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New flats problem...

ollypenrice

By ollypenrice
in Imaging - Discussion

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ollypenrice

ollypenrice

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This is a new one on me. For the 14 inch ODK we have an enormous Neumann panel. I use a smaller one like it for my refractors without issues. This is what we get as a Lum flat;

LUMFLAT-M.jpg

The bright columns on the left hand side and edge are spurious and should not be there. When applied to lights they create black columns by over-compensation.

Exposures are around 0.7 seconds which is longer than I use with my small panels on the refractors. (0.05 second is typical.) This puts the histogram peak right where it should be at around 25000 ADU. The camera is a Starlight Xpress H36 in Bin1.

However, the Ha flats needed 7 second exposures and produce excellent flats with no trace of the bright columns on the left. In fact I have used them to do a 'quick and dirty' on our present project and they work quite well.

In all cases the flats have been calibrated using a master bias from 100 stacked subs.

The obvious thing is to slow down the exposures in the LRGB to see if that will cure the problem. However, I wonder what is causing it? Anyone seen anything like this?

It's a bit frustrating because the data we have on M81/82 is drop dead gorgeous and we want to play with it!!!

Olly

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ollypenrice

ollypenrice

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Harry, hi. The capture is in Nebulosity and the stacking was in AA5, yes.

Yves, we can try both shorter and longer but it was the perfect Ha flats that made me think we had too much light in the LRGB. The columns are worst in the L which, of course, has the most light.

Olly

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vdb

vdb

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I would go for shorter, so histogram is loose from the left and then just a bit more ...

Longer will make the issue worse no? Ha is indeed less of an issue, I think it's because the chip is less sensitive in it ...

We are talking about the H36 indeed ...

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vdb

vdb

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Hmmm that would be interesting to know if we are doing sky flats if we have the same issue because it could be indeed pointing to noise from the electronics of the flat panel. Sky flats could rule that one out.

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harry page

harry page

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Hi

The reason I am asking is that the H35 ( my camera) and the H36 carry a lot of excess charge and need a good flushing before exposure. I had a lot of problems with my H35 on this .

Some software does not flush correctly on shorter exposure's :(

So try a longer exposure ( hence why your narrowband might work ) , of course you might need to lower the panel light level :)

Just a thought , Might not be this :p but worth a try

Harry

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ollypenrice

ollypenrice

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A big thanks fopr the input, everyone. Much appreciated.

Electronic noise from the panel? I've tried rotating the panel during a run of flats but the effect is always orthogonal to the chip. In other words the lines are not on the panel. I'm not sure if that's what folks meant but these are not, I don't think, 'photographs of lines on the panel.'

I haven't tried sky flats because I find them very difficult here. It may be the intensity of the light, I don't know.

I haven't tried very short flats with this camera.

What I like about Harry's idea is that it accords with the evidence of the long Ha subs working. I'll start by dimming the panel considerably and using long subs then I'll also try shorter ones.

What can we do about the flushing issue?

Olly

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Jessun

Jessun

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I'm not in your league chaps, but I use 4-6 seconds exposures for flats with my SX OSC. I use a Geoptic light panel and dim it accordingly to acheive a good flat.

When I go shorter and thus brighter all sorts of artefacts turn up, building from on corner, rendering the flat useless.

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ollypenrice

ollypenrice

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OK, it looks as if slowing down the panel is the answer. I've put 4 sheets of white fabric under it. Oddly I'm still on 0.9 secs to get 28000 ADU but there are no bright columns any more, or so it seems. I'm shooting now. Very odd.

Olly

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Jessun

Jessun

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How do you go about to flush?

I found that my sig cam is 'useless' on bright targets. The Moon for instance with 13% filter through only the centre dust cap of my SW120 at 0.000whatever second exposure still makes the cam go pretty much bananas.

For a good flat I have to look closely at my light panel to make sure it's even on...

I don't think it's a problem though, as you can dim accordingly.

On a sidenote I read in S@N mag that even the VLT spend the early morning hours taking flats! (Biiiig T-shirt) That's so reassuring that it's the way forward still and that any fancy software routine lags behind, and the flats need as much attention to perfect as the lights! Time well spent in other words nailing them - big or small scope. Might have to Google how the HST does flats now...

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ollypenrice

ollypenrice

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It's true that flats are super-important. In my early days I didn't appreciate this but as you become more and more interested in the faintest signal in your image (as you do!) then your flats are what sorts the... and now I'm stuck for a metaphor. The noise from the boys???

Olly

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