Another flats problem...

[upahill]

I have been trying to take some flats both last night and today but getting an odd gradient.

I have a t-shirt stretched over the lens, have tried 1 layer, 2 layers etc.

I have tried with and without an LED panel, with just sunlight through the t-shirt, without the t-shirt on a plain sky.

Have tried the panel at varying distances.

No matter what I try it still seems to create a gradient across the lower fifth of the image. The adjustments above can alter slightly the intensity but cant seem to get even illumination.

Im using APT, flats mode, camera in AV mode, ISO800

 

I literally have no idea how to diagnose this, I had it a few weeks ago and then it just started working ? Alas, the problem has returned.

Any tips gratefully received.

 

[happy-kat]

If you rotate the panel does the gradient move?

[upahill]

Just now, happy-kat said:

If you rotate the panel does the gradient move?

Nope, I was wondering if the sensor in the camera was misaligned after modding but last time I had this issue I actually tested it with a brand new 1200d and had the exact same result.

[geordie85]

How quick are your exposures? It could be that your exposures are too short 

[happy-kat]

If you rotate the camera does the gradient move?

[upahill]

8 minutes ago, geordie85 said:

How quick are your exposures? It could be that your exposures are too short 

It varies but between 1/4000 and 1/1000 on the last batch of tests. I don't have any control over exposure length in APT though.

[upahill]

8 minutes ago, happy-kat said:

If you rotate the camera does the gradient move?

Just checked and no. I've been looking for any mechanical problems around the focuser thinking maybe the t-ring was in at a weird angle. Only thing I noticed was this which doesnt look right...

[Demonperformer]

41 minutes ago, upahill said:

I literally have no idea how to diagnose this

You've tried with different cameras and the panel at different angles and it stays in the same place, so it surely has to be telescope-related (the thing that is not changing). If that is the case, the same (opposite?) problem should be occurring on your lights and the flats should remove it when everything is processed ... is that happening, or are you still getting a gradient on the processed result?

The exposures strike me as being a little on the short side, but if that is the figure that APT is coming up with, then it should be right.

[Skipper Billy]

30 minutes ago, upahill said:

Only thing I noticed was this which doesnt look right...

Put some kitchen foil over the holes and tape it down then try it again. Its likely that there is light leakage through those holes,

[upahill]

13 minutes ago, Demonperformer said:

You've tried with different cameras and the panel at different angles and it stays in the same place, so it surely has to be telescope-related (the thing that is not changing). If that is the case, the same (opposite?) problem should be occurring on your lights and the flats should remove it when everything is processed ... is that happening, or are you still getting a gradient on the processed result?

The exposures strike me as being a little on the short side, but if that is the figure that APT is coming up with, then it should be right.

If I use the flats then I get a severe gradient in the final image - if I leave them out I get my dust but a mostly even field.

The fact that the images are generally ok without the flats (except for dust) led me to believe the scope was ok, but like has been mentioned I should be able to rotate this gradient ?

Im going to have to take a laptop outside and try it again - controlling from indoors to see the result and being outdoors to adjust things is starting to feel a little too much like exercise.

The other thing to try i guess is to remove APT from the equation and try and take the flats directly on camera.

 

4 minutes ago, Skipper Billy said:

Put some kitchen foil over the holes and tape it down then try it again. Its likely that there is light leakage through those holes, 

Ill give this a go, any idea a) what its for and b) why the pins don't line up with the holes? I feel like they should. The scope was second hand so no idea if it came like this originally ?

[upahill]

Light leakage has got me thinking.... maybe the time I got it to work was at night - so there was less ambient light ?

Going to black bag the rear of the scope and try again I think.

[upahill]

Tried bagging up the camera and still happening ? Maybe its just too bright?

[alacant]

2 hours ago, upahill said:

Maybe its just too bright?

Yes, I think so. A (very) dim light panel in a darkened room with the viewfinder covered works for me. But read on...

