New Broom


Converting from a DSLR to the ZWO1600MM-Cool camera feels in part like I’m starting out all over again and is exciting.  I’m getting used to the new field-of-view and its implications for framing, which is complicated further by using separate mono filters that can often make it difficult to identify on screen the chosen imaging target.  I’m also learning to use Astro Photography Tool (APT) for image capture, which is turning out to be an excellent programme, though at times somewhat idiosyncratic in nature.  APT contains useful Histogram and associated Stretch tools, which when applied to test shots prior to data capture can reveal underlying target detail which is otherwise unseen and thus enables suitable framing to be chosen.

With a set mount location, better polar alignment, calibrated PHD2 and a basic star alignment model established in EQ-ASCOM early in the summer, the process of imaging has now become much more efficient. After adding a few supplementary alignment points local to the target and some other minor adjustments, I have recently been able to set-up and start imaging in much less than one hour; excluding the physical set-up, imaging is possible within 30-minutes.  Contrast this with one or two hours when previously using various Synscan handset procedures and setting up the DSLR camera, I think it’s fair to say I have at last crossed the proverbial Rubicon!  Using a cooled sensor and compiling a calibration library has also been very helpful in streamlining imaging sessions, which all-in-all has made my astrophotography much more productive – qualitatively and surprisingly quantitatively too, despite all the extra subs and calibration required.

I’m currently working through familiar targets with Ha-OIII-SII subs to produce Hubble Palette based images.  It’s true to say that the use of narrowband filters has also been nothing less than a revolution for my imaging, in terms of process and results.  I’m particularly pleased that I purchased the ZWO x8 EFW and matching LRGB + narrowband filters with the new camera – 31mm parfocal filters also help minimize the need to re-focus for different wavelengths.

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The Witch’s Broom (NGC 6960): WO GT81 & Canon 700D camera + 0.80x Focal Reducer | 20 x 90 sec + calibration @ ISO 800 | October 2014

It’s about 4-years since I embarked on my nascent astrophotography journey and soon thereafter I first attempted to image the notoriously difficult Veil Nebula – which of course is why I had to try.  At the time I did not know one end of the Veil from the other of this very large but faint and widely dispersed supernova and was pleased to achieve a recognizable image of the Western Veil or Witch’s Broom (NGC 6960).  This July I set out to re-image the same feature for the first time using the ZWO1600MM-Cool camera in narrowband.


The Veil Nebula AKA Cygnus Loop: Ultraviolet view ref. NASA

The full extent of the Veil is broadly demarcated by the Western and Eastern Veil Nebulae, with other generally more diffuse but related areas in between.  All-in-all the entire feature is some 3o or 110 light-years across.  The ZWO1600MM-Cool has two notable features that differentiate it from conventional CCD mono cameras, those being larger sensor size and high sensitivity when using only short exposures.  Unity of the sensor is 139 but like many other new users I’ve successfully been using a much higher Gain, in my case 300 with an Offset of 10; others have reported very good results as high as 600 Gain at just 30 second exposure, which though data heavy is very useful in helping to circumvent the UK’s fickle weather conditions and the need for perfect polar alignment, which was a major factor in deciding to purchase this type of mono camera.

Being still unfamiliar with the camera’s field-of-view using the William OpticsGT81 refractor and x0.80 focal reducer combination, on this occasion I centred the Broom just above the centre of the frame, thus adding the possibility of capturing other parts of the nebula located to the east and just below the Broom in this case.  As I’ve yet to master or even attempt mosaics or a Meridian flip with plate solving, for the moment my imaging is limited by the transit period defined from about 110o east to the Meridian or similarly to the west and between a 30o to 80o azimuth, which equates to just over 2 hours per target each side of the Meridian.  The nature of the object and lack of darkness at this time of the year can often restrict this available time even further.  Notwithstanding, on this occasion I was able to obtain 20 x Ha and 18 x OII 180 sec subs in order to produce a final bicolour image.


The Witch’s Broom NGC 6960 & Pickering’s Triangle NGC 6979 Ha-OIII-OIII Bicolour: WO GT81 & ZWO1600 + 0.80x Focal Reducer | 180 sec x 20 Ha & 18 OIII Subs + calibration @ Gain 300 , 10 Offset & -20C | 31st July 2017

With a good set of subs the stacked and collated Ha-OIII image has turned out well, with nebulous filament details typical of the Veil that had not previously been clear when using a modded-DSLR camera now clearly visible.  Furthermore, on this occasion the aforesaid framing included detail of both the Broom and as something of a bonus Pickering’s Triangle, altogether forming a very pleasing image.

