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Old 01-02-2011, 05:40 PM
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gregbradley
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Whats the best way to take flats?

I am finding flats are too often a problem for my image processing.

They become more vital with these large chip cameras. I am mainly using a 16803 chip.

I have been doing flats this way:

1. Typically a white T-shirt or white translucent foam plastic sheet, placed over the end of the scope.

2. Point towards the west area at about 20 degrees up.

3. Take flats with the scope in focus and cooled to usual light exposure temps.

4. Exposure time usually from 3 seconds minimum to maybe 40 seconds for Ha at 1x1. I shoot them at dusk when it is just about starting to get dark. If I shoot less than 3 seconds the mechanical shutter is seen in the image sometimes.

5. I do flats for each filter I use at the binning I am using when imaging.

6. I take a dark at the same exposure length and subtract that from the flat.

7. I usually take 3 flat exposures per filter. I move fast and get all of them done quickly as it is amazing how fast the light level drops. I find around 20.000 ADU gets good results. I have read where higher ADU gives lower noise but I find it overcompensates when used. Perhaps I am making a mistake in how I do them but thats what I find so I stick to arund 20,000 ADU exposure levels.

8. I then use CCDstack and use mean combine with the dark subtracted to create a master.

9. I shoot fresh flats on a regular basis and after a camera move.

10. I sometimes find these flats do not even out the image well and either overcompensate leaving outer dim corners too bright or not enough.

I also am finding that if I callibrate using bias subtraction as well as flats it gets a better result. But then I am also often using adaptive darks where there is not an exact match for the darks and the light exposures.

I have mainly a 10 minute -35 or -30C dark I use. In the recent heat my camera only went to -25C. Exact dark matches may be more important for best results here.

I am wondering if sky flats are the way to go. Also being more precise with the darks.

Any suggestions, experience with flats?

Greg.
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Old 01-02-2011, 06:02 PM
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bert (Brett)
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I use ccd autopilot for flats.

I set a target adu for sky flats at 20000adu. Ccdap starts taking the flats soon as the sun is 10 degrees below the horizon. Each flat is dithered so a median combine will subtract the stars out.

I am lazy so I just subtract bias rather than take darks. I should take darks but ...meh

Works for me.

Brett
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Old 01-02-2011, 06:36 PM
gbeal
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I relented and bought one of them new fangled light panels, blame Mr Multiweb and others. Now I religiously shoot flats at the end of the evenings imaging session, regardless of whether I think I have finished or not. That way if I move the camera or focuser before the next session I am sorted. It means about one second subs for the RGB or about five seconds for the Ha filter. mind you, I don't have a shutter, so one second is OK.
Gary
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Old 01-02-2011, 08:38 PM
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I'm watching this thread with interest. I still haven't got the hang of >3sec flats with my lightbox and CCD. Have been meaning to shoot skyflats, but so hard to get out that time of day with a young family.

Lately I've tended to just use Dynamic Background Extract in Pixinsight when I'm being particularly lazy. I knew I was in trouble when shots I took at Wiruna with very dark sky came out better without flats than corrected...
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Old 01-02-2011, 09:01 PM
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Tandum (Robin)
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It's around 4months since I actually did this but, I built light boxes for each scope out of foam board with 4 white LEDs inside. They push onto the end of the scopes. The one for the 10" is big.

The qhy9 has a slowish shutter so flats must be over 3 seconds or you'll see shutter artifacts in the flats. To get that long an exposure for Lum, I plug the light boxes into the dew controller and turn it way down low. I have each filter set to the right exposure times in a maxim sequence, so I just expose on the Lum filter and turn the light box down until it bells around 20K on the hystogram. If that filter is right, the rest will follow.

No fancy led sheets here, there's at least 3 cartons of crownies wasted with that stuff

Last edited by Tandum; 05-02-2011 at 05:09 PM.
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Old 01-02-2011, 09:06 PM
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I use one of Pete's light boxes he makes http://www.iceinspace.com.au/forum/s...ad.php?t=49736

with an adjustable light you can do away with the white t-shirt and take flats whenever you need to...Top job too, very robust.
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Old 01-02-2011, 09:20 PM
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Flats are a PITA

I use an EL panel now but why is it even getting an average ADU of around 35000 with my flats at .07 sec exposure when I apply them in DSS they really diminish the strength of the colour chanels in the histogram in DSS
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Old 01-02-2011, 10:32 PM
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Octane (Humayun)
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I've only just taken my first batch of flats with the STL-11000M using one of Pete's lightboxes cranked at 50% power.

