So this flat frame malarky.

[wuthton]

For me it boils down to this:

DSLR, set to AV, low iso, point at an evenly illuminated light source. take flats.

CCD, aim for ADU 15-25k, point at an evenly illuminated light source. take flats.

If you're not doing it immediately after your imaging session then you're probably wasting your time. The flats are only useful for that session, you can't reuse them.

[lensman57]

Hiya

Alas, I'm lucky if I can get 10 x 5min subs what with the window and Glasgow clouds . If it stays clear tonight I hope I'll be able to add to what I did last night - some of which were galaxies!  I've yet to get enough integration time to successfully image a galaxy - sigh. I got a 550mm pedestal mount recently which enables me to point higher in the sky . It was a lot of hassle getting Orion Optics UK to supply me with an azimuth post that would fit but it seems ok now.

Cheers

Louise

Hi Louise,

You can to some extent compensate for the lack of sub length with relatively short exposures but a lot of them to build the S/N. In particular as you are within an LP zone, 30 X 300s will do the trick. If you both Dither and stack with a lot of subs at least the noise will be diminished to a great degree.

Regards,

A.G

[Thalestris24]

Hi Louise,

You can to some extent compensate for the lack of sub length with relatively short exposures but a lot of them to build the S/N. In particular as you are within an LP zone, 30 X 300s will do the trick. If you both Dither and stack with a lot of subs at least the noise will be diminished to a great degree.

Regards,

A.G

Hiya

Yeah, still same problem - I can maybe do 10 x 5mins or 5 x 10mins. But I'd be lucky to get that - it's a limit of the window aperture

Cheers

Louise

[lensman57]

Hiya

Yeah, still same problem - I can maybe do 10 x 5mins or 5 x 10mins. But I'd be lucky to get that - it's a limit of the window aperture

Cheers

Louise

I feel your pain, you maybe able to image on different nights and combine the data later weather permiting ofcourse.

A.G

[ollypenrice]

If you're not doing it immediately after your imaging session then you're probably wasting your time. The flats are only useful for that session, you can't reuse them.

I don't agree. I use flats for months, even if I rotate the camera/filterwheel. Yes, I have high quality focusers which are nicely centered so the vignetting effects don't rotate eccentrically. As for the bunnies, they are from contamination close to the chip and rotate when I rotate the imaging rig. WIth parfocal filters I don't bother to take separate filter flats, either. I find luminance fits all. If ever I get a rogue filter I say to myself that it needs cleaning!

Olly

[Thalestris24]

I feel your pain, you maybe able to image on different nights and combine the data later weather permiting ofcourse.

A.G

Yep, that's my aim!

[wuthton]

In effect our flats photograph the imperfect light path through which our lights were captured. You must, therefore, recreate exactly that imperfect light path when taking flats. Same camera, same camera orientation, same filters, same point of focus (or your bunnies will contract or expand). It is the light path which must be duplicated, not the characteristics of the chip at the time. If you cannot recreate the same light path you cannot make useful flats.

I don't agree. I use flats for months, even if I rotate the camera/filterwheel. 

Olly

Is there anything that causes more confusion and debate than flats?

[Thalestris24]

Yep, that's my aim!

Damn clouds!!!  Oh well, got some 360s subs but nowhere near enough for a wee galaxy. Might be worth hanging on for a bit...

[Thalestris24]

Is there anything that causes more confusion and debate than flats?

Hopefully my ex-session flats should be ok as nothing's changed on the setup. The reason for needing a new set of flats is cause I added a coma corrector and an idas d1 a while ago so everything's optically different from before.

[wuthton]

Hopefully my ex-session flats should be ok as nothing's changed on the setup. The reason for needing a new set of flats is cause I added a coma corrector and an idas d1 a while ago so everything's optically different from before.

I do a set each time I'm out, it only takes 5 minutes.

Am I wasting my time? I've no idea but it works for me.

[Thalestris24]

I do a set each time I'm out, it only takes 5 minutes.

Am I wasting my time? I've no idea but it works for me.