At this stage I would usually crop the Witch’s Broom and finesse the resulting image in Photoshop.  However, following a prior line of thought and questioning on the SGL Forum, this time I tried to use the Drizzle_technique during stacking to enhance the quality of The Broom itself.  Unfortunately it didn’t take long to discover that, as so often is the case with astrophotography, drizzling is a good deal more complicated than just placing a check in the Drizzle box.

Originally developed for use with the Hubble Space Telescope, drizzle is a digital processing method for the linear reconstruction of under-sampled images, thus improving the apparent resolution of the image.  Deep Sky Stacker is an excellent piece of software and provides the facility of x2 and x3 Drizzle but unlike some commercial packages has limited memory that is used for this task.  As a result after attempting to use Drizzle whilst stacking a number of times in DSS, the process crashed at the end of each sequence.  Finally another SGL Forum query provided the answers: (i) DSS lacks memory required to stack and process the original sub using Drizzle, but (ii) Drizzle will work by applying the Custom Rectangle Mode in DSS to a select a specific, smaller area of the sub.  It took me a while to figure this out but eventually I manged to process the Witch’s Broom area of the image successfully.  I’ve concluded that Drizzle is certainly a feature worth deploying during stacking from time to time but only where the main target is poorly sampled and where the specific object will fit within the Custom Rectangle Mode defined by DSS – it should also be noted that the resulting data size also increases very substantially when using Drizzle.


Witch’s Broom (as above) + 2x Drizzle

All-in-all it continues to be a great surprise just how different and often complex the techniques are with a mono camera and filters compared to a one-shot DSLR camera, both during capturing and processing.  I was previously aware of these issues and some related shortcomings but so far the results have justified the additional effort; I’m not sure I would say the same about a conventional mono CCD camera, that requires much longer imaging times which in my opinion are not suitable for the average user and weather conditions in the UK.  I know there’s still much to master – Plate Solving + Mosaics + Meridian Flips + Sequence Generator Pro etc. – and I’ll soon need to start a completely new alignment star model for winter and recalibrate PHD2 guiding but the past few months have really been good fun and very productive.


A New Palette


It still remains early days with the new ZWO 1600MM-Cool camera but the initial signs using the narrowband filters have been very promising and frankly good fun.  I’ve been surprised at the benefits gained using these filters and especially the new and exciting possibilities it introduces to imaging. But there’s more.

When first using a digital camera for astrophotography it came as a surprise to discover that the colours come from an RGB Bayer Matrix placed in front of the sensor.  The so-called One Shot Colour camera or OSC, provided me with many decent images and when I recently started to think about getting a better camera another OSC seemed the way to go.  Their major benefit is that you are able to just take, as it says, one shot – or multiple colour shots for stacking.  Of course there’s still much to learn when starting out but the process is relatively simple and a decent set of subs and calibration frames can be obtained in a few hours or less; subsequent processing is also straightforward with one set of subs and no more than three sets of calibration frames.

Notwithstanding, when looking at more specialized OSC cameras, it soon became clear that the benefits of moving from DSLR were limited.  Another surprise with astrophotography is that the best images are obtained using a mono camera, in combination with colour and if required narrowband filters.  Hitherto, this has meant precise guiding in order to obtain long exposures using a very expensive CCD camera.  However, last year everything changed with the introduction of completely new technology in the form of a cooled CMOS camera, which I and the astrophotography community are currently getting to grips with – the aforementioned ZWO ASI1600MM-Cool.

Whilst more complex to use than a DSLR, this camera seems to be something of a game changer for astrophotographers in that: (a) it has a larger more sensitive sensor than a CCD camera, (b) it’s cheaper (though not cheap), (c) because of its sensitivity imaging requires only relatively short exposures, thus reducing the need for ultra-precise guiding as it gathers light more quickly than a conventional camera.  It was the latter feature that convinced me to go ahead with the camera and it is already clear this was not a mistake.

I often complain about the excessive cloud cover we suffer here at Fairvale Observatory, which can often prohibit imaging for weeks or even months on-end.  The advantage of imaging quickly when the clouds do eventually part is therefore a major factor for me.  If the camera has one problem it’s probably the vast quantity of data generated, putting huge demand on computer memory and processing power – but it’s worth it.


