Why flats are important but........................

[Ewan]

I know most of the SGL members already know why flats are needed but I thought I would post an image example why they are so needed when stacking your pictures.

After my 460EX 1st light I left the scope as it was until I got back from work at the end of the day, focus & filter was left as it was from the last image taken the night before, this way the optics were exactly the same when I was imaging.

I took 150 flat frames with a white carrier bag over the OTA's front cell held with an elastic band, I ended up using an exp of 0.005 seconds to get the data in the middle of the histogram, added all the flat frames when I chucked the lot in DSS, once all the frames were calibrated I then saved the Master Flat Frame with the Lum I shot.

Here's a with/without comparison :-

Without Flats :-

With Flats :-

As you can see I also had slight vignetting, it also depends on what 'filter' you use on the flat frames I have found, I used Kappa Sigma Clipping as it gave me the best results.

I do have a question though which is why I have the but.............. in the title.

As I have to setup each time I image, when I go out next time & do the same target I presume I have to redo new flats for that's nights Lum run as I would for subsequent channels I collect.

Now if I use the Lum/sub/flat data for M51 so far how do I combine with the next set, ie the new Lum flats will be different & the dust may have moved ?

Do I stack each set separately then combine in PS ?

Some advice with this would really help me out.

[Stargazer33]

Hi Ewan, nice post

To answer your question, yes you would need to do new flats. If you move anything in the image train or change focus you will need to do new flats.

[Ewan]

Hi Ewan, nice post

To answer your question, yes you would need to do new flats. If you move anything in the image train or change focus you will need to do new flats.

Thanks for confirming but what the best way to combine flats & lum subs with another set of flats & lum subs ?

In DSS somehow or PS ?

[tingting44]

you have groups in DSS at the bottom, you add each nights subs in its own group with each nights flats, job done lol

in the master group you can leave your bias in there as they will be the same for all groups

[Ewan]

you have groups in DSS at the bottom, you add each nights subs in its own group with each nights flats, job done lol

in the master group you can leave your bias in there as they will be the same for all groups

Thanks Martin,

Yes I see that works now, I should have copied & renamed my Master flat to test that out first before posting, very sorry for a daft mistake.

[swag72]

Can you not keep the camera and imaging train on the back of the scope when you pack up.... That way, with the camera orientation not moving I reckon you'd get away with one set of lum flats.

Can't help you on DSS combination I'm afraid..... But loois like its sorted now

[Ewan]

Can you not keep the camera and imaging train on the back of the scope when you pack up.... That way, with the camera orientation not moving I reckon you'd get away with one set of lum flats.

Can't help you on DSS combination I'm afraid..... But loois like its sorted now

I would Sara but as the UK weather is very unpredictable & I have a full house with children I can't take the risk of any damage.

How I long to move & have an Obsy

[ollypenrice]

Hi Ewan, nice post

To answer your question, yes you would need to do new flats. If you move anything in the image train or change focus you will need to do new flats.

In theory I'd agree. In practice I'm far from convinced, though my stuff does stay on the scope. The dust bunnies are created fairly close to the chip and may well be on the camera chip window. They can also be on the filters but on the three rigs I'm using they're not. Certainly chip window on two of them and chip window plus flattener for the third. SInce all of this rotates together I can turn from landscape to portrait and the flats are fine because the dust rotates with everything else.

If you do, say, 3 nights of shooting and you really do need different flats for each night then you have two options. Make three separate stacked results, one per night, and then combine those three together at the correct weighting based on their exposure time. Or, calibrate each separate image with darks and flats but don't combine them. Save them with a prefix or suffix to show they've been calibrated. Then run all these calibrated ones through a stacking programme but with no darks of flats because these have been done already.

Olly

[Ewan]

In theory I'd agree. In practice I'm far from convinced, though my stuff does stay on the scope. The dust bunnies are created fairly close to the chip and may well be on the camera chip window. They can also be on the filters but on the three rigs I'm using they're not. Certainly chip window on two of them and chip window plus flattener for the third. SInce all of this rotates together I can turn from landscape to portrait and the flats are fine because the dust rotates with everything else.

If you do, say, 3 nights of shooting and you really do need different flats for each night then you have two options. Make three separate stacked results, one per night, and then combine those three together at the correct weighting based on their exposure time. Or, calibrate each separate image with darks and flats but don't combine them. Save them with a prefix or suffix to show they've been calibrated. Then run all these calibrated ones through a stacking programme but with no darks of flats because these have been done already.

Olly

Thanks Olly,

I have been looking at the options & I think either will work but as I do take down my rig each time (& clean all the optical surfaces) I will probably go with flats for each session.

[perfrej]

150 flats? 15 is my magic number...

[Ewan]

150 flats? 15 is my magic number...

Got carried away with my cup o tea