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jamsgra
August 25, 2009, 19:58:15
Hi All,

I am new to imaging with a CCD, (I have used an SLR
previously), but now I'm looking for some basic information about the
cost/ benefit between Resolution and FPS.

I notice that the higher resolution DMK cams have slower frame rates.

So, what are the benefits of each?

I think I have decided to buy a mono (non IR-cut cam) as I will add
filters later when I get used to using it for mono astro photography.

So that just leaves me with which FPS/ Resolution combo to plump for.
Any advice would be much welcomed.

I am planning to use the cam in the UK with generally average seeing
conditions, with my 120mm Sky-Watcher retractor.

Thanks,

James

Stefan Geissler
August 26, 2009, 09:24:38
Hello James,

The frame rate of a camera is limited by its CCD read out speed and the bandwidth available on the used bus, e.g. USB or FireWire.
Since I am not an astronomer, I can only give the obvious technical answer. The higher the resolution, the more details you will see on your image. But with higher resolution the amount of pixel data rises, so you can not transfer as much images as in low resolution.

Thus if you want to see a high resolution images with as much pixels per star as possible, you need a high resolution camera. A low resolution camera as 640x480 pixel, may not show all stars you want to see. With a low resolution camera, you will receive much more image per second, so AVI stacking will give good results, the telescope must not be guided to far for many images.

I hope this explanation is not as bad :-)

nardes
September 3, 2009, 01:40:53
In my experience, the frame rate (fps) has only been important for high resolution imaging of the Moon, planets and the Sun (with full aperture Baader Solar Film filter). A high fps allows you to grab several 100's of frames of say, Jupiter in the relatively small 90 second window you have, before the rotation of Jupiter “smears” the features between 1st and last frames. High fps on the Moon and planets is also important so you can capture the fleeting moments of best seeing and make several recordings without having to wait too long between captures.

In terms of resolution, I have the DMK21AF04.AS and the DMK31AF04.AS with 5.6 and 4.65 micron pixels respectively.

Using the same hi-res optical system (‘scope + Barlow/PowerMate), my DMK31 images of say, Jupiter appear slightly larger on my Notebook display than the DMK21 images. So effectively, the smaller pixels produce a larger image.

The DMK31 (1024x768) is fabulous to use on the Moon, producing one shot large lunar scapes without the need to make mosaics with the smaller 640x480 DMK21.

If you are imaging deep space, then typically your exposures will be several seconds so fps is not an issue here. Generally, the larger the pixel (charge well) the more sensitive it is, so smaller pixels, like fine film grain, are not as sensitive.

Cheers

Dennis