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Are there real benefits of large pixel sizes with short exposures?


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Hello everyone,

I’ve been reading this forum for a few months on and off, but it is my first post :).

Common wisdom says large pixel (photosite) size is good because they have larger Full Well Capacity and therefore dynamic range. But should this really be a consideration for planetary imaging?

It is customary to work at f/20-30, high frame rate, and bump up gain to increase sensitivity. But gain means analog amplification of the signal read from the photosite, which means photosites are not saturated anyway and having larger Full Well Capacity has no benefits whatsoever. At least it is my reasoning. I would love to hear your thoughts on this matter.

Cheers,

Alex

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Hello everyone,

I’ve been reading this forum for a few months on and off, but it is my first post :).

Common wisdom says large pixel (photosite) size is good because they have larger Full Well Capacity and therefore dynamic range. But should this really be a consideration for planetary imaging?

It is customary to work at f/20-30, high frame rate, and bump up gain to increase sensitivity. But gain means analog amplification of the signal read from the photosite, which means photosites are not saturated anyway and having larger Full Well Capacity has no benefits whatsoever. At least it is my reasoning. I would love to hear your thoughts on this matter.

Cheers,

Alex

I would have thought that for planetary imaging the smaller pixels are preferred so that the very fine detail could be resolved unless you are imaging beyond 5 Meters of FL.

A.G

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@kalasinman, thanks for your comment. Sure, I meant all other things being equal.

Actually, my question was in the context of my deliberation between ASI120MC-S (3.75um) and ASI174MC (5.6um pixel sizes). The latter also features a global shutter and in general more modern sensor. I could not find detailed comparison of two sensors (better yet cameras), but there is very interesting thread about ASI174 here on SGZ (http://stargazerslounge.com/topic/235965-zwo-asi-174). After some research I understood there is marginal difference in QE and noise between the two cameras.

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@lensman57, as a matter of fact I am imaging with C11 (2800mm focal length, f/10). I have x2.5 barlow

In this case I think that you would benefit from the larger pixels. Point Grey Grasshopper 3 range is worth a look at if a bit pricy.

Regards,

A.G

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Thanks for the feedback. Grasshopper 3 seems to use exactly the same sensor as ASI174. And a guy in a shop where I usually buy my staff at says ASI174 is the best camera ever, period. And CCD is effectively dead for planetary imaging at least. I wonder if such a statement can be substantiated.

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Thanks for the feedback. Grasshopper 3 seems to use exactly the same sensor as ASI174. And a guy in a shop where I usually buy my staff at says ASI174 is the best camera ever, period. And CCD is effectively dead for planetary imaging at least. I wonder if such a statement can be substantiated.

Sales people tell you a lot of things such as the max magnification of a 60mm Achromat is 250x . There is a lot more to a camera than just the sensor, having said this I am very happy with my ASI120Mm but I would not go beyond that. American cameras seem to have much quieter electronics than some of the rivals and that is as far as I am going to stick my neck out.

A.G

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It may not be so much a question of pixel size related to exposure length as it is about the sampling rate - which is basically how much of the sky is "seen" by each individual photosite. From what I can gather - larger photosites work well with shorter focal lengths and smaller photosites work well with longer focal lengths however, sky conditions are ultimately the limiting factor. Here's a nifty little chart I ran across a while back published by Sky & Telescope Magazine who owns the copyright. If you position a straightedge through the corresponding pixel size (right) and focal length (center) the left edge should fall within the dark blue highlighted areas for best results - depending on the type of object being imaged (deep sky vs. planetary). I'm not sure it's quite this simple but it does seem to indicate smaller pixels/long FL's for planetary and larger pixels/short FL's for deep sky. :)

post-37916-0-75272300-1434768604.jpg

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It's a good chart, but really isn't sufficient. It doesn't take into account sensor size or size of the object being imaged. Some deep sky images fall outside of the range on both ends. A DSLR may have small pixels, but a relatively huge sensor. It concentrates on Kodak and TI sensors and omits Sony, implying that the data is quite old as well (1997). IMO relying on even modern charts, including the one from FLO, is another tool to aid making a decision, not a formula for making a decision.

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Don't believe I said the chart should be used as a formula for making a decision and yes - I agree it's somewhat dated. My point was that sampling has long been considered the primary consideration when determining what size photosites work best with various focal lengths based on the object being imaged (DSO's or Planets). Therefore, I would say if the OP is considering a sensor with larger photosites in order to get better results with short exposures at long FL's on planets this would not be in keeping with conventional wisdom. However, it could be that I have no idea what I'm talking about which is always a distinct possibility... :)

Edited by Scorpius
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