Perhaps not!
Hi,
I am not an expert, but over the last few years I have learned some very interesting things about digital astrophotography. There are some wonderful techniques that can be used to tame the noise generated in sensors during longer exposures. Much astrophotography is done with CCDs that can be pretty noisy. With the right techniques, you can get rid of the noise!
If you have not looked into this topic I can recommend the book:
The New CCD Astronomy by Ron Wodaski.
It is amazing what stacking a few image frames can do to reduce noise. Four stacked will make a significant difference in reducing noise. After 16 frames, the gains diminish quickly. Also, if you take the time and create a series of calibration frames (dark, bias, and flats) and apply them your image, quality will improve even more.
There are some fantastic software tools for both Mac and PC users that are free, or very inexpensive, that make processing digital images easy. One favorite is Nebulosity by Craig Stark of Stark Labs.
As an overview: you shoot your image files and put them in a folder. Shoot your dark frames, put them in a folder. And so on with your flat and bias frames (if you elect to use them). Then fire up a program like Nebulosity. The software will ask you to point to your dark frames. It will then stack them into a master dark frame (with low noise) and save it. You do the same thing with your flat frames and your bias frames. You end up with a master frame for dark, flat field, and bias.
Next, you tell the program the folder for your image files. Typically you will shoot 4 or more frames of a single subject on rig tracking the sky accurately like the AstoTrack mentioned in the original post. The program will work with you to align and stack the image frames. At this point the stacked photos will be nice. But then you tell the software to apply the master calibration frames for dark, flat, and bias, and everything gets even better. Noise should be all gone. Your image should be free of any natural vignetting. You will still have work to in order to stretch the image, correct color, sharpen, and so on, but your starting point should be massively better than if you had made single frame of the night sky.
One final point. It is important to keep the sensor as cool / cold as possible. Heat equals more noise recorded. Dedicated astro imaging cameras use active cooling systems to manage the heat. Big observatories will use liquid Nitrogen cooling. I have found shooting on cooler / colder nights vs on a warm summer night makes a difference in image noise.
Ray, I bet you can do some terrific asto work with your MFDB. With accurate tracking, good focus, and some free / inexpensive software I think you will be as surprised.
Cheers,
Paul
Thanks, Paul. This is all excellent advice; the sort I've been dispensing for a number of years
- I guess you didn't realise my background (research and lecturing in observational astronomy and astronomical data analysis; and over 20 years as an amateur astrophotographer).
I don't regret for a moment getting my MFDB, which I use for most of my photography; but I do berate myself for not doing better homework on my S/N expectations of MFDB-CCD vs. DSLR-CMOS. I'm afraid I took manufacturers' ISO claims much too literally
- not realising that they can correspond to
any arbitrary S/N in the shadow intensities. All those years I'd spent shooting film, where ISO was defined rigidly with respect to threshold detectability of density above base fog - and hence a minimum S/N - lulled me into this. And the concept of ISO sensitivities doesn't exist in research CCDs; we work directly with the sensor parameters for calculating S/N.
Well, what do you do when you get a result which is poorer than expected? You get to the bottom of the reason(s). I've plotted up noise models which explain exactly what's going on, for all sorts of sensors and exposure times. I find this really fascinating.
What these models show is that with high readnoise cameras (MFD systems), you really need
very long exposures - to accumulate lots of light per pixel, which pushes you above the lower intensities where readnoise dominates. The problem is that hardly any MFD systems allow exposures longer than 1 minute! But one scenario where you get high pixel intensities, even in 1 minute or less, is the high background of moonlit/twilit skies: this explains why MFD moonlight and deep twilight shots can be competitive with DSLR ones.
But OTOH, longer exposures also build up more thermal dark noise, which again is higher in MFD systems (especially the older generation CCDs with bigger pixels) than DSLRs. While this can be subtracted with dark frames (in-camera or afterwards), two particularly egregious issues can arise:
(1) the temperature of the sensor can keep rising as first the photo and then the dark frame are taken, so the dark frame subtracts
too much dark intensity;
(2) there are many cameras whose firmware subtracts the "bias level" before writing out the RAWs, and which rely on a masked border of reference pixels to measure that bias level in each exposure; the problem is that these reference pixels also accumulate their own dark noise or amp glow in long exposures, which skews the estimated bias level upwards! As a result,
too much bias offset is subtracted, and if the sky pixels had low counts, some or all of them can go negative, which means they are set to zero in the RAW file. I've seen all this happen with my Kodak back, and a Kodak DCS720x too.
Some active thermal control would make a huge difference: keep the temperature low, or at least keep it relatively stable. I gather that keeping it stable is the secret to the PhaseOne Xpose+ backs.
Not subtracting any Bias level (the Canon way) would also make a huge difference: no pixels end up being zeroed, and even if the sensor was hot, separate dark frames can always be matched to the photos for post-exposure dark subtraction in software. I've been trying to get sample RAW frames from people with different MFD backs/cameras, in order to see which ones subtract the bias and which don't. (Message to all: please PM me if you'd like to contribute a sample RAW!).
So you see, there are so many "gotchas" with MFD systems and astrophotography - I sometimes think they were put there deliberately to thwart it!
In contrast, it is absolutely effortless with my 5DII: true RAWs with no bias subtraction, really low readnoise, really low darknoise...
That doesn't stop me from keeping on trying to improve results with my Kodak though!
Ray