LRGB imaging is the main format used by most advanced astrophtographers, after all the resulting colours and detail achieved can be spectacular and it was now time for me to give it a try.  I was astonished to learn that some 80% of the detail in an image is in the luminance and only 20% colour, so the advantage of LRGB imaging becomes immediately apparent.  I also purchased the ZWO x8 EFW and matching 31mm filters with the camera and initially had some problems getting it to work.  However, once sorted it has been a pleasure to use and makes image sequencing with different filters a piece of cake.

At the moment I’m using Astro Photography Tool (APT) for capture and apart from my own lack of LRGB imaging experience and some misunderstandings of the software, it works very well.  A full LRGB sequence is easy to set-up beforehand and after that the imaging automatically looks after itself!  I carry out most of the calibration frames manually but this too is generally quite easy; I did struggle at first with the flats, which is a quite different method compared to a DSLR but once I got the hang of the APT Flats Aid – courtesy of the APT Forum members – all was OK.

Notwithstanding, at first I was concerned by large concentric light and dark halos in the flats.  I have long used the combination of a flowerpot, LED and T-shirt for taking flats, which has always worked well.  I therefore expected the flats from the new camera using APT would not be a problem.  However, as mentioned taking flats with this camera is a whole new ball game which has taken some time to master, or at least learn.

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The flower pot flats set-up has always worked well before.

Unlike the DSLR, which just requires changing to the AV mode, it is necessary to calculate an exact exposure – to five decimal places – using a mid-range ADU setting of between 20,000 and 25,000 for each different LRGB filter.  Sounds awful and it does generate a lot of frames but now I’ve got the hang of the Flats Aid it is much easier and very effective; on average I have so far found the required exposures vary from 0.00145 sec to 0.00792 sec.  I did discover some minor light leakage in the optical train when testing the flats taken in daylight but by doubling up the T-shirt material and adopting a more careful set-up of the flower pot directly in front of the telescope object lens, the halos now seem to have disappeared.

The other new ‘toy’ with this camera is cooling.  Despite the camera’s low noise and high sensitivity, like all digital cameras there is an inevitable amount of unwanted signal and currents created by the sensor.  Apart from applying calibration frames, this problem can be significantly reduced by cooling the sensor when imaging.   APT also has cooling and warming aid controls to avoid thermal shock but at first I struggled to get these working – turns out I forgot to plug the power lead in!  After that it has worked like a dream and I am now routinely imaging at -30oC.  The one drawback is that cooling down and warming up adds another 10 or 15 minutes to the set-up and take-down times, which can be annoying at 2.00 a.m. when you want to get to bed!

Unfortunately, now I have this camera working suitable imaging targets are in short supply for the moment.  At first I was able to carry out some narrowband imaging of the remaining late winter nebulae just before they disappeared for the season but I must now wait for a few months until later in the year for similar objects to appear again. Meanwhile, whilst the night sky in spring abounds with some wonderful galaxies, most of these are really too small for my set-up to resolve properly.  However, in the absence of larger DSO objects I have had to make do with these and have been pleasantly surprised by some of the results obtained.

The outcome of Leo Triplet and Markarian’s Chain were particularly good and demonstrate the superior abilities and power of the camera.  Compared to the Canon 550D, when used with the William Optics GT81 refractor and a x0.80 field flattener, the ZWO 1600MM-Cool’s sensor produces a notably larger image with much greater sensitivity.  As a result detail of all three galaxies that form the Leo Triplet M65, M66 and the end-on view of NGC 3628 is quite apparent and in another image, the spirals of the very small M61 galaxy can be clearly seen, though the quality is poor.


Leo Triplet – M65 (top right) M66 (centre right) NGC 3628 (lower left) Leo Constellation: William Optics GT81 & ZWO 1600MM-Cool + x0.80 field flattener | 60 sec 20 x L, 5 x RGB + full calibration, Gain 300 Offset 10 @ -20C | 21st March 2017

As expected the size of the data set from LRGB imaging with the camera is prodigious and furthermore, requires great care to organise before processing if mistakes are to be avoided.  In the case of Markarian’s Chain I experimented with binning – only to learn later that it has no effect with CMOS sensors – which reduced the file size of the RGB and calibration images.  Nonetheless, in all there were still 140 images with a combined file size of 1.6 Gigabytes!  Notwithstanding, the benefits of LRGB imaging with this camera and EFW outweigh such problems and nowadays an extra storage disk is relatively inexpensive.  Only processing power could perhaps be a problem if using an older PC but my 64-bit + i7 chip +16 GB RAM laptop computer has so far dealt easily with data processing very well.

