[Leica] Color Astronomy With Monochrom

[hlritter at bex.net (Howard Ritter)]

Bob?

What celestial objects are you planning to image? What equipment are you 
expecting to use? This astronomer?s opinion is that unless you?re planning 
to do the sort of color-critical imaging that?s usually done by 
battle-hardened astroimaging veterans with highly developed color 
sensibilities and idiosyncratic preferences, dedicated electronically cooled 
CCD astrocameras that cost as much as?to much more than?a prosumer DSLR, 
attached to an astronomical telescope with computer-controlled sky-tracking 
mount that each cost as much as the camera did, you?re not likely to see any 
advantage from using your M Monochrom to make (L)RGB images and combining 
them to get a final color image over using a color M to make a color image 
directly. Think of the difference between off-roading in your suburban SUV 
and those who do it in purpose-built Jeeps with stump-yanker gearing, tires 
with knobs the size of hen?s eggs, and winches front and rear.

First, there is no a priori reason that the Bayer color filters on a digital 
camera should produce a ?truer? or more pleasing color balance than 
dedicated narrowband filters as used in dedicated CCD astrocameras. The 
primary reason for using a panchromatic sensor with outboard color filters 
is to get better detail in the final image and to allow separate processing 
of each color channel for fine-tuning the final image. Given that each 
photosite in a Bayer-filtered sensor contributes to the structural detail of 
the image just as much as each photosite in an unfiltered sensor does, that 
the individual color channels can be accessed readily in raw or DNG files, 
and that the structural resolution of the final image is far more important 
esthetically than the color resolution, this practice has all but ceased to 
offer advantages. Modern higher-MPx sensors and sophisticated de-Baying 
algorithms have essentially eliminated any advantage to making and combining 
RGB images from a panchromatic sensor. Any small differences in color in the 
final images obtained through on-chip Bayer filters and through narrowband 
filters on a panchromatic CCD chip are less than the differences produced by 
different manufacturers? filter sets and different imagers? own personal 
processing techniques.

The following is in the event that you want to proceed anyway!

In general, the ?truest? (a highly subjective term of art!) astroimages in 
the age of CCD astrocameras have been obtained with panchromatic 
(monochrome) cameras and narrow-band filters, making two, three, or four 
exposures in different wavelengths and combining them in post-processing. 
Different exposure lengths may be used for each color, and in 
post-processing, each raw image can be mapped to a selected color. There is 
also a great deal of literature, and controversy, on what constitutes the 
?real? color of faint, nebulous deep-sky objects, which are seen visually in 
even the largest telescopes as shades of grey, and of how best to obtain 
that ?reality? in the final image. All of this is why I said that ?truest? 
is a subjective term. For images of the planets and the Moon, the advantages 
of taking and combining multiple monochrome images at different wavelengths 
are nonexistent, and the practice is a waste of time and effort. For these 
objects, a ?one-shot color? astrocamera will suffice, or alternatively, for 
the essentially colorless Moon, a single unfiltered image from a 
panchromatic camera will be perfectly satisfactory.

Most commonly, R, G, B, and L (a luminance ?filter? is a colorless piece of 
glass, used rather than no filter at all in order to preserve the same focus 
as with the colored filters, the purpose being to provide overall luminance 
information) filters are used in a panchromatic astrocamera, one that has a 
sensor with no integrated Bayer filter. For some gaseous nebulae that 
radiate primarily (or exclusively) by fluorescence, filters for, e.g., the 
hydrogen-alpha or doubly-ionized oxygen (O-III) filters are used. Since the 
image quality is most sensitive to physical resolution, whereas color may be 
applied rather crudely by comparison and still produce a visually excellent 
image, it is not uncommon practice, in the interest of optimizing the use of 
limited resources, to combine a luminance (i.e., B&W) image made with a 
large-aperture telescope (for best detail) with RGB images made with smaller 
instruments. 

However, the most modern one-shot color astrocameras, whose sensors have 
built-in Bayer filters, are said to produce color images that are equivalent 
to those obtained with panchromatic unfiltered sensors and outboard filters. 
Again, plentiful literature and controversy.

For the astroimaging community, the requisite filters are generally 
available in the two barrel diameters that are standard in astronomical 
telescopes, 1.25? and 2?. Some are available as 2? x 2? unmounted glass. 
Google ?color filters astroimaging? and look for articles and for links to 
manufacturers, for example the German Astronomik and the American AstroDon. 
And these filters are indeed associated with visible color: the R filter is 
very red, the G is green, and the B is blue, while the L is colorless.

Another option, which I assume is the one you?re considering, is to use what 
we?d all recognize as a ?camera?, usually a DSLR but in your case obviously 
the M Monochrom, which is the only B&W digital camera on the market. The 
evolution of high-MPx, low-noise sensors has allowed the performance of 
premium general-purpose consumer/prosumer 35mm cameras to rival that of 
purpose-made astrocameras, especially for imaging brighter objects that do 
not require hours of exposure and, consequently, active cooling with a 
built-in thermoelectric cooler. The ?hot hand?, no pun intended, in this 
respect is the Nikon D810A, a camera aimed at the astroimaging community, 
that lacks both the IR filter (to better capture deep-red and H-alpha 
wavelengths) and the anti-aliasing filter in order to improve structural 
resolution.

Bottom line: Unless you plan to put your Monochrom on an astronomical 
telescope with a computerized drive to make long-exposure astroimages, I?d 
stick with using an M9 or Typ 240 for images made with camera lenses. If you 
plan to make wide-field sky images with camera lenses, I don?t think that 
LRGB offers any advantages.

Coincidentally, I have gotten my scopes set up after a move from Ohio to NC, 
and have just received my M Monochrom in trade-up from my sensor-corroded 
M9. I?m going to be trying some astrophotography with it. In addition, the 
M8 would seem to be an excellent candidate for making images of nebulae that 
emit primarily H-alpha, so I?ll be trying that as well. I?ll post what I 
manage to get.

?howard



> On Feb 28, 2016, at 1:10 PM, Robert Adler <rgacpa at gmail.com> wrote:
> 
> I am hoping to get some guidance from the astronomy experts here. A little
> research shows that by using what are called "LRGB" (Luminance, red, green,
> and blue) filters with monochromatic sensors, one can combine the images
> taken with these filters to come up with astrophotos with very true color.
> 
> Does anyone know where to get these special filters (they are not
> associated with visible color, but more wavelength transmission). I would
> either need Series VII (for the 21/1,4) and E46 for the 35/1,4 or 4x4
> filters for a Lee holder (which I would somehow need to fit to my Leica
> (Seven5 filters would be great, but probably impossible to find).
> 
> Any leads or experience would be greatly appreciated.
> Thanks,
> Bob
> -- 
> Bob Adler
> www.robertadlerphotography.com
> 
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