Advice to newcomers to astroimaging regarding astro camera type and autoguiding

An article in the author’s Astronomy Digest  –

There is a much that can be done using a standard (or modified) DSLR or Mirrorless camera as many articles in this digest will testify.  At some point some may wish to purchase a dedicated astro camera such as an Altair Astro (AA) 294c Pro cooled camera having a Micro 4/3 seized sensor which appears to be identical to the ZWO ASI294MC.   The ‘c’ defines it as a RGB colour camera using a Bayer matrix to enable it to produce colour images.  They do have two advantages over a standard DSLR in that the red cut-off filter that reduces the sensitivity to H-alpha emission is not present and, if a Peltier cooled version were used it will have a far lower dark current reducing the noise in the image.

One then has a choice of a colour or mono camera (with its associated filters).

For many years I have happily used an SBIG STF-8300 CCD camera with filter wheel to do both LRGB and narrowband imaging on an autoguided mount.   But, having used the  AA 294 for some months now seriously wonder whether autoguiding or the use of a mono camera is really necessary for those of us who live under any but the most pristine skies or have very long focal length telescopes.  There is no doubt that a mono camera system can produce better images but at greater expense and image processing time, but it is surprising how well the latest CMOS colour cameras can perform.   

Autoguiding – is it now necessary when using CMOS cameras?

With the use of CCD astro cameras, autoguiding was usually required as, due to the relatively high read noise, longer sub frames were needed and, using a USB2 interface, image upload times of  ~7-10 seconds  meant that very short exposures would be inefficient in their use of imaging time.  With the latest CMOS cameras, their very low read noise means that the use of many short exposures will not harm the final image and, under light polluted skies will actually be better.  SharpCap Pro has a ‘brain’ which takes an image of the sky and, based on the sky brightness, suggests the optimum exposure.  Pointing my telescope over the City of Manchester, some 20 miles to the north, an exposure of 10 seconds was suggested!  So exposures of 30 to 60 seconds may well be optimum under light polluted skies. With their USB3 interface, download times are not a problem. Most mounts that are well aligned should be able to track sufficiently well so that star trailing will not be apparent when such short exposures are made.  The image of M13 was taken unguided taking 30 second exposures using a Vixen VC200L, 1,800 focal length, telescope – I think the stars look pretty good however the outer parts of the cluster have been masked by the light pollution at my urban location.  I hope this image makes a good case for not requiring the use of autoguiding when using telescopes of focal lengths under 2,000 mm. 

[Incidentally, I do not perfectly align the mount on the NCP (using a QHY PoleMaster) as I want the image to very slowly move across the sensor during the imaging period.  This removes what Tony Hallas calls ‘colour mottling’ due to uneven pixel sensitivity.  When autoguiding, ‘dithering’ is often employed to give the same result.] 

Imaging with a colour CMOS cameras and, when desired, a Tri-Band filter rather than a monochrome camera, LRGB and narrowband filters.

A CMOS colour camera will obviously provide a colour image directly with less post processing required.  Using a mono camera with LRGB filters will provide a better image given the same total exposure time but,  unless imaging from very dark skies, light pollution will limit the quality of the final image so giving, I suspect, similar final results.

What I have found very interesting is the use of a colour camera and  tri-band filter.  When producing an image of an emission nebula one really needs an RGB image to produce a star field in which the stars have appropriate colours.  The total exposure for this does not need to be long – in such images the stars do not want to overwhelm the nebula.  One can then take an image using, say, a  tri-band filter: typically they will have one pass band to capture the H-alpha spectral line and a second, broader band to capture two spectral line of O III and that of the H-beta spectral line.

The stars in this image will not look good so one can use the wonderful program StarNett++ to remove them.  The stars image can then be added using a ‘screen’ blending mode when, usefully, their brightness can be controlled using the ‘opacity’ slider.  The image of the Witch’s Broom nebula was taken using this technique.  The quality was limited by the light pollution at my urban location.

Together, the OIII and H-beta spectral lines should give a blue-green, (teal) colour as seen in my image.  I wondered why many of the images on the Web show this as blue but the consensus is that teal is the correct colour – which comes naturally when using a colour camera.

The third image was a surprise to me – and it is this that has really made me think that Mono cameras are perhaps no longer required for all but the most dedicated imagers.  Using a 90mm lens at f/3.5 and my AA 294c camera on the AA Nikon adapter mount, I first imaged in full colour (25 minutes total exposure using 60 second sub exposures) the region around  Deneb and Sadyr that emits much H-alpha emission including the North America, Pelican and Butterfly Nebulae.  I then mounted a 2 inch H-alpha filter within the mount and hoped to spend at least an hour narrow band imaging, but clouds limited the total exposure to just 38 minutes (again using 60 second exposures).  [One could just as easily use a triband filter and, if it is necessary to remove light pollution, simply process only the red channel of the image to remove light that is passed by the OIII/H-beta band.]  The RGB and H-alpha images were processed as above using Starnett++ to give the result shown.

To be honest, I was amazed at how well the Nebulae showed up – particularly the fainter Pelican nebula.  Could use of a Monochrome camera using LRGB and H-alpha filters have done any better given the ~1 hour total exposure time?  Theoretically yes, but I suspect that the difference will not be that great. 

So my advice is this; until  you get very experienced it is definitely better to simply buy a CMOS colour camera and, for emission nebula, a tri-band filter – and save a lot of money in doing so.

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