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Does Transparency or Seeing Hurt Global Clusters Severely?

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#1 WheezyGod

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Posted 17 September 2021 - 04:49 PM

It’s been 1-2 weeks since I saw both M13 and M22. For both I wasn’t able to resolve as many individual stars that I expected to (maybe 10-15 stars at 200x) using an AD10 under bortle 5/6 skies.

I’m unsure why I’m not seeing it pop like I’ve seen in some pictures (look at the one I’ve linked at the bottom for my expectations). I’m able to see the swirl of light for both globulars, but it’s as if I’m looking at many small, faint blobs of light. It’s not one giant one like others have described when looking though a small scope.

I know it’s not my collimation because I’ve done star tests and saw a lot of detail at 200x on Jupiter that I wouldn’t be able to see if my scope was poorly collimated.

M22 looked about as good from what I remember compared to M13 despite M22 being much lower to the horizon, and in a spot where I have more light pollution. I’ve read that both clusters are similar in size and brightness for a northern observer, but why would M22 look just as good as M13 unless it was because of transparency and/or seeing? The transparency was average both nights, but the seeing could have been poor.

https://images.app.g...SH9sgMqMrY9Pms6

Edited by WheezyGod, 17 September 2021 - 04:49 PM.


#2 havasman

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Posted 17 September 2021 - 05:23 PM

The linked sketch was made using a 400mm scope. That's a very poor predictor of what can be seen via 10" aperture. The author of the sketch is an experienced observer/sketcher almost certainly able to more fully use the capability of his aperture than most new observers. Aperture increase is often thought to most facilitate better globular cluster observations. IMO, M22 is the most impressive northern hemisphere GC so I am not at all surprised it retained more impact than M13. Additionally, EAA was in use for at least part of the 8 hours the author spent on the sketch. The coastal site in SW France could potentially bring very good conditions too.

 

The best way to get a better image in your eyepiece will be to go observe somewhere darker. Improved conditions at you site will also provide better opportunities. The best way to become disappointed with the views is to compare them to AP in general or to expert, accomplished sketches made using dissimilar gear at a superior site to what/where will be in use. Both transparency and seeing can degrade views of a GC but unreasonable expectations are worse.

 

It might be useful for you to attempt to sketch some objects to see the process that results in a likeness and to see the contribution time makes to all our observations.


Edited by havasman, 17 September 2021 - 05:28 PM.

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#3 slepage

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Posted 17 September 2021 - 05:27 PM

Expectations are hard to comment on as it is so subjective but I would not form an expectation based off a drawing or a image.  In order to try to help, what eyepieces are you using?  Poor eyepiece choice and cause disappointing views.

 

Steve



#4 WheezyGod

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Posted 17 September 2021 - 05:37 PM

Expectations are hard to comment on as it is so subjective but I would not form an expectation based off a drawing or a image. In order to try to help, what eyepieces are you using? Poor eyepiece choice and cause disappointing views.

Steve


For 208x I’m using the Svbony 6mm redline. I also have 9mm and 10mm plossls. I recently got a 2x Barlow, I was thinking of trying out my 9mm and 10mm with the 2x Barlow to see if it’s perhaps the 6mm eyepiece.

Although this 6mm eyepiece has given me great views of Jupiter and Saturn when I had finally had decent seeing earlier this week.

My expectations are based on what I’ve read observers say they can see using 8in or 10in scopes from sites that have more light pollution than mine. Where they can see hundreds of stars. Most came from the link here:
https://www.cloudyni...ight-pollution/

#5 slepage

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Posted 17 September 2021 - 05:58 PM

As you know, eyepieces can range from $35 to $650 and more.  I am most certainly not saying you have to go off and spend big dollars to get pleasing views, but the lower end eyepieces do suffer from all kinds of minor issues and one of the biggest issue is light transmission.  As such they can be wonderful for bight objects such as the planets, but might not provide you with all that can be had when viewing relatively dimmer objects.  My suggestion is try to hook up with other locals and see if you can borrow different eyepieces to try in your scope.  I think you will see what you need to do to get what you want out of your scope.

 

Steve.  



#6 WheezyGod

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Posted 17 September 2021 - 06:23 PM

As you know, eyepieces can range from $35 to $650 and more. I am most certainly not saying you have to go off and spend big dollars to get pleasing views, but the lower end eyepieces do suffer from all kinds of minor issues and one of the biggest issue is light transmission. As such they can be wonderful for bight objects such as the planets, but might not provide you with all that can be had when viewing relatively dimmer objects. My suggestion is try to hook up with other locals and see if you can borrow different eyepieces to try in your scope. I think you will see what you need to do to get what you want out of your scope.