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The difference between imaging and processing with a DSLR and the ZWO CMOS mono camera has been much greater than I anticipated and required more adjustment which I am still dealing with – the learning curve is steep!  Notwithstanding, the experience previously gained using a DSLR has proved invaluable and I would not like to take-on LRGB imaging from scratch; I’d like to think that my adage of walk before you run once again has paid off.  This camera is very exciting and I am confident that when the autumn and winter skies eventually return the new palette now in my hands will reap great rewards – can’t wait.

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Markarian’s Chain | Subs 60 sec – see table for details Gain 300 Offset -30C | 18th April 2017


M61 piral Galaxy in Virgo Cluster | William Optics GT81 & ZWO 1600MM-Cool + x0.80 field flattener | L 30 x 40 sec RGB 3×20 sec + full calibration Gain 75 Offset 15 -30C | 2nd April 2017

First Light


If nothing else, I have learned from life – walk before you run.  I have spent the past three years trying to learn about astronomy and astrophotography using a DSLR but late last year decided it was time to raise my game.  I was reluctant to go to a CCD mono camera as it involves greater complexity and, perhaps more significantly, during the aforesaid period clear skies have been in very short supply – making long, guided exposures over protracted periods something close to impossible where I live.  However, with the recent advent of the new CMOS cameras and their rave reviews, against my better judgement I took the plunge and bought a ZWO ASI 1600MM-Cool with a matching ZWO x8 EFW and 31mm LRGB, Ha, OIII and SII filter set.

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Wow! As expected, it’s a whole new world compared to DSLR and, as well as continuing week-after-week cloud cover, I have been battling with numerous set-up and processing issues. Still, rule number one with this hobby is patience and perseverance and I’m pleased to say I have just achieved First Light with the camera.

At first I set up the camera indoors on a tripod to test the equipment and quickly found that the camera and EFW would not work together; in addition to a USB 3.0 data socket, the camera also has two useful USB 2.0 sockets which can be used to power and control other equipment.  After more than 3-weeks, much help online and a mixed response from the manufacturer, ZWO suggested I try another USB driver which they sent me and it worked – so why not supply that in the first place or ensure users are aware of the issue and provide a solution from the outset?  Whilst the actual equipment is well specced, well made and well priced compared to CCD camera – though far from cheap, I found ZWO’s web-based technical guidance and general support poor and would expect better with such expensive equipment.  In retrospect, perhaps it is not a surprise as I had a USB problem when I purchased their ZWO ASI 120MC camera, with a similar response and outcome i.e. there is a theme here, which is a pity as their equipment itself is innovative and very well made.

During brief periods between the clouds I subsequently managed to try-out the camera in order to understand focus, capture and processing. Despite prior experience with the DSLR and software, this turned out to be new territory which I am still exploring. Briefly:

Focus – With the new camera I was faced with two fundamental focusing issues:

  • Establishing the correct optical train – I want to continue using the William Optics x0.80 focal reducer with the ZWO camera, thereby increasing the speed of the scope from f5.9 to f4.72. I therefore purchased a Canon EOS adapter to use with the existing EOS convertor which has previously worked successfully with the DSLR.  Despite the apparent complexity, the resulting set-up is within 0.50mm of the optimum distance and seems to work OK – with one exception. Whilst the locking pin on the EOS adapter works i.e. it locks, there is some unacceptable lateral play, which for now I have solved with the addition of a piece of electrical tape! I’m advised this is normal for such adapters but it seems like a poor product to me if this is the case.    ZWO cam_EFW_FF_annotated (Large)
  • Operating focus – after months of battling with focus when I started out DSLR imaging, I eventually discovered the Bahtinov mask and assumed this wonderfully simple method would work just as well with the new ZWO camera; of course, after my initial trials it was apparent this wasn’t going to be the case.  Not that the mask doesn’t work but in order to achieve good focus with the smaller pixels of the ZWO ASI 1600 requires much greater accuracy, which I’m pleased to say has now been achieved by using APT’s Bahtinov Aid (based on Neils Noordhoek’s Bahtinov Grabber), so that I am now getting much better results.  However, as the focus point can change with seeing conditions and when using different filters, it is apparent that I’ll need to return to this matter again to finesse the operation, probably by using an electronic focuser.

