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RASA 8 question about effective FL with ASI183 sensor

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#1 jprideaux

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Posted 13 January 2019 - 04:55 PM

I have a question about the effective focal length of the RASA 8 if using the ASI183 sensor.  Please let me know if my reasoning is correct below about the RASA 8 and the use of different cameras.

 

RASA 8

Aperture:  203 mm.

Focal Length 400 mm (Focal ration about 2.0)

Image circle without vignetting :  22 mm.

 

As it turns out a 4/3 size chip (like the ASI294) has a diagonal of 23 mm which pretty much matches the 22 mm image circle without vignetting.  Some people point out that there will be some significant under-sampling with using the ASI294 and also 400 mm of focal length is a bit wide for many targets.  Therefore, many people recommend the ASI183 which has a 15.9 mm imaging diagonal and smaller pixels.  There will be quite a bit of unused light (from the RASA imaging circle) falling outside the collection zone of the ASI183, though. 

  

In playing around with a field-of-view calculator, the RASA 8 with the ASI183 can frame the Orion nebula fine.  I get the exact same framing with a "custom scope" of aperture 203 mm and focal length of 588 mm pared with the ASI294.  This custom scope has a focal ratio of about f2.9.

 

Would it therefore be correct to think of the RASA 8 pared with the ASI183 camera as effectively operating at focal length 588 mm with a focal ratio of f2.9?  

 

RASA-FOV.jpg

 

 



#2 Alien Observatory

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Posted 13 January 2019 - 08:29 PM

Not quite sure how you got there...The Rasa F2 is the same for both Cams, the only difference is the Field of View (183 is smaller as it has a smaller sensor compared to the 294)....and the 183 has a better arc/sec per pixel value (as the pixels are smaller than the 294)....Pat Utah

 

 

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#3 GaryShaw

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Posted 13 January 2019 - 11:26 PM

Hi

Unless I'm brain-dead after a rough weekend, here's what the math says about the two FOV's:

 

   FOV294= 3436* (D/L) [D is 23mm and L is 400mm] = 198 arc-minutes = 3.3 degrees

 

   FOV183=3436* (D/L) [D is 15.9mm and L is 588mm] = 92.9 arc-mins =  1.6 degrees

 

I can't help but notice the 2x factor in the above but, unless there are some other optics in the system, it will take someone more alert than I am to see how these two conditions could end up with the same FOV. 



#4 Alien Observatory

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Posted 13 January 2019 - 11:50 PM

The FOV is dependent on the Sensor Size which equates to an apparent FOV.  They do not have the same FOV (183 v.s. 294)...not sure what is not equated to basic science...FOV for any telescope is dependent on Sensor Size and nothing else if the F ratio is the same (and F ratio does not change with sensor size)...Pat Utah :)



#5 OleCuss

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Posted 14 January 2019 - 06:07 AM

Right.  The FOV depends on the focal length and sensor size.

 

The focal length of the RASA will not change.  Technically someone might be able to change the effective focal length but I doubt that anyone will ever try to do that - IMHO that would be changing it into a different sort of beast which would not be pleasing to anyone.

 

The IMX183 camera will yield a much smaller FOV but better sampling of the light from the target.

 

The IMX294 will yield a larger FOV but its larger pixels mean you will have very significant under-sampling.  I suspect I would find this frustrating.

 

Your usable image scale is going to depend on your preference, display, viewing distance, tolerance, etc.

 

I'd really like to have the 8" RASA with sensor which was either 4/3rd or APS-C size but with a pixel size about that of the IMX183 sensor.  However, that sensor would be putting out a lot of data and just saving the images might prove problematic and the processing would be very demanding on the RAM and CPU/GPU of the computer.  I don't think I could afford this imaginary sensor and even though I have some computers which are relatively fast and running 32-64 GB of RAM - I'm not sure I'd have enough to do OAP with my computers while using such a sensor.

 

And even if I could afford the sensor/camera and my computers were up to the task?  My displays could not show me the entirety of the image.  I'd either have to view just a small portion of the image in order to see all the detail or I'd have to do software binning (or similar) just in order to fit the entirety of the image onto my screen - and that means a loss of detail.

 

Give us a few years and we might have the tech available to do better for my purposes, but right now I'd not be able to take full advantage of the 8" RASA.  That doesn't mean I wouldn't be very happy using it, but I'd have to accept the compromise.