Steve.


Yea I want to try my 9mm and 10mm plossls with my 2x Barlow and see if the view is the same. The FOV will be poor but the brightness shouldn’t suffer (could in the 9mm but not in the 10mm I bought separately).

You may have a good point about a dimmed view which I believe I had read about for my 6mm. I was thinking only of planets/moon at the time.

I am planning to go to a bortle 3 site in 8 days weather permits. Hoping to take a look at M13 and other objects before the moon starts having an impact.

#7 sanbai

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Posted 17 September 2021 - 06:58 PM

For star clusters that can be resolved, light pollution could be partially compensated with a somehow higher magnification (because the stars are punctual, non-extended objects).

Low transparency, however, is going to hit the contrast and the limiting magnitudes. This is a common issue here in wet Louisiana. This summer was terrible in that regard.

Seeing could also affect if it's noticeable at the working magnification. Optically the stars won't be punctual anymore, so magnification will dilute their image and lower the contrast.

So, both transparency and seeing can transform the big bunch of diamonds on black velvet into a fuzzy soup.

I've also seen the difference between my 8" SCT and a 15" dob. Huge.

#8 Blint

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Posted 17 September 2021 - 07:28 PM

Check out m15, in my sqm 19.0 and ad10 hundreds of stars were resolved. I have yet to observe m13 since it’s low in the horizon when I’m able to set up the scope.
Alex

#9 Tony Flanders

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Posted 17 September 2021 - 08:24 PM

It’s been 1-2 weeks since I saw both M13 and M22. For both I wasn’t able to resolve as many individual stars that I expected to (maybe 10-15 stars at 200x) using an AD10 under bortle 5/6 skies.


I think the main culprit here is light pollution. It doesn't hurt globulars as much as it hurts galaxies, but it still hurts them plenty. Not only does it hide the individual stars -- many of which are already fairly close to the limit for a 10-inch scope -- it also totally blots out the extended halo, which is generally where stars are easiest to resolve.

Bad seeing and transparency can definitely be problems, too. And of course you will be able to see more stars with more experience.
 

M22 looked about as good from what I remember compared to M13 despite M22 being much lower to the horizon, and in a spot where I have more light pollution. I’ve read that both clusters are similar in size and brightness for a northern observer, but why would M22 look just as good as M13 unless it was because of transparency and/or seeing? The transparency was average both nights, but the seeing could have been poor.

Aside from the fact that it's lower in the sky, M22 is much easier to resolve than M13, because it's only 40% as distant. That means that on average its stars are 6 times brighter than M13's. In addition, M22 isn't as rich as M13, meaning that the stars are farther apart.

 

The apparent similarity between the two clusters when viewing at low magnifications in small instruments is deceptive. M13 is much richer and more massive -- as it must be to appear nearly as bright as M22 despite being 2.5X more distant.


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#10 Jon Isaacs

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Posted 18 September 2021 - 04:54 AM

As you know, eyepieces can range from $35 to $650 and more.  I am most certainly not saying you have to go off and spend big dollars to get pleasing views, but the lower end eyepieces do suffer from all kinds of minor issues and one of the biggest issue is light transmission.  As such they can be wonderful for bight objects such as the planets, but might not provide you with all that can be had when viewing relatively dimmer objects.  My suggestion is try to hook up with other locals and see if you can borrow different eyepieces to try in your scope.  I think you will see what you need to do to get what you want out of your scope.

 

Steve.  

 

I own the 6mm Svbony Redline though it's currently on loan.  In my experience, eyepiece transmission is not a big issue.  I think Tony and others have hit the nail on the head, light pollution, transparency, seeing.  When I compare eyepieces like the 6mm Redline or the 5mm Astro-Tech Paradigm the type 6 Naglers and Ethos's, there are some issues but the inexpensive eyepieces show the globulars quite well.. 

 

I would not recommend buying more eyepieces.. 

 

Globulars are best viewed under dark skies.. 

 

Jon


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#11 sanbai

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Posted 18 September 2021 - 08:35 AM

I own the 6mm Svbony Redline though it's currently on loan. In my experience, eyepiece transmission is not a big issue. I think Tony and others have hit the nail on the head, light pollution, transparency, seeing. When I compare eyepieces like the 6mm Redline or the 5mm Astro-Tech Paradigm the type 6 Naglers and Ethos's, there are some issues but the inexpensive eyepieces show the globulars quite well..

I would not recommend buying more eyepieces..

Globulars are best viewed under dark skies..

Jon

Totally agree. There are excellent eyepieces, and others less so, but very difficult to find a bad one.

