Re: Exposure comparisons with CCD cameras

Daniel Borcard

Hi Rolando,

Sorry if I rarely chime in. I follow the group closely and am a happy owner of an AP1200 GTO and a Traveler.

Your comparison is very interesting, but rather extreme. In narrowband imaging with CCD cameras one rarely takes 1 minute subs, and few of us are enjoying plane- or satellite-less skies allowing 1 hour subs :-)

If you continue this experiment it would be interesting to also take intermediate-length subs: 6 x 10 minutes, 4 x 15 minutes or 3 x 20 minutes. These will likely be long enough for the signal to overcome the various sources of noise. I would actually be surprised if three 20 minute subs show less signal than an unique 60 minute exposure does.

With my AP Traveler and the AP Mach 1 I had at the time I used to go up to 30 minute subs, but I saw almost no improvement over 15 or 20 minutes. And I image through the worst of the light pollution dome of Montreal, Canada...

Clear skies!


Hello Astronuts,

Last night was a good night to try some experiments with the Mach2 mount and my 160 EDF refractor. I have been shooting the Veil nebula for the last couple of nights, normally using 10 minute subs and stacking them. I have not been guiding, but using modeling of the path to get sharp round stars.

Last night i did one exposure of 60 minutes and 6 exposures of 10 minutes each (60 minute stack). I wanted to see how the faint detail and noise levels compare. It turns out that the single 60 minute shot has much lower noise and shows more fainter details than the 60 minute stacked image. In fact, it took 120 minutes of stacked images to equal the single 60 minute one. You can see the result here:

As noted, the images were stretched to bring up the faintest detail and to show the noise levels. It appears to me that longer exposures for narrowband produce better results faster. There are two drawbacks. The image can be ruined by satellites or airplane trails. A 1 hour exposure requires some guiding. 

Both images had the model running in the background, which was good for round stars in a 10 minute time interval. However for the 1 hour exposure I wanted to make sure the stars would be round and sharp, so I set up my Lodestar off-axis guider. The guider was set to take a 2 sec exposure every 10 seconds to nudge the two axes. The image below shows how well the mount guides when it is also being modeled:
Daniel Borcard
Observatoire du Geai Bleu
Les faits sont têtus. Les nier ne les fait pas disparaître.

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