More considerations for Novices whoare Choosing Accessories and Cameras for astrophotography:
•Special Planetary Imaging Considerations
•Cooling and Dark Current
•Guiding and Polar AJignment
•Adapters and Accessories
Putting it all together
Of the several possible sources of noise in an astronomical image, most of us focus on reducing one or more of the 3 chief sources: dark current, read noise and sky background Of the three, obvious ly, only the first two can be blamed on the camera! But even the third source of noise, sky background, could be handled at the camera without needing to move to the Andes mountains to find dark skies.
- Read Noi se
Read noise is noise that’s introduced into the picture since the sensor is read out after exposure. Unlike dark current or sky background noise, read noise doesn’t increase with integration time. It is 1 dose of noise per exposure whether it is a one second vulnerability or a hour-long exposure. It
Will increase as you add multiple exposures to make a final image
If a camera has high read noise, the imaging strateS)’. Is to raise the integ ration time before read noise becomes insignificant compared to other sources of sound Iike dark current or sky background. If you can increase your integration period by simply holding off the consequences of sky background (by state using a long focal length and imaging throug h narrowband filters) then dim c urrent becomes the limiting noise factor. Cooling reduces this offender, so by employing these several means, long deep exposures even in comparatively light polluted skjes are possible. Modern CMOS detectors have such low read noise (and such low dark current) that is not much of an issue any more.
It is fair to state that, today, the appearance of sensitive, low noise, CMOS sensors has altered the landscape when it comes to imaging philosophy(and directing ). QHYCCD creates both CCD and CMOS cameras using a number of popular sensors. Taking a look at the graph, below, it’s apparent that even at cheapest gain our CMOS cameras generally have much lower read noise than our CCD cameras. At high gain, where many of our CMOS cameras achieve around 1e-of read sound, this difference is much greater.
A few years ago, when CCDs were the detector of choice, imagers worked really difficult to build a guiding system that could monitor correctly forhours, not moments. The idea was to”bake in”the exposures to find the most of each framework and thus lessen the contribution of read sound when adding several sub-exposures. Some CCD cameras have read sound as large as 15 or 20 electrons. Even the best struggle to get read sound much under 9 or 10 electrons and nearly none have read sound below 5 electrons.
- The AVERAGE read sound (at large profit ) of those eight CMOS cameras in graph, above, is 1 electron! At lowest gain exactly the exact same eight models average only 3.6e-. With this kind of low read noise, shooting multiple shorter exposures is becoming commonplace, particularly in planetary imaging.
Shorter exposures mean less stress on the directing system but in addition requires sensors to have high QE to succeed in less time. This is currently the case too. Compare, by way of example, the QE of the CMOS cameras as well as the CCD cameras at the quantum efficiency graph shown in Part 1. The average peak QE for its three CCD sensors in that graph is approximately 56% while the average QE for its four CMOS detectors is approximately 85%. Even discarding the greatest CMOS and cheapest CCD, the distinction remains about 60% vs 80 percent and I think it’s fair to say this is pretty typical when you compare modern CMOS vs. older CCDs. Of course, there are exceptions, but generally it is the case that contemporary CMOS sensors are more sensitive than average CCD of days ago.
- Special Planetary Imaging Considerations
This image of Jupiter was shot several years back by Ed Grafton with an ST-6 camera and C-14 telescope. Read noise was 23e-and dark current was 10e-/p/s at-30C. Download time for a single frame was 22 seconds. The ST-6 had a monochrome sensor so this colour picture was created with separate RGB frames taken through color filters, each one taking 22 minutes to download.
The camera offered for approximately $3000(20 years ago).
Based on everything that’s been said in this guide, there’s nothing about this camera that would cause you to pick it for planetary imaging(or any other kind of imaging for that matter) but that picture is not too bad! I wanted to incorporate it here to illustrate two things: First, planetary imaging ain’t what it was and, second, that despite figuring out all of the fine details of what’s best, the individual carrying the picture along with his location play the most important functions in the outcomes. One should not get overly obsessed with specs and technical specifics.
