CCD v DSLR pixel size

[telecasterguru]

I have just purchased an ATIK 320E CCD colour camera as I thought it would be a good camera to learn CCD imaging on.

It has small pixels, 4.4um, which from what I have read should be good for my ED127 and my ED80 but not my 10" RC. My first imaging attemps with the RC looked burnt out. When I bined at 2x2 they looked even further burnt out. This was at 300 second exposures.

My question is about pixel size and focal length as I also have a 350D modded. I can't find any literature on the pixel size of the 350D but I assume that they are also small like the ATIK camera.

If the ATIK is not really suitable for long focal length image capture because of its small pixel size, why does the 350D work well with long focal length if it has small pixels?

As you can see I am only just learning about CCD imaging and appreciate any information.

Thanks

Frank

[Octane]

Frank,

350D has large pixels in comparison, 6.4 microns.

H

[telecasterguru]

H,

What size pixels does the 5D MarkII have?

I bought the 320 as a way of learning CCD capture without breaking the bank and I think once I have come to terms with focus and software capture that I will really enjoy it.

Frank

[Octane]

Frank,

The same, 6.4 microns.

H

[multiweb]

Good for you Frank. You'll have fun with your new toy.
What's important is your image scale:

206.265/600mm * 4.4um ~ 1.5 [ED80] [Atik]
206.265/600mm * 6.4um ~ 2.2 [ED80] [Canon]

On your ED80 at prime focus you see your image scale drops to 1.5 opposed to 2.2 with the canon. So you'll capture more fine details with your Atik.

Just start with short exposures then increase to figure out the full well capacity on your CCD. Your 10" RC will collect a lot of light in a 5min sub depending on the target. Try 1min first.

[telecasterguru]

Marc,

Thanks for the info.

I am just glad that I finally made a decision on the camera. I had to take the plunge and now there's no looking back.

Would the ATIK be a good match for the RC10" and longer subs imaging planetary nebula?

Frank

[multiweb]

Just plug your RC FL in mm in the formula below and it will give you your image scale. Anything around 1-1.5asp is good. 2-3 good for widefields. Under 1 to 0.5 need AO or very good seeing.

[bmitchell82]

The other way you can do it is download a small program

Ron Wodaski's CCD Calc
housed at New Astronomy Press.

I use it all the time to see what effect things have.

within that you put the information of your telescope, camera and you get the image size, arc per pixel (which is what marc is talking about).

Good luck!

[Paul Haese]

Frank you would be well over sampled but aside from stars being bloated you will have plenty of data for sharpening. I would just try it anyway and see what the results come from the use of the camera.

[RobF]

I've been surprised with the QHY9 how quickly the 8" (F5) Newt saturates it. I know its known for smallish well depth, but didn't expect 5 mins would be an issue. Mind you I'm still very much in the learning stages - I've got the gain set pretty low though.

All this time I've been fighting to get my guiding good enough to go over 5mins, and now I find it might be safer to stay short. ;)

[mithrandir]

Now a QHY8 on the back of a C8 gives:

206.265/2032*7.8 ~= 0.8



So I guess that explains the fuzzies, even when the Bahtinov mask says focus is excellent. Would I have more luck with the 0.63 or 0.33 focal reducer installed:

206.265/(2032*0.63)*7.8 ~= 1.3

206.265/(2032*0.33)*7.8 ~= 2.4

[jmelquist]

For posterity's sake: the following is a chart I compiled while deciding on which ATIK CCD to get:

314e = 12812 e-
314L = 17500 e-
320e = 14000 e- (same sensor as http://www.framos.eu/skynyx22.html)

-Jas

[rat156]

Hi Paul,

Why would the stars bloat?

Cheers
Stuart

[Paul Haese]

Stuart, my thinking is that with small pixel dimensions the well depth will be quite low and this will provide a tendency to be overwhelmed at even modest sub exposures. Similar to what I currently get with the QSI and the RC. If the pixel size is around 9 microns then this issue goes away. The well depth is generally around 100,000e as opposed to 25,000e.

It may well be that the Atik has large pixel sizes but those indicated seemed to suggest otherwise. I am happy to be corrected. That said, I get bloated stars with small pixels on the QSI with the RC. It can be controlled to some extent by removing stars and replace with shorter subs where the stars are not bloated. I think overall though the better way around the issue is larger pixels. :)

[rat156]

Hi Paul,

I'm still not getting why your stars would bloat.

I can understand that the shallow well depth will saturate the stars quickly, but this would be offset by the smaller surface area of the pixel itself, swings and roundabouts for me.

Given that the stars are saturated, how does this show up as bloat, most of you guys use cameras with ABGs don't you? I thought it was just me struggling with blooms.

I find that my leading cause of bloated stars poor seeing, closely followed by poor focus (that's two reasons, I'll come in again), then guiding. If you're in any doubt about any of these, drop a Ha filter into the imaging train, do a quick exposure, then use a blue filter for the same area. Compare the FWHM's. Can't do much about the atmosphere...

