me flats hav gorn orf!

[mindburner]

I just got an Astronomik CLS LP clip filter for my full spectrum Canon 1100D camera

I have been taking a few subs of the triangulum galaxy ro try out the new filter. It successfully removed the blue/violet star halos from my images. Which was great.

I shot 20 or so flats. My usual way. Scope pointed infront of a laptop screen with notepad open. Camera in AV mode. Snapping away

My issue is that when I combine the flats with the lights the resulting image has huges swathes of red blotchiness and blocks of bright red colour!

The image is ok, if dust bunnied etc, without using them

The flats looked ok to me, just a bit darker and blue due to the new filter.

I attach an image of hone of them. If anyone has any advice, it's much appreciated as usual

[Bizibilder]

I wonder if the selective blocking of the LP filter is also blocking the "spectrum" from the lappy screen. I doubt if a lappy screen is a pure and complete spectral range - far more like three basic colours that are mixed to give "white". No doubt someone will have the difinitive answer for you soon.

[Stephen]

Have you changed the exposure length to compensate for the new filter (where's the histogram peak on the flats?)

I had blue tints with the Astronomik (never red) but they were processed out easily enough.

Don't worry about the flat being "bluer" : I'm almost sure (waits to be proved wrong) it's the intensity not the colour that's the important thing.

[Uranium235]

Did you first measure how much exposure you needed? If you can find the well depth for the ISO you are working at, then aim for about 1/3rd of that - eg: the well depth for a 1000d @ ISO800 is just over 4000, so I aimed for 1500 at the average level of illumination.

Secondly, have a go at monoising your master flat before applying it as the blue tint with CLS filters flats can mess things up a bit.

[mindburner]

Have you changed the exposure length to compensate for the new filter (where's the histogram peak on the flats?)

I had blue tints with the Astronomik (never red) but they were processed out easily enough.

Don't worry about the flat being "bluer" : I'm almost sure (waits to be proved wrong) it's the intensity not the colour that's the important thing.

hi no I just set it to AV and used the default settings. I must try and increase the exposure a little.

[mindburner]

Did you first measure how much exposure you needed? If you can find the well depth for the ISO you are working at, then aim for about 1/3rd of that - eg: the well depth for a 1000d @ ISO800 is just over 4000, so I aimed for 1500 at the average level of illumination.

Secondly, have a go at monoising your master flat before applying it as the blue tint with CLS filters flats can mess things up a bit.

HI the well depth. Is that the electrons for digital noise ratio? I will investigate this. I have been just checjing the histogram in DSLR remote pro,which i use to control the camera and making sure the histogram was about a third of the way from the left

Here's a sample of good old M31. ABout 30x 2min subs and darks. The one on the right is with flats with the red blotches.

[mindburner]

meant to say, processing was just with DSS

[Uranium235]

Looking at your blotchy version was Id say it was more than levels or the colour. Try taking the flats again, making sure the screen is exactly flat against the telescope, but not actually touching it. Im just thinking it might be an effect of the scope touching the screen and causing the same effect as would touching the screen of your laptop or LCD monitor.

[Uranium235]

... with your finger.

lol... damn this non editing!

[ollypenrice]

Hmmm, that ring in the second image is interesting. I suspect some kind of optical effect from the filter, maybe an internal reflection. Or maybe the angle of the light cone through the filter affects what passes. Just thinking aloud here.

I don't think laptop screens are that great for taking flats. I always found (using mono CCD) that at least one of the filters would get a null result. Blue, from memory, got nothing at all from the lappy. This may be disguised in a one shot colour camera but the effect is still there. I'd try a full spectrum light source like a Neumann panel or a home made lightbox with a full spectrum bulb. The choice of bulb makes a hell of a difference, believe me.

Olly

[mindburner]

Many thanks Unanium and Olly

The 127APO was plonked on two spare skywatcher counterbalance weights, pointing about an inch from the lappy screen. The alptop screen is old and not exactle stellar in brightness, so I will have a play with other sources.

I tried the suggestion of convering the master flat to mono and applying it in DSS. It has produced a much better result.

I was at least pleased that the dreaded blue halos that seem to plague imaging with this scope have been much reduced.

As for panels, you are right Olly, a panel is my next investment.

[Uranium235]

I tried the suggestion of convering the master flat to mono and applying it in DSS. It has produced a much better result.

Sorted

It seems to have done the trick for now, but its only a temporary fix until you get a proper flatfield panel. For now, try VLB as it allows you to adjust the brightness so you can experiment with flats of different length.

Virtual light box: http://www.iceinspace.com.au/forum/showthread.php?t=30188

[rfdesigner]

Hi

I just split your jpeg flat into R G B. I took at look at the R channel

here it is

as you can see, it looks awful, and whilst the G & B channels have levels of 164 and 223 the R channel has a level of 2 (almost no signal), hence the contours in the above image, even if your 'real' flat is a 12/14/16 bit image it still is needs a lot more signal.

Yes mono is one way to go.. the best way is a new light source with red that doesn't get removed by your filter.

Derek

[mindburner]

Guys , that's great. I think I have this sorted, or at least I will when I get my new panel. Rfdesigner, that is very cool. It shows I still have a lot to learn about imaging. With the red signal almost nil, the pattern then is apparent.

I am still deciding weather to use the new astronomik filter, which cures the blue star halo effect, at the expense of longer subs OR go back to my Baader Neodynium LP filter, which leaves the halos (I can remove then in post) but needs shorter subs.

Overall I think I will stick with the astronomik and get more subs to combine, plus a new light box for flats.

I was impressed witht he astronomik as I managed to bag M33 for 3x5 min subs and it looks like it could look ok with a lot more subs

Many thanks again

[mindburner]