Telescopes

How to deal with Newton’s rings in CMOS imaging?

Whilst I cannot say that I’ve done a very in-depth research on the subject, I’ll try to provide some sort of guidance and advice about how to deal with Newton’s rings in CMOS imaging.

Newton’s rings are generally accepted as an interference phenomenon, appearing on surfaces where the light is being reflected from two nearly parallel surfaces (i.e. bottom and top of a thin film). Note, that I said “nearly” parallel, means not exactly parallel, which is an important moment in the explanation. But as I mentioned I don’t really want to get into the depth of the theory, so lets move on.

In conjunction with CMOS imaging, I’ve also heard that one of the reasons of appearance of Newton’s rings might lay in the way CMOS imagers read out the information from the sensor, i.e. the use of rolling shutter, and the same phenomenon is therefore non existent in cameras with global shutter, like the Celestron Skyris planetary imagers.

UPDATE (8 Sep 2015): ZWO now also came out with a new range of cameras that feature global readout and thus don’t suffer from this phenomenon. These are the ZWO ASI174, ASI178, ASI185 and ASI224 planetary imager cameras.

Newton’s rings in imaging might appear as a result of reflection of light from the two surfaces (inner and outer) of the optical window that covers the sensor or when used for H-alpha solar imaging, possibly a result of similar reflection from the surfaces of the H-alpha filter. Never-the-less we have quite a few customers using the ASI120MM cameras for solar imaging.

Some experienced users recommend to use various techniques, like a tilting device. Tilting seems to be a good idea, but I think there is also another, possibly easier solution that should work well for planets, if you let the object slowly drift through the imaging session, the Newton rings should not appear or should disappear by the end of processing.

I’ve used AVIStack 2 myself, although I haven’t tried the camera on planets yet, I’ve only used it for solar imaging with a Lunt Calcium-K telescope.

I’m not sure if there is any difference between ASI120MM cameras in a way that some would provide Newton’s rings and others won’t, but certainly we haven’t heard about this issue yet from other customers and we sold quite a few of the ASI120MM cameras.

As I mentioned above, from a theoretical point of view, CMOS cameras are more likely to produce these rings than CCD cameras, but CCD cameras of similar specifications are usually about twice as expensive… Interestingly, Damien Peach didn’t mention in his review in the Astronomy Now magazine (September 2013) that he had any problems with Newton’s rings with the ZWO ASI120MM camera that he tested, so I assume that if he had he would have certainly mentioned. I think I’ll drop him an email and ask him to confirm this…

Now, although ZWO advises to use tilting for H-alpha solar imaging,¬† lets not jump into it straight away… lets try couple more simple things first:
1. make sure that you are not plugging the camera into a USB3.0 port
2. you might try drifting the object
3. you might try recording videos with various setting sets; it seems that only a certain combination of settings will produce Newton’s rings, as if some sort of resonance would occur (like when you drive a car and it starts vibrate at a certain speed when your wheels are not well balanced). Most likely you will be able to get rid of Newton’s rings by playing with the gain.

I believe these above are simple solutions that can be achieved/tried easily, but if you will not be able to improve your imaging you could also look at other alternative cameras, i.e. Moravian Instrument’s planetary imager CCD cameras or the new Celestron Skyris cameras that are CCD based and reportedly lack any Newton’s rings as the readout happens momentarily in those CCD imagers due to the use of global shutters, not sequentially like in the CMOS based cameras of ZWO where they use a rolling shutter.

Update from Mar 2015:
ZWO just came out with a new, exciting CMOS camera, the ZWO ASI174 that features a global shutter that was featured only in CCD cameras until now. This fully eliminates Newton’s rings and due to the large sensor and high frame rate, it is recommended for imaging the Moon and the Sun (don’t forget to use a solar filter when imaging¬† the Sun!).

Check out also David Cortner’s blog about his success and how he solved this issue: The Slow Blog

It would be really great to hear from other users about their experiences of these otherwise excellent CMOS cameras from ZWO.

 

Article by Zoltan Trenovszki
www.365Astronomy.com

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Posted on October 8th, 2013.