Are flats really required?

[RugbyRene (Rene)]

Hi all,

I've heard that flats are required to fix vignetting and dust donuts. The thing is, with the system I have (SW 100ED + 2-inch filters + ZWO2600MC) I don't have any vignetting and I keep my optical train clean so there's no obvious dust donuts.

So, having said that is there any need for flats?

Rene

[RB (Andrew)]

Yes.

[rmuhlack (Richard)]

You sure you don't have ANY vignetting? If you take a flat frame and stretch it, do you really have completely uniform brightness across the entire image? If not, then you probably need flats. Kstars/Ekos has an automated flat capture module, so if it is as easy to use as the equivalent feature on NINA, it will only take a few minutes to collect the flat frames, so what's the harm in collecting them...?

[Drac0 (Mark)]

There's more to flats than just vignetting & "obvious dust donuts". Can you be sure you have no dust or anything else in your imaging train? Camera, filters, reducers, lenses (inside or out). What about water spots forming on the lens during an imaging session from moisture in the air? You can take all the precautions you want but you will never get everything. So yes, do the flats - a few minutes that will save pain later.

Cheers,
Mark

[Nikolas (Nik)]

I don't always use flats in fact rarely ever do.

[Startrek (Martin)]

Flats are a must with my Newts and have made a significant improvement in my images
They take no time at all , I take them at the end of each session ( as with my Bias frames )
Also imaging multiple nights even if you don’t touch your set up, Flats still provide the best outcome for your images

Cheers
M

[raymo]

If anyone is interested, my experience has been somewhat different to
most of the members here. I was already in my seventies when digital
imaging started to appear in AP. I had been doing film AP for over 50 years,
and having never taken to computers [I don't even have a mobile phone], struggled with digital AP from the outset.
I changed over to a DSLR, and tried delving into the dark art of digital AP,
but found Registax, Photoshop, Irfanview, DSS, etc etc overwhelming. I
was forced to stick with the most basic method of doing tracked AP, which was max 90 secs subs [no autoguider, and had had enough of manual guiding]; ISO 1600 with Canon 1100D [sweet spot], or 800 with 600D; High ISO noise reduction on high, Long Exposure Noise Reduction on high, JPEGs [couldn't fathom using RAWS]. Stacking and processing in DSS, that's it, no separate darks, flats, biases, or in depth processing.
This method produced results pretty much as good as the digital newbies
were getting in those days, but as equipment and software evolved I soon
got left behind in the mud at the bottom of the IIS pond. [ I have managed to get several images published in Astro mags, so they couldn't have been that bad I suppose].
On the plus side, my experience in film AP and other forms of photography,
has enabled me to help out some of the newbies with their various [mostly non digital] questions and/or problems.
raymo

[AnakChan (Sean)]

I guess it varies from one person’s experience to another but to me, flats & flat-darks are more important than darks especially with current BSI-based CMOS sensors.

CMOS are quite sensitive with much more manageable noise & minimal hot pixels that PixInsight can address more easily.

But flats/flat darks to me fixes things that is much harder to try to fix in post - not impossible but just a lot more effort.

[The_bluester (Paul)]

First up, if you are happy with your results without flats, no one should step in and say that you MUST take them.

From my own equipment, both scopes use a ZWOASI2600, one MC and the other MM. I find flats essential with my newtonian both for any dust bunnies that creep in, and it has some vignetting related to the secondary mirror. My refractor on the other hand has quite a flat field illumination wise over my APS-C sensor, but flats do still make a difference that is visible. I always use flats calibrated with a master bias (I shoot sky flats so exposure times can vary from about 0.5 to 20 seconds, dark-flats would be no good to me) and I calibrate the lights with the master flats and a master dark of the appropriate length. I find without the master dark that the master flat tends to introduce brightening in the corners as the master flat acts on the dark current related "signal". It is minor but the longer the exposure the more pronounced the effect.

[ronson]

Majority of the comments mentioned flats correct for vignetting and dust, however there is more:

1. Correcting for Vignetting: Vignetting refers to the darkening of the corners or edges of an image caused by light falloff in the optical system. Flat frames capture the evenly illuminated sky or a light source, allowing you to measure and correct for the vignetting effect. By applying the calibration information from flat frames, the uneven illumination across the image can be normalized, resulting in a more uniform background.
   
2. Dust and Debris Removal: Flat frames also help in identifying and removing dust particles, smudges, and other debris present on the camera sensor or optical elements. These imperfections can cast shadows or create dark spots in the images. By capturing a flat frame with the same exposure settings as the light frames but with the telescope or camera lens capped, the imperfections become visible as dark spots. These spots can then be subtracted from the light frames during the calibration process.
   