The access to sensors that have both high QE and very low read noise has eased a different approach to imaging planets. In deep space imaging one normally needs longer exposures to capture the subtle aspect of very dim objects. Butfor planets, that can be much brighter, the key is to take exposures as short as possible to”suspend’ the viewing and stack thousands or hundreds of those pictures to extract the subtle detail.
Today, more often than not, planetary imaging is performed with a small, fast, uncooled camera like the QHY5I1462C. When compared with this ST-6 it’s 23X more pixels,46X lower read noise, and it can capture almost 3,000 images in the time that it took the ST-6 to obtain one frame. Plus it costs 1/10th too much. The 462C additionally has 2.9um pixels in comparison with the ST-6’s huge 23x27um pixels. To achieve the identical pixel scale with all the 462C camera, one needs a focal length about 1/10th of Eds-about 2400mm, or quite roughly the typical focal length of a C11 at prime focus. As stated earlier, the emphasis today is taking hundreds or thousands of pictures, thengrading and stacking them to bring detail out, like application to”lucky imaging. “This was simply not possible with CCD cameras that had high read noise and took 22 minutes to download a single frame to boot. Both utilize C-14 scopes with their planetary cameras. Christopher Go utilizes aQHY5III290M or even QHY5III462C. Damien Peach uses several cameras also has only recently turned in some extraordinary images using a QHY5l462C.
In most situations the pixel scale is roughly 0.15 arcsec.
While this might appear to violate the formulation for optimal pixel dimensions per focal length used for deep space imaging, their results clearly illustrate that planetary imaging is an exception that has its own set of rules and resolution is king.
Cooling and Dark Current Noise Cooling and dark current noise increase with exposure time and are therefore more significant issues in deep space imaging. They are mentionedtogether because one is dependent on the other.
Dark current is the generation of electrons in the sensor itself just by virtue of being turned on. It is called dark current because it will produce these electrons in the pixels even if you are not exposing the sensor to light during an integration(i.e., taking a exposure in complete darkness).
Dark current is usually expressed as electrons per pixel per second at a specific temperature.(e.g., e-/p/s @-15C).
One fortunate property of dark current is that it is greater at higher temperature and is reduced at lower temperature. This is why cooling CCD and CMOS sensors is a common design feature of cameras intended to take long exposures. Another fortunate property of dark current is that it creates a pattern that is quite repeatable. This means that you can take an image of just the dark current(a “dark frame”) and subtract the result from a light frame to remove the dark current pattern from an exposure of long duration, leaving only the random noise. Of course, the less dark current there is, the less noise will remain after subtracting the dark frame. Noise associated with dark current is also sometimes referred to as “thermal noise.”Dark current noise follows Poisson statistics, the rms dark current noise is the square root of the dark current.
Since dark currellt can be reduced in CCD and CMOS sensors by reducing the temperature of the sensor, nearly every astronomical camera intended to be used for long exposures features thermoelectric cooling of the sensor. Typically, the dark current present in the sensor is reduced by 50% for about every 6 to 7 degrees C of cooling. In other words, if the sensor has 10e-/pixel/second of dark current at 25 degrees C, and the temperature of the is lowered to 18 or 19 degrees C then the dark current will be reduced to only 5e-/pixel/second, and if the temperature is lowered another 6 or 7 degrees to around 12 or 13 degrees C then the dark current will be 2.5e-/pixel/second, and so forth.
When this article was first written, cameras having less than about 0.1e-of dark current at zero degrees C was considered pretty low. It meant that dramatic cooling of the sensor was not required to get very low dark current under typical operating conditions. Cooling an 8300 sensor to-20C, for example, reduced the dark current toonly 0.01e-/pixel/second. To reach comparable dark current with, say,a KAF-3200 sensor, it would require cooling to -40C and for a KAF-1001 with its large 24um pixels, such low dark current could not be reached even if the sensor was cooled to-50C.
Again, all of this has changed with the current level of CMOS technology. The chart below compares the effect of cooling on dark current of an 8300 sensor and Sony’s new IMX571 used in the new QHY268 cameras.
