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I live in a sububan area and take backyard photos. I'm shooting at unity gain on an old 1600MM Pro mono camera at -10 degrees. I'm curently taking exposures as follows with an F4.5 scope: Narrowband (7nm) 300 secs RGB 90 secs Lum 30 secs Given that I think I am in a bortle 5 region, does this sound about right? |
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Hi Andy, From a Bortle 5 region your chosen exposures are definitely a great starting point. Unfortunately, there is a lot more that goes into it than just a Bortle number. Sky transparency, elevation of the object you are imaging, nearby light domes, etc., etc. all have an effect. The best way I have found of getting an exact "best" exposure time for any given night and object is to use the sensor analysis within the SharpCap image acquisition program (sharpcap.co.uk). You need to purchase the Pro version to do this but it is very inexpensive ($20/yr). Once you have analyzed your sensor (I have the ZWO ASI6200 MM Pro), you just point your scope towards your intended object, SharpCap takes a few quick exposures, does a whole lot of behind the scenes calculations and gives you the perfect exposure time. For instance, from my Bortle 3 home it told me that imaging an object to the north at 45 degrees elevation required a luminance exposure of 96 seconds to record the faintest object possible under those conditions, and then I can stack 96 second exposures for as however long I want to get the total integration time desired. But pointing to the south, where there is a small light dome, on the same night and elevation, required an exposure no longer than 54 seconds. It gives me different exposure values when the transparency changes with the seasons. At my Bortle 1 dark sky site in the Oregon high desert with no humidity, excellent transparency and no light domes I could do 300 second luminance subs. Once you have done the sensor analysis, it will give you values for any filter you are imaging with. I use it at the start of each night's imaging to get an idea of what the conditions are like, and then choose the closest exposure to one for which I already have a dark library. I can't recommend it highly enough. Hope this helps, Arnie |
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Arnie: I have SharpCap Pro and have run the sensor analysis on all my cameras, but I've never understood how to use the exposure calculator tool. Is there a good video of how this is done? |
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I use NINA exposure calculator with a setup similar to yours (F4 newt, 1600mm). My average sky is 18.5, and I use 40s exposure with L and RGB filter at gain 75, 240s at gain 139 with Ha and OIII filters and 300s at gain 139 with SII filter. My times are an approximation of the calculator, taken in order to use a limitate number of exposure times and built a reliable dark library. Sharpcap tool is more sofisticated for sure, but as far as I know if you are over the read noise with your median value, and you do not have too much burned stars you are ok. (ps. if the stars are the problem, you can remove them from your L and replace them at the very end with the RGB version only) |
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Stuart Taylor: @Stuart Taylor If you've already done the sensor analysis you're 90% of the way there. I don't know of any videos off-hand, but here's a quick summary of what you need to do. With an analyzed camera attached, open the Smart Histogram tool, then click on the "Brain" icon in the upper left corner. In the window that opens, choose the filter you are measuring in the upper right, and make sure to uncheck the "use a value of" box. Under Options, leave Min and Max exposures at 5 sec and 5 minutes respectively, Read Noise at 10%, Max Dynamic Range. Set the total exposure time you plan on taking. Point the scope at your target and hit the Measure button at top right. That's it! Your magic number (best exposure time for the camera, filter and conditions) will appear in the middle. If you want SharpCap to automatically apply these to your session, there is a button for that at middle right. The rest of the graphs at the bottom are informational only. Let me know if you want to go over those. Arnie The image below shows these steps, with credit and gratitude to Robin Glover.  |
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Arnie:Stuart Taylor: Thanks SO much for this. I'll give it a try! So far I have basically just guessed exposure times |
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| @Stuart Taylor My pleasure! Let me know if you want to discuss any other parts of this. "So far I have basically just guessed exposure times" I know what you mean. I was shocked the first time I ran this and found out I was wasting so much time taking three minute luminance exposures when 96 seconds would get me the same result. Arnie |
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There are some basic maths things here: Luminance means approx 300nm bandwidth RGB means approx 100nm bandwidth and narrowband is anything between 3nm and 7nm bandwidth It's like looking through ever stronger sunglasses, so clearly the exposure times stretch accordingly. I actually think that my L/RGB exposures are way too long at 30 secs and 90 secs respectively. Even my 300 sec narrowband exposures are probably too long. I think I will dial them all back ... shame, because I'll need to do a whole new set of darks. |
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I feel your pain. I've had to re-do darks for every camera I use and for every filter with each camera. Unfortunately, with the number of cloudy nights here in the Pacific NW, there is plenty of time to obtain them.  |
