Planetary imagers

jogdish

Hi,
Thinking of buying a planetary imager, ZWO ASI120MC-S USB3.0 colour camera
Anyone have experience of using these ? Worth it for somone that will use it sporadically at the same time enjoy going out.

tjhook

I was reluctant to post a reply here, because my opinion is just that - my opinion. But anyway, here it is. Others may have good opposing arguments.

For a number of years, that camera (or its mono sibling) would have been the obvious choice for anybody looking for a planetary camera. Since it's colour rather than mono, there's no faffing around with filters and wheels. So it's relatively painless to use. The sensor is well capable of producing good results, and while other manufacturers also use the same sensor, ZWO's build quality would be as good as any of them.

So what has changed since? Probably the biggest change is the variety now available in cameras of that level. Depending on one's needs, it's possible to choose a camera (even from the same manufacturer) with a larger sensor, or a sensor with smaller pixels, or USB3 (which this camera has - it facilitates more frames per second, or larger resolutions, or both) or even powered cooling. So it's no longer as simple as saying there's one obvious choice.

But the ASI120 has produced plenty of amazing images for people, and would still do so. I'd be pretty confident that for 99% of people, such a camera would not be the limiting factor in the quality of their images.

So in my opinion, unless you have a specific requirement that causes you to look elsewhre, a good deal on a ASI120MC-S would be a nice solution.

jogdish

Thanks for the reply, you seem to have some more experience than myself was hoping I might ask a question.

Currently I hook my DSLR to my scope and I see an image that is a reduced magnification compared to my (26mm) eyepiece. Ideally I would love to just use the afocal(?) method of pointing a camera down the eyepeice - but after many many attempts its annoying so fiddly and annoying. I know the eyepeice / and scope focal lenght determine mag. factor.
Should I be worried that the ZWO doesnt have a facility to look down the eyepeice? - Im just worried about another bright blob situation again given that there wont be any eyepeice.

tjhook

jogdish wrote: »

Currently I hook my DSLR to my scope and I see an image that is a reduced magnification compared to my (26mm) eyepiece. Ideally I would love to just use the afocal(?) method of pointing a camera down the eyepeice - but after many many attempts its annoying so fiddly and annoying. I know the eyepeice / and scope focal lenght determine mag. factor.
Should I be worried that the ZWO doesnt have a facility to look down the eyepeice? - Im just worried about another bright blob situation again given that there wont be any eyepeice.

Ideally one should interface the camera to the telescope directly, without an eyepiece. Putting an eyepiece in the middle doesn't help image quality or stability.

When imaging, the field of view you capture depends on 2 main things; the focal length of your scope and the size of your imaging sensor. The wider the sensor, the wider the field of view. The shorter the focal length of the scope, the wider the field of view.

You can use a barlow with your camera to increase the zoom by increasing the focal length. But for a given sensor, the longer the focal length of the imaging train (i.e. the higher the "zoom"), the fewer photons of light will hit each pixel of the sensor, and so the dimmer the image. Meaning longer "exposures" are needed. So a DSLR and telescope with a wide field of view is not a bad thing at all, especially for somebody who's not an expert.

Keep in mind that it's not the case that wide fields of view produce "better" images or vice versa. Small targets (e.g. some planetary nebula or very distant galaxies) might be best with a narrow field of view, while others (e.g. some galaxies or emission nebula) look best with wide fields of view.

The difficulty with putting a camera directly into the telescope is that finding the target can be tricky. Especially with a planetary camera, where the field of view is likely to be very small. You can put in an eyepiece, find the target, then replace the eyepiece with a camera and discover that you can no longer see the target. To help with this, you can get a thing called a "flip mirror" that allows both a camera and eyepiece to be plugged in together, and helps with the finding. But you might want to see how you get on before spending more on something like that.

An alternative approach might be to use an eyepiece to find your target, then replace the eyepiece with a higher magnification one, and recentre the target. You might do this a few times with increasing magnification eyepieces before inserting the planetary camera. Think of the planetary camera as a *really* high magnification eyepiece.

Sometimes I purposely de-focus the telescope to make it easier to find bright targets outside the field of view. A well-focused target is small and sharp. When unfocused, it's a wide doughnut, and even if the target is not in the field of view, part of the halo (doughnut) can sometimes be seen to the top or side of the camera's field of view.

When you have found the target through the camera, you can view it on a laptop screen in order to get focused.