#6 DonBoy

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Posted 14 January 2019 - 04:11 PM

As a compromise you may want to consider are those cameras that use the Panasonic sensor that's in the ASI1600 -  The color version cameras are still available from several manufacturers.

 

A ASI1600 is 4/3 size with a 1.96 arcsec/pixel image scale.



#7 jprideaux

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Posted 15 January 2019 - 12:50 AM

I was just thinking like with the "crop sensor" lingo people use in photography when discussing digital cameras and sharing lenses between full sensor cameras and smaller 4/3 sensor cameras.  For example, with the crop factor, using a 50mm F2 lens from a full frame camera on a 4/3 camera will yield an effective 100mm at F4.   Perhaps the analogy breaks down between camera lens and telescopes.  I don't know.

 

I was kind-of using that analogy with the RASA really being designed for 400mm at F2 with a 4/3 sensor but best used perhaps with a smaller (cropped relative to 4/3) 1" sensor for the various reasons people have pointed out.  Thus, relative to the 4/3 sensor, one experiences an effective local-length increase just like my original example with a camera lens going from attached to a full frame camera to attached to a to a 4/3 camera.   Yeah, I know the mirror isn't changing just like the camera lens isn't changing.   You are just getting a smaller field of view in both cases.    But isn't that really just another way of saying an effective increase in focal length relative to using the larger sensor?   I'm just trying to understand.



#8 jprideaux

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Posted 15 January 2019 - 01:13 AM

 

 

FOV294= 3436* (D/L) [D is 23mm and L is 400mm] = 198 arc-minutes = 3.3 degrees

 

FOV183=3436* (D/L) [D is 15.9mm and L is 588mm] = 92.9 arc-mins =  1.6 degrees

I was actually suggesting to compare the RASA using 183 with a different scope with FL of 588 using the 294.

 

so 

 

FOV294 = 3436*(D/L) [D is 23mm    and L is 588mm] =  134.4 arc-minutes

FOV183 = 3436*(D/L) [D is 15.9mm and L is 400mm] =  136.6 arc minutes

 

So using the RASA with the 183 gives about the same FOV as using a 588 MM scope with the 294.

That is all I was saying.  And if the aperture of that other scope was 200mm, then the f ratio would be about 2.9.

 

I was looking at things like the RASA with the 1" sensor was behaving like it was a 588mm scope if that scope was using a 4/3 sensor.  I was asking if this way of looking at it was correct.  



#9 OleCuss

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Posted 15 January 2019 - 05:37 AM

The difficulty is that if you start thinking of shrinking the sensor as increasing the focal length it can mess up the way that you look at other things.

 

A longer effective focal length means you have a change in sampling, increased sensitivity to tracking errors, and increased sensitivity to atmospheric turbulence.  You may also have a different size for your image circle?

 

Even when done by us typical amateur astronomers, AP (be it Observational AP or Conventional AP) is considerably more technically demanding than is your typical consumer terrestrial photography.  So we try not to confuse things about such an important parameter in our rig as the focal length.

 

You can, of course, choose to think about it any way which works for you but there may be a bit of confusion at times.


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#10 jprideaux

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Posted 15 January 2019 - 02:42 PM

Old Cuss,  Is it not true that if you shrink the sensor way down (say a high-resolution half-inch sensor) that you will be quite a bit more susceptible to tracking issues and atmospheric conditions just like if you had used a different longer focal-length scope illuminating a larger image circle working with a larger sensor?  But anyway, thanks for pointing out that focusing exclusively on just the FOV may cause one to overlook other considerations.  FOV is just the thing that I currently have the best understanding of so I am a bit FOV centric right now.  As I learn more aspects of imaging, my perspective will probably change.



#11 OleCuss

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Posted 15 January 2019 - 05:02 PM

Old Cuss,  Is it not true that if you shrink the sensor way down (say a high-resolution half-inch sensor) that you will be quite a bit more susceptible to tracking issues and atmospheric conditions just like if you had used a different longer focal-length scope illuminating a larger image circle working with a larger sensor?  But anyway, thanks for pointing out that focusing exclusively on just the FOV may cause one to overlook other considerations.  FOV is just the thing that I currently have the best understanding of so I am a bit FOV centric right now.  As I learn more aspects of imaging, my perspective will probably change.

The smaller sensor makes tracking more problematic because the smaller FOV combined with the tracking errors may result in the photons from the target no longer reaching the target.  For those of us who like the wider FOV and framing the target artistically rather than simply centered in the image this is more of a problem than for those who just center the target and go from there.

