is fact or fiction. 

The idea of Dr. Okano's LRGB method is to let the "luminance" image provide the detail and let the RGB, or chrominance, color the luminance.  This is why we take the high-resolution "luminance" image unbinned at the highest resolution of our CCD and take the RGB binned 2x2 at lower resolution for faster imaging and often slightly "blur" the chrominance.  We then combine the high resolution "luminance" image and the RGB chrominance image in separate layers in programs such as Adobe Photoshop resulting in a vibrant, high resolution color image. The question becomes what is best for the "luminance" portion of the the image - C vs L? 

There are two schools of thought on this subject that is still hotly debated within the imaging community:
  1. Choose the NIR-blocked L filter that matches the RGB filters for best color integration, but sacrifice from 25% (STL11000, ST2000XM)  to 45% (ST10XME) of the luminance signal from the NIR.  This assumes that the additional NIR signal shown above is not "pure signal" but contributes some "noise" to the LRGB because there are no RGB photons to directly match the NIR photons.  Adding more "noise" to an image does not improve its quality.
  2. Choose the non-blocked C filter for the greater signal response to get all the photons possible in the time allotted.  Any impairment of color is slight and can be processed out. 

 

DATA

These data were collected by Ken Crawford at his Rancho del Sol observatory at 3000' in the Sierra Nevada foothills of Northern California with his 20" RCOS Ritchey-Cretien telescope, an SBIG ST10XME CCD and Astrodon® Tru-Balance LCRGB filters. 60 minutes of L and C were taken unbinned and 42 minutes of each R, G and B binned 2x2 were taken. The RGB was processed.  The L and C images were also processed as identically as possible.  LRGB and CRGB images were composed in Photoshop CS as identically as possible 

DISCUSSION

For as long as I have been imaging, there has been a debate over whether to use a near-infrared-blocked luminance filter for our high-resolution portion of our  LRGB data, or a clear filter, which does not have ultraviolet (UV) or near-infrared (NIR) blocking. It only has anti-reflective coating.  A poll on the SBIG Yahoo Group indicated an evenly split community. As with many issues in our hobby, there are alot of opinions, heresay and expectations, but not much in the way of hard data to make an informed decision. 

Before I go further, I recommend that we use consistent terminology.  I propose that we use the Luminance (L) for a NIR-Blocked filter and Clear (C) for unblocked.  Alternatively, we could used blocked or unblocked luminance.

I have my own perspective on this issue.  I have stated for several years now that I believe L filters, matching the extents of the RGB filters will provide the best overall color and detail, especially for galaxies.  The C filter provides a 45% improvement in signal from NIR photons for an ST10XME and about 24% for an STL11000.  Since, NIR photons don't have direct analogs within the RGB space, the extra signal could be considered "noise", akin to sky glow.  Further, the signal will mostly affect reddish objects, so the 45% improvement will not be seen "across the board". 

Note.  I recognize that the NIR photons are just part of the blackbody curve generated by stars in the galaxy that are present in the RGB filters, with cooler stars contributing relatively more. So, there is some relationship.

I tried to show some of this with Hubble's Variable Nebula, where, with the exception of some fatter red stars, there was little or no improvement in the nebula using a C filter over an L filter.   Click here for the link to that report.

The limitation of that study was that is was done on a bluish object rather than on a galaxy having a yellowish core, bright HII regions and extended blue spirals.  Fortunately, my close imaging friend, Ken Crawford, took an image of NGC2403 in Camalopardalis in December and took RGB, L and C data.  His finished image combined both L and C into the "luminance" layer.  Ken provided me with his master L, C, R, G,  and B FITs files. These were all taken on the same evening. My goal was to process the RGB TIF file and then as best I can process the L and C files similarly, and make LRGB and CRGB composites.  The results are shown below.

The immediate overall impression in comparing the LRGB and CRBG images above is that the CRGB galaxy seems to have a "glow" around the galaxy that washes out the color and appears to diminish contrast.  The opacities of the L or C layer in Photoshop CS (luminosity blending mode) were set to 60%.

The next comparison is of the two luminance images using MaximDL's information window. Identical rectangles were drawn at the same place in each image; a place in the core and another within the background without including any stars.  The average ADU for L and C in the core was 508 and 829, respectively.  I have shown in the Hubble's Variable Nebula report that the Standard Deviation  (Std. Dev.) value provided in the Maxim Information window is a good approximation of signl-to-noise (S/N).  The std. dev. values for these core samples are 98 and 166, respectively.  So it appears that the C filter delivers much better S/N.

MaximDL Analysis of S/N in the Luminance (left) and Clear (right) Filter Images

 

 

  (click on image above to enlarge)

The background areas were also analyzed similarly within identical rectangles near the upper right portion of each image. The average ADU values for L and C are  216 and 379 and std. dev. values of 10 and 15, respectively. 

So, the C filter provides the predicted (from the quantum efficiency [QE] curve of the CCD detector [KAF3200ME]) improvement of of ~1.5 over the L filter. It provides a higher S/N, albeit over a brighter background.  This creates the "glow" described above as can be clearly seen in these "luminance" images. 

Comparison of Equally Stretched Luminance (left) and Clear (right) in Photoshop

 

The greater S/N from the C filter sits atop a brighter sky background, or "galaxy glow" due to the extra NIR photons.  To me this is like imaging the Horsehead Nebula with RGB filters from a large city.  Fainter detail will tend to be washed out.  This is not so much a problem with a bright galactic core.  It will be noticed more as a loss of detail in fainter regions, e.g. between the spiral arms. I think this effect can be seen above.

The plate scale of Ken's 20" RCOS RC with an Astro-Physics' 0.67x focal reducer is 0.44"/pixel in the ST10XME.  Using that setting in MaximDL, a number of stars in each "luminance" image were analyzed with Maxim's Information Window. Those data are summarized in the table above.  There is a systematic increase in the full-width-at-half-maximum (FWHM) of the stars from the C filter in comparison to the L filter. Overall, there was a 0.08" increase in star size in the C image.  Of course seeing could change within the course of imaging throughout the evening, but there are reasons to expect this, as discussed next.

Analysis of Full-Width at Half Maximum Intensity (FWHM) for Stars

 

It could be argued that the focal reducer is the cause.  However, in discussing this with Roland Christen at Astro-Physics (personal communication), he mentioned that the Airy disk in the NIR (e.g at 1000 nm) is nearly twice that in the visible.  So, even if we had perfect optics, there would be a difference.  The C filter would degrade contrast. Refractive optics will just make it worse. This is the reason why L filters are strongly recommended for people using telephoto lenses on digital cameras (e.g. Canon 20D) to minimize this refractive optics dispersion problem. 

SUMMARY

If your goal is to bring out magnitude 22 galaxies by combining many long "luminance" exposures, then it may be best to use a C filter or no filter at all.

If your goal is to bring out vibrant color in galaxies, these tests suggest that the L filter, matched spectrally to the RGB filters, is a better choice to eliminate the NIR "galaxy glow" contributed by a C filter.   This becomes even more important in systems with refractive optics (SCTs, refractors).

If you have a refractor or telephoto lens, remember that a C filter also lets in UV. This is why less color-corrected refractors often need "minus violet" filters to eliminate the optical dispersion in the short wavelength region (blue halos around bright stars). An L filter does cut out some of this. For these systems, an L filter is likely to be a better choice. 

ACKNOWLEDGEMENT

Thanks to Ken Crawford for making his data available and Roland Christen for discussion of the optics.