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Garethr

Dark bar down 1 side of flats.

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Can anyone help? I'm getting a dark bar down one side of my flats. It's def the camera because when I rotate the camera the bar moves as well. I'm using a canon100d on SW 200 p

Have checked the optical train and all seems tight and square, using at T mount and coma corrector nowt else.

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Could you drop a JPEG into the thread to show the effect?

And where is the histogram peak for these flats?

Thirdly, did you dark-subtract your flats? A master bias or set of bias is all you need as a 'dark for flats.'.

Olly

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Olly

As ever thanks for the high speed response that's why I love SGL.

Will post a JPEG when I get home, my concern was just taking a flat and having this dark bar down one side meant I wouldn't really be sorting out any vingetting at the corners and it makes processing anything a right pain.

Histogram was about 1/3 from left and no I haven't subtracted anything as I didn't realise I had to. DOH this imaging game just gets harder and harder.

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Sounds like a band caused by the camera shutter,  When taking flats you may have to adjust the exposure to avoid these bands.  One way is to set the camera to Av and allow it to find its own speed.  then if you have bands just set the speed "one faster" than the Av setting used and try that.  it really is trial and error.  I use a dimmable light panel for my flats and i can usually persuade the camera to take good flats by a combination of the panel brightness setting and the shutter speed.

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Bizbuilder,

Might be a daft question but I am seeing this bar on live view. Or do you mean the edge of the shutter in the field of view.

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What is your light source for your flats?  You shouldn't be seeing a dark bar in live view - very odd?

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I got a dark bar on my dslr flats when the light source was too bright and i needed. Really fast (1/4000th) shutter speed and it still put the peak about 75% along the histogram.

My money is on it being the shutter.

James

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Shutter would seem most likely to me too - what light source are you using? I suspect that you could do with a slower shutter speed so placing two or three sheets of white paper over  your telescope front might just attenuate the light source sufficiently.

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Another who suspects the shutter. It would be good to get your exposure time down to about 2 seconds to be sure to avoid shutter wipe, I would have thought.

Flats don't look like yours. Expect something more like this.

O%20FLAT%20web-S.jpg

Olly

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Garethr, are you able to post a picture of your own flats so we can see if your dark bar is like the one I've encountered.

Olly, how far along the histogram is a flat like that? It looks like it can only be 25% along. Might the histogram for a OSC camera be different to a mono camera with filters?

James

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No AP expert, but I have used a film SLR a lot and that looks like the mirror failing to lock up completely - dark unfocused fbar at the bottom. The shutter moves across not up and down.

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Garethr, are you able to post a picture of your own flats so we can see if your dark bar is like the one I've encountered.

Olly, how far along the histogram is a flat like that? It looks like it can only be 25% along. Might the histogram for a OSC camera be different to a mono camera with filters?

James

That's about 23000 counts out of 65000. This is what works best for me but, of course 'your mileage may vary...'

Olly

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A too fast exposure can definitely make black bars and weird effects, but it should not affect liveview. What can affect liveview is the mirror since it obstructs parts of the light cone when used with telescopes.

With very short exposures on a DSLR you get a rolling shutter effect caused by the 2 parts of the shutter just opening a small slit for the light to enter through which rolls over the sensor.

screenshot3.jpg

Here you can see how the mirror and shutter moves in slow motion

Edited by Xplode

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post-32109-0-02002600-1427922601_thumb.jGuys

sorry for delay busy at work 

Attached is a JPEG sample of my flat 

Canon 1100D

ISO 800

Exposure 1/4000 in AV mode

scope pointed at a bright sky with a diffuser fitted over scope giving a nice even flat light.

looking through eyepiece all looks ok

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Your flat is overexposed and with too fast shutter.

First you can do is use ISO100, but in your case i don't think that is enough so you either need to use another dimmer light source or something in front of the scope that gives you a darker image.

AV mode doesn't always work great so if it still overexposes use manual settings and adjust till you get a flat that looks good.

When you got good flats you should see slight vignetting towards the corners and usually a black bar on the bottom that comes from the mirror.

This is one of my flats with no processing.

post-17296-0-91216600-1427941431_thumb.j

Here's the same flat stretched to the extreme to show vignetting and dust bunnies.

post-17296-0-63724800-1427941632_thumb.j

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Garethr, yes yours looks just like mine. Light source is too bright so needing a very fast shutter speed.

James

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> I'm pretty certain that the 1100D shutter moves vertically...

Oops, sorry for any confusion - I have had very little experience with DSLRs rather than SLRs.

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