Noise vs Mosaics....Rule of thumb

[Dimitris Platis]

Does anyone know a general rule of thumb regarding exposure time in mosaics?......meaning that surely we can get away with less integration time per pane the more panes u get in mosaics since the noise is obscured in a way by the oversampling of the imaged area compared to the actual resolution of the complete photo.

 

I hope its clear what I mean...

[chrisvdberge]

well.. not really unless you will be scaling down the final image to the resolution of your regular image. But then you loose a lot of the benefits of a mosaic in my humble opinion..

[Dimitris Platis]

Yes...the way I see it...u can either shoot a mosaic so u can have a high resolution widefield image....OR....u can shoot a mosaic because of relatively high focal length in which case u can downsample. In most cases, u wont be able to take advantage of the high resolution because most images are depicted on a PC screen or even when printed u will never have a large enough image to really display the true resolution of your image.

It is a shame.....but in many cases a mosaic its just a way to get over the obstacle of long focal length.

[mschuster]

Dimitris, if rendered at the same physical size on screen, and if equal SNR is desired, then I believe rule of thumb is (maybe surprisingly) equal total time.

Example, a single frame image and a 2x2 mosaic rendered at the same physical size. Downscaling the mosaic by 2x increases SNR by 2x. Don't need this 2x SNR increase, so expose panes 4x shorter to end up with equal SNR. So as a result total time is equal.

This is theoretical. Problems may be not enough subs for good integration rejection, may be harder to process at full resolution due to lower SNR, and (not sure about this) limiting magnitude may be less (a point source issue).

Thanks,
Mike

[Dimitris Platis]

Yes Mike....that was my original suspicion and indeed there must be a lower integration time required to avoid problems...eg alignment

[chris.bailey]

If you are only doing a mosaic to get a wider field of view then why not bin the images in acquisition? Shorter duration sub frames for same SNR, Smaller files = faster processing, which is  major bonus with large mosaics

The problem I have found with mosaics is that even if the same integration time is used in each frame, if taken over multiple sessions, differing sky conditions can lead to differing noise levels in the panel integrations.

[Dimitris Platis]

In that case I guess its a matter whether i prefer higher snr with lower resolution or lower snr with higher resolution.