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Trouble with flats

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#1 Hilmi

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Posted 19 January 2013 - 02:35 AM

I'm having a weird problem with flats, and I can't explain it. I noticed it after I added my Astronomik CLS filter to my imaging train.

So far, I have observed this problem with my Ha images and my Luminance images. I did not get this problem with my OIII and SII images.

My flats seem to have a very clearly defined hot spot, which is not there in my light frames. My flats are around half way up my histogram, which when I looked at the specs of the STT-8300, should be within the linear range of the camera and far from saturating the pixels. Problem is, when I use the flats, I can't stretch the image by much because as soon as I stretch the image to show the faint details, that nasty hot spot becomes clearly visible.

I'm using MaximDL to capture and calibrate if that has any impact on things.

I have attached my image from last Thursday, unfortunately it doesn't show so clearly in this image after I converted it to 8 bit JPEG, but if your monitor has good contrast you should be able to spot it on the right side of m66

Posted Image

Any ideas?

#2 HunterofPhotons

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Posted 19 January 2013 - 05:14 PM


Tell us how you are taking your flats.
Your procedure is obviously flawed.

dan k.

#3 shawnhar

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Posted 19 January 2013 - 06:04 PM

Take the CLS filter out, take an image, replace the filter, take an image process both, that will tell you if the filter is the cause.

#4 zerro1

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Posted 19 January 2013 - 06:51 PM

I'm assuming that you are aware that these 8300 cameras require a somewhat extended duration flat frame to eliminate the shadow from the shutter? somewhere above 3 or 4 seconds... :shocked:

#5 JoseBorrero

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Posted 20 January 2013 - 12:31 AM

here starizona explain how to take flats with maxim. http://starizona.com...Flats/Flats.htm

Also I used warren keller soup to nuts dvd. he teach maxim techniques very well. http://www.ip4ap.com/soup2nuts.htm

#6 CounterWeight

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Posted 20 January 2013 - 08:24 AM

Just double checked my own flats, I usually use 2 seconds. This is why I bought the 'FlatMan' panel... I can adjust the brightness to just right for that long of a flat exposure, and I use a very low setting for the FlatMan.

I use very mild flats, maybe not for everyone but works very well for me.

If you experiment a bit, at faster(than 2 seconds) time settings you'll see there is a distortion of the center to edge illumination caused by the shutter, so what I do is go a safe distance past that for certainty. This may prove a bit difficult if not using an illum panel. No way could i pull it off using dawn/dusk but maybe you can.

I know there are other panel solutions out there, I've only used the Flatman and though it's pricy it is well constructed and the software for controlling it works very well for this sort of thing (I can re-use settings) or change if I choose.

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#7 Hilmi

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Posted 20 January 2013 - 11:19 AM

I use Gerd Numan flat panel with a couple of shirts to dim it. I used 4 sec exposures.

Sky flats are not practical for me due to timing restraints. Comeback from work almost when dark. Give the wife and son company, Start imaging very late and go to bed So I can wake up for work. Dawn flats? I have not seen the dawn in weeks

#8 CounterWeight

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Posted 20 January 2013 - 03:44 PM

That hot spot can also happen from improper flats, too agressive. Also on another tack, have you checked your darks you are using? (not the flat darks, the light dark frames - are they good?

#9 Hilmi

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Posted 20 January 2013 - 11:58 PM

I think I will post some lights and calibration files on google docs. I have no idea how to make them public?

#10 John Wunderlin

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Posted 21 January 2013 - 03:43 PM

Be sure to also use bias frames. If you skip bias frames, the flats can over or under correct (I found this out the hard way!).

#11 CounterWeight

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Posted 22 January 2013 - 06:19 AM

John,

Are you sure about that?, I've never used bias frames for my ST-8300 work and have managed quite well using only darks and flats (with darks for the flats as well).

#12 Hilmi

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Posted 22 January 2013 - 08:39 AM

I have Bias frames too. I think the best way is to wait till I travel back home and post all the files online. Then you can all see how I have done it.

#13 John Wunderlin

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Posted 22 January 2013 - 10:56 AM

John,

Are you sure about that?, I've never used bias frames for my ST-8300 work and have managed quite well using only darks and flats (with darks for the flats as well).


I didn't used to use bias frames and didn't notice a problem until I had one picture a couple of years back where I noticed my dust motes were 'inverted' and visible in my final picture when I stretched all the way. After doing some googling I found other people suggesting bias frames would fix 'overcorrecting flats' and indeed this solved my problem. I've used them ever since.

Theortically, you can substitute dark flats, so that's probably why it's working for you. I prefer to just use bias frames so I don't have to take the dark flats each time I shoot flats.

