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Recretos

True power of a 6" reflector?

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Hello again after a few months.

 

I did not had any opportunity to observe since may, except for the moon eclipse, which was nothing to write home about. Today I was observing in likely the best weather conditions so far, with very weak winds in the entire troposphere column, very low moisture aswell and nice transparency (low water vapour and dust particles). So far my observations were during spring on my balcony towards S/SE only. Today I took the scope (Skywatcher 150P classic Dobson) outside in the backyard, which was basically just a carry out the house. The whole setup of carry outside and with the eyepieces prepared, took no more than 30 seconds. i then left the scope to cool for about an hour, of course with all caps on. I had a go across some pre-chosen targets and one huge surprise. After I was done, there were two questions that bothered me, and a third one that kinda makes me sad. 

 

First things first, I will do a quick recap on what has sparked my questions. This is a one-time only that I do this report type post, so it can be for future reference what one can expect from a 6" f8 reflector. It sure was beyond my expectations. If it is in contradiction with any terms, please let me know and I will try to edit the post in some way.  

As mentioned before, the weather conditions were good. I live on a hill at 3450ft (1060m) above sea level, so I am out of the main haze and smog, and also out of some atmosphere :D . The area where I live is a Bortle 3, with some Bortle 2/low 3 nights during winter when the valley is filled with fog that blocks all light sources, so the sky got filled with stars soon as my eyes got used to the dark. 

Tonight Andromeda was really standing out, the Hercules cluster was visible with naked eyes, the M33 barely with averted vision as it got a bit higher, and the milky way was nice, with the dark patches seen at Deneb, and the split down towards S/SW.

 

My main target of the night was M57 (Ring nebula). I was not expecting much to be honest, given the images, videos, and reports. And this was already the first surprise of the night. To me, the ring looked very bright. It was very nice at 48x, and even better at 92x (13mm TeleVue plossl). The disk was very bright (subjective of course), with a clear dark center and oval shaped. It was a gray doughnut, and when I used the UHC filter I bought from China (SvBony), there wasn't a huge difference, but the contrast was noticeably better. The small star next to the ring was also visible and a few small around, which stellarium shows to be at magnitude 13.1 to 13.5 extinct. So I guess that they were kinda at the limit magnitude of this scope near zenith.

 

I then went on to the nearby M13 Hercules cluster. Also an amazing view. Bright with more stars than you can count. Quickly jumped up to M92 just above which was less impressive, but still a nice sight. I do really like globular clusters. 

 

Next target was obviously M31 Andromeda. Here there was the second surprise. The core was bright, at least to my eyes, and there was enough contrast to see "cloudiness" extended outwards to the edge of the eyepiece ( GSO 32mm - 37,5x zoom). Especially when I wiggled the scope a bit up and down so my eyes grabbed on to the contrast even more. I only expected to see the core. The neighbour M110 and M32 were also easily and distinctly visible as separate galaxies. A very awesome trio, despite being far apart in reality. A great sight that I will try to include in every session. Quickly jumped down to M33 Triangulum galaxy, which was of course less impressive since it was also lower down the horizon, but it still looked good, with hints in contrast of the darker patches (in between spiral arms?), not looking just like one fuzzy disk.

 

Next were M81 and M82. Both looked great, the Bodes galaxy looked like an actual galaxy, tho one could mistake its fuzziness for a nebula, and the cigar galaxy really stood out and had nice contrast and the obviously seen shape, being rounded a bit on both edges, kinda like a wave.

 

Next came the big surprise of the evening. I turned towards south and wanted to do a quick glance at Saturn. When I was looking through the finder (6x30), I noticed something under Saturn. Of course in reality its above Saturn since the finder flips the image. l targeted that gray patch, and looked into the eyepiece. I was blown away by the view. It was a nebula. Looking at the star charts it was M20, Trifid nebula. It was amazing, with 4 patches of nebulosity, separated by dark streaks. I quickly added the UHC filter to the 25mm, and the contrast was even better, with nicer separation of the 4 patches. 

 

l looked at Saturn then, but it was a mistake, since it kinda hurt my dark adapted vision. A rookie mistake i guess, and a harsh lesson to leave planets for last.

 

I finished off with the omega nebula, M17, which I now know why they call the swan nebula. The shape of the swan was perfect, with both the head and body nicely seen and it had good contrast, and the UHC filter added a bit more of the latter. Last one was M16, eagle nebula which was also a nice patch of nebulosity, with narrow extensions towards upper right. Kinda like a T turned by 45 degrees. Here also the UHC filter improved the contrast. 

