Flats.......but when?

[Shelster1973]

Am just thunking to myself...when is the correct time to take flats?

Having moved over to CCD imaging along with the various filters involved, am happy that each filter will require it's own set of flats, but was wondering when is th ebest time to shoot the flat?

Am thinking that I get myself setup, sort out my focus, then shoot the flat and then the data collection as...with it becoming increasingly cold, when my data collection sequence is complete, all I need to do it go back outside and then pack up...as opposed to collecting data and then heading out at silly late and cold o'clock and doing the flats and then packing up.

Also, just found out that SGP has a flats wizard.....who knew?  Am learing so many new things with moving to CCD....just hope it doesn't push the old info out of my head.

[geordie85]

Flats can be taken at any time as long a the imaging train hasn't changed. I always leave my CCD attached to my scope at the correct focus point so flats can be taken the next day or when it's cloudy.

[Filroden]

I do likewise but I've just treated myself to a light box which is powered by USB so I may run flats at the end of a night.

[SkyBound]

Do flats have to be taken through each filter when doing RGB or NB ?? I did not realise that... ?

[Filroden]

Yes, as dust can accumulate on each filter and this needs to be calibrated out. That said, if you find they are dust free, you could apply a single set of flats but the only way to check they are dust free is to ... take flats!

[ollypenrice]

4 hours ago, Filroden said:

Yes, as dust can accumulate on each filter and this needs to be calibrated out. That said, if you find they are dust free, you could apply a single set of flats but the only way to check they are dust free is to ... take flats!

It can, but it rarely has much effect. Most of the relevant dust is not on the filters but on the chip window. I quite often use only one flat for all filters and when it works that's what I do. If I do get a significant horror on one filter then I need dedicatednew flats.

I don't touch my rigs between nights so I find my flats can be very durable, lasting a month or two with luck, but then a new contaminant will get you or an old one will move slightly - then it still gets you!

My real point is that flats are not a form of religious ritual to be performed in a certain way every night. Think through what the are, how they work, what they do, and then look at how they are working. If they are working don't fix them.

Olly

[Shelster1973]

Wise words all

Unfortunately I remove my camera from the scope each night. On the plus side it remains attached to the filter wheel. Do try and get same alignment by using registration marks but cannot guarantee it. 

Have decided to only go for one target at the moment ( bubble nebula) so will be concentrating on one channel per night, so a set of flats for each channel is not such a chore

Am just thankful I don't need darks....

[wimvb]

Just to add:

flats are for correcting the optical deficiencies of your setup. As such, they should not be affected by temperature fluctuations.

(In the same way, darks and bias frames are for correcting a cameras electronic deficiencies, and as such depend on camera settings and temperature, and are independent of optics. You don't have to leave your camera attached to the optics when taking these.)

As Olly noted, dust in sub frames is only from dust on or very near the sensor. Any dust more than a few mm from the sensor will spread its shadow across the entire frame. There was a very interesting reply to a post about flats/dust bunnies here recently, that proved this point elegantly.

On the other hand, vignetting is due to the entire optical train. In the worst case, this can vary between filters. But if it does, you should probably check your filter box. If you reorientate your camera, this will cause the vignetting to rotate, and that's the reason why you should take flats without changing the optical train.

Personally I would take flats after the imaging session, simply because you never know when clouds move in, and you'd want to make every minute with clear skies count.

Cheers,