Well, my stuff is more or less permanently set up so nothing usually changes from one day to the next. I find even the focusser on the 150pds is remarkably stable. But as I've made changes and it's been a couple of months since I last did any imaging, I need to take some flats. I guess if you are setting up every time and changing focus/camera orientation etc then you may need to take fresh flats each time?

Louise

[wuthton]

Well, my stuff is more or less permanently set up so nothing usually changes from one day to the next. I find even the focusser on the 150pds is remarkably stable. But as I've made changes and it's been a couple of months since I last did any imaging, I need to take some flats. I guess if you are setting up every time and changing focus/camera orientation etc then you may need to take fresh flats each time?

Louise

My understanding of flats is that temperature with a reflector (>focus point) has a much greater effect on the quality and influence of your flats than with a small refractor. So even more reason to do them regularly.

[carastro]

If you have a permanent set up and haven't moved the camera then of course you can re-use your flats, but I wouldn't risk re-using them if I had moved the camera. 

I have also done the flats later provided the camera is still attached to the scope and not moved - I did this once when I was at camp and had started to dismantle everything and realised just in time I had forgotten to do my flats, so I packed the scope carefully with the camera still in position and did the flats when I got home.  

Flats are not temperature dependent, and do not need to be done at the same ISO as the light frames. 

If you are doing them at dusk using the sky, make sure to put a T shirt or something over the end of the scope or you could capture stars.  I always cover the scope with a piece of typing paper if doing sky flats.  

Finally, doing day time flats as Olly says, before of light leaks.  Reflectors can leak light, as can manual filter wheels which i found out to my cost!!!!!  I have never had a problem with the DSLR viewer leaking light, but it seems some cameras can, so beware.

Carole 

[ollypenrice]

Is there anything that causes more confusion and debate than flats?

Heheh, you got me! However, in the first instance I was really trying to point out the purpose of flats and give the stricter plan of action, by the book. In the second I was saying that, because I have high end focusers, I can get away with bending the rules. If you are using a budget focuser prone to a bit of droop and assymetry then you'd be better off sticking to the book.

Whether or not you stick to the book or economise on effort comes down to what works for you.

Sorry to add to the confusion!

Olly

[carastro]

Correction:

before of   should have been beware of.  

Carole 

[wuthton]

I stick by what I said last night.

The amount of conflicting information is the problem for most beginners scouring this forum for a definitive answer on how to take flats. How many beginners have a permanently mounted scope? 

For me it boils down to this:

DSLR, set to AV, low iso, point at an evenly illuminated light source. take flats.

CCD, aim for ADU 15-25k, point at an evenly illuminated light source. take flats.

If you're not doing it immediately after your imaging session then you're probably wasting your time. The flats are only useful for that session, you can't reuse them.

My apologies for calling you out Olly, it was a little rude.

[ChrisLX200]

A perfect flat will enable you to reveal every last bit of faint detail in an image - because it allows the strongest stretch without uneven illumination ruining the result. However, even a less than perfect flat will still markedly improve matters by (mostly) removing gross vignetting and the bulk of the dust bunnie doughnuts, and you can perform a histogram stretch to a degree which the quality of the flat you have allows.

Personally, my camera stays in the focuser all the time and is only removed when I reconfigure with a focal reducer or extender. It never has an eyepiece inserted instead of the camera, and mount polar alignment is done via the guidescope. I take the OTA off the mount for storage intact - complete with with camera etc., and guidscope. It is no lightweight! But that does allow flats to be taken at any time (not necessarily on the night of imaging) and also off the mount with it on my bench pointed at an EL panel on a wall.

All the dust bunnies and filter artifacts will likely be local to the camera and filterwheel (and possibly reducer) because anything further away from the sensor will be so far out of focus as not to produce any definition. So if all these peices of hardware move as one then camera rotation will not affect the flat and it can be rotated with imputnity - provided the focuser keeps these items central and on axis. You should still make every effort to clean the optical train to the best of your ability, then keep everything sealed with lens caps etc., (or in my case simply don't remove them from the scope). Under these circumstances flats will remain valid idefinitely, it is only an additional dust bunnie (or perhaps one less!) that will lessen their effectiveness. My current set of flats are three months old and I see no requirement yet to generate a new set.