 

The physical size of the sensor will likely make little difference in regards to the effects of the atmospheric turbulence.

 

The atmospheric turbulence should be considered mostly in the following ways:

1.  A longer focal length will tend to make the target occupy a larger portion of the sensor.  This means that atmospheric effects will also have more potential to be seen in the final image.

2.  However, if your sampling is appropriate for your atmospheric turbulence then the atmospheric turbulence will not be an evident problem.

3.  If you are under-sampling then atmospheric turbulence will not be an evident problem.

4.  If you are over-sampling the atmospheric turbulence will be evident if you are viewing the image in its highest possible detail.

5.  If the problem is not high-level atmospheric turbulence but rather the wind is blowing your OTA around, then your image may be messed up but that should probably be classified as a tracking error.

 

So when we choose sampling rates (a matter of focal length of the OTA and the size of the pixels in your sensor) we choose with attention to the expected "seeing" conditions or atmospheric turbulence.

 

I think a lot of us simply assume that we will have 2 arc-second seeing conditions and will thus want sampling in the range of 0.67-1 APP (Arc-seconds Per Pixel).  Sometimes this assumption is optimistic and sometimes pessimistic.

 

But some of us pretty much refuse to guide and that means our tracking is a little problematic and one could argue that choosing sampling of 1-1.5 APP might be more realistic in that situation.

 

 

It is important to remember that you simply cannot choose the perfect system.  No matter how you choose there will be something(s) which is/are not ideal or best suited to what you are doing.  A lot of systems will do a pretty good job and that is usually where we go with our choices - "pretty good" rather than "ideal".


Edited by OleCuss, 15 January 2019 - 05:04 PM.

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#12 Tattooeddeputy

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Posted 08 May 2019 - 02:25 AM

For what its worth...   I have the RASA8 with the 183mm pro and filter drawer and it works great.


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#13 Rickster

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Posted 08 May 2019 - 03:02 AM

jprideaux,

I get what you are saying, and I agree from the crop factor perspective.  That is, the images from both scope/camera combos would be the same size and be the product of the same number of photons.  But I can't totally agree because both combos wouldn't have the same resolution.  The ASI183/400mm combo yields 1.24arc sec/px and the ASI294/588mm combo yields 1.62 arc sec/px (per astronomy tools).



#14 Astrojedi

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Posted 11 May 2019 - 11:00 PM

Old Cuss,  Is it not true that if you shrink the sensor way down (say a high-resolution half-inch sensor) that you will be quite a bit more susceptible to tracking issues and atmospheric conditions just like if you had used a different longer focal-length scope illuminating a larger image circle working with a larger sensor?  But anyway, thanks for pointing out that focusing exclusively on just the FOV may cause one to overlook other considerations.  FOV is just the thing that I currently have the best understanding of so I am a bit FOV centric right now.  As I learn more aspects of imaging, my perspective will probably change.

No a smaller sensor will not be more susceptible to tracking issues... a sensor with smaller pixels will be more susceptible due to its higher resolution.

 

The photography way of thinking is technically incorrect but helps non-scientific users use the equipment better. There is no such thing as effective focal ratio.

 

The focal ratio and aperture together determine F ratio and focal length and pixel size determine the sampling of the system. Hence it is best to think of the question you are asking in terms of fov which is determined by sensor size alone - all else being equal. Otherwise it will cause you more confusion.


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#15 lucutes

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Posted 18 May 2019 - 04:17 PM

I have to agree with the above post. I use the ASI071 with my RASA8 and I have to crop the image to something close to 4/3 size. What’s interesting is the pixel size of the IMX071 is about the same as the IMX294 and therefore I wouldn’t really be gaining anything by purchasing the 294. I briefly was considering the 1600 colour version but the IMX193 would be more appropriate (QHY247C) as the pixels are the same but better anti-reflection coatings. 

 

I have been seriously considered the 183 but more from the sampling point of view. All those tiny pixels would benefit greatly from the powerful light gathering of the RASA. The crop factor is like someone trying to explain turbulence to a passenger and calling it an airpocket? It gets the idea across but people tend to lean on that and go no further. My above mention of the 071 vs the 294 is a good example IMHO of FOV vs sampling. I can get same done with the 071 I just have to crop it in PS. 



#16 jprideaux

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Posted 18 May 2019 - 10:39 PM

Thanks for all the explanations.


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