Optimum FWHM vs Seeing

Discussion in 'My Astrophotos' started by Mike Hambrick, Apr 27, 2021.

  1. Mike Hambrick

    Mike Hambrick Cyanogen Customer

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    I posted this question on another forum but thought I would also ask here:

    What is the conventional wisdom on the finding the optimum focus point based on the lowest FWHM value at different degrees of seeing ? I am assuming that this is being done with a good scope, mount, and camera, and relatively short (< 10 second) focusing images. The best I have been able to get in my focusing images is a FWHM in the range of 1.20 to 1.30, but more often it is in the range of 1.50 to 1.80.

    As a follow-up, is there an upper limit for FWHM above which there is no point in trying to do any imaging ?

    Mike
     
  2. Doug

    Doug Staff Member

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    You didn't mention the units you are using. Are you talking 1.2 arc-seconds, or 1.2 pixels.

    In general, to maximize resolution you should have enough sampling to get 2.5 to 3 pixels across the FWHM of the star. If you have less sampling than that you will end up losing resolution due to pixelation (aliasing).

    Where I live, the best seeing I've ever seen was 1.8 arc-seconds FWHM. My focal length and pixel size gave me a 0.6 arc-seconds per pixel, which was a good match to the seeing.
     
  3. Mike Hambrick

    Mike Hambrick Cyanogen Customer

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    Hi Doug

    I believe the units are pixels, but I can see that I need to do some cleanup in my setup. The information screen shows a scale of 1.06" / pixel, but that is for my other scope. I forgot to check the FITS scale box. For this particular scope (Astro-Physics 92 mm stowaway f.l. 644 mm) and camera (STXL 16200), PinPoint calculates an image scale of 1.922" / pixel.

    On a slightly different topic I have several scope and camera combinations for which I have set up different configurations in MaxIm DL. If I set up the information screen the way I want it for a particular scope & camera combination and go to the Save Configuration command will the Information screen details be saved in that particular configuration ?

    Mike

    upload_2021-4-28_8-9-57.png
     
  4. Doug

    Doug Staff Member

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    Your pixels are pretty large - the seeing shouldn't affect your images unless it's really bad!

    On a good night in my back yard, the seeing is in the 2.0 - 2.5 range. Assuming you are in similar conditions, that means your pixel is as large as the FWHM of your seeing disk. You are on the verge of having little square stars instead of round ones.

    For my seeing conditions, I like to have a pixel scale of 0.7 arc-seconds per pixel. I would not want to go above 1 arc-second per pixel.
     
  5. Tim Povlick

    Tim Povlick Cyanogen Customer

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  6. Tim Povlick

    Tim Povlick Cyanogen Customer

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    This is an interesting topic. One question (if I may) - what is the preferred procedure for taking the image for FWHM for evaluating Seeing? Should the exposure be as short as possible or use the time exposure that would be used for capturing the image sequence? The former should yield 'optimal' results while the later will average in bad & good seeing.
    As a test, a 130 mm f/6.3 refractor was teamed with an STC-7 (image scale=1.12965 arcSec/pixel) for a 1-second exposure using high gain mode. FWHM measured 2.25arc-sec with a maximum pixel value of 1931 and SNR of 166.

    Using a longer exposure the quality of guiding and mount performance would seem to effect the FWHM value. If the goal is to measure seeing, would short exposures to form an ensemble of FWHM measurements be a preferred approach?

    Attached is the FITS file from which the measurements were taken. The scope really is a 130mm and not the AP-RH per the FITS header.

    Thanks for any expert advice.

    _..--
    Tim
     

    Attached Files:

  7. Doug

    Doug Staff Member

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    If you want something representative of your actual imaging, then I wouldn't use a very short exposure to evaluate seeing. Assuming good tracking/guiding I would use at least 1 second. If in doubt take a full length exposure.

    You have quite good seeing, and your image is actually a little undersampled! You don't even have 2 pixels across the FWHM. I would recommend 2.5 -3 for best resolution.

    Of course that's the equipment you actually have, so it may be hard to change. It's not terrible, but it is limiting your resolution a little. On the other hand it gives you a bigger FOV. It would be a good setup for imaging larger objects, but perhaps not for small planetary nebulae.
     
  8. Tim Povlick

    Tim Povlick Cyanogen Customer

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    Very good. I will try a set of 1-sec exposures and calculate the FHWM over a sequence. Windy conditions had prevailed in previous days (25 knot gusts) so the results were a welcome surprise. Should've taken some test FHWM readings at the end of the session as conditions seemed to improve (SQM was 21.2).

    The Astro-Physics Riccardi-Honders Astrograph (305mm aperture, 1120mm fl @ f/3.67) seems an ideal match for the STC-7.

    Thank you for the expert advice...

    _..--
    Tim
     

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