ChrisH

[Alien 13]

The frequency in which you have to update flats depends a lot on tha camera scope combo if like me you have a DSLR that has an ultrasonic sensor cleaner then in theory a flat would have to be taken every time the camera is powered up and would only be valid until the camera was switched off again.

The way i get around this is to produce a master flat that has all the sensor defects removed in PS so that all that it is correcting for is the scope/flattener vignetting this of course can be used over and over again.

The dust bunny problem (thats if the sensor clean hasn`t removed them) is tackled in the final image by means of an inverted layre mask where all star detail is removed leaving just the bunnies and then added to the original image.

Alan

[dph1nm]

but I wouldnbut I wouldn't risk re-using them if I had moved the camera.

I take only two of three sets a year (always on twilight sky), and happily take the camera on and off every time I observe. I do make sure it is roughly at the same rotation relative to the OTA when I replace it though.  Absolutely no problem, they work fine. I also have dust removal on the camera and that isn't a problem either.

NigelM

[ollypenrice]

I stick by what I said last night.

 

The amount of conflicting information is the problem for most beginners scouring this forum for a definitive answer on how to take flats. How many beginners have a permanently mounted scope? 

 

 

My apologies for calling you out Olly, it was a little rude.

Not rude at all and not taken as such. I did contradict myself, for reasons later explained. I don't think there is a definitive way of doing flats. Best to do it by the book to start with and then see if re-using older ones works for you. It works fine for me till one day it doesn't, and then I redo them. There is no way in the world I'm going to lose more hours' sleep if I don't need to, though, especially running three rigs.

A good number of our guests have stopped doing multi filter flats and just use Lumlinance for all.

Now why don't flats cure gradients even on dark site images? I haven't a clue, but they don't.

Olly

[Christopher Davenport]

Whoaa, not sure if I want to wade in here! I do agree that flats can be a real pain to get right. 

I have stopped doing them because I am lazy and the amount of benefit I get is not much better than what I can achieve post processing.

i.e gradient removal and the clone tool to get rid of dust bunnies.

Having said this, my pictures are not publish worthy So maybe one day when I grow up I might learn to do them right.

One thing I have always pondered with flats is, why take more than one, per session?

(i.e only read noise is present, which the bias will remove.)

[ollypenrice]

How much flats matter varies a lot with a) your optics and b ) how hard you are going after the faint stuff. Our TEC140 at F7 doesn't vignette through 2 inch filters. Our Tak FSQ at F5 does. We should have bigger filters!

If you are going after the Integrated FLux Nebula or the Leo Tidal Tail you need flats. Yes siree! Separating the IFN from the background is a game of soldiers and only possible with good flats.

http://ollypenrice.smugmug.com/Other/Best-of-Les-Granges/i-frXBgjm/0/X3/IFN%20FINAL4web-X3.jpg

Olly

[tingting44]

some people say take flats at lowest ISO, some people say take flats at same ISO as lights? which works best from those who have done flats? im going to be getting an aurora flat panel soon and just reading up on taking flats for now

http://www.gerdneumann.net/english/astrofotografie-parts-astrophotography/aurora-flatfield-panels/uebersicht-aurora-flatfield-panels-overview.html

[ollypenrice]

some people say take flats at lowest ISO, some people say take flats at same ISO as lights? which works best from those who have done flats? im going to be getting an aurora flat panel soon and just reading up on taking flats for now

http://www.gerdneumann.net/english/astrofotografie-parts-astrophotography/aurora-flatfield-panels/uebersicht-aurora-flatfield-panels-overview.html

As soon as you get the panel try to protect the wires that go into the panel and into the power supply. I would add a blob of epoxy at both ends to support them. They are very fragile.

Olly

[tingting44]

As soon as you get the panel try to protect the wires that go into the panel and into the power supply. I would add a blob of epoxy at both ends to support them. They are very fragile.

Olly

Thanks Olly, that will be my 1`st port of call