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Thread: Short note on viewing the galaxies under the light polluted skies

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    Default Short note on viewing the galaxies under the light polluted skies



    Hello Kyw0277, hello all,

    that's the frequently asked question by the suburban-to-urban observers, and covered to some extent in the Urban Observing Lists https://www.astroleague.org/al/obscl...ban/urban.html

    Well, there are two essential tricks when hunting the galaxies under the light polluted skies:
    1) the choice of the eyepiece exit pupil (and of the eyepiece type itself, given the telescope)
    2) the choice of the galaxies types.

    ad1) What works for me best whenever the night skies are not clear and dark, is the exit pupil around 1.5mm, which is historically one of the exit pupils prefered by Charles Messier.
    What matters technically, is the contrast of the eyepiece, and here are the 'simple' eyepiece designs, like Plössl or orthoscopics, the choice well of considering. The new classic ortho f=10mm from Baader has been reported to conquer in contrast and in light transmission the Pentax XW and the TeleVue eyepieces, but for a fraction of price.

    ad) You can choose the galaxies according to their surface bightness (the equivalent stellar magnitude across the angle of 1 arcminute squared), and the galaxies with the highest surface brightness are best candidates. The break even is around 13.0mag/arcmin² .
    Very good candidates are the elliptical galaxies with bright cores, the galaxies with active cores Seyfert galaxy - Wikipedia, the free encyclopedia, and the galaxies with the bright star birth regions, like the NGC 604 in M33, HubbleSite - NewsCenter - Firestorm of Star Birth Seen in a Local Galaxy (12/04/2003) - Release Images . The NGC 604 can be easily seen through an UHC filter at the magnification around 70x and larger, even under the light polluted skies, when nothing else from the M33 galaxy is observable.

    There are also important recommendations how to protect your observing site, how to shield your scope your head and your eyes, from the stray light sources in the suburban and urban environments.

    Best,

    JG
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    Default Re: Short note on viewing the galaxies under the light polluted skies

    Excellent input, JG. Thank you so much!

    Clear skies!

    Bob
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    Default Re: Short note on viewing the galaxies under the light polluted skies

    Really interesting many thanks. I thought the ideal "magic" exit pupil for galaxies was 2 so interesting to hear that the contrast could be improved further going to 1.5.
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    Default Re: Short note on viewing the galaxies under the light polluted skies

    Very good info, JG. Thank You!

    I think too often we (including me) have a tenancy to use a larger exit pupil... in my case, it's usually around 2.8 with an 18mm Ep in an f/6.5 refactor. To your point, dropping down to a 14mm (exit pupil 2.2) does seem to make certain galaxies brighter, and obviously with better contrast against the background.

    If the skies ever clear again, I'll follow your suggestion and drop to a 10mm in the same scope. I think I never tried it because I was "satisfied" with what I was observing at 2.2.

    Good thread, Friend
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    Default Re: Short note on viewing the galaxies under the light polluted skies

    Hello Dave, hello John,

    the exit pupils in the range 1.4mm (Pentax XW 7mm, magnification 107x) up to 2mm (Pentax XW 10mm and Swarovski 10.2mm, magnification 75x) have increased my reach towards the fainter and more distant galaxies even under my best suburban-to-country skies.
    Herewith, I have been cured from the horror from magnification, and from the large exit pupil fever, when it is about the galaxies.

    The M33: well below 40x and above 4mm exit pupil - it is often just a smudge through the 6" refractor in my backyard, with and without the UHC filter.
    Around 50x (3mm exit pupil) it gets a washed away S-form, eventually with a hint of the not well defined core.
    At 70x (2mm exit pupil) plus UHC, the NE spiral arm becomes visible, and the NGC 604 is looking like a 'planetary nebula'.
    At 107x (1.4 exit pupil) plus UHC, further spiral arms pop up, the Frog (=the curly NW spiral arms with IC 139, IC 140, IC 137, and A48, A14) and some more IC condensations become visible. See also M33 HII Regions and Star Clouds

    Happy hunting,

    JG
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    Default Re: Short note on viewing the galaxies under the light polluted skies

    You've convinced me... I didn't have a 10mm and First Light Optics had an ex demo Baader Classic 10mm. i was looking for a plossl of 10mm anyway for planetary and thinking of the TV but baulking at the price...
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    Default Re: Short note on viewing the galaxies under the light polluted skies

    Thank You for This interesting insight!

    For my 10" a magnification of 170x gives an exit pupil of 1.5 mm.
    This means barlowing the ES 14mm 82 degree eye piece.
    I do have some good views in This combination and now Will make sure to try it more often.
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    Default Re: Short note on viewing the galaxies under the light polluted skies

    JG your wisdom is very appreciated. Is there a list of the Seyfert galaxies I did not notice any on the links. thanks ag
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    Default Re: Short note on viewing the galaxies under the light polluted skies

    Excellent points as usual JG. Though I use my ES 18mm as my galaxy hunting EP in the 10 and 12 inch dobs, I rarely make substantative observations at that level (exit pupil 3.7mm at 69x and 3.6mm at 84x respectively). My best views typically are with the 14mm (2.9mm at 89x and 2.8mm at 108x respectively), or 11mm (2.2mm at 113x and 2.2mm at 138x respectively), or 8.8mm (1.8mm at 142x and 1.8mm at 172x respectively). The variability of which galaxies respond best at a particular exit pupil/magnification level are typically dictated by a combination of the nature of the galaxy and its surface brightness. However, one cannot discount a galaxy's apparent magnitude entirely, as that also needs to be considered in the mix.

    I find often that lenticular galaxies are brighter than spirals and can punch through LP better. Examples of nice lenticulars are M84, M85, M86, NGC 5102, NGC 5866 and NGC 3115 (to name just a few). As JG mentions, elliptical galaxies, with their bright core and lack of dimmer extended arm structure are also often better targets. JG also mentioned galaxies with an Active Galactic Nucleus (AGN), such as the Seyfert types. They can have exceedingly bright cores that punch through LP well. Examples of some nice Seyfert types are M51, M66, M77, M81, M87, M88 and M106 (to name just a few).

    Sometimes the prevailing conditions can severely limit what can be seen with galaxies. Whether that be your LP conditions, or a night of poor transparency, or both. But everytime you observe a galaxy you should always play with your assortment of EPs to see which one(s) gives you the best views. There is always a point that is too far, where the views start to break down. But there is also that point where the exit pupil and magnification give the best image for the specific target under the current conditions. Always experiment.
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    Default Re: Short note on viewing the galaxies under the light polluted skies

    Quote Originally Posted by allen g View Post
    JG your wisdom is very appreciated. Is there a list of the Seyfert galaxies I did not notice any on the links. thanks ag
    Allen, you can find a list of some Seyfert's at the bottom of the page of the link below:

    Seyfert galaxy - Wikipedia, the free encyclopedia
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