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Old 05-08-2018, 09:12 AM
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Archseconds per pixel????

Hi,


I am trying to understand the best resolution to capture planetary images. I have come across formulas out every internet orifice and find it difficult to understand. 0.25" (arcseconds) /pixel seems to be the optimum. But what does this mean in real terms?????



Ok, heres my setup (not an exhaustive list, ony the relevant ones for this thread):
  • Skywacher Newtonian 1200mm / 250mm (MAX 500x magnification)
  • ZWO 174mm camera
  • 5x Televue Powermate (increasing focal length to 6000mm (F24)) (don't know what this means in magnification except for the obvious 5x)
I've taken pretty reasonable images with this setup, but i have seen far better images taken with the same camera and know there must be a better way.


So, my query is how do I work out what is best with the equipment I have, how do know the target 0.25" (arcseconds) / pixel is met (even if this is optimum). Let's take Saturn as an example. It's around 21 arcseconds in diameter, how do I magnify this to reach the optimum.


I find this all very confusing.

Last edited by cyberblitz; 05-08-2018 at 04:23 PM.
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Old 05-08-2018, 09:41 AM
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Merlin66 (Ken)
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Arc sec/ pixel is a measure of the plate scale for your set-up.
Based on effective focal length and the pixel size of the camera...

If the object you want to image is say 20 arc/sec in size and the arc sec/ pix is 0.5 then the object would cover 40 pixel on the image.

The optimum arc sec/ pixel really depends on your seeing conditions. Somewhere around 0.5 arc sec/ pixel is a good starting point.

To answer you other questions download CCDCalc and enter your data - it will give you arc/sec pix (plate scale), field of view and comparison with various objects.
I've entered your data into CDCCalc and get 0.19 arc sec/ pixel (Ignore the telescope label - I entered 250 f5 with x5)
HTH
http://www.newastro.com/book_new/camera_app.html
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Old 05-08-2018, 10:05 AM
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Thanks, i get 0.19 arcseconds per pixel. I don't know if this is good or bad. It seems i'm 0.06" below optimum.



I've read using a barlow with telescopes less than 2000mm is not a good idea, using projected imaging is better. Not sure which is better, not tried projected imaging.
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Old 05-08-2018, 10:17 AM
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Although I know very little about planetary imaging, I think that one needs to also take into account diffraction limit for a given aperture. 250mm aperture has a diffraction limit of about 0.5", so I believe there would be little advantage in going lower than that with arcseconds per pixel for this aperture, even in perfect seeing. Accurate and precise collimation is also a definite must for achieving highly detailed images.
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Old 05-08-2018, 10:21 AM
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Don't know where you read that.....barlows (and Powermates) can be successfully used on all focal lengths. Much easy to set-up and use than the eyepiece projection.
At 0.19 arc sec/ pixel you're probably over sampling which realistically will add nothing to the image...targeting around 0.3 to 0.5 would probably be better.
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Old 05-08-2018, 10:29 AM
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Considering the camera has 2.3Mega pixels, Saturn would only cover a very small area of the sensor, 110 pixels (going by 0.19"/pixel), it is a very small area indeed.



Thought using more pixels would be better?? But i'm probalby limited by the telescope to magnify any more.



Would adding a Atmosphere Dispersion Corrector help with increasing this? If only a little
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Old 05-08-2018, 10:32 AM
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An ADC won't add more data.

Bigger telescopes longer focal lengths more sensitive cameras....endless circle!!

Practise with what you have, you may be pleasantly surprised with the outcome.
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Old 05-08-2018, 10:34 AM
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Read it here: https://starizona.com/tutorial/planetary-imaging/


Although i may not totally understand their meaning.



Quote:
The most common accessories for amplifying the image are Barlows and eyepiece-projection adapters. If your telescope has a fairly long focal length (2000mm or more) a Barlow will probably be sufficient. The exceptions to this might be if you have a CCD with very large pixels (16-microns or more), or if you are seeking the most detail possible and have very good seeing conditions. For smaller (shorter-focal-length) telescopes, or for large-pixel CCDs, the usual method of magnification is eyepiece projection. By shooting through an eyepiece, more magnification is provided than a Barlow can give
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Old 05-08-2018, 10:42 AM
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No idea what oversampling means
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Old 05-08-2018, 10:47 AM
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Quote:
Originally Posted by Merlin66 View Post
An ADC won't add more data.

Bigger telescopes longer focal lengths more sensitive cameras....endless circle!!

Practise with what you have, you may be pleasantly surprised with the outcome.
I totally agree. This person gets interesting results with a mere 135mm aperture: https://www.cloudynights.com/topic/5.../#entry8716678

The reason I mentioned diffraction limit and aperture size is that from what I understand there is little benefit in imaging high resolution way beyond telescope’s aperture limit.
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Old 05-08-2018, 10:49 AM
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Over sampling is a bit like excess magnification - it doesn't add any info to the image.
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Old 05-08-2018, 04:26 PM
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Lowering the magnification reduces the image on the sensor, producing a video file containing a pretty small object. I suspect I need to improve on post-processing techniques to both clean it up and increase its size to be of any use.


I understand the diffraction issue; it happens with any camera situation if the f-stop is reduced enough. I suppose this explains over-sampling in a way, but with magnification.


I could do with reading a good book on this if anyone has any recommendations?
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Old 06-08-2018, 11:14 AM
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For planetary you need a camera with smaller pixel size NOT more pixels.
You should be capturing at smaller size (not full) so its not such a tiny dot in the blackness, but you're wasting processing and capture speed on data you have to throw away. By dropping the recording size the planet is still the same number of pixels across but you gain frame rate which is very important. Planetary you only have a few minutes to record before the planet's rotation starts to smear your stacked result (you ARE stacking right?!).
no books, just experiment and learn, it takes time and patience. ADC wont help much if at all, every piece of glass you add to the optical train reduces the photons that reach the camera and adds extra distortions. No such thing as 100.0% transparent glass with 0.0% distortions so a 5x televue sound great but youre often magnifying the atmospheric distortions anyway. There are practical magnification limits of OTAs .
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Old 06-08-2018, 12:09 PM
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Its starting to make a lot more sense now. I'll try less magnification and see were i go. And yes, i do stack - i use Autostakkart 3.

Attached: an example of Saturn with my current setup
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