Nikon D750 DSLR takes only black and white images


#1

Used SGP for the first time last night and noticed the resulting .FITS light images are black and white only. E.g. https://www.dropbox.com/s/uchiqfhurtufxqd/Antares%20HIP80763_30sec_1x1__frame8.fit?dl=0

Why is the colour missing? The only settings I can see in SGP for my Nikon are FITS and 1x1 AF Binning, which were the defaults.

  1. I connected the Nikon camera
  2. Ran the sequence with several repeats, ISO 1600.
  3. The .fits files were generated as expected, but were greyscale only?

#2

SGP captures and displays the raw data as it comes off the camera, where each (monochrome) pixel is covered by a colour dye, allowing them to capture the amount of red/green/blue light. From each four adjacent pixels two are covered with a green dye, one with red and one with blue. If you zoom into the image in SGP you will see the grid pattern caused by this. Your image processing/stacking application will convert the raw images to color when calibrating and stacking using a process known as debayering. Unfortunately SGP doesn’t do this for preview purposes.


#3

Yes I noticed the grid pattern in the photos and was curious about that. I’m still confused however. My understanding is that a .fit/.nef raw file can be in one channel (mono) or have all rggb channels (colour). I’m expecting SGP to be creating colour images from my Nikon, just like e.g. Backyard Nikon does. I’m not using filter wheels.

I understand that when processing colour .nef images from my Nikon (e.g. captured via Backyard Nikon) in Pixinsight I have to tick the debayer option. All one shot colour camera images need to be debayerred. My Nikon is a one shot colour camera. I was expecting to be following the same workflow with SGP. But SGP is creating one channel (mono) from my OSC Nikon. Please examine my previously attached .fit file to see what I mean. Surely SGP supports creating OSC .fit files?

Sorry if I am missing something!


#4

There is a difference between true monochrome and Bayered images (which are grayscale). SGP does not do any debayering. As such all images viewed in SGP will be grayscale. However OSC images will always have color data associated with them. If you open up PI and debayer the images you will indeed get color from a “single channel” OSC image. SGP does not output 3 channel (Red, Green Blue) FITs as they would be 3x the size (and are honestly obnoxious to process…)

If you would prefer to work in NEF rather than FITs, SGP can save off the untouched .NEF files as well. Just choose the NEF option in the Nikon Settings Dialog.

Might help to better understand how “OSC” cameras actually work. The CCD/CMOS itself is a monochrome sensor (technically it reads light levels and not color…but let’s not go there). On top of the sensor is a Bayer Matrix which consists of pixel sized filters. This Bayer Matrix is really the only difference between a monochrome and a OSC camera using the same CMOS/CCD chip. Each pixel has a single color filter overlaid on it. Generally in a block of 4 pixels you’ll generally have 1 blue, 1 red and 2 green. So you take an image and each of those pixels captures light through a single filter. The output is an image that has intensity data for each of those pixels, but no actual color to it (as the sensor is monochrome). However there actually is color information in the image as each pixel has a color filter in front of it.

Debayering is the process of matching the pixel with the correct color filter, then combining near by neighboring pixels to create a RGB image.

Thanks,
Jared


#5

Hi Jared - thanks for the detailed explanation.
Good to know that I can save one shot colour images using SGP using Nikon .NEF format instead of what I was doing, which was using .FIT format.

You see, when I originally imported the .fit files produced by my Nikon (via SGP) into Pixinsight and told it to debayer (by checking the “CFA images” checkbox in the batch preprocessing script), Pixinsight refused and eventually gave an error. Only when I unchecked that box did PI proceed to completion, in which case, after integration, I ended up with a greyscale image with a kind of ‘dotty’/streaky vibe about it.

So this seems to suggest that even when I upgrade to a proper cooled OSC camera like as ZWO ASI071 I will still have to avoid .fit format because as you said " SGP does not output 3 channel (Red, Green Blue) FITs". That would mean saving in some sort of raw format that ZWO supports - I’m not sure what that would be till I buy that camera.

Seems strange that, in this day where OSC cameras are rising in popularity, that SGP by default seems to support a non bayered .fit file format = mono. Its like SGP workflow favours people using filters. I think what I have run into might be a common trap for new users with OSC/DSLR cameras and there should be some sort of warning in SGP that if they select .fit format for a DSLR that they are not going to get colour.


#6

Good to know that I can save one shot colour images using SGP using Nikon .NEF format instead of what I was doing, which was using .FIT format.

Yes, but useful information which is saved in the FITS header will not be available then.

You see, when I originally imported the .fit files produced by my Nikon (via SGP) into Pixinsight and told it to debayer (by checking the “CFA images” checkbox in the batch preprocessing script), Pixinsight refused and eventually gave an error. Only when I unchecked that box did PI proceed to completion, in which case, after integration, I ended up with a greyscale image with a kind of ‘dotty’/streaky vibe about it.

The checkbox ‘CFA images’ has to be checked. Additionally you will have to set the correct bayer pattern for your camera (BatchPreprocessing, Tab ‘Lights’, section: DeBayer, ‘Bayer/mosaic pattern’) in order to avoid the error. PixInsight is able to detect it from raw (NEF) files, but this information is missing in the FITS files.

Bernd


#7

Yes just set the correct debayer format. Once you go back to NEF, everything will make more sense.

Another important issue is for this session though your calibrations (darks/bias/flat) from NEF files won’t work well with the light FIT files. So if you want to calibrate for this session only you should make new calibration with FIT files in SGP. Then after it’s done switch back to NEF moving forward. So the calibrations in FIT would be for this single image or session you did.


#8

I don’t think you completely understood my reply. In short SGP works great with OSC cameras. I have personally used SGP with a ZWO ASI1600 OSC as well as mutliple DSLRs. All in FITs format and they work just fine. SGP has worked with OSC cameras since it was first released. And while there is an influx of inexpensive CMOS cameras, OSC is not a new technology in astrophotography by any means.

The images coming out of SGP appear grayscale if no additional processing is done, but they are not. And once you Debayer them they are color images. SGP does not create 3 channel fits as the images are massive and provide 0 value since the color channels are interpolated, the exact same data is stored in a CFA FITs… We store the raw data off of the chip in a FITs file. After you debayer you’re left with a color image. You do need to select the correct Debayer pattern in PI or whatever you’re using to process.

In the FITs standard you’re really given 2 choices about how to store color images:
CFA (Color Filter Array) - what SGP and every other AP capture application does.
3 Channel Color - 3x the size and stores one interpolated color per plate per resolution (more similar to a mono camera with filters)

Thanks,
Jared


www.mainsequencesoftware.com