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ZWOASI120MC as a DSO guide camera?

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#1 Naptown Larry

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Posted 23 October 2021 - 10:26 AM

I'm just putting together all the components of my rig. So after purchasing the high ticket items, Eq6-r Pro mount and SW 200p quattro scope. I wanted to give my wallet a break by purchasing an inexpensive ZWO ASI120MC with the idea that I could use it initially for planetary imaging, and then potentially after getting a better imaging camera, I could use it as my guide camera.

But now I'm reading that a DSO guide camera needs to be able to deal with up to 10 minute exposures and that mono is also much better for this purpose.

Did I waste my money on the 120MC? And what would be a reasonably budget friendly ZWO* guide camera that can deal with long DSO exposures?

*I'm staying in the ZWO line because at some point I plan to get an ASI Air Plus when they become available. And my understanding is that the ASI Air doesn't support non-ZWO cameras.

Edited by Naptown Larry, 23 October 2021 - 10:27 AM.

#2 Ken Sturrock

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Posted 23 October 2021 - 10:36 AM

I'm not sure from your post if you get the process, so please forgive me if you know all of this.


Some DSO images, especially when shooting narrow band, may require quite long duration exposures on the IMAGING camera. The guide camera shoots a much shorter exposure as it repeatedly takes measurements of a bright star for the purpose of correcting the mount's tracking in order to minimize tracking error while the IMAGING camera shoots its long exposure.


From a practical standpoint: Many people have used the ASI-120 family as guide cameras. In fact, one version of the camera is sold for exactly that purpose, but it is a relatively low-end camera in terms of sensitivity and works better in some situations (eg: a properly sampled separate guide scope) than in others (eg: long focal length OAG). Moreover, while some people do successfully guide with OSC cameras, they are inheretly less sensitive and also suffer from sensitivity differences between adjacent pixels which make them less desirable for calculating a centroid and guiding than a monochrome camera.


Since you already have the camera, by all means try it. It may work just fine. If it doesn't work then you can cross that bridge when you come to it by buying something like an ASI-290 mm Mini or an ASI-174 mm Mini depending on the resolution of your guiding optics and FOV needs*.



* = Don't get sucked into the "ratio myth" that says that your guide camera's resolution should be related to that of your imaging camera. Due to sub-pixel centroid calculation algorithms, a well-sampled guide camera should be able to guide your mount as best as it can be guided, regardless of the imaging train's imaging scale.

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#3 bobzeq25



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Posted 23 October 2021 - 10:45 AM

Color cameras are suboptimal for guiding, mono is better.  Splitting color among neighboring pixels reduces sensitivity, has no value for guiding.  Doesn't mean they can't work, just means if you are buying a guide camera you should go with mono.


It has little to do with main camera exposure time, it's a more general thing.

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#4 michael8554


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Posted 23 October 2021 - 11:01 AM

Hi Larry


For guiding you need a guidecam that will expose at 1 to 4 seconds.


Long enough to capture stars to guide on, while taking 10 minute exposures with the other imaging camera.


Try your MC, if it turns out to be too insensitive you still have a Planetary camera.

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#5 iantaylor2uk


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Posted 23 October 2021 - 11:26 AM

I have a ZWO ASI 120MM Mini USB 2.0 Mono Camera which I use as a guide camera, which is quite cheap, and works well as a guide camera through a Starwave 50 mm finderscope

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