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Old 02-06-2016, 11:19 AM
glend (Glen)
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Narrow Band Flats -Necessary & How?

Are narrow band flats a necessity? If so how are they done given the bandwidth restriction of the filters, or do you just use a broadband flat?
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Old 02-06-2016, 11:25 AM
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Octane (Humayun)
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Sky flats or light panel flats will do the trick. Obviously, they take longer to make.

I have had great success (with automation) using dawn flats.

H
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Old 02-06-2016, 01:02 PM
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multiweb (Marc)
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Quote:
Originally Posted by glend View Post
Are narrow band flats a necessity? If so how are they done given the bandwidth restriction of the filters, or do you just use a broadband flat?
Always used my EL panel for Ha/Sii/Oiii flats as well. Just got to up the exposure time a little to register enough ADU.
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Old 02-06-2016, 04:59 PM
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Hi Glen,

That would probably depend on light pollution in your area, moon phase, bandwidth of your narrowband filter, the length of exposure and brightness of your target.

I use 3nm filters and flats are essential because of heavy light pollution in my area; however, with some bright targets like the Carina nebula and shorter 2-minute exposures I can get away without using flats.

I use one of these and it is gold: http://www.iceinspace.com.au/forum/s...ad.php?t=49736
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Old 02-06-2016, 05:40 PM
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Atmos (Colin)
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I second Suavi, very useful light box! My Ha flat takes 14s while my Lum flat takes 0.1s
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Old 02-06-2016, 06:04 PM
glend (Glen)
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Whoops a double post for some reason, so cleaning this one out. See next response. Mods can dekete thus one.

Last edited by glend; 02-06-2016 at 06:17 PM. Reason: double post
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Old 02-06-2016, 06:12 PM
glend (Glen)
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Thanks guys. I think Ray is building a flat box for his ASI1600 so i might try and copy his when its complete.
Re the exposure times in narrowband, my filters are Ha 7nm, OIII 8,5nm, & SII 8nm; is there a way to work out required exposure? Obviously they should probably be shot at the Gain and Offset settings used to capture Lights, right?
As to light pollution, i am in a pretty dark location, above 20 on the SQM but with some skyglow from Newcastle, but most of my imaging is done on targets to the east, so i shoot out over the ocean - pretty dark out there.
I try to avoid full moon periods and either side, and always wait for the moon to be well away from the front of the scope, 45 degrees is my norm. I do not shoot any Oiii when the moon is up.
Typical targets right now with my new camera are emission nebs so they tend to be fairly bright. Light sub lengths with this camera are curently 60", relying on alot of them stacked. I have been doing all my testing without flats and was pretty happy with the outcome. Being a new camera, the sensor and cover glass are both clean. Filters were cleaned prior to install in the filter wheel and still appear clean.
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Old 02-06-2016, 06:33 PM
mountainjoo (Jerome)
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Glen, if you know what ADU values to aim for with your flats, you can use the flats calibration wizard in SGP to determine the correct exposure lengths for each of your filters.
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Old 02-06-2016, 07:52 PM
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I find around 20-30,000 adu gives me good flats that correct vignetting without overcorrecting.

I take either dusk flats with a netural white cloth cover over the scope or I do the same but with the roof closed during the day and enough light gets in but not too much.

4 second exposures up to 14 work usually. Dusk flats are a little harder in that if you are doing 7 filters you have to move fast so that you don't have lots of different exposure lengths should you do flat darks. Its simpler if there are only one or two different exposure lengths.

Greg.
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Old 02-06-2016, 08:42 PM
glend (Glen)
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Thanks Greg, i have done broadband flats before using the old white t-shirt technique after sunset or early next morning using 2" exposures - but that for my dslr images. I was not sure if narrowband flats were a different beast.
Since i am using SGP i had better read the manual section on calibration/flats.
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Old 02-06-2016, 09:03 PM
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I just do my exposure time based on trying to get 30,000 ADU. This for me equates to 0.1s L, 0.2s R, 0.25s G&B, 14s Ha, 17s OIII 16s SII. This just gives a bit of an indication as to the exposure time difference with 3nm narrowband and broadband filters.