At zero C where the 8300 has about 0. Le-of dark current, the Sony sensor has less than 0.005e-About 20X lower! And where the 8300 reaches 0.01e-at-20C the Sony part reaches this exact same level of dark current at +10C.
This means is that in modern CMOS astro cameras, striking cooling is not as critical a necessity as it was for noisier CCDs in days ago. Cooling of CMOS detectors to-20C or -30C is sufficient to decrease the dark current to nearly insignificant amounts. At -20C by way of example, that the 8300 sensor has 0.0le-of dark current whereas the IMX571 has an incredibly low 0.0005e-.
4.Sky Background Noise
Sky background lighting or brightness is the number of counts in the image in regions free of stars or nebulosity and is due to city lights and sky glow. Elevated levels of sky background can increase the noise in pictures the same as dark present.
The majority of us live near or in urban areas where skies background is higher than it’s out in the country.
The sky background is frequently the limiting factor in taking images pictures, unless one has very dark skies or is imaging through narrowband filters. In our area, here in Santa Barbara, in f/6, we are typically limited to about 10-15 minutes of time before sky background overwhelms the dark current sound.
The maps of Europe and North America on the preceding page are coloured according to the brightness of the sky background along with the legend describes the brightness in terms of decreased visual perception of the night sky:
Blue-Degraded Close to the horizon
Green-Degraded to the zenith
Yellow-Natural sky lost
Red-Milky Way missing
The sky background spectrum(based on where you reside ) has a substantial spike in intensity across 5577 angstroms(approximately 558nm) right between the red and green filter passbands within an RGB filter set. Several years back, my spouse at the time, Alan Holmes, designed LRGB filters with a gap between the green and red filters to balance the high degree of O-III and H-a from emission nebula whilst still correctly rendering the continuum of background stars. When introduced, this design has been criticized by a few (who were making their own filter layouts ) but the results obtained using this layout were spectacular and it is intriguing to see today several top manufacturers using a similarapproach into LRGB filter transmission design(See for instance the graph of RGB filters exhibited in the section filter wheels and filters).
Another means to reduce sky background is to just use a red filter or LPR filter for monochrome imaging or narrowband filters whenimaging particular kinds of objects.Imaging using narrowband filters considerably reduces sky background by permitting just a narrow passband at selected wavelengths for nebula that emit light from the wavelengths of H-alpha and/or O-l. Having an H-alpha filter, for instance, exposures of half an hour to an hour aren’t a problem in our location at which 15 to 20 minutes would be the limit with no filters.Guiding and Polar Alignment
The need for guiding is frequently overlooked-or thought of just after everything else-when initially constructing a imaging system.Guiding isan extremely important function in astronomical imaging that should not be trivialized.Without good guiding you won’t get very good images.
Good guiding begins with a fantastic polar alignment of your range. If your polar orientation of off, it will introduce image rotation in pictures of extended duration. Even when the last image is made up of multiple short exposures, none of which appear to have much rotation, the result will demonstrate the shift in star places with time.
Picture rotation manifests as celebrity images appearing like little arcs rather than single points.Getting good polar alignment can be a tedious job when you do it without aid.
One of the matters that QHYCCD is famed for introducing to astrophotographers is the PoleMaster accessory that makes becoming highly accurate polar alignment a comparatively straightforward undertaking.
Simple enough to perform before each imaging session.The PoleMaster is a necessary accessory to improve your alignment.
Since the resolution of detectors increases with more and smaller pixels, guiding becomes more critical. Many imagers use either off-axis guiding or guiding through a separate guide scope. Ofthese two solutions, the off-axis arrangement provides the best accuracy as independent guide scopes are subject to differential deflection that may cause directing errors.A word of warning, however.
