Flats (episode two)

[MikeWilson]

episode one was here

Hello everyone,

I've only had a few partial imaging nights since my last imaging session to experiment with getting the perfect flat. I'm getting there, but still way (way!) out.

I'm using an incredibly bright Gerd Nuemann EL panel and a sensitive QHY8L OSC CCD. I took a variety of flat images from 1ms (gave me nothing but bias signal), 5ms, 10ms, 30ms and 50ms. The 30ms flats give me a mean pixel value of 23695 - (the closest to the magical 20-22k that seems to be optimal) and so I'm using that. I'm going to pick up some opaque white perspex from eBay and fix it to the front of the EL panel to make it dimmer and go for longer flats.. just in case I'm running into some artefacts caused by the super short exposure time on the QHY8L.

I present to you - a couple of frames of M16, including darks and bias with and without flats.

I have found that these flat frames deal reasonably well with the lighter dust bunnies but the darker ones while dulled down are still annoyingly present. The flats work to reduce vignetting to an acceptable level so I know they're working this time! I suspect my earlier flats were over exposed.

The semi-circle cut out at the top left I strongly suspect comes from light ingress into the filter wheel. It's got a whopping big gap in it where you change the filter over so on the next imaging run I'll make sure that I cover this gap while taking flats.

Also while researching the cause of the super-bunnies on the image and considering that the main culprits are in the same place on every test image over the past fortnight, I checked the sensor - and lo and behold, it had dust on it. A quick and incredibly careful sideways blast from a compressed air can cleared the sensor chamber out with 'clean' air and I resealed the chamber with the dessicant tube. Is it possible that dust on or very close to the sensor may not be adequately removed by flat images?

Any ideas or suggestions would be warmly welcome.

Full frame without & with flats:

Crop without & with flats:

Many thanks,

Mike

[MikeWilson]

I think it might be a good idea to attach some gear photos so ya'll know what I'm talking about...

Calibration...

Imaging..

[rfdesigner]

If you've got light ingress that could be affecting your flats as well as your lights. I'd get some tape on that gap before going any further.

Derek

[roundycat]

You certainly should not tolerate any light leaks whatsoever.

Two to three seconds is a good exposure time for a flat although the time is not important in itself. Ultra short exposures could suffer from strobing effects from some EL panels. I use a Neumann panel and it has no such issues.

Anything in the light path that should not be there will be eradicated with a good master flat. It doesn't matter where it is.

Dennis

[George]

Have you tried a natural daylight flat instead of artificial?

[MikeWilson]

If you've got light ingress that could be affecting your flats as well as your lights. I'd get some tape on that gap before going any further.

Derek

Tape won't work (as I need the gap to change filter), but you've got a good point here. If light is entering sensor via a second vector, that could very well explain why the resulting flat frames are not properly eliminating all of the dust bunnies from the light frames. I'll improve something later with black felt and velcro.

You certainly should not tolerate any light leaks whatsoever.

Two to three seconds is a good exposure time for a flat although the time is not important in itself. Ultra short exposures could suffer from strobing effects from some EL panels. I use a Neumann panel and it has no such issues.

Anything in the light path that should not be there will be eradicated with a good master flat. It doesn't matter where it is.

Dennis

I'm not sure that I can get two or three seconds on the CLS filter. The Ha filter takes a lot longer, the OII filter less, but the CLS is relatively clear by comparison and needs 1/20th of a second at most. If I can somehow dim the panel, I'll get longer exposures and hopefully get away from any read noise on the camera and still get the same ADU/mean pixel level.

Have you tried a natural daylight flat instead of artificial?

Not with the filter wheel on it. I had planned to, but reading my son a story before bed ended the opportunity for twilight flats. This was the main reason why I wanted an EL panel :-)

Mike

[MikeWilson]

Solved with a sock and some electrical tape.

Here's tonight's NGC7000 test - two subs, plus flats:

Uploaded with ImageShack.us

Absolutely no sign of dust bunnies or vignetting. I finally feel like I'm getting somewhere!

[rfdesigner]

nicely done.. much better.... what's the permanent fix?

Derek

[MikeWilson]

Temporary fix is this (pic). Permanent fix will have to be a bag with drawstring or something which can live over the filter wheel to block out light.

Sent from my mobile using TapaTalk (so please excuse bad grammar & spelling!) :-)