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IMX178 vs. IMX183 - capturing detail on modest focal length

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#1 jtrezzo

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Posted 14 December 2018 - 04:22 PM

My goal here is capturing detail in galaxies at a modest focal length. I have an 11" RASA on the way, which has a focal length of 620mm. Looking at the 11" RASA, the specs are a Dawes limit of 0.42" and Rayleigh limit of 0.49". Supposedly, unlike Hyperstar setup, the RASA might be diffraction limited, according to Celestron catalog I had it said "43.3 mm optimized image circle (with a 52 mm useable field) maintains diffraction limited, pinpoint stars to the far corners of even the largest astroimaging sensors". Though maybe this is not the case as I see they no longer have the "diffraction limited" line on their product website now.

 

Both cameras would produce an image scale of 0.8"/pixel. I am hoping to do short exposures in the range of 5 second or less to overcome seeing effects which I think should work well with RASA.

 

I have been looking at these two sensors, trying to distinguish any difference besides the obvious one - sensor size. Both are small and should fit most galaxies fine, but to summarize, the differences are:

 

 

                                        IMX178                     IMX183
Sensor Size                7.4mm x 5mm        13.2mm x 8.8mm

QE% Peak                         81%                           84%

ADC                                 14 bit                          12 bit
Read Noise (HG-LG)     1.4e - 2.4e                 1.6e - 3e

 

Both are BSI, both have 2.4um pixels, and both have a maximum 15K fullwell. The 183 is approximately $250 more than the 178 for a cooled version, $300 more for uncooled (which may be an option if I just do short exposure).

 

 

Has anyone had any experience with either of these on a RASA or Hyperstar themselves? Or any comparison between the two at all on any scope. I know I've seen Mark Holbrook's incredible galaxy images with the 178 and 183 with a 14" Hyperstar setup which has a 690mm focal length which has been the inspiration here, but that is the pinnacle of greatness, so looking for more experiences. I guess my key questions are just wondering if the extra cost of the 183 is worth it over the 178 for a bit bigger FOV, or if the 14 bit ADC of the 178 puts it ahead a little bit in terms of dynamic range. 

I currently have a QHY5III178M (uncooled version I use as a guide camera), which I plan to test out to begin with to see how this goes before buying anything else once I get it...but that brings up another concern, there is kind of an odd diffraction pattern on brighter stars that I do not like, though I did not see that on Mark's images with it.



#2 AdamJ

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Posted 14 December 2018 - 05:02 PM

The 178 is the better chip for galaxies, its lower noise due to the 14bit A/D and you dont want to be taking 100's of short exposures with a 20mp chip it will be difficult to stack and store.



#3 ks__observer

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Posted 14 December 2018 - 06:45 PM

Not sure you can beat seeing, which is on order of 100th second.

The noise difference seems rather small, particularly at unity gain or higher on the 183.



#4 jtrezzo

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Posted 14 December 2018 - 10:48 PM

Not sure you can beat seeing, which is on order of 100th second.

The noise difference seems rather small, particularly at unity gain or higher on the 183.

Well, not beat it literally, but overcome a lot of the blurring. I agree the noise difference seems pretty irrelevant. 



#5 ks__observer

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Posted 15 December 2018 - 04:50 AM

If you integrate over an hour, you will get same total seeing blur no matter if you break the hour into 2 subs or 2000 subs.

The shorter sub would let you better cull out the worst pics.


Edited by ks__observer, 15 December 2018 - 04:51 AM.



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