Jump to content


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Class 6 sky’s to class 4 how much difference in seeing conditions!?

  • Please log in to reply
8 replies to this topic

#1 Supernova74



  • -----
  • topic starter
  • Posts: 1,175
  • Joined: 25 May 2020
  • Loc: Epsom surrey near (London)

Posted 21 January 2021 - 01:56 PM

Good evening all hope everyone is keeping safe and not to bored!?

Anyway I may have an opportunity in the very near furture in relocating to semi rural location approximately 40miles away from my present location according to the bortal scale it’s a 4 and my current location is a 6 and was woundering how much darker it would be theoretically speaking of course depending on sky conditions and transparency etc.


apart from the planets I’m interested in all aspects of Astronomy and I’m asking for exsperiances from other forum members with similer sky conditions.the weapon of choice at the moment is a Meade 12”ACF sct and I’m mainly into visual observation.so for just an exsample we all know that galaxies do demand dark sky sites to detect more contrast and structures.however for an exsample M51 is quite a challenging object to observe and in a class 6 sky’s I’m able to detect nucleus type glow similer to a nebula with a hint of structure so would a class 4 sky’s allow to see even more structure in the spiral arms etc!?

#2 petert913



  • *****
  • Posts: 4,531
  • Joined: 27 May 2013
  • Loc: Silverton, OR

Posted 21 January 2021 - 02:22 PM

It should be noticeably better.  You will see the Milky Way easily as a band in the sky.  The background sky will be much blacker, rather than gray.

Overall contrast while viewing and imaging will be improved. 


Check this info out:



#3 Pcbessa



  • -----
  • Posts: 496
  • Joined: 26 Jan 2019
  • Loc: Forres, NE Scotland, UK

Posted 21 January 2021 - 02:42 PM

The jump from Bortle 8 (terrible) to Bortle 6 (so so) is similar to that experienced from Bortle 6 (so so) to Bortle 4 (reasonably fine).

Under Bortle 6 skies most Messier galaxies are detectable but uninteresting.

Under Bortle 4 skies, you could see M51 and most galaxies as a faint extended fuzzies and M51and a few other Messier galaxies would only show hints of arms in exceptional nights.

By contrast under Bortle 2 skies you should have really nice views of M51 with a mottled structure visible and arms seen when seeing and transparency is good. Under the same skies you can see hints of arms for a few other brighter galaxies.


I mean with excellent skies I was able to trace the structure of maybe around 20-25 galaxies with my 10", so you should see even more. In many galaxies I guessed the structure before I saw any photo. Only M51 really shows as an archetypical spiral and M33 and M101 in the very best nights.

Edited by Pcbessa, 21 January 2021 - 02:45 PM.

#4 Redbetter


    Voyager 1

  • *****
  • Posts: 10,280
  • Joined: 16 Feb 2016
  • Loc: Central Valley, CA

Posted 21 January 2021 - 08:50 PM

My experience:

  • Bortle 4 entry level for galaxy observing.  Brighter than this and I take a pass.  
  • Bortle 6 is rather miserable by comparison, challenging even for experienced observers and a lot less is even discernible (mostly just the centers of some higher surface brightness galaxies.)
  • Bortle 3 is decent.
  • Bortle 2 is quite good.

  • Astro-Master likes this

#5 Allan Wade

Allan Wade


  • *****
  • Posts: 4,912
  • Joined: 27 Jan 2013
  • Loc: Newcastle, Australia

Posted 21 January 2021 - 09:18 PM


My experience:

  • Bortle 4 entry level for galaxy observing.  Brighter than this and I take a pass. 


Same here. My city home is Bortle 5, and I never look at galaxies from there. Well, except the LMC and SMC, but they’re a different case. I think Bortle 4 is when galaxies make it onto the observing menu. 

  • Redbetter and Astro-Master like this

#6 Tony Flanders

Tony Flanders


  • *****
  • Posts: 19,359
  • Joined: 18 May 2006
  • Loc: Cambridge, MA, USA

Posted 22 January 2021 - 06:48 AM

First of all, I will assume that we're talking about genuine Bortle 6 versus genuine Bortle 4. The only way to determine that is to go to the site and see for yourself how the skies rate according to John Bortle's criteria. Figures taken from some map or database are not reliable.


But I would say that for observing galaxies in particular, the difference between Bortle 4 and 6 is enormous. Bortle 4 is sort of the worst of the acceptable; Bortle 5 or 6 are the best of the unacceptable.


My astronomy club's site in the outer suburbs of Boston, MA, is somewhere between Bortle 5 and 6, depending which criteria you use. It is possible to see M51's spiral arms -- fairly well, even -- through some of the club's giant scopes, like the 24-incher. But M51 is a bit of a freak; its arms are much brighter and bolder than 95% of all spiral galaxies. Forget about seeing M31's dust lanes, no matter what scope you're using.


My country home is Bortle 4 by most criteria. There, I can see M51's spiral arms through my 7-inch Dob with some effort and easily through my 12.5-inch Dob. M31's dust lanes are a bit harder, but they're not a serious challenge. Most of the stuff that I can see under dark skies I can also see from my Bortle-4 backyard, though it typically takes a good deal more effort.

  • brentknight, Redbetter and Voyager 3 like this

#7 Pcbessa



  • -----
  • Posts: 496
  • Joined: 26 Jan 2019
  • Loc: Forres, NE Scotland, UK

Posted 22 January 2021 - 05:47 PM

M31 dust lanes are actually a good indicator
And M33 spiral arms are another good indicator.

I usually struggle to see both of them under my home Bortle 4 skies. But in Bortle 2 they are beautiful sights. It makes a huge difference. Still I can see many galaxies from my home site.

#8 Love Cowboy

Love Cowboy


  • -----
  • Posts: 1,061
  • Joined: 05 May 2014
  • Loc: Houston, TX USA

Posted 23 January 2021 - 06:41 PM

My club's dark site isn't much better than Bortle 4 these days, and I can see spiral arms in M33 and M51 on nights of good seeing, and I've seen hundreds of fainter galaxies. People who have access to better won't like Bortle 4 much, but if you're used to worse, you will love it.    

Edited by Love Cowboy, 23 January 2021 - 06:42 PM.

#9 brentknight



  • *****
  • Posts: 2,544
  • Joined: 29 Dec 2014
  • Loc: Foley, Alabama

Posted 23 January 2021 - 07:51 PM

If moving from B6 to B4 is doable for you, then you should do that.


A couple things to consider though - how long is the B4 site going to remain that way?  There are a couple good areas within 20 miles of my home, and until recently they were fairly affordable.  Developers have snatched the land up, and probably fairly soon they won't be much better than where I live now.


Also, most B4 sites have neighbors few and far between.  No more fretting over the people in the backyard announcing their homestead at night to everyone else around them...

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Recent Topics

Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics