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Old 20-04-2016, 12:38 PM
glend (Glen)
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Narrowband Imaging into Astro Dawn - How Far?

Seeking advice from the narrowband imaging experts concerning how far i can image into the astronomical dawn? For example, in the early hours of tomorrow the astro dawn begins at 4:56am, the moon sets at 4:17am. I plan to run Ha and Sii from say 1am, while the moon is still up and Oiii once it sets. Obviously Oiii is going to be impacted by dawn but how 'deep' into astro dawn can i go without influencing the Oiii? Likewise, as the moon is setting can i start Oiii before it is fully set, assuming my target is in the east and the moon is on the western horizon? I am looking for at least an hour of Oiii in this period. BTW there is at least 90 degrees of separation between the moon and the target (M8).

Last edited by glend; 20-04-2016 at 01:14 PM.
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Old 20-04-2016, 01:57 PM
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Atmos (Colin)
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I have personally found that it starts having an effect from 45-60 minutes before sunrise. Your best bet is simply to keep imaging and then remove the final sub (pushing time wise) if the background becomes too bright.
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Old 20-04-2016, 02:32 PM
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Shiraz (Ray)
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I have personally found that it starts having an effect from 45-60 minutes before sunrise. Your best bet is simply to keep imaging and then remove the final sub (pushing time wise) if the background becomes too bright.
agree. depends on how much haze there is, latitude and the filter bandwidth. With Ha, I have gained an extra 3/4 hour or so (I didn't measure exactly how much, just threw out the bright subs) by starting imaging as soon as the guider would work after dusk and then stopping around the end of twilight. I guess you should get roughly the same in the morning and it would be worth trying while the moon is low down. O3 will be more sensitive to skylight, but if you use PI it can adjust the stacking to account for noisy images as the sky brightens. Please let us know how you get on.
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Old 20-04-2016, 07:22 PM
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RobF (Rob)
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Yes, I'd be interested to hear other people's experience too. From memory was able to get 15-20mins extra Ha into morning twilight at astrofest last year (just judging by brightening of subs in a series).
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Old 21-04-2016, 02:31 AM
glend (Glen)
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I was unable to get the imaging running this morning. Two hours trying but was defeated by inability to pickup a guidestar at the target ( M16). The moon was washing out everything nearby. I am obsessive but not crazy, bed is a better choice this time.
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Old 22-04-2016, 11:19 AM
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strongmanmike (Michael)
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Since NB imaging is done quite successfully from bright urban skies, my approach to NB imaging into twilight has been to stop once the sky looks worse then say Sydney's would at night ie. once I can't see any traces of the Milky Way and only the brighter stars.. I finish up

Not a scientific approach but it seems to produce usable data

Mike
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Old 22-04-2016, 11:26 AM
glend (Glen)
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Thanks Mike, I am going to give it another go in a couple of days when this latest weather clears away, and the full moon is past. Unfortunately the moon rise times are placing it too close to the Lagoon area, and thus light can shine into the scope. My understanding is that at least 45 degrees off the target object is minimal.
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Old 22-04-2016, 11:43 AM
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strongmanmike (Michael)
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Thanks Mike, I am going to give it another go in a couple of days when this latest weather clears away, and the full moon is past. Unfortunately the moon rise times are placing it too close to the Lagoon area, and thus light can shine into the scope. My understanding is that at least 45 degrees off the target object is minimal.
Yes, as far as the moon goes, it should be far enough away to keep its light from shining inside the tube/dew shield as much as possible (preferably not at all)

Mike
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Old 23-04-2016, 10:29 AM
phomer (Paul)
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I keep imaging and when I review the images I can clearly see when the dark level suddenly increases. I then toss (obviously not literally) out all those which are impacted.

Paul
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Old 24-04-2016, 09:40 AM
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Also when you get too low to the horizon the quality of the subs starts to really go off and stars become fuzzier and larger.

I usually only want to image an object when its about 35 degrees up in the east and 35 degrees up from the west. That's at a dark site. It may be well less than that if you have to allow for light pollution on one side.

5nm narrowband filters work well when not pointing close to the moon.
Close to the moon it would seem 5nm is still not enough. Not sure if 3nm would work but I doubt even then they would.

Image until the subexposures go off (too much light pollution or stars too fat). You can simply flick through the resulting images and delete those that are not useful. Its fairly obvious which are usable and which are not when you review them.

Greg.
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Old 24-04-2016, 12:55 PM
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Atmos (Colin)
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Last night I was imaging NGC 6334 with a 3nn Ha, very close to the moon so not good BUT I could image until half an hour before dawn. By that I mean, the background sky didn't become brighter than the moon cast until then
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