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Smaller pixels or longer focal length for the same pixel scale?

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#1 DynamicUniverse

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Posted 08 May 2021 - 04:31 PM

Hypothetical Situation

Imagine you're imaging Jupiter with your C8.  The seeing and transparency are pristine. Which would you do:

 

Use a mono camera with 2 micron pixels or 

Use a mono camera with 4 micron pixels and a 2x Barlow 

 

I just want a straight answer that doesn't involve discussion of the other camera's specs or capture parameters which can be adjusted in the capturing software anyway.

 

Chris

 

 

 

 

 

 

 

 

 



#2 PiotrM

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Posted 08 May 2021 - 05:02 PM

There should be no difference if small and big pixels are imaging at max resolution. Barlows aren't a problem unless you want the best UV imaging (although SCTs aren't best for UV). Also for Moon you may want bit less than max resolution to limit light diffraction effects on high contrast features.



#3 Tulloch

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Posted 08 May 2021 - 05:03 PM

Hi Chris, from what I learned on the subject, here's my take.

 

Generally speaking, the less glass you have in your system the better, less chance of dispersion, dust bunnies, transmission loss etc so the 2 micron camera would win here. However, top quality barlows/powermates are extremely good, not too expensive and almost impact free if kept clean.

 

The biggest assumption in your question is that the two cameras are identical in their specifications; same sensitivity, same losses, same noise etc. However, speaking in general again, this is not the case. Cameras with smaller pixels are usually less sensitive and produce higher noise levels than larger pixel cameras, requiring higher gain levels in the software which again increases noise. So the large pixel camera wins here.

 

So, if you can find a 2 micron camera with the same read noise and sensitivity of the 4 micron camera, that would be preferred - however, if you cannot, then a 4 micron camera with a good barlow would be my suggestion. (eg a ASI224MC with 2x Barlow/PowerMate, like what I use smile.gif).

 

IMHO

 

Andrew


Edited by Tulloch, 08 May 2021 - 05:09 PM.

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#4 Tom Glenn

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Posted 08 May 2021 - 05:06 PM

I just want a straight answer that doesn't involve discussion of the other camera's specs or capture parameters which can be adjusted in the capturing software anyway.

 

Image scale will be identical, and the posts above are correct.  However, it is difficult to address a hypothetical question without including a discussion of important parameters related to that question (ie which cameras?)  From your list of cameras, however, there are multiple that would all produce equivalent images if adjusted to identical image scales.  



#5 Ittaku

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Posted 08 May 2021 - 07:14 PM

Here's a radical take on it: Use the barlow version. The slower focal ratio will make it easier to get fine focus.


Edited by Ittaku, 08 May 2021 - 07:14 PM.


#6 Prudentis

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Posted 08 May 2021 - 08:38 PM

Use a mono camera with 2 micron pixels or 

Use a mono camera with 4 micron pixels and a 2x Barlow 

I'm assuming, both systems will fit Jupiter inside the chip area.

Sampling stays the same between both. Resolution stays the same.

F/Ratio will change but the total amount of light gathered from Jupiter, will stay the same.

Since you double the area Jupiter is occupying on the chip on the 4µm sensor, the light gathered per pixel will also stay the same.

 

With the given parameters there are only two things, that can be consiered relevant:

1. tracking

2. the number of air-to-glass surfaces

 

Both are better without the barlow.

However, even though this answer is theoretically correct, I almost feel bad giving it, since I know, someone will equate my answer as "barlo=always bad" hich is obviously ridiculous.

Now what would I do? I'd take the picture with 2µm pixels AND the barlow. Why? because then with your hypothetical "seeing and transparency are pristine", I'd get double the resolution.

 

 

I just want a straight answer that doesn't involve discussion of the other camera's specs.

The problem with this approach is, you can change so much with "other specs" that the whole question becomes unanswerable. From the answers to such a hypothetical question, you can't derive any meaningful conclusions.


Edited by Prudentis, 08 May 2021 - 08:43 PM.


#7 Tulloch

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Posted 08 May 2021 - 09:05 PM

Now what would I do? I'd take the picture with 2µm pixels AND the barlow. Why? because then with your hypothetical "seeing and transparency are pristine", I'd get double the resolution.

 

As has been discussed many times here amongst people who image the planets using the prime focus method, optimal focal length is approximately 5x the pixel size of the camera - f/10 for a 2 micron pixel camera, f/20 for a 4 micron pixel camera.

