As a complement to an earlier broadband analysis, have completed a Halpha analysis to determine optimum narrowband sub length. Turned out to be a bit of a nugatory exercise, since the basic rule of thumb for narrowband is “make the subs as long as you possibly can” – and that’s it. The limiting factor that determines how long your subs can be will probably be one of:
• Tracking consistency,
• Polar alignment accuracy/drift,
• Meridian flip/altitude,
• Satellites, clouds, wind, seeing variability
• Cable snags etc.
In doing the analysis to come to this conclusion, the starting assumptions were:
• the narrow band sky brightness is 5 p/s/m2/arcsec2 , which corresponds to about a 10nm filter and a fairly dark sky,
• the narrow band brightness of the target nebula in Ha is 10 p/s/m2/arcsec2 - this is probably realistic, but the value is not critical to the analysis in any case.
Three approaches to determining appropriate sub lengths were investigated:
1. Initially tried the standard broadband sub-exposure strategy of “read noise = 5% of total background noise” (See the earlier broadband discussion and the following websites:
http://starizona.com/acb/ccd/advtheoryexp.aspx http://www.starrywonders.com/snr.html http://www.hiddenloft.com/notes/SubExposures.pdf ). This strategy can be applied to narrowband imaging with the proviso that total background noise includes dark current as well as Poisson noise. This approach can lead to extremely long optimum Ha sub exposures for low QE CCDs – to the point of impracticality.
2. As a more practical alternative, the second approach is based on the idea that sub exposures can be significantly reduced from the “5% noise” optimum by accepting a tolerable 20% increase in overall imaging time - if you use the specified shorter subs you will have to image for 20% longer to get the same SNR as from optimum subs. This is called the “20% more” sub length in the following comments. You could use some other SNR/time penalty, but 20% seemed reasonable.
3. The third measure shows the effects of a much shorter short sub length, where read noise equals other noise – this is the point where overall imaging time must be doubled to restore the “5%noise” SNR. This is called the “double” sub length.
The SNR and headroom plots vs sub exposure for the three demo systems used previously and also a KAF3200ME based system (also ~1 arc sec) are shown in the attached figures. Note that the SNR used here is the ratio between the nominal object signal and the background noise (rather than the object noise) – this measure was chosen to represents the visual quality of an image. As an illustrative analogy for the measure, imagine a grove of trees that is flooded for a new dam – the high SNR systems will have a low water level (noise), with most of the trees (nebulae) sticking out of the water, whereas the low SNR systems will have high water level and only the highest treetops (brightest nebulae regions) will be visible. ie the SNR used here indicates how deep a system will image before it runs into the noise floor.
The attached table shows the”5% noise”, the “20% more” and the “double” sub exposure times for each system - plus the associated headroom and SNR after 3 hours total exposure for the “20%more” option. In summary:
• The KAI11002M system has a relatively low QE at Ha and relatively high noise, so it requires long sub exposures – the standard “5% noise” sub is 3.9 hours, the “20% more” sub length is over an hour and the “double” sub time is 24 minutes.
• The KAF8300 system has moderately low noise and intermediate QE. The “5% noise” sub exposure is 2.2 hours, and the “20% more” sub is 47 minutes. The “double” sub is 18 minutes.
• The icx694 system has low noise and high QE. The “5% noise” sub exposure is 0.9 hours, but the low read noise of the CCD brings the “20% more” sub length back to 23 minutes and the ”double” sub time is 5 minutes.
• The KAF3200ME system has very high QE, but relatively high dark current. It requires 1.1 hours for “5% noise” subs, 28 minutes for “20% more” subs and the “double” sub time is 7 minutes.
Any of the systems will benefit from subs of an hour or so and the 11002 system would do best if the nominal 3 hour total exposure was a single sub. Of course, you could use short subs with any of the systems, but at the expense of significantly increased overall imaging time or decreased imaging depth (in effect, short subs throw away some of the advantage of narrowband imaging – narrowband gets rid of most of the background Poisson noise, but multiple short subs just put read noise in its place).
Note that the 11002 and 3200 systems are both heavily influenced by dark current at the assumed sky background level. I took the dark current specs from commercially available cameras, but better performance would be possible if these two chips were more deeply cooled. The 3200 system has the best headroom, but all systems probably have sufficient for most purposes due to the greatly reduced star brightness in narrowband.
I would greatly appreciate any feedback from anyone who has done some narrowband imaging with systems anything like those mentioned.
Thanks for taking the time to read. Regards Ray