When and how do I make flats?

[Stub Mandrel]

In my experience, few things cause as much confusion and odd problems as flats, the extra set of frames you take to compensate for any defects in your imaging train like dust, vignetting or optical oddities.

Flats are images that capture these 'defects' without any otehr content, and when applied to stacked images they can make the most ugly of shadows or dust doughnuts disappear like magic.

This post was inspired by a presentation by @Whistlin Bob that covered the subject really well and made me realise why I was getting a particular defect in some of my images!

Flats are simply a set of images made using a plain background and exposed so as no part of the image is under or over exposed (so the histogram should be near the middle). With a DSLR the 'Av' mode will do the job reliably every time, with a CCD or COS camera you need to pay attention to the histogram display.

The target can be any smoothly illuminated surface, I have used teh following, all with success:

• A plain painted wall, illuminated by diffuse light.*
• A white t-shirt over the end of the scope pointed at the sky (away from the sun).
• An LED tracing panel.*
• The sky (as long as it hasn't got any clouds as these will focus and affect the background).

For the marked sources* I found it best to rotate the scope (or source) to avoid any gradient affecting the flat.

Witha DSLR I take 16 to 24 flats. With a dedicated astro camera, I take 64, vbeacuse it is easy to do so.

You can combine flats, DSS does this automatically the first time you use a set, then you can delete the originals and just use the master flat.

SharpCap has a special routine which takes flats for you. It always checks 'apply automatically' - I always make sure this is unchecked as you end up with (wrong) flats being unexpectedly applied to future sessions. It also allows you to apply a different flat (if soem dust appears during a session or something moves).

Which leads up to 'when should I make flats'?

Some people make them for every session. This is the 'gold standard' and is what i do when working with Sharpcap as it only takes a few minutes to get a good 'sky flat' while you are waiting for darkness.

Otehrwsise, it's possible to make flats before or after your session with the key thing being that you make no (or as little as possible) change to the setup.

If, like me, you leave camera and scope set up and are meticulous about cleaning dust from sensors and filters, then a set of flats could last you several sessions, even a month!

But sometimes it becaomes apparent that dust has got in and you need a new set of flats or master flat. I've learned that it is easiest in the long run to get theat master flat at the start of each session (or at the end if you use something like a panel). Flats taken the following day are fine - but only if nothing changes. The problem is moving scopes around can dislodge dust.

Optical changes. Many people create a flat for each filter they use. The presentation made me realise that filters do have an effect. I have a filter wheel and usually make my flats using the L filter, which is parfoical with my (Baader) narrowband filters. For a good while I have seen odd, circular artefacts on my (ZWO) RGB filtered images and it's finally dawned on me why. For my last few sessions I created a master flat using the Ha filter, which worked perfectly on all my narrowband images. The RGB ones showed a dark,circular artefact near the top left. As everything was still set up, I created a new flat using the red filter and an evenly illumineted blanket. This worked perfectly.

The lesson is that flats can work for multiple filters but only if they as parfocal, or nearly so.

I hope this is useful for those beginning with imaging and helps you get your head around how and when to use flats faster than I did!

[carastro]

A good thread Neil. 

I normally do flats for every filter, though on the odd occasion, I found I had missed one and was forced to use the flat from a different filter, and so far I have got away with it.  But I would not risk doing this as routine. 

Don't forget if you bin filters you need binned flats.

Generally I do flats the following day if I can and use the sky with something over the aperture, typing paper or a white sheet (well pillow case actually).  But I do have a tracing board I use as well if I can't do sky flats for any reason (i.e. rain). The tracing board needing typing paper over it as the light is in dot form.

For CCD cameras you need to check the ADU (and it will vary between filters, especially narrowband).  Anything between 21,000 and 28,000 seems to work for me.  You should be able to find the ADU showing in your capture software somewhere.  Otherwise as Neil suggests, check the histogram, should be 1/3 to halfway.

If your flat is too bright, shorten the exposure, or use more layers of typing paper or sheet and vice versa. 

Agreed, if you don't move your kit or camera you can re-use flats.

Carole 

 

Edited August 10, 2020 by carastro

[ollypenrice]

I routinely use L flats for everything, without issue, ninety-odd percent of the time. I don't find bunnies are generated by my filters. I also find they can last a lot longer than a month. However, my setups are permanent and all sealed.

What about calibration? For CCD you can use a master bias as a dark for flats. WIth CMOS you will need 'same settings' darks for flats. There's a danger of over-correction if you don't calibrate them.

Newt users might find issues with sky flats when light enters the rear of the scope.

Olly

[Stub Mandrel]

Some good points!

I didn't explore other calibration frames. I use bias for DSLR and dark flats for CMOS images.

2 hours ago, ollypenrice said:

I routinely use L flats for everything, without issue, ninety-odd percent of the time.

The difference between my RGB and narrowband filters is really noticeable. These images both use the same G-channel data. The first one uses a flat made with the Ha filter and the second with the R filter. Incidentally my L filter, a Baader, is pretty close for focus to the narrowband filters, rather than the RGB ones. I've noticed this problem several times before but never realised the cause. It may actually be that the amount of vignetting is different between filters, rather than a focus issue.

 

ZWO Green image stacked with Baader Ha flat:

 

ZWO green image stacked with ZWO Red flat:

[ollypenrice]

2 hours ago, Stub Mandrel said:

Some good points!

I didn't explore other calibration frames. I use bias for DSLR and dark flats for CMOS images.

The difference between my RGB and narrowband filters is really noticeable. These images both use the same G-channel data. The first one uses a flat made with the Ha filter and the second with the R filter. Incidentally my L filter, a Baader, is pretty close for focus to the narrowband filters, rather than the RGB ones. I've noticed this problem several times before but never realised the cause. It may actually be that the amount of vignetting is different between filters, rather than a focus issue.

 

ZWO Green image stacked with Baader Ha flat:

 

ZWO green image stacked with ZWO Red flat:

Sure. Whether or not 'one flat fits all' is specific to the setup I'm sure.

One test you can do is to apply one flat to another.  (I do this in tutorials to demonstrate what flats actually do.) The result, of course, should be perfect flatness.

Olly