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What is the issue with my flats?

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#1 hoa101

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Posted 02 February 2013 - 11:51 AM

I'm trying to get some better flats, and I have an odd result no matter what I try.

With my 50mm lens, I've used t-shirt method with overcast sky, double white paper, white LCD computer screen and t-shirt, computer screen and paper. But, I always get these strange vertical "ripples". Initially I thought the t-shrits had wrinkles, hence the paper tests. But I get the same result.

I've looked at other people's flats, and do not see anything quite like it. Is there some kind of issue with my method, or maybe the camera? Thought this was a bit strange...

Here is low quality jpg of one that I quickly stretched.

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#2 Skyshooter

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Posted 02 February 2013 - 12:01 PM

Looks to me like uneven illumination caused by the lamp on the LCD display on a laptop. Have you tried twilight flats? A quick shot of an evenly illuminated blue sky will tell you if it's a sensor problem or not.

Good Luck,
Ed

#3 hoa101

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Posted 02 February 2013 - 12:08 PM

That's what I tried initially, but I got a noticeable gradient from uneven illumination of the sunlight. I think the 50mm is just too wide.

If there's something wrong with the sensor I need to decide asap... my 14-day return period to KEH will run out in a few days.

#4 hoa101

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Posted 02 February 2013 - 12:19 PM

Okay, another test method. This has to be the camera... I wish I had another lens to test. The other ones are all Nikon for our day-use camera.

I took a photo of overcast sky at f/22 while moving the camera side to side. Ripples still there.

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#5 Tonk

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Posted 02 February 2013 - 12:44 PM

How long are these exposures - can you show us the histogram?

Until I see the info it is looking to me like you are grossly under exposing these and the banding is coming up after stretching from the underlying bias pattern that all sensors have

Are you also shooting flat darks (darks for the flats). Without these included in the calibration the bias pattern issue will remain regardless of correct flat exposure

Can I suggest using AV mode to measure the correct exposure length and then switching to manual and dial the required exposure in. Then its easy to rattle off flats and then flat darks

#6 zerro1

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Posted 02 February 2013 - 12:44 PM

the features look to be in the same locations. this would be virtually impossible to replicate using paper for one and using a T-shirt for another.

try a flat without a lens?

#7 hoa101

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Posted 02 February 2013 - 12:47 PM

This is indeed AV mode. The histogram is very close to 0.0 peak in GIMP. I will attempt some with no lens.

#8 hoa101

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Posted 02 February 2013 - 12:56 PM

Here is a quick and dirty version - AV mode, no lens, light of white LCD diffused through two layers of white paper. I was forced to clip the black a bit because the peak is really spread out. It's not perfect, but you can still see those ripples.

I'm thinking I should return this body, if nothing else than for piece of mind. Flats could potentially take this effect out of my images, but this cannot be normal behavior of the sensor...

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#9 zerro1

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Posted 02 February 2013 - 01:05 PM

This is indeed AV mode. The histogram is very close to 0.0 peak in GIMP. I will attempt some with no lens.


How AV mode "exposes" is adjustable... you can increase the exposure of AV mode. (On my older Canon 1000D) you press the AV button on back of camera wich toggles the scale, then adjust up or down by using the "wheel". Not familiar with newer models so it may be different. I usually have mine set just under 1/2 of the scale

#10 Sean13

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Posted 02 February 2013 - 01:08 PM

It still looks under exposed to me. How far away from the lens/body are you holding your paper or computer screen?

#11 hoa101

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Posted 02 February 2013 - 01:11 PM

From DPP using the raw image - this one with no lens actually looks overexposed. The others were right at 0.0.

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#12 hoa101

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Posted 02 February 2013 - 01:15 PM

Here's a histogram from one of the flats taken with a lens, facing the overcast sky, with double-layer t-shirt fabric over the lens shield, focused to infinity, AV mode, ISO 100.

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#13 hoa101

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Posted 02 February 2013 - 03:08 PM

Thanks for the help guys.

I just sent it back. Not worth the worry, and I can get a full refund. If something is really wrong, bullet-dodged. If not, then oh well, I am only out the cost of shipping.

