Flats question

[peter_4059]

I have a question regarding taking flats. I've read Eddie T's article and understand I'm aiming for the average flat pixel value to be 30% of the of the maximum pixel value but was wondering if I am better to change the camera iso setting or shutter speed to achieve this?

[Dennis]

Hi Peter

I’m guessing here, but if I were using a DSLR, I would use the same ISO for lights, darks and flats. If the ISO “value” is a property of some on board circuitry processing/software algorithm, then I would try to ensure that all the incoming data, whether lights, darks or flats, are subjected to the same processing.

Therefore, I would vary the exposure time. However, I could be wrong!:)

Cheers

Dennis

[kinetic]

Peter, Dennis,

FWIW, here is a good laymans overview of the whole process
that I keep as a bookmark for self reference every now and then.
It's easily forgotten by me over months and years and so a
good refresh every now and then is very helpful.

It mentions not only Flats but also Dark flats towards
the bottom.

http://www.aavso.org/observing/programs/ccd/manual/3.shtml

Steve

[Hagar]

Hi Peter, Keep your ISO. and every thing identical to capture except you can vary the exposure time. You shouldn't shift focus or camera or adapters etc as you are imaging the full imaging train and nothing else in this process. You may also find you get better results by also taking Bias frames and using them in the calibration process. Bias are easy. Just take as if you are taking darks at as close to the same temprature but at the fastest shutter speed available. This will reduce the read electronic noise from both your calibration files and your light files.

All pretty easy when you get it wored out and the improvement to your images will be imeadiately noticable.

Good luck with it all.

[peter_4059]

thanks for the advice guys. Hopefully I'm in for some good weather later this week to give it a try.

[White Rabbit]

Soryy to jump in here but rather than start a new thread I thought I tag onto this one.

After reading another thread about 30 mins ago i think it was in the ATM forum someone posted a link to this web page explain how to correct vignetting with photoshop, http://www.covingtoninnovations.com/dslr/Vignetting.html .

Could someone explain the beniffits of using a light box to do flats as opposed to correcting in photoshop. It seemed to work really well when I tried it and it seems to be much much quicker than taking flats manually.

Thanks and I hope you dont mind me jumping in here.

Sandy

[RB]

Sandy, when you take a proper flat, you are imaging through the actual optical train.
This allows you to correct for more than just vignetting, you also correct for dust and other optical imperfections.
Applying proper flats allows you to stretch the data much more when processing.

Using PS to correct vignetting is really only suitable for single snapshot type images.
You are much better off applying proper flats to your data when calibrating your subs.

[Dennis]

Hi Sandy

Vignetting is just one of the imaging system defects that flat fielding attempts to remove.

However, depending on what else you have in the optical train, such as filters for example, when these other optical surfaces are close to the sensor, any dust on them will show up as a doughnut or “O” ring and flat fields will also remove these.

Here are some examples, taken through separate RGB filters, along with a guess at the possible sources of the dust.

Cheers

Dennis

[White Rabbit]

Thanks for that guys, so let me get this right here.

To take flats you need to do it on the night you take the images, beause of temp and also camera angle.
The iso, focus and camera angle should be the same, but exposure should be as short as possible.

Seems easy enough, I might do it with an lcd screen and some white paper. Seems easier than constructing a light box.

Thanks
Sandy

[RB]

Not quite, exposure should be enough to give you a histogram with a spike at around 1/3rd to 1/2 way over on the preview screen on your dslr.
This will depend on the type of light source you use for your flats, eg blue sky.

See attachment.

[Dennis]

Hi Sandy

Although in practice your Flat Fields will generally be taken at the same temperature as your Light Frames, I don’t think that temperature is quite as critical for taking Flats, other than your tube and focuser may expand/contract with large temperature variations and hence may alter the focus slightly.

However, the same temperature is critical for Light Frames and Dark Frames; that is, your Dark Frames should (ideally) be taken at the same temperature and duration (exposure) as your Light Frames.

Dark Frames characterize the thermal noise and other unwanted sources of electrical noise of the sensor and associated circuitry.

Light Frames characterize the overall optical “quality” of your imaging train; that is, vignetting, gradients, dust spots, etc.

Cheers

Dennis

[White Rabbit]

thanks guys.

This is great info, keep it coming.

Andrew.

Thanks for the pic, I was wondering how to work out the 30% thing, never thought about looking at the histogram. It all seems to come back to the histogram doesnt it.


Dennis.

I assume that over time a camera will develop more hot pixels so making a darks library is pointless or at least limited by time???

Again thanks for the time.

Sandy .