This is how it works for me anyway. With my KAF-8300 I was working with sometimes 25s exposures.
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Old 03-06-2016, 07:24 AM
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I find with flats there is a little bit of art to it. Sometimes I find a flat can slightly overcorrect vignetting, other times not enough. Sometimes calibration leaves the lines in the ccd other times not.

So I don't think a rote approach is the way to go. You need to test what you have for its results when calibrating otherwise you're going to get some misses.

After doing the tutorial with Rick about PI it seems PI has the best calibration routine. CCDstack is quite good as well but is very memory intensive. Occassionally it will fail to align images. It does not like colour images. It does not tell you how many subs you just opened - they are not numbered, so you have to manually count them every time to keep track of total exposure.

Also I find its better to reduce vignetting physically and keep filters and ccd window clean rather than rely on flats to clean up your dust donuts and vignetting.

Depending on your camera and the type of shutter it has you can't do exposures of less than about 3 seconds with leaf shutters. If its a KAI chip then you can take flats without a shutter if the camera allows it like the SX cameras. SBIG have an even illumination shutter which is a good thing. But for leaf shutters which perhaps have other advantages (perhaps more light proof) they require 3 second minimum exposures otherwise you get shutter artifacts in the image that wreck the flat.

Temperature of the ccd should be the same as the lights when doing flats. That can make it hard to take a flat during the day.

Greg.
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Old 03-06-2016, 08:34 AM
glend (Glen)
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I believe the ASI1600 has a Global shutter there is certainly no mechanism i can detect, and the sensor is visible at all times. It can take 0" Bias frames when driven by SGPro, so i have to conclude no shutter. With -40C of cooling available i don't see dayyime flats as a problem especially at this time of year. I normally shoot lights at -25C.
Vignetting is not a problem with my 36mm filters and the camera screwed directly to the wheel. There is no more than maybe 12-15mm of separation. Illumination is equal across the field.
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Old 03-06-2016, 11:20 AM
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Quote:
Originally Posted by glend View Post
Thanks Greg, i have done broadband flats before using the old white t-shirt technique after sunset or early next morning using 2" exposures - but that for my dslr images. I was not sure if narrowband flats were a different beast.
Since i am using SGP i had better read the manual section on calibration/flats.
SGP has a flat calibration wizard that does the job for you. Just run that, selecting which filters/binning you want, and it'll automatically work out what exposure times needed, then save those settings so when you do a sequence with flats, it fills in the exposure for you.
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Old 03-06-2016, 02:04 PM
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I believe the ASI1600 has a Global shutter there is certainly no mechanism i can detect, and the sensor is visible at all times. It can take 0" Bias frames when driven by SGPro, so i have to conclude no shutter. With -40C of cooling available i don't see dayyime flats as a problem especially at this time of year. I normally shoot lights at -25C.
Vignetting is not a problem with my 36mm filters and the camera screwed directly to the wheel. There is no more than maybe 12-15mm of separation. Illumination is equal across the field.
That's very good then. Vignetting also comes from the scope and the F ratio of the scope, whether there are any reducers/flatteners/adapters, off axis guiders etc etc. What aperture scope are you using?

Greg.
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Old 03-06-2016, 02:52 PM
glend (Glen)
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That's very good then. Vignetting also comes from the scope and the F ratio of the scope, whether there are any reducers/flatteners/adapters, off axis guiders etc etc. What aperture scope are you using?

Greg.
Greg, no reducers, flatteners, adaptors, or OAG. I use a dedicated guide scope. So far i am imaging with this camera exclusively on my Skywatcher MN190 Mak-Newt, f5.3, 1000mm focal length, as it best suits the camera pixel size (so i believe).
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