Many inexpensive radial off-axis guiders have a serious problem in a little prism or mirror is used to pick off a tiny portion of the light to direct to the eyepiece. Guide stars tend to be dim, and one is forced to rotate the assembly to discover a guide star. When one moves the meeting, the star motion directions(in response to guiding inputs) also rotate, and one is made to recalibrate the autoguider rather often. Also, the dark stars induce some autoguiders to require long exposures, devoting their ability to compensate for periodic errors and induce hops. In a nutshell, many radial guiders are awkward to use.
QHYCCD offers several sizes of off-axis guidersto accommodate various camera and sensor dimensions. These OAGs use big enough prisms to get around the issue mentioned above and the method of attachment into the camera or filter wheel is rock solid.
The other main alternative is to use a separate guide scope. The issue here is differential deflection-slight tilts or wobbles of the principal mirror can alter a star position on the imaging CCD. The mirror tends to shift since the gravity loads change as the telescope counteracts the earth’s rotation.
So,how does all of this affect guiding decisions?
Well, very low noise and high QE make several short exposures a workable option to single long guided exposures. In this case good polar alignment is still critical but it is easier to handle good directing via an exposure of a few minutes instead of 1 hour. And if you really do getsome end or other unexpected bump in directing, its less painful to throw out a short bad frame than to learn after a long night of guiding that you had a problem!
I should start this part with a caveat. It was true that if you wanted to shoot decent color images, you would obviously choose a mono camera and take through LRGB filters. It alsoused to be the case that placing an automatic transmission in a high-performance sports car was akin to wearing tennis shoes with a tuxedo. So called”one-shot”colour cameras were for novices or the idle. Color CCD cameras were normally much less sensitive than their monochrome counterparts.
At my former company, maybe 1% of cameras sold with the favorite 35mm sized 11002 detectors were the one-shot colour version. The same is true of this very popular 8300 that came incolor. In both cases the peak QE for RGB was between 30% to 40%. Imagers only preferred to take LRGB to receive the best outcomes.
This has now changed. It’s still true that using afilter wheel with select filters, one has more control over the passbands and color balance, and of course the ability to utilize technical filters for certain kinds of imaging. But just as most high-performance sports cars now have automatic transmissions as standard equipment that the incredible sensitivity of back-illuminated color detectors, driven by the luxury consumer cameramarket, has generated imaging using colour sensor a lot more commonplace and rather respectable. The latest QHY410C is a back-illuminated variant of the QHY128C. With 5.94um pixels it is predicted to be the most sensitive colour camera we’ ve ever made.
OK,having said this,why buy a filter wheel with filters instead of a color camera? There are lots of reasons why you may like to possess external filters: Firstyou can pick filters with the passbands that you would like, and you can freely change them. Secondly, filters made for astronomy generally have greater transmission ratios than the filters constructed onto a color detector. Third, custom filters, like emission line filters, IR filters and photometric filters may be used without interference by the built-in RGB filters over the sensor. Filter wheels come in a variety of capacities, usually 5 places for LRGB and clear, or 7 places (or much more ) for LRGB and narrowband filters, etc..
Though the Bayer matrix coating of RGB filters on sensors have made improvements through the years, as CMOS sensors have enhanced, theexample shown under a KAF-16200 CCD detector that is available in both monochrome and color highlights several factors in favor of working with a mono camera and filter wheel for advanced color imaging. The QE chart for both the mono and colour edition of the sensor are out of the sensor manufacturer and the outside filter transmission features are from Antlia, an astronomy filter maker of high quality filters.
It is clear that the on-chip filters have a significant effect on the General QE of the sensor.Using external filters in this Situation Seems to Enhance this
By 30 percent or more(only eyeballing). Additionally, while both kinds of filters capture the blue-green O-II emission lines around 500nm both with the blue and green passbands, the external filter set does so with much higher efficacy. Another obvious difference is the gap I mentioned before involving the green and red filters of the external filter set. This means that the Bayer filters would catch the sky background(light pollution) around 578nm with both the red and green filters, but the external filter set wouldn’t find this section of the sky background at all. Adapters and Accessories It may also be extremely frustrating, especially when you’re building a system made from bits from various manufacturers who are each trying to make their bit fit as many distinct configurations as possible.