#14 John Miele

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Posted 22 January 2013 - 01:45 PM

Does a flat have to be taken at the same temperature as the light frames? I don't think it does but wanted to check. Thanks...John

#15 CounterWeight

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Posted 22 January 2013 - 02:10 PM

Does a flat have to be taken at the same temperature as the light frames?


That's how I've always tried to do it just so everything relating to the noise 'floor' or the iamge matches up as close as possible. If you don't want to do agressive manipulation (like a DDP stretch) don't think it would matter as much?


Then you can all see how I have done it

Hilmi, I posted that image from my ST-8300 and you can see the numbers there... are yours very different? I tried using same levels as I had with the SSP way back after just getting the 8300 and had to re-establish what worked as a baseline. Also are you flatting for each filter or using one set for all?

oops! forgot last thing... the original ST-8300 it is possible for light to leak inside the camera housing(this I verified with their tech support) and it is possible to get funny flats and darks4flats if the camera in lighted area - so now i just loosely wrap with tin foil (all but exhaust fan) for the few minutes it takes, or take them in dark during bad weather.

#16 Bill W.

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Posted 23 January 2013 - 09:26 AM

oops! forgot last thing... the original ST-8300 it is possible for light to leak inside the camera housing(this I verified with their tech support) and it is possible to get funny flats and darks4flats if the camera in lighted area - so now i just loosely wrap with tin foil (all but exhaust fan) for the few minutes it takes, or take them in dark during bad weather.


Did tech support say what area on the 8300 the light came in? Thanks!

-Bill

#17 John Wunderlin

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Posted 23 January 2013 - 11:34 AM

Does a flat have to be taken at the same temperature as the light frames? I don't think it does but wanted to check. Thanks...John


I'm pretty sure that they do not- the flats are taken at a much shorter duration so thermal noise is not much of an issue.

#18 Warhen

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Posted 30 January 2013 - 04:41 PM

Thank you Jose for the thumbs up. We work hard on our tutorials and your acknowledgement is appreciated!

#19 Jared

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Posted 31 January 2013 - 01:45 PM

John,

Are you sure about that?, I've never used bias frames for my ST-8300 work and have managed quite well using only darks and flats (with darks for the flats as well).


If you are using darks for your flats, then you don't need (or want) bias frames for the flats. It's an "either/or" situation. The flats will typically have a high enough ADU that the pattern noise in the bias frames doesn't matter, but it is critical that the pedestal be removed, and either bias frames or flat darks will accomplish that.

#20 Jared

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Posted 31 January 2013 - 01:56 PM

Does a flat have to be taken at the same temperature as the light frames? I don't think it does but wanted to check. Thanks...John


There is no requirement that it be taken at the same temperature. If, however, your flats are more than a few seconds dark current can start to matter, so in those situations I would want to dark subtract my flats. The flat-darks need to match the flats for temperature, but neither needs to match the lights. If your flats are less than a few seconds in duration, you probably don't need to cool your camera at all and can just bias subtract to get rid of the pedestal. The temperature of the bias frames won't matter either.

It's hard to give an exact answer on when you need to cool your flats (in terms of exposure duration), and when you need to dark subtract rather than just bias subtract. Depending on your camera's dark current and, in particular, on how hard you "push" when you are stretching your results will vary. Personally, I use Kodak based cameras that do not have particularly good dark current characteristics, and I can't tell a difference in my images with cooled vs. not cooled flats or dark subtracted vs. bias subtracted flats with 4s exposures. YMMV.

#21 Jared

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Posted 31 January 2013 - 01:57 PM

Does a flat have to be taken at the same temperature as the light frames? I don't think it does but wanted to check. Thanks...John


There is no requirement that it be taken at the same temperature. If, however, your flats are more than a few seconds dark current can start to matter, so in those situations I would want to dark subtract my flats. The flat-darks need to match the flats for temperature, but neither needs to match the lights. If your flats are less than a few seconds in duration, you probably don't need to cool your camera at all and can just bias subtract to get rid of the pedestal. The temperature of the bias frames won't matter either.

It's hard to give an exact answer on when you need to cool your flats (in terms of exposure duration), and when you need to dark subtract rather than just bias subtract. Depending on your camera's dark current and, in particular, on how hard you "push" when you are stretching your results will vary. Personally, I use Kodak based cameras that do not have particularly good dark current characteristics, and I can't tell a difference in my images with cooled vs. not cooled flats or dark subtracted vs. bias subtracted flats with 4s exposures. YMMV.

#22 Hilmi

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Posted 01 February 2013 - 06:30 AM

Ok. Since I posted this message I have been busy repairing my mount. But last night I found the reasons my flats don't work. Its a reflection from the CLS filter. The brightness of the EL panel makes the reflection stronger and it spoils the flats

#23 shawnhar

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Posted 01 February 2013 - 12:51 PM

:waytogo:

#24 CounterWeight

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Posted 02 February 2013 - 09:32 AM

Congrat's on the detective work! :)






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