 

Don't use a 2x barlow on a 2 micron pixel camera.


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#8 Kokatha man

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Posted 08 May 2021 - 09:42 PM

As has been discussed many times here amongst people who image the planets using the prime focus method, optimal focal length is approximately 5x the pixel size of the camera - f/10 for a 2 micron pixel camera, f/20 for a 4 micron pixel camera.

 

Don't use a 2x barlow on a 2 micron pixel camera.

But "Hey!" didn't you read the previous poster's qualifier about "seeing & transparency are pristine" which the OP postulated on initially Andrew - in the land of "make believe" anything is possible, so I'd say "give it a go..!" :rofl:


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#9 Tulloch

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Posted 08 May 2021 - 11:00 PM

But "Hey!" didn't you read the previous poster's qualifier about "seeing & transparency are pristine" which the OP postulated on initially Andrew - in the land of "make believe" anything is possible, so I'd say "give it a go..!" rofl2.gif

I stand corrected - next time I'm imaging in pristine seeing I'll be sure to stack all my barlows together for the maximum resolution possible lol.gif


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#10 Tom Glenn

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Posted 08 May 2021 - 11:38 PM

Don't use a 2x barlow on a 2 micron pixel camera.

Unless, of course, the scope was much faster than f/10 (which it isn't in this case).  FWIW, I wouldn't go above f/10 with a 2um camera ever.  But the question is, what is the 2um camera the OP is referring to?  I see a 2.4um camera in the gear list (ASI183).  And for that matter, what is the 4um pixel camera in question, as the OP specifically said monochrome cameras?  ASI224 is 3.75um but color, and the Atik 428ex is 4.5um.  


Edited by Tom Glenn, 08 May 2021 - 11:39 PM.

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#11 Tulloch

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Posted 08 May 2021 - 11:43 PM

Unless, of course, the scope was much faster than f/10 (which it isn't in this case).

Sure, that was kinda implied (since the OP was talking about a C8). If it was a 8" f/5 then yes, the 2x barlow would be preferred (I guess).

 

But the question is, what is the 2um camera the OP is referring to?  

I just took it as the perfectly valid hypothetical, is it better to image with a smaller pixel size and no barlow, or a larger pixel size and with a barlow. And the answer is, of course, it depends smile.gif.


Edited by Tulloch, 08 May 2021 - 11:50 PM.

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#12 Tom Glenn

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Posted 08 May 2021 - 11:50 PM

The other issue is the use of an ADC, which for many of us in the northern hemisphere, will continue to be absolutely required this season for Jupiter and Saturn.  If we are talking purely hypothetical, which it sounds like we are, then the ADC will perform better with an f/20 light path than f/10, although the real world difference here is very small.  But of course, the OP specified monochrome cameras, which makes the implementation of the ADC more difficult, yet still beneficial for low altitude planets.  I think the main takeaway here for the OP should be that pixel size versus focal length is largely irrelevant, as long as the final image scale is matched (during capture), but that there are numerous other practical considerations that override almost every hypothetical situation.  


Edited by Tom Glenn, 08 May 2021 - 11:51 PM.

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#13 RedLionNJ

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Posted 09 May 2021 - 11:30 AM

I'm glad Tom brought up the ADC. He is correct - a larger f-ratio is preferred for optimal use of the ADC.  So we'd want to include the barlow and use larger pixels.

 

 

And as for the misplaced comment somewhere above "I'd take the picture with 2µm pixels AND the barlow. Why? because then with your hypothetical "seeing and transparency are pristine", I'd get double the resolution." -  you would get double the potential resolution, for sure - but you wouldn't get double the information captured.  Even under perfect conditions, you're limited by the aperture and the wave nature of light. You can't just infinitely stack barlows to get finer and finer detail.



#14 John Boudreau

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Posted 09 May 2021 - 01:09 PM

Here's a radical take on it: Use the barlow version. The slower focal ratio will make it easier to get fine focus.

Unless the auxiliary optic... be it a Barlow or focal reducer is actually part of the telescope's design and doesn't move relative to the scope's main optics (primary and secondary if a reflector, objective if a refractor) the depth of focus doesn't change. A C8 used with a 2x Barlow will still have a depth of focus of an approximately f/10 system (the C8's advertised f-ratio changes a bit when the primary is moved in relation to the secondary). The reason is that while focusing, the C8's native light cone is moving in relation to the Barlow and so the Barlow's resulting focal point is subjected to the f/10 based depth of focus.


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