Flats

Taking flat frames was one of my clearer improvements. And I didn’t do so for some years, … so I decided to write a little about this.

The theory was clear for me from the beginning: flat frames was THE way to go, as they help you correct illumination problems and get rid of those nasty donuts which appear in some images due to dust in the optical chain. But I wasn’t sure about the mechanism to use to get good flats, … so I simply skipped this piece of theory for a while! (and that was a mistake!)
What are flat frames good for?
Flat frames take care of some of the most common defects in astrophotography. One of these defects has to be with uneven illumination across the field, and the other is dust.
As far as I know, all the optical systems present some degree of illumination uneveness (sometimes called vignetting). This is apparent when you use elements as focal reducers in your optical chain. The thing is that the light is more intense in the “middle” of the field created in the focal plane, and it weakens as you move out of this center.
Although this problem not only affects photography, its effects are tinny in visual observation and can be neglected. But in astrophotography, the illumination defects show pretty easily. The problem gets worse while you increase the surface of the detector (the CCD).
If you look at the image, if the detector is small, it will probably lay into the center of the frame, so it will be free from the extreme effects that you see around the edges. But as you increase the size of the sensor (which is what you aim for, to get more resolution and more field) things get worse.
I must say that even with my humble DSI Pro, I had severe illumination effects, in part due to my use of a F3.3 focal reducer. This is an image of the Ring Nebula (it appears near the center, overexposed as a big ball, so that the vignetting effect can be easily seen), taken with the Meade LX200 8″ at F3.3 (focal reducer) and the DSI Pro:
vignetting with the DSI and F3.3
As you can imagine, vignetting is a nightmare while processing the images, specially if they contain subtel ad delicate details (as it’s always the case in astrophotography!). Not only it’s very difficult to get rid of for great looking pictures, but it’s a real problem as it hides details of nebulas and galaxies.
This “old” picture it’s also good to show the second common problem that the flat frames try to fix: dust.
As your optical system age (some days old are enough for this!), every part in it begins to collect dust and other nasty little things. This problem worsen with manipulation, so (as it’s my case) if you mount and assemble pieces every night, you force dust into the system.
The dust, diposited on the mirrors of a SCT, is NOT a serious problem. But if the dust live on the elements which are close to the detector (CCD), they appear in the images as uggly donuts (hey, look for some of these nasty critters near the center of the previous image!). The preferred places for dust to be and ruin you pictures is on the color filters (first option)  and in the focal reducer. Of course, as you change filters for astrophotography, you change also the “configuration” of the dust traces in your images.
These traces change also with focusing, and from night to night if you vary the orientation of the CCD (for example, if you mount and dismount it).
The miracle of the flat frames
Enter flat frames.
Flats are the antidote for vignetting and dust traces in your images.
Put simply, a flat frame is an image of the imperfections of your system, which can then “substracted” from the light images. Most processing programs manage the flats and “calibrate” your images with the flats.
So, a flat frame is produced taking an image to a source of light, as even as possible, so that the imperfections are exposed (vignetting and dust). The trick is to achieve an evenly iluminated source to image.
There are many ways to do so. There are some devices (called light boxes), and one can always manufacture his/her own box (internet is full of examples). And there is also a very humble mechanism, called the T-shirt way.
The T-shirt method is what I’m being using so far. You cover the scope with a WHITE t-shirt (please, no colours or drawings on it!), you point the scope towards the day sky (the afternoon/evening sky, before it gets dark with no stars yet), and you image with the SAME optical chain you’ll use that night (same focus, or at least approximate), at the SAME binning, and with ALL of the filters you’ll use.
You need some testing (pretty quick and dirt), as you aim to get frames with more or less half the light you CCD can take. If its a 16-bit CCD, this means getting about 32.000 count reading for the maximum illuminated pixels. So, my routine is taking some test images for eeach filter, and inspecting the pictures with Maxim DL to see it they get close to this 32K count. I play with the exposure time until I get what I want.
With my configuration, and taking flats as the sun sets (care! point your scope towards the East, as you want to avoid injecting light gradients in your flats (the gradient of light produced by the Sun setting), I usually use exposures of about 0,30 seconds for filters and 0,10 seconds for clear luminance. But it always depend on the day.
This is what a typical flat frame looks like:
Look for the dust traces (donuts) and see the vignetting effect.
My routine
Every evening, before my imaging session, I take my flats.
As I dismount the optical chain each time, I try to keep the focus of my system from the last imaging session, hoping it will not be mucg different that day.
As I said, I use a white T-shirt, and point towards the evening east horizon while the Sun is setting or it has already just set. I do my testing (playing with exposures), wait until the temperature in my CCD has reached the programmed point, and then I take at leat 3 flats for every filter I’ll use (usually, clear Luminance, R, G, B, and H-alpha). I also take DARKS for my flats (using, of course, the same exposures and at the same temperature). Everything is binned 2X2 in my case.
These flats will be used by Maxim DL (or any other software) to create a master flat for each filter, and then the master will be “substracted” authomatically from each image produced that night.
Flats at work
This is a picture, with and without flat frames applied.
without flat with flat

Can you tell? Yes, first one is without flats, and the second one is THE SAME picture after applying flat correction. The fine detail of the Sharpless 2-115 Nebula was hidden behind the uneven illuminated field in the first image. Of course, processing may improve things a little, specially using advanced tools for light gradient removals. But these tools, while getting rid of the gradient they also destroy part of the fine detail in the Nebula.

An analysis of the existing gradients in picture number 1 (without flats) is pretty revealing, and clearly shows the amount of vignetting:
without flat analysis
As you can see, it measures as much as a 38% drop in light in the upper right corner!
The same analysis, but after flats have been applied, gives this also revealing result:
with flat analysis
The field is homogeneus and almost flat in terms of light. The maximum drop in light is a 5%.

My future flats

The T-shirt method is “quick and dirt”: it’s very easy, straightforward, but not perfect due to its simplicity. It’s somehow prone to capture small gradients in the evening sky. Flats cannot be obtained immediately after the imaging session, which would assure the same focus. And, in my case, forces me to mount everything well before dark.

But even with that simplicity, from the previous analysis one can test the great job that these T-shirt flats do. The question would be: does it make sense to try to improve them using other techniques?

The answer, for me, is yes. Not because that 5% which can be improved regarding light flatness (maybe to achieve 2-3% gain), but because the practical inconveniences that the T-shirt method has for me: not always can I mount the equipment during daylight time, specially in winter. And I probably will be changing focus a lot from now on if I dive into planetary imaging (this implies another CCD, with barlow, … and with a completely different focus): I won’t be able to keep focus from session to session, and that focus can be only regained after dark during the beginning of the session.

There ara a lot of people who take the flats with the first light of the following day, AFTER the imaging session. This is great if you are ready for it. But I don’t want to keep the equipment, which is not protected by any shed or housing, set until dawn and take any risk about an unexpected weather change damaging it.

So, I’m waiting for an improvement. I’ve ordered a flat pannel, made of electro-luminiscence material, which gives an even field, and that I will be able to use whenever I need, just covering the scope with it. It comes from www.gerdneumann.net.

aurora_flatfield_folie_auf_astrograph_web

Looking forward to it!

October update: it works wonderfully. This is my first flat taken with the Neumann panel:

Panel Flat

 

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