CCD Camera

CCD Camera

You can test the CCD camera at twilight or in a dimly lit room, but not in the daytime outside. The shortest exposure is 0.1 seconds and the CCD camera is very sensitive and the telescope collects ~100 times as much light as a small camera (which would need just 1/60th second exposure or shorter during daylight). If the resulting image in SIPS is black and shows pixel values of 65,535 (the maximum), then you have overexposed. When you see your first successful image it might lookawful, with noise and gradients across the image. That’s why you need to take “darks” and “flats”to subtract noise and create a uniform final image.

  1. Connect the CCD camera to the battery
  2. Connect the CCD camera to your computer using the USB cable 
  3. Start the camera software (SIPS) on the laptop, under tools menu, select the ccd camera G2 as the imaging camera (it should show up in the list once you click ‘scan’)

     

  4. Set the CCD temperature to cool down to 20 degrees below the ambient temperature

     

  5. Focus using the “focus tab” by first selecting a region of the frame with contains a visible object or bright star and turn the focus wheels on the tube to bring the image into focus.

    This takes patience since if it is a long way from focus the image will appear just black and noisy or you may see a single white filled circle – this is the out of focus tube view. You can speed up the focusing by clicking on the ‘binning’ tab and choosing 3×3 for example. For fine adjustment, use the Bahtinov Mask on the front of the tube, and bring the three diffraction spikes of a star into alignment: see https://en.wikipedia.org/wiki/Bahtinov_mask

     

  6. If you wish to refine the accuracy of the alignment you should now follow the steps at theend of these notes for accurate polar alignment using the “declination drift technique”. Ifyou have made a reasonably accurate polar alignment and use the autoguider correctly, you will not need to do this but the telescope will track more accurately if you do. And if you cannot get the autoguiding to work properly, this step is essential.

Troubleshooting

The provided laptops have two standard USB ports and one USB-C port. You may need to use the USB multi-port adapter. Very often, cameras are not recognised, the goto software fails to recognise the mount or connections are lost. That’s typical with USB devices and these problems are usually solved by unplugging/replugging the cables and/or restarting the laptop or software.

 

Flat field frames

Flat field frames are used to correct problems in the optical path, including dust, vignetting and internal reflections. Any change in the optical path, such as changing the camera orientation, will require a new flat field frame. To obtain the best possible flat field frames would mean taking it at the same conditions as the light and dark frames. To take flat field frames, first focus and then point the telescope to an evenly illuminated surface. Flat field frames can be taken from the twilight sky or with a clean white cloth placed over the tube to reduce light or with the led illuminated white panels. The exposure time has to be chosen in such a way that the intensity level reached approximately 50% of the maximum. Do not over expose and take several flat field images at different exposure times. These typically need short exposure times – if the photo count is over 65,535 you have overexposed.

Dark frames

The purpose of a dark frame is to remove the non-image related thermal and electronic noise that the CCD generated when capturing photons. It also removes imperfections of the chip, such as dead pixels. Since we are capturing the build up of thermal and electronic noise, it is important to match the dark frame to the same conditions as the light frame, this should include the same duration of exposure, chip temperature, gain, brightness and contrast. Dark frames can be taken by covering the telescope with its lens cap and take at least 10 images with the same exposure length as the light frames. With these dark frames you can generate a master dark frame.

Your final calibrated image is then equal to (raw – dark)/[(flat-dark)*(mean flat – mean dark)]