[DavidLJ (David)]

I'm planning my first foray into narrowband imaging taking the Rosette Nebula as my target. But I'm having difficulty in deciding what exposure time to use. I understand that it is inevitable that narrowband exposures will generally require more exposure time than the LRGB filters that I have been using up to now. With 3 narrowband filters for the Hubble palette and, say, 6 subframes for each filter the total integration time increases by a factor of 18 for each additional minute of exposure time. Therefore the total integration time for a 5 minute exposure would be 1.5 hours. For a 10 minute exposure it would be 3 hours. For a 15 minute exposure, 4.5 hours and for a 20 minute exposure, 6 hours. So, like everyone I guess, I want to balance the quality of the end result with the need to avoid unnecessary exposure.


Of course one can always simply “suck it and see” and make a full exposure run using a guessed-at exposure time but I would prefer to take a more considered approach. I therefore did a test run of just 4 exposures with the Ha filter – 1 each for 5, 10, 15 and 20 minutes to see what resulted after stretching using the levels tool in Photoshop. I already had dark frames for 5 and 10 minutes but as yet I have no flat frames for Ha. But I reckoned that that didn't really matter as hot pixels and any vignetting, etc. would not be much, if any, of an issue in comparing the results and coming to a decision as to which exposure time to use. So I simply subtracted the 5 minute dark from the 5 minute light and the 10 minute dark from the other 3 exposures.


Attached is a screenshot of each stretched image showing the histogram settings used in the levels tool. As to be expected, the light distribution curve widens with increasing exposure. For simple comparison purposes I have only used the white slider button and set it at the right hand end of the distribution curve. It is fairly obvious that the 5 minute exposure is not as good as the others and can be discarded immediately. But there seems to be very little, if any, difference in quality between the other 3 exposures. Do I play it safe and use 20 minute exposures? Take a chance and use 10 minute exposures? Or maybe have a bet each way and use 15 minute exposures?



Any comment that those experienced in this field would care to make would be most welcome.