 

Slowly the high level clouds moved in, noticed by how the stars started disappearing towards W and SW, without actually seeing the clouds, and also as seen later on the IR satellite imagery.

 

Now this session was relatively short, but very awesome, and the main two questions are following:

 

1- Are these views in Bortle 3 with my 6" kinda what one would see with an 8-10" in bortle 7-8? Or does the aperture still win and could produce even better views despite higher light pollution? (This is why I wrote the description above of what I saw, so there is a better understanding of the views to compare to)

 

2- I read a lot how eyes loose their dark adaptation with age, so does this mean that in 20-40 years, I will need an 8" for these same views?

 

And the sad question that arose today, deep down inside, is an obvious one. I began to wonder what views i would get here with a 10". Oh well, one day.....

 

Best regards,

Andrew

Edited by Recretos
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Thats a great report, and sounds like you had great fun!

Its often said that a 6" scope can work better than an 8" scope, looking at the same targets, if the 6" worked out,  under a better sky, so no man-made light pollution.

A bigger aperture will collect more light, therefore effectively providing more detail.

And as for the 10" or bigger....................one day maybe, after you have peeked through a friends scope?

 

Edited by Charic
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Thanks for the reply. 

Yes I have heard that too, but I never heard any specific bortle scale difference, about when the 6" and 8" might show the same views, like 2-3 bortle scale difference (6" bortle 3 = 8" bortle 6 for example), or more..?

None of my friends have a scope, at least not anything over 50$ from the hardware store, so looking through a 10" I might spare for entirely myself. :) But on such night it does make you wonder even more, just what more detail one could see, if the small 6" already shows so much. But that "so much" is fully subjective. Someones "so much" in another ones "barely anything". Especially if that another one likes to look at Hubble images a lot. :D 

regards

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1. A larger scope lets you see more but the improvement from a darker sky wins.

2. Maybe the average people lose some eye sensitivity with age but frequent stargazers increase it. We also see finer features and lower contrast with practice.

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By the way, I've been into stargazing for a lifetime but my memory refuses to learn the Bortle scale. It's simpler and more reliable to state your observed local limiting magnitude at the zenith, or give the star count inside the square of Pegasus, or the Pleiades.

According to my city Bortle number I shouldn't see this or that, but I do if I my shield my eyes, or if I simply go to the large common driveway yard surrounded by walls and trees that block street lamps. My club's country site Bortle number is supposed to permit certain sights and preclude others but no, the Bortle scale is not accurate enough. It cuts the landscape into very large blocks that don't reflect real local blackness. The reading is false because of a nearby village and a more distant city.

Lastly, a friend's holiday home in southern Italy is supposed to be just good on the light pollution map, but it's really exceptional, with M33 visible to the naked eye from a lit village. Walking a few hundred meters into the meadows makes the view clearly better thanks to no direct street light. But the light pollution map lacks the resolution to show that.

So, when stating local conditions, it's best to say the real observed limiting magnitude.

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39 minutes ago, Ben the Ignorant said:

It's simpler and more reliable to state your observed local limiting magnitude at the zenith, or give the star count inside the square of Pegasus, or the Pleiades.

Now for me that's three to choose from?
I've often read that I should use the Stars from Ursa Minor, whilst observing in the Northern Hemisphere, and that Ursa Minor was the standard! 

Having said that, even if we both looked at the same constellation, it would still depend on our visual acuity. Your eyes may be younger/sharper than mine and see things  differently.

Even the Bortle system with its 9 levels could still be interpreted differently, reading from the same sheet under the same sky ?

Edited by Charic

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1 hour ago, Charic said:

Now for me that's three to choose from?

If you can judge magnitude by eye, state the magnitude; if you can't, count the stars in Ursa Minor, the Square of Pegasus or the Pleiades and others can look up the magnitudes on a star map. The rectangle in Cepheus, the pentagon in Auriga or any area will do, what matters is you give the true local limit in a fashion everyone can understand.

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5 hours ago, Charic said:

Having said that, even if we both looked at the same constellation, it would still depend on our visual acuity. Your eyes may be younger/sharper than mine and see things  differently.

