Blue Flats?

[zedds]

Hello all.

I`m struggling with flats making any difference with vignetting and removing marks from the mirror or objective lens depending on which scope i use.

I thought i was getting it right as when capturing the flats the histogram is always between a third and half way using ZoomBrowser EX.

The flats have always shown up, when captured, as blue. Is this right?

Here`s an image of one taken last weekend. I`ve also included what PS Camera Raw shows if it`s of any use.

Any thoughts?

[swag72]

Not a problem with them being blue - Mine were always blue with the clip in filter - Looks like a vignette showing as well, these look like decent flats, should work fine. I'd perhaps have been inclined to lower the shutter speed a little so that the histogram is a little more to the left - But I'm sure these'll work.

[Photosbykev]

The colour doesn't matter as the information needed is the luminosity which is a the greyscale.

[Ant]

I would agree that the peaks should be further to the left IMO, apart from that they look ok.

If you stretch them do you see the dust bunnies (assuming that you have them)?

Ant

[Cath]

Still plenty of red and green in the image ..

[zedds]

Thanks.

Actually looking at the flats in ZoomBrowser EX the histogram is between halfway and three quaters across. Not sure why i thought they were between a third and halfway????

Here is a screenshot of the capture in ZoonBrowser EX. Does this histogram mean that the flat is too over exposed. Should it be alot more over to the left.

Also Ant, the flat is picking up the dust bunnies. I`ve circled them to show after a little levels tweaking.

When i run the flats with DSS the dust bunnies and vignetting are still present.

And thanks Cath but not sure what those images mean.

[zedds]

Looking at ZoomBrowser EX the second flat capture i took looks ok histogram wise. Would you say that`s the way to go. Histogram is a good way to the left.

[kirkster501]

I am having same issues Zedds. My flats show same imperfections in optical train- taken immediately after the lights - but the flats are not removing all the imperfections and dust bunnies in DSS and its spoiling my final pictures. I am at a loss as what to do..... Tried different DSS stacking and registering options to no avail.

Only thing I can think of is that the iPad method I use is not working to create the proper flats in terms of the light spectrum. But my master flat - created in DSS in the flats directory I use - certainly looks OK and the bunnies and smudges etc seem aligned enough to the lights in terms of the imperfections in the image.

is this the same with you Zedds?

Lots of beers to anyone who can help me with this.

[Uranium235]

Convert your master flat to mono, job done

[Cath]

Their is an anomaly in the blue channel Andy of your flat, no idea if you get the same when taking normal images of the sky, but if it's not present when you take images then I guess your flat won't be doing what you need it to do ..

http://www.youtube.com/watch?v=D9MLwUcU99w

[billhinge]

Great thread, I got blue flats using a dslr and a Gerd Neumann panel , thought it was just me, useful info, thanks

[spectre]

Cath, That is an amazing image. What program is that and is it available?

[andrewluck]

Andy

Can you upload a flat and a light frame to Dropbox. Both should be unaltered, as they come off the camera.

Cheers

Andrew

[zedds]

Andy

Can you upload a flat and a light frame to Dropbox. Both should be unaltered, as they come off the camera.

Cheers

Andrew

Sorry for the late reply Andrew.

Here are the two links for the unaltered light and flat. Both took at ISO 800.

The light is 210 seconds and the flat is 1/40th second.

LIGHT

FLAT

[andrewluck]

Hi Andy

Here's a link to the flattened image. As you can see, there's no problem with the flattened image so the issue must be with the processing. What software are you using to do the calibration?

All I did to the flat was load it into Maxim DL, convert to a fit and add a header entry to identify it as a flat. Calibrated the image, colour converted it. I've stretched to show the background and saved as a jpeg.

Hope this is of some help

Andrew

[zedds]

Thank you for that Andrew. Shows the flats work.

I use Deep Sky Stacker to get my final image.

I need to look into my technique (i use the term loosely ) when stacking.

Thanks again.

[andrewluck]

Another tip here to help ensure the flat calibration is correct:

If you're subtracting darks from the light frames then subtract a master bias frame from the flats. Bias frames should be the shortest exposure you can take (with the lens cap on). Aim for at least 40. The more the merrier; they're quick and easy to obtain. Take as many flat frames as you can. Both these points are critical as noise in these calibration frames will be imprinted on every calibrated light frame.

Good luck with DSS. I've no experience with that but lots of people here use it.

Cheers

Andrew