Capture – At the heart of my philosophy is the KISS principal – Keep It Simple Stupid! When working with the DSLR I therefore only ever used the Canon EOS Utilities software for image capture – it is simple, did what I needed and worked.  Moving to a mono camera with filters, the increase in complexity is exponential and inevitably requires more sophisticated image capture software.  Judging by the experience of others Sequence Generator Pro seems to be one of the best low cost programmes that will do this job and I have purchased a copy for US$99 and the accompanying Framing and Mosaic Wizard for an additional US$39.  However, in applying the KISS principal during the early stages of getting to know and understand the new equipment and processes, for now I’m using Astro Photography Tool (APT) – something I’ve had for a while but not used before.  It is a very capable programme that manages sequencing, cooling, filter management etc. well, with excellent support from its author Ivo but importantly seems easier to use than SGP, albeit inevitably with its own idiosyncrasies.  It’s early days but so far so good.

Processing – Mono images differ in a number of fundamental ways to DSLR other than just colour, which requires a quite different approach to processing and post-processing, in particular:

  • the images are FITS not RAW;
  • mono images are taken with a variety of filters which subsequently need to be compiled.

Despite successfully working with Deep Sky Stacker (DSS) for some time, I was not aware of any changes required when processing FITS files, which are the product of the ZWO 1600 camera; useful pre-assessment of the files can be carried out using the ESA/NASA free Fits Liberator software.  As a result my first try of the Beehive Nebula based on just Luminance subs was covered in bizarre ‘green spider-like’ artefacts after stacking in DSS.  These disappeared when transferred into Photoshop but then became covered in Bayer matrix-like coloured squares!

Thanks to help from the SGL Forum it was apparent that I had failed to turn-off the FITS colour option in Settings before stacking – unchecking this and restacking immediately resulted in a half-decent image of the open star cluster.  Meanwhile, since purchasing the ZWO camera I have read and watched numerous videos on post-processing and using LRGB files available online for practice, I have been able to start experimenting with this technique prior to obtaining my own data from the new camera.  It is much more complex and I’ve got a long way to go but the experience gained from DSLR processing has nevertheless helped immensely; walk before you run pays off in the end!

And so last week I managed my first reasonable image of the Rosette Nebula (NGC 2244) using only Ha-subs, which this object has in abundance.  I consider this marks the camera’s First Light and am pleased with the result but realise there’s still much more to learn, weather permitting!

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Rosette Nebula in Ha | William Optics GT81 + ZWO ASI 1600MM-Cool & x0.80 focal reducer + guided | 15 x 180 secs + darks & bias calibration Gain 300, Offset 10 | 21st March 2017

  • This guided image is 15 x 180sec Ha-subs + darks and bias, gain 300, offset 10 + minor stretching in Photoshop. The corners – particularly the bottom right – look like there may be some vignetting in the stacked image?  I haven’t managed to achieve any decent flats yet (another story) but I assume these would help eliminate this effect? However, I am surprised as I’m using the ZWO x8 EFW with larger 31mm filters, which with the focal length of 382mm (f4.72) should not result in vignetting.
  • Clearly my experimentation is ongoing with this new technology and I particularly need to understand better what is the ‘best’ gain and offset setting for different types of objects.
  • Whilst the CMOS chip is bigger than standard CCD sensors it is still smaller than a standard DSLR and with smaller pixels too (3.80nm v 4.30nm), the result when used with the focal reducer is a 29% reduction of the field-of view from 3.34o x 2.23o to 2.65o x 2.0o, which on-screen translates to increased magnification and allows me to get at some of the galaxies which hitherto have been too small; the benefit is minor but is worth having nonetheless.




From Cloudy Nights Forum – compiled by Jon Rista

All-in-all the ZWO ASI 1600mm-Cool is an excellent piece of kit that has the potential to open up new vistas for my astroimaging.  At a fundamental level it does all the basic stuff very well and the addition of cooling is a major improvement which  reduces noise still further.  I’m particularly looking forwards to experimenting with narrowband and bi-colour imaging – not least in order to keep working when the Moon’s about, unlike broadband imaging.  However, the cameras intrinsically low read noise and ability to capture fine detail using only short exposures is surely set to mark the next revolution in astrophotography and furthermore reduces the need for very precise guiding; it’s clear the other manufacturers are scrambling to catch-up with this leading edge technology.  Timing is everything in life and I’m pleased to be part of this hobby at such an exciting moment.