Taking Flats

[TrevorW]

How do you get the perfect flat

I read somewhre you need to focus your scope as normal on a star and then take the flats, why is this

I've taken flats from .5 to 10 seconds in duration and then when stacking these in DSS with lights etc the colour seems to get stripped out in ths histogram why is this

[Octane (Humayun)]

Because if the telescope is not at the same focus point as when you took your image, then, any aberrations such as dust bunnies, fibres on the optics, etc., will either be smaller than when you imaged, or larger.

You need to take flats at the same camera angle orientation and focus distance to ensure any anomalies are removed in calibration.

H

[TrevorW]

H, that I can understand, so effectively you are focusing at infinity on a flat source of uniform light, thus dust motes etc on the ccd which block out the reflected light will be at the same scale as when you focus on the target

[dugnsuz (Doug)]

I do rough and ready flats Trevor - at dusk on a clear night, point scope
towards sky, high in East (opposite setting Sun), focused @ infinity. I take 20-30 shots on the automatic "P" setting on the 40D then make a master flat in Images Plus.
Don't know if it's the best way to do it, but hasn't caused any problems for me during processing.

[TrevorW]

Thanks Doug, I'm using a EL panel now so it's trying to get that right balance

[allan gould]

Look at your histogram while taking flats and make sure that your range of values is not clipped at either end of the histogram ie somewhere in the middle. I use 4 secs with my light box so that the effect of the camera shutter in the filter wheel does not become part of n inaccurate flat due to the time being too short and the shutter being part of the vignetting pattern.

[Octane (Humayun)]

Allan's spot on the money.

Personally, with the DSLR, after taking a RAW image, I would check the histogram in Digital Photo Professional. I try to ensure that the histogram peaks along the x-axis somewhere between the 1/2-way to 2/3rds of the way across. Calibrating them with appropriate length dark frames and then median combining has worked a treat for me.

Of course, with the STL this is going to be a new trial and error experience for me; Pete's beautiful lightbox will be of help here.

H

[TrevorW]

With the EL panel to acheive an average ADU, in the Maxim info window of around 30000, I only need very short exposures 0.25 of a second, pushing the time to around 4 seconds would make the ADU the full extent 65000 odd, so where am I going wrong

Thanks

[Octane (Humayun)]

Can you turn the intensity of the EL panel down, or, perhaps diffuse it even further through perspex or white t-shirts?

H

[davewaldo]

I remember reading on the qhyccd forums that the QHY8 only uses 27000 of the possible 65000 possible tones in 16bit due to the sony sensors well depth.
It is also recommended to have a flat brightness value of 1/3 of the dynamic range. So this means you want your flats to read an average ADU of 9000.

I'll try to find the thread in question.

Also do you use bias frames?

[TrevorW]

Can't adjust the intensity on the panel may need too add some sheets of white paper

9000 adu would make it a dim flat ??

[davewaldo]

This is an interesting thread: http://www.iceinspace.com.au/forum/s...ad.php?t=62368

I see Multiweb uses flats around 9000

[davewaldo]

Also this thread if you haven't seen it... http://www.iceinspace.com.au/forum/s...escopic+spacer

What software do you use for capture?

[multiweb (Marc)]

Originally Posted by TrevorW

How do you get the perfect flat

I'm still looking for the perfect flat. I don't think it exists.
With the QHY8 10000ADU is a good place to start then increase exposure time to get brighter and test.

[Hagar (Doug)]

The QHY8 and 8Pro were not a big problem with flats but the 9 was so sensitive I had to use a few sheets of A3 white paper to get the luminance down so I could use a reasonable shutter length exposure when using the EL sheet.

I also aimed for an ADU level around the 20000 mark as the camera itself only has a well depth of 27K while the images you are correcting have already passed through the Analogue to digital converter and are in effect 16 bit images with a max level before saturation of 65K. I also found that the flat must be corrected with a bias frame or at least a flat dark in order to get them to work correctly.

Give it a try and see how you go. As for focus, you must have the focus, camera adjustment and every other thing in the imaging train in exactly the same position to get a set of flats that will work. It is always much easier to take them at the start or end of an imaging run to ensure all is the same. With a mono camera you have to take a set for each filter but just one set of bias files is enough.

Flats are a pain in the backside but they do make a huge difference.
Good luck Trevor.