Flats

[Steve91]

Hi guys,

Only just getting into astrophotography so lots to learn but i'm slowing getting there.

I understand what Lights/Darks/Bias/Flats are, and I understand that Darks and Bias need to be taken under the same settings and temperature as the Lights, but as for the Flats could I just set the scope up in the day time and take a load of Flats at various ISO's and save them on my laptop for use on future images?

Also how much affect do flats have on the images? Is it noticable or would I be fine without them whilst learning and experimenting?

Thanks

Steve

[Langy]

I only ever do the Lights, Darks and Bias frames.

I think that they are the absolute minimum you should go for.

Obviously the more you can supply the software the better it will be.

[Davey-T]

Flats help remove dust bunnies and vignetting.

Dave

[johnrt]

Lights image the actual optical train of your camera, flattener (if you have one) and optics. You do not need to vary ISO settings. Flats should be shot with the same camera orientation and focus point as the light images - don't move anything!!! Aim for the peak in the histogram on your flat exposures to be about 1/3rd of the way in from the left (about 23k ADU) and take lots, I aim for around 50 normally. Use an evenly illuminated surface (I use an iMac screen) or some point at the sky in daylight and place a white T shirt over the end of the telescope.

After you have shot your flats you must subtract the bias from them using you master bias frame *don't forget to do this*

Then you are free to stack up all your flats in to a master for use in calibrating your light frames. The flat will correct any uneven illumination on the chip, vignetting and dust bunnies on your optics.

Here's a master flat from my 6"RC and mono Atik 314l+.

Hmmmmm, donuts.

[Steve91]

Wow thanks for a very in depth reply. So you do have to shoot flats every session, as obviously the focus point and orientation would change.

As for stacking flats to make a master, can DSS do this?

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[dph1nm]

It does depend on your optical setup, but in general I would says that if you are trying to use the full  field of your DSLR then flats are essential in order to remove vignetting. Otherwise when you stretch the image (which you will want to if you are trying for DSOs!) you will just end up with a bright circle in the middle. You can try to correct in software, but honestly it is much easier if you take flats.

NigelM

[dph1nm]

So you do have to shoot flats every session, as obviously the focus point and orientation would change.

I don't bother - I only take flats a few times each year and they work fine. But I do make sure I always put the camera in same orientation wrt the OTA (as best ine can do by eye) when taking the lights. And my 1000D doesn't suffer from dust bunnies (seems to have a remarkably good sensor cleaning routine).

NigelM

[johnrt]

I don't take new flats every session as my camera lives on the OTA. But if I remove it for collimation, cleaning etc then I shoot a new set of flats. You focus point won't change between sessions if you are using a DSLR, it only tends to need to be adjusted by those using mono cameras with LRGB, Ha or other filters.

[idigitize]

Surely the dust on your camera sensor could change as often as you change lenses/scopes, or move the camera about, so if the flats are used to remove dust, it would be wise to take them fairly often too?

[Steve91]

Ok so to my understanding it would be ideal to shoot flats as often as possible, but if I were to shoot just one set every so often it would be better than having non at all?

If I were to set my scope up this afternoon and try to take some, how would I obtain the correct focal point that would match what it would be while shooting astro images?

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[Alien 13]

Surely the dust on your camera sensor could change as often as you change lenses/scopes, or move the camera about, so if the flats are used to remove dust, it would be wise to take them fairly often too?

My camera has an auto sensor cleaning function and the dust data changes all the time so i only use flats for vignetting (i stretch them and manually remove dust data for use as masters) any dust etc that does move during a session can add more artifacts than it removes.

Alan

[IanL]

You can (and should) turn of the automatic sensor cleaning if possible.  On Canons the option is buried somewhere in the on-camera menus, plus there is an option to manually initiate the sensor cleaning.  I tend to leave the cleaning off and manually run it before the start of an imaging session (otherwise it is too easy to forget and have the camera auto-clean when you don't want it to).

Dust and fibres do migrate around even with the routine turned off.  I've been halfway through taking a sequences of flats and seen a big dust donut vanish from the middle of the screen, so it pays to do them as soon as possible after (or before) imaging. How noticeable the dust bunnies are will depend on your target and how much you stretch the image, but I have yet to take an image where flattening them out wasn't required (and lost a couple of images where the flats didn't match the dust pattern or similar).

You can end up with dark spots / donuts or bright spots / donuts depending on where the dust has disappeared or appeared between lights and flats.

[ollypenrice]

Ok so to my understanding it would be ideal to shoot flats as often as possible, but if I were to shoot just one set every so often it would be better than having non at all?

If I were to set my scope up this afternoon and try to take some, how would I obtain the correct focal point that would match what it would be while shooting astro images?

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You can't because you don't know where your focus point is going to be. If using reflectors you also have to be careful to block light entering from behind the primary. Once you do know the focus point you can take flats provided the camera is exactly orientated. Mine also live on the scopes so that makes re-using flats easy. Tippex is a good alignment marker.

To give you an idea of why they matter, look at these images which are stretched so the effects are exaggerated, but your final images will also be stretched, remember. The first image is a flat from a full frame CCD.

It's an image of an even light source but look how inaccurate it is, with dark corners, a bright middle and bouncing bunnies. All this irregulariy of illuminanation is present in my raw data from this rig.

Now I apply a flat to this image. In the case this flat is applied to itself to make the point, but what comes out looks like a picture of an even light panel, just as it should. Phew, great relief!! The same corrections are made to your real images when you apply flats. Do they matter? They matter!!!

You should take at least 15 flats and stack them, using a set of bias frames as 'darks for flats' so that your flats don't bring their own noise into you picture.

Olly

[Pete Presland]

flats are one the easiest and quickest parts of imaging, ignore them at your peril!