Jeff, good question. A CCD responds to light in a linear fashion, thus the longer the exposure time, the fainter the details you’ll acquire and theoretically, better the signal to noise ratio. Depending on the target, the difference between a 30min sub and a 10min sub can be significant especially when working with filters that have a small bandpass window such as Ha filter.
However, to specifically address your question, you need to determine the quality of one 30min sub compared to three 10min subs. If the one sub contains satellite trails, cosmic ray hits and other anomalies, it can only be corrected through combining more data with a form of an outlier pixel rejection algorithm (sure you can chase them in PS, but why bother). Thus it’s difficult for one 30min sub to equal the quality of three 10min subs. Having stated this, you’ll find that the three 10min subs will not be as deep as the one 30min sub because you’ve only allowed 10mins of light collection compared to 30mins - remember CCD's respond in a linear fashion. Combining more and more 10min subs will improve the signal to noise ratio, but it doesn’t make the image any more deeper…you are still limited to 10mins of light collection on the sensor. You can try using the sum combine function in an attempt to make the image deeper and bring out those faint wisps of nebulosity, but from my experience it’s not the same as collecting 30min subs – certainly when comparing background ADU counts vs object in question.
As you combine more subs you combine, the data will be become smoother and you’ll be able to stretch it harder without introducing noise. Technically, two subs data is barely workable. You’ll find that data combine algorithms usually require a minimum of three to function, and all work significantly better with much more.
So, now that I’ve told you going deep with 30min subs is the way to go assuming you are willing to also collect three or more 30min subs, there are some other things you should know.
Firstly, your ST2k has an ABG chip, thus a gate bleeds the pixel wells when they reach a saturation level as designed by the manufacturer. This prevents blooms, which is the full saturation of a pixel well that subsequently spills charge into surrounding pixels. Clearly, the exposure times for an NABG chip will be shorter than that of an ABG to counteract blooming of bright stars in the FOV. For now, this is something you don’t need to worry about and is what makes an ABG chip somewhat easier to use. What is important to note however is as there is no bleeding with NABG chips, they are typically more sensitive than their ABG relatives. The signal to noise ratio of a 3min sub taken with a NABG chip will yield a greater signal to noise ratio than that of an ABG of the same exposure time. Hence, you need to use long subs with an ABG chip to obtain a similar signal to noise ratio. i.e. make up of the lack of sensitivity.
Secondly, going deep with long subs is great for nebulosity and faint details such as dim structure in spiral arms, however watch out for the stars. Stars are bright and don’t need much exposure time to reach saturation. Saturated stars aren’t too much of an issue with ABG camera as there is no blooming as mentioned above, however you’ll still find they’ll loose colour the deeper you go. This is more pronounced with LRGB blends as the strong luminance data de-saturates the colour data. You can specifically process the stars in an attempt to bring them back, but sometimes it’s difficult to achieve. Star management i.e. managing the stellar profiles in processing is a different topic in which I’m sure others can share their experiences. To counter act this, one method is to take some shorter RGB subs and blending them into the deeper data. Russ Croman did a presentation at AIC2004 on preserving star colours and profiles, this maybe of interest -
http://www.rc-astro.com/resources/ta...StarColors.htm
Finally, as Steve mentions, the longer the subs, the more demanding it can be on your equipment. Good polar alignment is a must, but equally good tracking/guiding. It’s frustrating if you keep loosing 30min subs due to guiding errors and the like. Its probably just a frustrating as collecting three hours of data to only realise your focus is slightly out or shifted due to temp. changes. The duration can also depend on your environmental conditions and equipment. Light pollution never helps. Also imaging at F/5 compared to F/9 can make a difference to exposure times.
If there is a single piece of advice to give, then it would be to monitor your subs closely. After you’ve calibrated them, review the background ADU count and the target in question. Check the overall quality of stars, if they are bloated due to poor seeing; remove them from the combine function. Extremely faint details on short subs will be difficult to measure their intensity in relation to the background. This is an indication that you could go longer. Longer subs will show a marked improvement, unless of course hindered by sky glow such as pollution. In short, go long/deep, but work within your boundaries. You don’t want to reduce your acquisition efficiency by throwing away every second sub due issues associated with long sub exposures.