We may have to start a new topic if we keep going though.

Cheers
Stuart

[Paul Haese]

Hmmm, it is probably more to do with the sampling. The sensor is ABG but with the sampling the wells fill quickly and there are more pixels for each star and this results in bloat, not bloom. I don't generally have bad seeing down at the observatory due to its location, and focus is usually spot on, so the other thing that makes stars large and bloated is the above discourse.

[rat156]

OK, but why does sampling result in bloat?

The way I see it, or more correctly my camera sees it, is that if a star is 4 pixels FWHM and I change the camera so the pixels are twice as big, the the star will be 2 pixels FWHM.

Now for my system the 4 equates to about 3" FWHM, which for a 5-10 minute exposure from Melbourne is about as good as it gets, seeingwise. Yeah, sure we get some nights with 2" seeing, but over a longish exposure and 1-2" PEC, you'll get around 3" FWHM.

So if I swap my camera out for one with bigger pixels, the arc-second per pixel goes up (to around 1.5), but the star will still be 3" FWHM, unless the pixels are so big so that the star image only falls on one pixel, this happens with widefield imaging and results in square stars.

Have a look at the stars in the images below, both are from the same imaging run on the same night. The Ha is unbinned, the red is binned 2x2. Are the star profiles that much different? Once you blow them up so you can see each pixel, you can see that the oversampled image contains more profile information for each star, but overall the stars are the same size.

Cheers
Stuart

[Paul Haese]

I am gonna assume you are using the St10 as your imaging camera which has fairly large pixels from memory. So it is really not oversampling a lot. My setup oversamples at 0.68" per pixel. That means my brighter stars have 12-16 pixels coverage and so they appear to be bloated. Also when talking about bloat I am not talking about bloom. These concepts at quite different. Mathematically I am sure you are quite correct in what you are saying, but in real terms using a sensor with 5.4 pixels at 1600mm or higher is going to result in hugely over sampled images and bloated brighter stars when imaging at say 10 minute subs. Your system would have less trouble with this issue mainly because of you pixel size and well depth. Maybe I am looking at this all wrong but that is what I have understood so far. :)

[rat156]

Hi Paul,

I'm imaging at 0.76"/pixel, also seriously oversampled. FL of 1854mm and pixel size of 6.8um. I also suffer from somewhat bloated stars, so I'd like to get to the bottom of this.

I still don't understand the link between sampling and bloat. Just so that we make sure we're on the same page, by bloat I mean inflated stars, not blooming, which I mentioned because you mentioned well depth. This particularly affects the brighter stars.

So far you've just said that oversampling results in bloat, I'm struggling to understand why. I always though it to be a direct consequence of imaging at long FL, rather than image resolution. For mine, the causes of bloat are there for everyone, whether they are doing widefield or narrow, it's just much more noticeable in the narrow filed image because of image scale as displayed on screen. AFAIK the star's apparent size depends on the brightness, mainly because of the photon flux. Take a perfect scope and perfect seeing and the star's light will fall on one pixel, it's a point source. The fact that it doesn't is due to many things, chiefly our atmosphere, tracking errors and the diffraction limit of the scope. Once the image is produced it is faithfully recorded by the CCD. Now, as the star's light is wobbling around on the CCD, each pixel will record the number of photons that hit it, assuming a time based probability function spreading from the theoretical point source, the brighter a star (higher number of photons/second) the more highly populated the outer regions of the probability function, or for us, bigger stars.

So I can see that high resolution imaging can result in bloating of stars, but due to the FL of the imaging scope more than the pixel size of the CCD, unless you're seriously undersampled. Long FL leads to the familiar problems of battling seeing, mount tracking etc. all of which get significantly harder the longer the FL.

That's my take on it anyway.

Cheers
Stuart

[Peter Ward]

The physics is pretty simple.

Small pixels = small full-well capacities. As far as I am aware there are no 16bit small pixel devices as they simply don't have the well capacities to achieve high dynamic ranges. Saturation levels are also reached quite quickly.

Optical scattering within the CCD/CMOS substrate also becomes more obvious as pixel size decreases.

Apogee instruments cover much of this here

http://www.ccd.com/ccd111.html

[Paul Haese]

Thanks Peter, that explains it so well and succinctly.

Stuart, small pixels and long focal length means the wells fill quickly and there are more wells being filled per brighter star, that results in bloated looking stars. That combined with as you correctly pointed out guiding, atmospheric distortions etc makes for quite large stars. By way of example; if you were to use my RC with a 9 micron sensor your stars will appear much smaller than using a 5.4 micron sensor. Wells will take longer to fill and there are fewer pixels per the brighter stars with the 9 micron sensor. That is why using the QSI with the Tak produces such nice tight little stars generally as opposed to the narrow field scope.

I like you are talking about bloat and not bloom as you. :)