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Remove Focal Reducer From Optical Train For Smaller Objects?



Continuing a thread wrongly posted in the app support section, 

s3igell wrote:



AP Imaging with most any DSLR - especially the current Canon / Nikon 18-24MP Models - is already capable of reaching Seeing-limited Resolutions.  There is nothing more in terms of actual Detail which can be dredged up by using a Barlow, only the Image Scale and Focal Ratio are magnified - meaning the need for Longer Exposures and More of them - placing stress on the Mount Tracking and AutoGuiding System.


Except for Planetary Video Imaging, where Video Stacking can recover Detail beyond the normal Atmospheric Seeing Limits, the Best Solution is to take High Quality Exposures at Prime Focus (without any Barlow or Tele-Extender) and then to Crop the Resulting Image once all other Calibration and Stacking and Processing is completed.  That is how to produce the Best Detail and Large Image Scale.


Eyepiece Projection and Afocal Imaging and Barlows were more pertinent in the days of Film AP Imaging, when there was no ability to perform Calibration / Stacking / Stretching 



So a similar question, I'm imaging through a focal reducer at 0.89 ArcSec per pixel. If I remove the focal reducer I'll be at 0.54 ArcSec per pixel, both well below the seeing limit.


I was thinking of trying some of the smaller objects, say M1, without the focal reducer to get more pixels on the target. Based on the above quote, and assuming my mount can handle it, is that a waste of time?





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It won't be a "Wasted Effort" exactly, but it is one of "Diminishing Returns".  You'll be in the condition of Over-Sampling the Available Resolution / Seeing Conditions.


The Incoming Light will be Distorted by the Atmospheric Seeing Conditions, which are surely limited to >1arcsec (often 2-3as depending on your locality).  At your proposed 0.54as/pixel Resolution, you will be capturing substantial amounts of this Blurring effect.  You will capture some instances of Clearer Seeing which will tend to add up to some additional Image Signal Strength at that "Higher Clarity" - so Imaging at the Over-Sampled Condition isn't all for Naught.


This isn't very Analogous to the practice of Video Stacking Imaging for Planetary Photography, as that uses the very-short Exposures of the individual AVI Frames to capture instances of Improved Clarity and to Isolate and Stack just the Quality Portions of just the High Quality Frames.  Long Exposure DSO Imaging would instead be summing up any instances of Higher Clarity within the Individual Sensor Pixels, providing no methodology for Isolating such "Clarity" during the later Stacking and Processing and instead works to only average-away the "Noise" of that Distortion - although modern Sharpening and Deconvolution Software can work wonders with just the small amounts of stronger Input Signal.

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