My 1x1 luminance flat lights were exposing around 20,000 ADU at 0.4 seconds. 2x2 were 0.2 seconds.

I'm processing my very first image at the moment and it looks the goods. I think I hit the jackpot first time around. : )

H
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Old 01-02-2011, 11:24 PM
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Quote:
Originally Posted by TrevorW View Post
Flats are a PITA

I use an EL panel now but why is it even getting an average ADU of around 35000 with my flats at .07 sec exposure when I apply them in DSS they really diminish the strength of the colour chanels in the histogram in DSS
Knock back the adu to 20K odd Trevor. I find over 30K is too high. Can you turn the light output down? Run the thing off a dew controller like I do maybe? Flats made the biggest difference to my OSC images, way more than LP filters or anything else. They got rid of most gradients and crap. Really cleaned the raw image up.
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Old 01-02-2011, 11:32 PM
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Rigel003 (Graeme)
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I was also having the shutter pattern appear in short flat exposures with my ML8300 so got an adjustable lightbox from Pete. They're great. Also did lots of reading on the FLI Yahoo group and asked a few questions. I'm now doing the following which seems to be working well:

No RBI, and use High speed mode for flats (using Maxim)
Exposures of about 6 - 10 secs (longer for narrowband filters)
About 2/3 full well depth (around 35000adu for the ML8300)
Same length flat darks
About 15 flats and 20 flat darks - doesn't take long.
Sigma reject mean for master darks and master flats.
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Old 02-02-2011, 07:06 AM
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Greg

I have been reading this with interest - flats were driving me mad a few months back. I have them more under control now (I think).

I use another of Peter's lightboxes. To avoid shutter artifact, I asked Peter to include a potentiometer to step down the intensity. I have also used a t-shirt between scope and box.

My routine is as follows:

1. Take a flat using box - I aim for 20k ADU except for Blue filter which needs 25k. I get the exposure right using the Sky Flats plug in for Maxim.

Be careful with filter wheels - sometimes Maxim uses the same filter even though I have told it to move to a different filter. That throws a big spanner into the works.

2. Manually take another 9 or so flats with the same exposure time. The plug in does shift the exposure time slightly between flats so I do it manually to keep it fixed for dark subtraction.

3. Repeat for each filter using the same binning as I am imaging.

4. Take darks with same exposure length.

5. Take a few bias frames.

6. Calibrate flats with darks and bias.

7. Then take long darks - or use a library dark. I had problems relying on the scaling function in Maxim. For some reason, the scaling works for light frames but introduces noise when applied to flats.

I do all this at the end of an imaging run or begining if I am early setting up.

The other lesson I have learned is that flats don't help much if you have a low signal to noise ratio in your light frames. I was stacking 3 x 5 minutes light frames of galaxies and wondering why the flat cured the dust bunnies but didn't quite fix the apparent uneven exposure and in fact made it worse by highlighting the edges at the expense of the centre - especially in colour. The real issue was there wasn't enough signal to play with.

Hope that helps

Pete
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Old 02-02-2011, 06:08 PM
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flatssss

I concur. Flats are a current issue I'm in the process of refining and neutralising. Brett has been at me to use CCDautopilot - I do have it loaded and use some of its capability - but I am still torn between sky flats and flat boxes. My current EL panel flat box doesn't quite seem to be cutting it for my 12" RC. Another box attempt is in the mill.

Also Gary's scenario of flats everytime you get the scope out is making more and more sense. I've had a lot of good data let down by tardy flat workflow or an inadequate flat library.

Dithering is the other attraction of CCDAP, as Brett mentions, plus a very powerful guiding routine, and the rest of the automation stuff. Greg, there is a freeware app called 'autodither' which might be worthwhile trying. It's a sort of dithering plug-in for CCDsoft - assuming you have CCDsoft guiding tamed with the PME and Planewave, it might be a good way to get dithering into your processes. I haven't tried it as yet

guy

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Old 02-02-2011, 06:42 PM
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I have the camera at -10C and in broad daylight with the Sun either rising or setting take flats at 200ISO opposite the Sun. I also take darks to get rid of all the inherent noise. Further I take about forty or fifty flats. The exposure are in the range of 1/200 or 1/320 th of a second.

My needs are not normal as I take wide fields.

A hint here is that any flats not taken with exactly the same optic configuration is a waste of time!