True. But surely, when describing what you saw during a particular session, the important thing is how many stars you saw in the constellation. That gives an indication of what I would see if I saw the same number of stars in the constellation. The fact that I might (on this particular night) see more or fewer stars is not really significant.

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5 minutes ago, Demonperformer said:

........the important thing is how many stars you saw in the constellation.........

Yes I would agree. 
I wrote late, but missed out the word magnitude?

Its not like were holding up  our fingers, say  7 or 9 of them, which would a clear cut decision, some eyes would still see some stars clearer than others, based on their magnitude, so do the two not go hand-in-hand, magnitude and how many? 

If you and I were  under the same sky, same garden even, and you saw 5 more Stars than I, then your interpretation of the night sky has to be regarded as better than mine,  whereas, I'd be asking, "how come you can see all that" 😀
 

 

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6 hours ago, Recretos said:

I was blown away by the view

Hi and that sounds like a great session. It's sad that having a camera permanently attached to my telescopes, I've forgotten what looking through one is like:( But hey, don't be disappointed with not having tt bigger telescope. My 6" f8 is by far the purest I have; my 10" needs a coma corrector. The latter introduces glass into the light path and false colour and a glow around bright stars like you get with a refractor. So yeah, don't knock your telescope. It's likely amongst the purest views you're gonna get:)

12 minutes ago, Demonperformer said:

how many stars you saw in the constellation

Yeah, I think scales and numbers are misleading. I'm at bortle 23 or my sky is 107 or whatever. Some nights I can see the milky way easily. Others not. That doesn't give much useful information because there's nothing to say that when I can see the glow of our own galaxy, the seeing will match...

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Yes I did forgot to include the limit magnitude. I didnt put much note to this, but the ursa minor was visible easily of course, and as a reference in ursa minor I used the star next to the the one in the top right corner of the squared part, whic I saw in stellarium is called 20 Umi, and has the extinct magnitude of 6.51. 

Across the sky there were many that looked fainter over wider zenith area, but it was impossible to identified them by any means on any star map I use, to check their magnitude. 

Btw, when you give your limiting magnitude, you gice the stars original magnitude or the extinct magnitude?

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Nice report Andrew, sounds like you have some cracking skies to observe under. First things first, the skies are the most important thing to have right, if the sky background is brighter than the object you are trying to see, then you won’t see it regardless of the scope you use.

If you have a play around with this calculator you will see these effects clearly, a 6” scope will show faint objects more easily under a mag 21.5 sky, than an 8” scope under a mag 18.5 sky.

http://www.bbastrodesigns.com/VisualDetectionCalculator.htm

Once you are under a good sky, aperture counts as it allows you to increase the image scale whilst maintaining the surface brightness. Larger objects allow your eye and brain to detect contrast changes more easily, so the object is easier to detect.

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On ‎13‎/‎09‎/‎2018 at 09:28, Recretos said:

ursa minor was visible easily of course, and as a reference in ursa minor I used the star next to the the one in the top right corner of the squared part, whic I saw in stellarium is called 20 Umi, and has the extinct magnitude of 6.51. 

Across the sky there were many that looked fainter over wider zenith area

Great, with a smaller scope you can see more than most of us.

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On 13/09/2018 at 06:46, Charic said:

If you and I were  under the same sky, same garden even, and you saw 5 more Stars than I, then your interpretation of the night sky has to be regarded as better than mine,  whereas, I'd be asking, "how come you can see all that"

I see what you are saying, but tend to come at it from a slightly different perspective. Not so much what I would have seen had I been standing on the same spot at the same time (which would indeed be different), but what I can expect to see when conditions appear to be the same to me.

So, if on a particular night you can see 4 Umi (mag 4.82), but not η Umi (mag 4.96) and go on to describe your observing session, on a night when I can see 4 Umi, but not η Umi and go on to view the same objects, I can compare how I see those objects with how you did and make a meaningful comparison (assuming similar equipment). Did you see details that I am not seeing? This would suggest that my observing technique has room for improvement. Or, if you were using a bigger scope, it would give me some indication of how much more I could see if I were to upgrade my scope (a very slippery slope!).

I guess my point would be that, the stars in UMi, always being at roughly the same altitude, provide a more objective comparison between observations made on/by different nights/observers.

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On 13/09/2018 at 01:07, Charic said:

I've often read that I should use the Stars from Ursa Minor, whilst observing in the Northern Hemisphere, and that Ursa Minor was the standard! 