#14 Tonk

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Posted 02 February 2013 - 04:12 PM

Your flat with no lens just looks totally wrong - why is the whole thing eliptical???

It looks like the light source is off axis and/or your paper sheets were tilted

Flats taken though a good optical system should have a circular gradient

Can we have a photo of your rig when taking these flats?

#15 Sean13

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Posted 02 February 2013 - 04:13 PM

Yea I would tend to agree, best just to send it back and replace it. That is a really strange effect.

#16 nganga

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Posted 02 February 2013 - 04:26 PM

I'm trying to get some better flats, and I have an odd result no matter what I try.

With my 50mm lens, I've used t-shirt method with overcast sky, double white paper, white LCD computer screen and t-shirt, computer screen and paper. But, I always get these strange vertical "ripples". Initially I thought the t-shrits had wrinkles, hence the paper tests. But I get the same result.

I've looked at other people's flats, and do not see anything quite like it. Is there some kind of issue with my method, or maybe the camera? Thought this was a bit strange...

Here is low quality jpg of one that I quickly stretched.


I wonder if you are not seeing a shutter artifact? Before you return anything, try increasing the exposure. Either add one or 2 stops to the exposure compensation if you want to use AV mode, or use manual exposure, again adding one stop or 2 to the autoexposure reading.

Clem

#17 hoa101

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Posted 02 February 2013 - 05:09 PM

The ellipse was because I have no way to evenly expose a flat with no lens. It's snowing outside, so I covered it with paper and fired away at the computer screen.

I did think of the exposure length issue. I tried everything from 1/400s to multiple seconds, no change.

Anyway, it's moot now. Camera is going back. I'll buy a new one instead that has a warranty. Amazon has them with the kit lens for $400.

Thanks all for the help.

#18 Toxic Coolaid

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Posted 02 February 2013 - 05:20 PM

Cover it with a sheet of paper, go outside and shoot up, with or without a lens. You cant ask for a more even light source

#19 Tonk

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Posted 02 February 2013 - 05:43 PM

The ellipse was because I have no way to evenly expose a flat with no lens. It's snowing outside, so I covered it with paper and fired away at the computer screen.


Which is a bad way to expose flats. Don't be supprised if the next camera does the same

#20 Sean13

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Posted 02 February 2013 - 06:22 PM

I see also that your taking your flats at f/22 in the pic up there. I could be wrong but I'd think you want the largest aperture you can get.

I shoot my flats thru the scope at f/6.3, ISO 100, with an LED Laptop screen full brightness white background virtually touching the end of the scope. I don't use paper or anything to difuse it, as the focus does that pretty well for me and an LED is already evenly illuminated. My shutter speed escapes me at the moment, but this will vary for you as well based on how bright your light source is and your f/stop.

#21 Tonk

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Posted 02 February 2013 - 07:18 PM

The essential feature of flats is you change nothing - focus, apperture, lens elements or camera orientation on scope must not be changed

#22 Mike Clemens

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Posted 02 February 2013 - 07:24 PM

> The essential feature of flats is you change nothing -

Which I still haven't come to terms with. If I am shooting 4 basic sets a night of a target, (2) sets 0 and 90 degrees rotated before and after the meridian, the only way to get good flats would be to have a robotic thing which flips an illuminated panel in front of and out of the way of the refractor.

#23 Tonk

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Posted 02 February 2013 - 07:37 PM

Meridian flips do not change camera-to-scope orientation

#24 Mike Clemens

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Posted 02 February 2013 - 07:43 PM

Even on a 2 meter long 80 pound refractor with a 12 pound focuser hanging 4 feet away from the mount? My pointing correction terms sure feel it.

#25 pfile

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Posted 02 February 2013 - 10:25 PM

well then your sensor is no longer square with the optical path, so forget about flats... that's a bigger problem.

but are you actually rotating the camera? like loosening it and rotating it and retightening it? in that case you do need new flats. there are in fact robotic flat things like you just described - the flatman is one.

if you never change the position angle of the camera then one set of flats is fine.






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