Saturday, April 20, 2013

QHY8L camera: Dark, Flat, and Bias images

The QHY8L is a low cost, entry level, scientific camera equipped with an APS-C size color chip. It has a good sensitivity also in Ha emission line. It is cooled and it has been primarily designed for astronomical use. The resulted photos go deep, e.g. 21th magnitude celestial objects can be captured in only 3 hours exposures at f/5.3 and 130mm aperture.

Image 1: QHY8L one shot color astrocamera

For those people making scientific work on CCD sensors without any commercial interesst, I am making available the dark, flat and bias images of a QHY8L camera: => click here to download them now <=
 Image 2: Against any specification, both QHY8L cameras I have used so far
were able to cool down to -45°C under ambient

As usual, every image coming down from the camera must be calibrated. I do that in MaximDL by using flat field images made with Gerd Neumann's "Aurora flatfield panel", since it is also suitable for OSC CCD sensors. Flat fields remove artifacts caused by inhomogeneities of the CCD sensor, dust particles on both the sensor and the optics, and vignetting. A master flat field should be calibrated by subtracting a master Bias frame.
Image 3: Some tips how to calibrate QHY8L images in MaximDL

The dark frames (download link) will completely remove the so called "amplifier glow" artifact in your QHY8L light frames. The "amplifier glow" is a small bright area at top-left edge of your light/dark images. The bias frames will remove the noise of the electronics.

Image 4: How to convert the QHY8L raw images in color in MaximDL.

Process your QHY8L images strictly in the following way in MaximDL:
  1. image acquisition with this OSC camera => RGBRAW
  2. image calibration RGBRAW => RGBCAL (using dark, bias, flat frames) 
  3. color conversion (RGBCAL => RGB)
  4. color split (RGB => R,G,B)
  5. remove gradient (i.e. the light pollution) in R channel => Rclean
  6. remove gradient in G channel  => Gclean
  7. remove gradient in B channel  => Bclean
  8. color combine (Rclean,Gclean,Bclean => RGBclean)
  9. repeat from step (2) for all your images 
  10. align all images   
  11. stack all images   
  12. ...   
  13. further steps  
Download an example here. Do NOT align your images before the color conversion, since this will move the RGGB map and you will loose the color information. The according procedure is also explained here.

The 2D Fast Fourier Transformation (FFT) of the master BIAS averaged of 30 Bias gives information about the electronics. Periodical patterns indicate badly shielded electronics. A point in the middle of the image with perfect noise around it without any pattern is a very good sign. See also here.
Image 5: Fast fourier transformation (FFT) of the master BIAS frame

There some slight differences between the BIAS of both QHY8L cameras I have used so far.
Image 6: Comparison of the BIAS frames of two different QHY8L cameras

I calculated the read noise of my QHY8L according to the instructions in and I found it to be 10.96e-, assuming a system gain of 0.5e- /ADU  as shown in and 

Image 7: Processed image of a QHY8L (5x1200s, 660mm, f/5.2)

These are the gain and offset settings I use for the QHY8L.
Image 8: Gain and Offset settings for the QHY8L camera

Finally, I would like to present you a collection of my QHY8L photos at:

Thanks for reading.

Panagiotis Xipteras

1 comment:

Unknown said...


Let me first say this is a very useful post and also a very interesting blog with great images.

I am thinking of getting an Astrolumina ALCCD8L which is almost identical with the QHY8L, just i think is made in Germany..

Would you recommend this camera ? Because i see very nice images taken with it. Did you have any major issues with it that you recall?

Thanks a lot,