If I can see more than Kochab and Polaris, it's an exceptional night!

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Very nice report and some great objects observed.  It's always interesting to hear what can be see under certain skies with certain scopes as once you have your first , you spend a lot of time dreaming about owning something else - and your thinking about a 10" dob - I have to constantly push thoughts of a new scope out of my head and focus on what I have otherwise I find my observing sessions a little frustrating...  (But it's always fun to dream) 🙄

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Great report!  Some nights just seem to stand out.

Ouch, I didn’t know that dark adaptation also deteriorates with age!  I hope that’s not the case as there are enough things to struggle with as it is.  A delaying of dark adaptation by a few extra minutes is not going to have a functional impact on observing over several hours.  I choose to ignore it!  Let’s face it:  age will have an impact on our sight.  But hopefully, experience in observations can overcome some of these handicaps.

On the topic of measuring quality of skies, there’s a lot of layers to the onion.  Some methods are useful for different things, e.g. the Bortle Scale seems to get very detailed at the highest levels where a key factor is whether you are detecting Zodiacal light or Gegenschein.  I also find NELM indicative of the base conditions of the location being observed from and add to this the factors of local seeing and transparency.  Just to make things more confusing, there are SQM measurements too ;) 

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Contrast (clarity of the sky) and freedom from high level cloud is key.  Here in scarborough it seems always hazy. Just v occasionally do you get a night where you go outside and go wow.  

 

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I seem to be lucky here in that my skies have got a little darker since the LED street lights were installed. They used to be about 4.5 NELM but are now around 5. I regularly get SQM readings of about 19.05 ish and can see all seven stars in Ursa Minor.

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The famous British observer W.F. Denning wrote over a hundred years ago, "What one man sees through a 5" glass, another needs a 10"." That is still the case today, but at least part of the personal equation can be linked to experience, so generally the more experienced observer will see more through a given aperture than a less experienced observer. It is amazing just how much detail a 6" reflector will show, so don't be in a hurry to jump to a bigger scope, as the 6" is more than capable of giving spectacular views and is easy to use. The bigger they get the more cumbersom they become, and you might possibly find you observe less.

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29 minutes ago, mikeDnight said:

The famous British observer W.F. Denning wrote over a hundred years ago, "What one man sees through a 5" glass, another needs a 10"." That is still the case today, but at least part of the personal equation can be linked to experience, so generally the more experienced observer will see more through a given aperture than a less experienced observer. It is amazing just how much detail a 6" reflector will show, so don't be in a hurry to jump to a bigger scope, as the 6" is more than capable of giving spectacular views and is easy to use. The bigger they get the more cumbersom they become, and you might possibly find you observe less

Quite so, Mike - perhaps this advice should be a "sticky" at the top of SGL!

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6 hours ago, Stu said:

I seem to be lucky here in that my skies have got a little darker since the LED street lights were installed. They used to be about 4.5 NELM but are now around 5. I regularly get SQM readings of about 19.05 ish and can see all seven stars in Ursa Minor.

I’ve heard this quite a few times now and looking forward to LED!  We’re still on the orange ones here but my tshirt-on-a-stick trick works a treat for the worst lamp across the street!

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They are certainly more directional, which is a good thing in general, but makes experiences very individual. Personally, from my front garden (where I image) my SQM readings are (on average ... whatever that means!) about 0.75 mag better than under the old lights. Some people have reported that when there is excessive moisture in the atmosphere  they are much worse than the old lights, but then I don't tend to be out imaging when that is the case!

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On 13/09/2018 at 08:28, Recretos said:

Yes I did forgot to include the limit magnitude. I didnt put much note to this, but the ursa minor was visible easily of course, and as a reference in ursa minor I used the star next to the the one in the top right corner of the squared part, whic I saw in stellarium is called 20 Umi, and has the extinct magnitude of 6.51. 

Across the sky there were many that looked fainter over wider zenith area, but it was impossible to identified them by any means on any star map I use, to check their magnitude. 

Btw, when you give your limiting magnitude, you gice the stars original magnitude or the extinct magnitude?

Great report. When I'm noting the sky quality I do the same and  just look for the faintest star I can definitely see in the area of sky  I'm observing and look it up in Sky Safari. The limiting magnitude does vary from one bit of sky to the next e.g. due to twilight, the light domes of towns and cities, etc.

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