3. Sensitivity Variations: Camera sensors often have variations in sensitivity across the image area, resulting in different levels of brightness. Flat frames allow you to measure these variations and apply corrections to compensate for the sensor's non-uniform response. This ensures that the final image accurately represents the true brightness and colors of the celestial objects being captured.
   
4. Optical Imperfections: Even high-quality optics can introduce subtle imperfections like diffraction patterns or ghosting. Flat frames help in identifying these imperfections by capturing the uniform illumination of the sky or a light source. They provide a reference frame to measure and correct for any optical artifacts that may affect the final image quality.

Here are a few reputable sources that discuss the significance of flat frames in astrophotography:

• "The Astrophotography Manual" by Chris Woodhouse: This book covers various aspects of astrophotography and provides guidance on image calibration techniques, including the use of flat frames.
  
• "Deep-Sky Astrophotography" by Jerry Lodriguss: This comprehensive guide explores the techniques and processes involved in deep-sky astrophotography, including a section on image calibration that emphasizes the importance of flat frames.

Answer generated using ChatGPT.

If your workflow (hardware+software) doesn't experience or doesn't depend on the highlighted issues, if you like the results your workflow produces then you probably don't need flats. Otherwise, consider using them.

[glend (Glen)]

Based on my years of imaging I would say Flats are not required. I briefly tried them but found they were not worth the extra effort, for the negligible return on image quality.

If you keep your optics clean. I have beautiful prints on my wall, none of which were made with Flats.

That said, the obsessives will follow any rabbit hole chasing the alledged path to perfection; and they will rant about straying from " the standard model", which endorses Flats.
As my mother used to say " Misery loves company".

[AnakChan (Sean)]

Originally Posted by glend

As my mother used to say " Misery loves company".

[OT]
But “ Laugh and the world laughs with you; weep and you weep alone.”

https://www.psychologytoday.com/au/b...ict-each-other

Sorry I had to throw that one in
[/OT]

[TrevorW]

Don't use flats or darks with my QHY268c

[Peter Ward]

Just to dispel the myths, all optical systems vignette to some degree.

Saying yours doesn't, is a bit like saying gravity is a myth.

But, if the level of vignetting in you system doesn't bother you, then sure, don't apply flats.

If you are performing photometry, or simply striving for excellent image fidelity and accuracy, then flats are required.

While there are some excellent software tools that approximate then reduce all manner of vignetting and gradients, I have yet to see a better method than a properly calibrating (i.e. flats and darks) a frame.

Everything else is just noise.

[ChrisD (Chris)]

Originally Posted by RugbyRene

Hi all,

I've heard that flats are required to fix vignetting and dust donuts. The thing is, with the system I have (SW 100ED + 2-inch filters + ZWO2600MC) I don't have any vignetting and I keep my optical train clean so there's no obvious dust donuts.

So, having said that is there any need for flats?

Rene

Can I suggest that you stack the same set of subs with and without flats and if you don't see any appreciable difference then you have your answer.

Chris

[RB (Andrew)]

Originally Posted by Peter Ward

Just to dispel the myths, all optical systems vignette to some degree.

Saying yours doesn't, is a bit like saying gravity is a myth.

But, if the level of vignetting in you system doesn't bother you, then sure, don't apply flats.

If you are performing photometry, or simply striving for excellent image fidelity and accuracy, then flats are required.

While there are some excellent software tools that approximate then reduce all manner of vignetting and gradients, I have yet to see a better method than a properly calibrating (i.e. flats and darks) a frame.

Everything else is just noise.

That's exactly what I said, expect my answer was shorter.
Welcome to the 'misery club' Peter.

[Peter Ward]

Originally Posted by RB

That's exactly what I said, expect my answer was shorter.
Welcome to the 'misery club' Peter.

Reminds me of the old gag, "money (flats) can't buy you happiness, but it does get you a much better class of misery (images) ".

[RB (Andrew)]

Originally Posted by Peter Ward

Reminds me of the old gag, "money (flats) can't buy you happiness, but it does get you a much better class of misery (images) ".

[TrevorW]

A I have tried to use darks with the Qhy268c just made the image worse so I don't bother - also flats, if the optics are clean and you have little in the way of vignetting, maybe not necessary depending on the total integration and number of frames stacking particular at the size and format of images posted on most web sites. Granted if you are creating images destined to be an APOD or win a major competition, go for it, although there are numerous ingredients which determine the quality of the final product one of which may be flats

[Premordial (Max)]

For my setups (see sig), flats are invaluable.