Astroblog

Astrophotographic results with the ASI 1600 mono cooled:

October 23rd, 2017 by Wolfi

NGC 891 with ASI1600MMC & TS 16" f/8 RC

Image author: Michael Pagitz (AMOS Observatory)

Telescope: TS 16" f/8 Ritchey Chretien Astrograph

Camera: ASI1600MM-Cool - gain 200

Exposure: 20 x 120 sek / binning 2x

More Infos about the ASI 1600 cooled cameras

Customer comment:
Yesterday, we have installed everything and got it running and, quite frankly, I have NEVER made a quick and dirty photograph which was that good. We have snapshot "cross-country" just for fun and taken NGC 891 with 10x2 minutes. The image is unguided, no darks, no bias, just stacked and the contrast adjusted very slightly, nothing else.

The ASI1600MMC works like a DSLR, attach, pull the trigger, ready.

ASI 1600 cooled - a declaration of love

Unfortunately I have too little time for astrophotography and also not the desire anymore to work through the night during the week.

That is why, for me, astrophotography unfortunately was in "pause mode" - until I have worked with the ZWO AI 1600 cooled for the first time. For me, it is plain fun with this camera.

Just yesterday, I have taken a picture of NGC 7331 with my 12" TS RC (focal ratio f/5.4). Instead of using approximately 300 seconds per integration and to stay until 2 am as in the past, I have exposed only 40 seconds per integration with the ASI 1600 (Gain 300). I finished at 10 pm and made a beautiful picture - everything recorded - even the color information. Now I am looking forward to editing ... thus astrophotography is just fun ...

Wolfi Ransburg - Munich, 12.10.2017

Deep-sky photography with cooled ASI cameras with high resolution - without guiding - "lucky imaging" principle

The cooled cameras from ZWO offer new and extremely exciting opportunities in the field of deep-sky photography. Finally highly resolving, smooth deep-sky images without great effort!

With conventional CCDs, there is still a race taking place to the longest single exposure times (some of us know that from imaging with film). But long exposure times for each single frame place various demands and come with a few problems:

Perfect guiding / autoguiding

Calm air for fine details

Furthermore, airplane and satellite tracks on long exposure single frames are very annoying.

With the new CMOS sensors which are have been installed in the cooled ASI cameras from ZWO, you can tread indeed entirely new paths. It does not count anymore how long you expose, but how often!

Two of the biggest advantages of these new cameras are the extremely low readout noise, despite the high frame rates, and the enormous sensitivity of the sensors. In combination, these advantages are the key to impressive high-resolution images. The signal-to-noise ratio is excellent. The more frames are recorded, the more noiseless and deeper the summary image will become. Basically it is the same as with planetary imaging. Hundreds or even thousands frames are stacked.

The limit is set mainly by the capacity of the hard disk. The following should be kept in mind: while a conventional 16 megapixel CCD camera need up to 15 seconds for transfering a single frame to the PC, a CMOS camera like the ASI1600 can transfer up to 23 frames per second (of course depending on the exposure time). At an exposure time of 5 seconds, the CCD would achieve just under 3 images per minute, an ASI1600 approximately 12 images! So we can take 300% more exposures in the same time.

There are several free programs for imaging and editing (stacking) which can help us here, for example Firecapture. Many know this software already from planetary photography, but Firecapture can also be used for deep-sky images. It can control the cooling of the camera, create a dark and also a flat and subtract automatically, save in FITS format, and even do autoguiding. But we only need the latter for this type of deep-sky photography if the mount is not polar aligned properly or other extreme tracking errors are foreseeable.

Apart from that, we now just have to: set the exposure time to a value between 1 and 15 seconds. Set gain not too high but also not overcautiously low (approximately between 100 and 350), and take at least 200 single images.

A concrete example (Fig. 1):

We use a ZWO ASI1600MMC, a 300/1500 Newtonian telescope plus coma corrector on an EQ6. A fast(er) aperture ratio promotes short exposure times, of course.

Firecapture is started and the image is focused on a bright star with still short exposure times (<1s). Normally, a usable star can be found in the preset image field.

A check mark is set at 16Bit (top left) and the file type FITS is chosen. Exposure time is set to 5 seconds, gain set to 321 (without particular reason). Thereby the image noise still stays within acceptable limits. Cooling can be set at pleasure or matching the power supply. Gamma stays at 50. USB traffic is not relevant in this case.

Now set the mark at Dark Frame. Firecapture then takes the selescted number od dark frames (don´t forget to cover the telescope aperture) and subtracts their mean value from the single images. The few hot pixels disappear. Amplifier glow is no longer an issue.

Set as exposure number limit are, for example 1000 frames. However, it should be kept in mind that, in the case of the ASI1600, a single image has a size of 16 MB. A few hundred single images quickly result in a few gigabytes of data. It should always be ensured that a storage location is selected which is able to handle this amount of data.



Abb. 1


When the sleep-stealing part is over and the hard disk filled with raw images, it is recommended (for example with the program Fitswork) to classify them and to sort out possible "duds" with satellites, airplanes, wobbles, etc. One could also stack the frames with Fitswork, but with Autostakkert!, stars and details become finer. However, stacking with Fitswork makes sense if manual intervention is necessary, for example if much displacement between the single frames.

Attention - if you want to load the single images in Autostakkert!, the file type "Image files" has to be chosen. Then a bright star is selected as anchor point and clicked on "Surface" under "Image stabilisation" (just under the "open" button). "Planet (COG)" can also work with bright objects which have an obvious centre of brightness, but mostly, "Surface" is more succesfull. At this point, you click the button "Analyse". (Fig. 2)



Fig. 2


When this has been completed, a bright star is selected as "alignment point" again. You can set several ones, but it is not necessary (except with image rotation or similar problems).

Time to click on the "Stack" button. Now the single images are stacked.

The final result is saved by Autostakkert! in the chosen image file format (in any case, choose a 16-bit format like TIF or FITS).

The difference between a single frame and the summarized image becomes astonishing. (Fig. 3)



Fig. 3 histogram enhanced



At the beginning of the processing, the summary image will seem quite unremarkable and dark, but with the help of a histogramm adjustment, the object of desire emerges quickly. (Fig.4)



Fig. 4


Important! For further processing, a program should be used which can handle 16 bit and the corresponding formats! Once again, Fitswork is well suited for that. However, many amateur astronomers will use Photoshop for the final touches.

We suspect that one could intensely debate how long the maximal single exposure time should be. However this rule applies: the shorter, the crispier. Short exposures are less disturbed by seeing. Guiding errors carry no weight. Therefore, the attainable accuracy is accuracy is significantly better than with exposures of several minutes.

On the other hand, the (areal) brightness of the object must be taken into account, after all we need a bit of signal on the sensor.

Of course, you can also expose several minutes with the cooled cameras from ZWO. There is nothing to stop you from taking short focal length photographs, for example of nebula regions. One can definitely use the ASIs like a ctandard CCD camera. But this slightly different approach with short exposure times offers many amateurs astronomers the possibility for imaging nebulae and galaxies with additional details. Given the increasing light pollution in many places, this might possibly the long-awaited life belt for this wonderful hobby.

This photo technique is only just beginning and still offers many opportunities. We will continue, get involved!

Your Teleskop-Service Team





The experience of a customer might be helpful:

1. With the Starlight software, the Starlight filter wheel runs trouble-free under Win 7 (64 bit).

2. A USB stick from Intenso (64 GB) can be plugged at the same time and is recognised immediately in the laptop!