QHYCCD produces a set of adapter rings that may space pieces of a system just right in a variety of configurations. Lately,a type of”standard”has evolved that needs 55mm of backfocus for field flatteners or other back optical elements of several poplar types of scopes.In response to thisQHYCCD now comprises a pair of adapter rings using each camera/filter wheel and/or OAG configuration to permit the user to achieve this magic number without having to figure out what adapters are necessary. The most essential thinghere is to do one’s homework before hand and make sure that all the pieces you would like to combineare harmonious with all the optics you intend to use.
For instance, Canon and Nikon camera lens adapters fit various other items, even filter brakes. However, certain camera, OAG and filter wheel mixes need more space than the backfocus need of this lens permits and infinity focus might not be possible though the components can all be mechanically screwed together.
So planning in advance can save you a bothersome gotcha!Software compatibility
Unlike DSLRs, virtually every astronomical camera is operated with an external computer of some type. So, no matter how great your camera might be, if it does not have great control applications, it is just a costly bunch of wires and metal. Getting focused, framing a thing that is difficult to see, calculating the results, etc, all go into the finished result. Fantastic software makes theseand other tasks simpler to get right. To earn QHY cameras compatible with the broadest variety of third part software, we provide both native andASCOM drivers in addition to drivers for TheSky. As of this writing, we are working on new drivers for Software Bisque’s Linux based FusionSystem and are about to launch a new version of the compact StarMaster controller. Warranty
Probably the last thing you want to think about is what happens if my camera fails? QHYCCD cameras possess a two-year warranty. However, other things can occur, too. ; gremlins indoors (no, not literally). The point is you wish to be familiar with your purchase and know that if lightning does strike, you’ve got some expedient way to getting a fix or replacement without flying into Timbuktu.
QHYCCD is aware of this concern and also for this very reason we have created a stock of fresh cameras at the US for easy and quick replacements when the delivery drops off your camera the back of the truck. Additionally, we are also setting up a complete repair facility here in the US for in warranty and out of warranty repairs therefore cameras do not need to be sent overseas for routine repairs and maintenance. This repair facility ought to be operational by vacation time this year.
Putting It All together Do not obsess over quantity too much. Use them for you at the ball park and then PLAY BALL!
Make a few big decisions up front, just like do you wish to do color imaging using a color camera with a mono camera and filter wheel. The color camera is simpler and cheaper-the monocamera and filter wheel filters is more equipment, more to go wrong and much more costly. But it’s also more flexible and offers benefits particularly if you’re in a light polluted area and/or wish to use narrowband filters. The best way is off-axis. Additionally, consider a PoleMaster for great polar alignment.This will save you ours of frustration
Attempt to narrow down what it is that you want to know more about imaging. If you would like to start just imaging planets, then save your money and get a planetary camera.
They’re small and relatively affordable and good. You may always use it to get a guider also once you choose to do deep space imaging. If your interest is solar or lunar imaging, rememberthat these are both about 1/2 degree in diameter and then select a sensor/scope combination that may accommodate a full disk.Then you can use a barlow for greater magnification of surface features.The moon and sun are also relatively bright objects along with the requirement to maximize the sensitivity of your camera by simply matching the pixel dimensions to the focal length is slightly meaningless.
Should you know you want to picture deep space,use the charts in Part 1 to select a sensor size and pixel size that’s fantastic for your intended targets and your scope. If you don’t understand, buy as much sensor because you are able to afford. You’ll receive higher resolution and shed very little sensitivity if you sample FWHM stellar pictures with three or four pixels instead of two. Many beautiful wide field images are taken with cameras that have pixels that are”too big”and many fine deep space pictures of spiral galaxies are taken with pixels which are”too small. “A number of the very beautiful astro pictures are taken with camera lenses. They’re a great way to begin and I strongly suggest it. They are less difficult to direct and create very pleasing results while you’re learning how to use your equipment.
And for planets, over sampling is now the order of the afternoon when piling hundreds or even thousands of pictures to tease out detail. Most importantly, have fun and enjoy the night skies!