According to the guy who modified my 700d, at short shutter speeds the gradient occurs along the swivel edge of the mirror;

**EDIT over time/wear and tear/cheeepo cameras, the mirror doesn't get out of the way quick enough. </EDIT>

The reason it doesn't match your light frames is that these are taken with the shutter open for much longer and so the percentage of time whilst the mirror swivel exposes the sensor unevenly to the light source is negligible. The best way to combat this is to arrange for a light source which gives you a shutter speed of 2s or more when the camera is set on Av.

HTH

[upahill]

2 minutes ago, alacant said:

Yes, I think so. A (very) dim light panel in a darkened room with the viewfinder covered works for me. But read on...

According to the guy who modified my 700d, at short shutter speeds the gradient occurs along the swivel edge of the mirror. The reason it doesn't match your light frames is that these are taken with the shutter open for much longer and so the percentage of time whilst the mirror swivel exposes the sensor unevenly to the light source is negligible. The best way to combat this is to arrange for a light source which gives you a shutter speed of 2s or more when the camera is set on Av.

HTH

Ah that would make sense, I managed to get something reasonable with 4 layers of t-shirt. Even useable.

I looked into commercial products, specifically motorised flats/dark shutter with an EL panel. After picking myself up I found some panels on eBay for £20 so might try making a dimmable version of that.

[RichLD]

12 minutes ago, upahill said:

After picking myself up I found some panels on eBay for £20 so might try making a dimmable version of that.

If your going to use an EL or LED panel, I would dim the light source with layers of white paper or translucent plastic - pwm dimmers can create odd artefacts in flats IIRC. My QHY9m has a mechanical shutter - I generally aim for flats that are >= 2s or I get a similar gradient to yours.

HTH

Rich

[david_taurus83]

I use APT for flats on my 600D with a flats panel. I use AV mode but with a exposure value of +2. Generally get good flats with 0.5s exposures.

[upahill]

1 minute ago, david_taurus83 said:

I use APT for flats on my 600D with a flats panel. I use AV mode but with a exposure value of +2. Generally get good flats with 0.5s exposures.

Where do you set the exposure value? on the camera?

I feel like im missing something ?

[Jonk]

My first thought is it's a rolling shutter effect. EDIT - it could also be the shutter due to the fast exposure.

If you're doing DSLR flats at 1/1000s then to me that's way too fast.

If possible, dim the panel right down, or if using sky flats, wait until the sky is a lot dimmer then aim for say a 1s exposure.

This is easy on a DSLR as you can use aperture priority to see the histogram in the middle during live view.

 

[david_taurus83]

3 minutes ago, upahill said:

Where do you set the exposure value? on the camera?

I feel like im missing something ?

Hold down the AV button and scroll the wheel on top near the shutter button. It increases the exposure/brightness value. So when in AV mode it takes a longer exposure.

[upahill]

Cool, thought it was a setting in APT i had missed. Ill still try a dim panel as well since it doesnt require manual intervention each time I want to take flats.

I was looking at it the wrong way round, thinking more light was probably better for increasing shutter speed - but might have confused myself with bias frames.

Thanks for the help everyone! Ill report back with any success

 

[Atreta]

Hi, if I'm not mistaken i think you are using the wrong setting to take the flats. Are you setting it to AV mode? 1/4000 shutter speed is used to take bias frames( with the lens cover on), for flats you set the camera to AV mode and you place your light panel over it with the usual t-shirt trick and it should work. 

[david_taurus83]

Just to add to this, if I saturate the sensor with a pure white flat, 30s say, the max ADU that Iris software reads is 15300 or so. After a bit of testing I got that value down to 6500/7200 by taking flats at the +2 exposure value. APT shows this on the histogram way over to the right. I've read on another forum that APT and BYEOS take their histogram values from an 8 bit jpeg thumbnail of the actual raw image and not the actual full 14 bit image. The OP of that forum recommended the +2 exposure value so I've gone with that and it works for me. You may need to play with a light source but try this method and the camera should aim to expose to half full well depth. Try and find a light source that generates a 0.5 to 2 second exposure and see how you get on.

 

Edit: link below

 

http://www.astronomyforum.net/astronomy-digital-cameras-forum/178452-dslr-histograms-flats-warning.html