Imaging Session - what goes on

For those interested in imaging and image processing with DSLR, CCD, webcam or film.
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Imaging Session - what goes on

Post by DRatledge » Fri Apr 17, 2020 7:32 pm

In these strange times I thought I would explain how one of my imaging sessions works. Attached is a screen dump of my observatory computer viewed from my indoor computer! I have 3 programs running in the observatory, the first that controls the cameras; another that maps the sky showing where the telescope is pointing and enables me to send the telescope to a selected object; the third that controls the telescope, adjusting dew heaters, fans and electric focus. This screen dump is just the camera control software.

So what is going on? I am taking a series (sequence) of images of a pair of galaxies (the Mice). The main image on the screen is image 22. This has completed and been displayed on screen. There will be 24 identical images that eventually will be added together to make a more detailed and less noisy image. Not quite identical because between each of those 24 images the telescope will move a small amount randomly. This is called dithering and ensures every image falls on a slightly different position on the camera chip. Great for resolution.

The top right window with what looks like a white dot on a black square is the guide camera image of a star. This is a second camera that keeps the telescope locked onto its target. It is from what is called an off axis guider – the only way to guide believe me. This camera takes a picture every 4 seconds of a star – if the star has moved off centre then it sends a command to the mount to drive the telescope to bring the star back to the centre. This is autoguiding and takes the pain out of imaging. The image is only a small black square around the star as the rest of its image is not needed – I just use a small region. The guide square resets itself after that random dithering movement referred to above - important the guide camera knows this.

Below this window is something called screen stretch. It shows the brightness values of the guide star square. If the minimum number (sky background) suddenly goes up then clouds are coming! This needs to be watched in Lancashire for obvious reasons.

The window below is a graph of the guiding error over time – how much that guide star has moved off centre. X-error is in RA and Y-error is declination. It is watching this that gives you grey hairs. In this run errors (quoted as RMS) are both below 0.2 units. The units are pixels which equates to an error of around 0.5 arcseconds which is acceptable. Remember Lancashire seeing is typically as bad as 3 arcseconds so tonight’s guiding is more than adequate.

The final window is about the sequence of images that the main camera is undertaking. For camera 1 (the image taking camera) it shows we are 159 seconds into the next 600 second image – number 23. The cooling of the main camera is at -20 degrees C. This will help reduce noise in the image. For camera 2 (the guiding camera) it lists the errors every 4 seconds.

So in an imaging session this is what I spend hours looking at. I also have to check the other 2 programmes too. The sky mapping one will tell me when I need to do a meridian flip – when the telescope gets to due south. The third controlling the telescope lets me know the temperatures of the primary mirror, the secondary mirror and ambient. I can then adjust heater or fans as necessary.

When the session is complete I have to slowly bring up the temperature of the main camera up from -20 degrees to ambient so as to avoid thermal shock – this is a drag when you want to go to bed. Then walk down the garden to shut the observatory and finally I can go to bed.
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