Flats question ....:O)

[WaveSoarer]

Olly,

Many thanks.

Dave

[IanL]

I'm thinking of doing some flats too. How often should these be done? The reason I ask is that the CCD detectors I work with for X-ray crystallography, which are peltier cooled to -45 C with all the cold parts held under high vacuum (they are extremely expensive pieces of kit by the way) have their flat images (flood fields) measured once in the factory. These last the life-time of the detector unless a repair is required. The dark frames are recollected on a regular basis, though, and we have a stack of these for different exposure times. I'm just wondering if it's really necessary to acquire flat images for every imaging session. Unless there is a significant change to the CCD sensor I would find it surprising that flats would need to be taken all that frequently. I expect dust on the CCD sensor would be the most likely issue unless the optics of the telescope has an effect that can vary between sessions (such as the orientation of the camera on the focuser). What experience do people have with using sets of old flat images?

The difference is that your lab kit is measuring X-rays. Unless you are grating a lump of lead on to the surface of the detector, I doubt very much that any other kind of contaminant (such as common-or-garden dust) is going to have an appreciable effect on transmission of the X-rays to the detector! I don't know about any other effects that might occur due to the changes in the set up of the equipment, but it sounds like it is not an issue.

Therefore the purpose of the factory created flats is simply to measure and allow correction for differences in sensitivity of the individual sensor elements on the CCD.

This is also one of the purposes of taking flats in an optical imaging set-up, as the sensor elements will respond differently to incoming photons (including some that are dead and some that are hot, i.e. overly bright or saturated, due to manufacturing defects).

Optical flats also correct for uneven illumination due to the lens or telescope (a circular vignette or similar gradient). It is valid to take one set of flats for this purpose, provided you are absolutely sure that you do not change the orientation of the camera or re-focus significantly and you do not change filters. I think you can use a filter wheel and take a set of flats for each (but I am not a CCD imager so someone else will advise). Manual changing of screw-in filters would not work though.

The big problem is dust. This settles on the surface of the sensor and/or the optical window, and on filters. This will vary between imaging sessions, depending on how clean/well sealed your kit is you may get away with it, or you may not. You can calculate where the dust is by measuring the diameter (in pixels) of the circular shadows it creates and plugging them in to this calculator:

http://www.wilmslowastro.com/software/formulae.htm#Dust

This will tell you where the dust is settling.

[WaveSoarer]

Thanks Ian,

Our X-ray detectors are held at very stabe temperatures and the optical light train is constant. The X-ray photons are converted to visible photons on a thin sheet of fluorescent material just after the opening apperture and the resulting image is demagnified onto the smaller CCD chip via a fibre optic taper. There are a whole bunch of calibration files associated with the performance of the taper and the fluorescent screen and there are also corrections for the distortions produced by the taper. All of these are fixed, though, and it's only the dark frames that need to be updated regularly but that's comparatively easy to do. From what you've said I can more easily appreciate what the flats are trying to achieve: they compensate for any non-uniformity in performance across the surface of the CCD and any inhomegeneity in the light path, including dust. I have noticed a couple of circles on some of my images and I can see that I now do need to think about flats.

thanks again

Dave

[IanL]

Dave,

That's a really interesting process there. I hadn't heard of a fibre-optic taper before, but looks like they are bonded to the CCD surface? What about the phosphor at the other end, is that bonded or just some kind of air gap? I guess it limits the opportunities for environmental changes to the light path with all factors capable of being measured once? Interested to know why it is necessary to repeatedly take darks assuming that the CCD is cooled (or is it?) If a set temperature is maintained I'd have thought a single master dark would be all that you would need.

If you are seeing circles on your astro-images it would definitely be worth taking flats after each session. The more you stretch the histogram/curves in the dark regions the more noticeable the dust becomes. You only need a few percent difference in illumination for them to be very noticeable on a fully processed deep-sky image. Less of a problem on lunar images, but again can be a problem on planetary if you are using a lot of magnification since it only needs one well-placed dust speck to obscure a big chunk of the disk.

[WaveSoarer]

Ian,

The fibre optic taper, as its name suggests, is broad at the end where the fluorescent screen is placed and it necks down to the CCD sensor which is smaller than the screen. The demagnification ratio depends on the ratio of the taper but it's usuallly on the order of 1:2 - though this depends on the application (and the budget). The taper is quite something as the fibres must maintain their position in the bundle so that the image isn't scrambled. From what I've heard, the process is extremely difficult and there are not many manufacturers that can do it. The fluorescent screen is replaceable if it gets damaged, and this can happen if something accidentally strikes the light-tight (but X-ray transparent) window in front of it. The window also helps to keep the screen flush with the taper. The narrow end of the taper is usually bonded to the CCD sensor, although some maufacturers use an oil filled membrane to light-couple the two together as this is less liable to fail during cycles of cooling. The chip is peltier cooled to about -40 C, though some of the very expensive detectors go down to ~-80 C, and the hot side of the peltier is cooled via a water-chilled heat exchanger. The whole assembly is kept under vacuum to pevent icing and condensation and the detectors are kept running for months on end, usually, without any intervention. The only regular calibrations are the dark images as nothing else changes. The flat images, corrections for the taper distortions and the unused regions at the corners of the CCD (the taper is round and the CCD is square) are all factory set and aren't expected to alter - though at some point one of our users dinged the X-ray window on our most heavily used detectors and every diffraction image from then on has a small region of fewer counts. It would take a lot of effort (and expense) to do anything about this. Sorry if this is a bit dull.

I'll investigate taking some flats. I expect that a white cloth, such as a t-shirt, stretched over the end of the scope, and some uniform illumination on to it is one way of achieving this.

thanks again

Dave

[IanL]

Most of us start with the t-shirt method, but for best results you would need to do it on a dull cloudy day outside. Various people tinker with sky-flats and twilight flats or image a flat surface. I have gone for a light box. If you take a look in the DIY section of the forum you'll see lots of people have built light boxes or EL-panel type devices which are convenient as you can just chuck them on the scope straight away and get your flats in a couple of minutes.