Bert
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Old 02-02-2011, 07:14 PM
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Quote:
Originally Posted by gregbradley View Post
I am wondering if sky flats are the way to go.
Greg.
mmm, GRAS used (uses?) sky flats for automated convienience (everyday), but they needed tedious manual weeding. I use a flat box, not so convienient, but at least predictable. If the exposures are in seconds you dont have to bother with darks at all.
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Old 02-02-2011, 07:53 PM
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Just to add on. I take flats with every data set, I found this to be very advantagous. Especially now that I use a rotator.

Brett
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Old 02-02-2011, 10:15 PM
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Sky flats can be a problem if you have lots of filters/binnings you need to take while the sky brightness is dropping fast.

You have to work in strict order of sensitivity (least sensitive filter/bin combination first, upto most sensitive), but even then, you are forced to compromise on how many images you can take.

For a median combine to reduce noise, you really need a fair number of images. 5 is not very effective.

I now use a homemade EL panel, with several layers of white paper in front to reduce the level to something that lets me use a reasonable exposure time (typically between 2s and 8s).

I will take at least 25 images (along with matching darks). Since they are all at the same intensity, the differences are essentially the gaussian noise, so median combining really cleans them up nicely.

I've analysed the flats images for artifacts produced by the panel/paper, and can't find any. It's working well for me for precision photometry.

Agreed, always take flats after the session, otherwise you cannot change ANYTHING in the optical train.

Ivan
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Old 02-02-2011, 10:19 PM
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Quote:
Originally Posted by Bassnut View Post
mmm, GRAS used (uses?) sky flats for automated convienience (everyday), but they needed tedious manual weeding. I use a flat box, not so convienient, but at least predictable. If the exposures are in seconds you dont have to bother with darks at all.
Sure, but you still need to subtract the bias.
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Old 05-02-2011, 03:03 PM
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Thanks very much for the responses.

Flats are an important part of processing and with some setups more important than with others. Some of my refractors barely need flats. The CDK can't live without them. Same with refractors using reducers and large chip cameras.

Looking at the responses it seems I need to:

1. Conquer sky flats and dither between flats.
2. Take a lot more than just 3 and perhaps just one for luminance and one for RGB. I don't know that each individual RGB flat varies that much. Doing 30 for each filter probably would not be practical with dusk flats as the light level falls too quickly.
I tend to clean my filters regularly to avoid dust bunnies but I do get them occassionally. Cleaning is better than using flats.

I wonder if black material would enable doing flats during the daylight hours. I actually got some nice flats once through a desert storm cover I had over my refractor during daylight hours!

The auto dither plugin for CCDsoft is flawed. I installed it a few months ago and my autoguiding went to hell. It took me a couple of weeks to work out why. The autodither was being applied to the autoguider as well as the main CCD. Every guide exposure then of course had a command to shift a couple of pixels and the autoguider was trying to guide that command back out and it was a losing battle. It does not seem to accept only being turned on for the main CCD. I'll try again but I don't think so.

An EL panel sounds good or a large lightbox. Or perhaps just a lot more t-shirt type flats taken and only one for luminance and one for all RGB.

Thanks for the advice.

I haven't found 35,000 ADU flats to work for me so far. They seem too bright and make the dull vignetted corners now too bright after application. 20,000 seems to work.

I think also I need a larger dark/bias library for different temps. This hot spell has meant even the Proline is maxed at -25C. Not bad really when the air temp is close to 30C or more.

Greg.
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Old 05-02-2011, 05:33 PM
Hagar (Doug)
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Greg, I also use one of Pete's (EXFSO) light boxes with a variable light output. I use it to get good exposure time and suitable ADU limits that I am after(20000). The glory of this system is flats can be taken each session and time is no longer a problem. The variable light output is a wonderful addition to a great light box.
I also shoot bias frames or darks and put them together as you do in CCDStack.
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Old 05-02-2011, 05:45 PM
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Greg, I also use one of Pete's (EXFSO) light boxes with a variable light output. I use it to get good exposure time and suitable ADU limits that I am after(20000). The glory of this system is flats can be taken each session and time is no longer a problem. The variable light output is a wonderful addition to a great light box.
I also shoot bias frames or darks and put them together as you do in CCDStack.
Yes, light boxes are better than sky flats imo too. Why do you dither flats?, no need to with a light box. You can set the light box up in a set place and set the mount "park" to it, that way you can automate flats if need be. In my experience, sky flats are dodgy, you run out of time (min 10 each filter really), you have to iether automate them for exactly the right time or be there at an awkward time manually, and then clouds appear after a clear image run.

I use guider only dithering whilst autoguiding, works well.

If your optical train is uniform and clean (no dust donuts), you can use a flat library for each filter, even if you change the cam rotation, ive seen that way, but your opticals must be uniform and clean.
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