The big elephant in the room is not the equipment but the atmosphere. Good that we have it (no life without it), but it's a huge variable filter in front of our scopes.

#12 WheezyGod

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Posted 18 September 2021 - 11:09 AM

I own the 6mm Svbony Redline though it's currently on loan. In my experience, eyepiece transmission is not a big issue. I think Tony and others have hit the nail on the head, light pollution, transparency, seeing. When I compare eyepieces like the 6mm Redline or the 5mm Astro-Tech Paradigm the type 6 Naglers and Ethos's, there are some issues but the inexpensive eyepieces show the globulars quite well..

I would not recommend buying more eyepieces..

Globulars are best viewed under dark skies..

Jon


Thanks appreciate the clarity. I’ll definitely keep trying on better nights and see if I get better results. Dark site in the next 1-2 weeks and I’ll see how much of a difference that makes.

#13 Starman1

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Posted 18 September 2021 - 02:54 PM

It’s been 1-2 weeks since I saw both M13 and M22. For both I wasn’t able to resolve as many individual stars that I expected to (maybe 10-15 stars at 200x) using an AD10 under bortle 5/6 skies.

I’m unsure why I’m not seeing it pop like I’ve seen in some pictures (look at the one I’ve linked at the bottom for my expectations). I’m able to see the swirl of light for both globulars, but it’s as if I’m looking at many small, faint blobs of light. It’s not one giant one like others have described when looking though a small scope.

I know it’s not my collimation because I’ve done star tests and saw a lot of detail at 200x on Jupiter that I wouldn’t be able to see if my scope was poorly collimated.

M22 looked about as good from what I remember compared to M13 despite M22 being much lower to the horizon, and in a spot where I have more light pollution. I’ve read that both clusters are similar in size and brightness for a northern observer, but why would M22 look just as good as M13 unless it was because of transparency and/or seeing? The transparency was average both nights, but the seeing could have been poor.

https://images.app.g...SH9sgMqMrY9Pms6

Seeing really hurts the visibility of faint stars.

Bright skies hurt the visibility of faint stars

Transparency hurts the visibility of faint stars, but less than the other two.

You can get around the light pollution by increasing the magnification to darken the skies (the stars don't dim) and improve contrast.

But if seeing doesn't allow the higher magnifications (150x-250x is about right for globulars in a 10"), then....

 

The best views will always be where teh atmosphere is 1-2 atmospheres thick (i.e. above a 30° altitude).

If an object never rises above 30°, you will be looking through some thick air, and the image will never have the quality of a higher object.

 

P.S. Practice using averted vision.


Edited by Starman1, 18 September 2021 - 02:55 PM.


#14 Rodgerraubach

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Posted 30 September 2021 - 04:41 PM

Globular clusters are my favorite DSOs, and I have observed a large number of them that are visible in the Northern hemisphere, including 5 of the Palomar clusters and about a dozen in M31.

 

That said, I was in a very dark observing site with very clear skies when doing my observations. I would say that atmospheric transparency is the killer of good observations, much more so than seeing. Light pollution doesn't seem to affect globulars too much, but haze, smoke, and a generally cruddy atmosphere will will finish the night before it starts.



#15 Inkswitch

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Posted 01 October 2021 - 11:09 AM

Last night, backyard, 300mm reflector, SQM-L 20.8, transparency 3.5/5, seeing 2/5.  The seeing gave very soft focus at 375X.

 

I was working in western Aquarius and I noticed that M72 was nearby.  After twenty years of visual DSO observing I have not yet seen all the Messier objects and this was one of the unseen, so I had to look.  The GC was obvious at 45X but would not resolve at any magnification, I credit the poor seeing.  The GC's extent could be seen as a glow and size estimation was relatively straightforward.  At all magnifications it showed as "grainy".  I could tell it was a GC but couldn't resolve any stars.  Finally, in desperation, I went to 375X magnification which provides me with a .8mm exit pupil.  At this mag, even though it was soft, I saw about six stars flashing in and out.  My conclusion, you need both transparency and seeing to get the most out of a globular.

 

My transparency was very good last night, I was observing low surface brightness MGC galaxies in the area but failed to see the Aquarius dwarf.  I must admit the AQU dwarf may have been impossible for my aperture, but never let that stop you.


Edited by Inkswitch, 01 October 2021 - 11:10 AM.


#16 Starman1

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Posted 01 October 2021 - 01:33 PM

The problem is not insufficient aperture, it's your bright skies.

20.8 on the SQM-L is a fairly bright sky.

The brightest stars in M72 are around magnitude 14, only a magnitude fainter than M13.

But the horizontal branch, where hundreds of stars become visible, is around mag.16.9, almost 2 magnitudes

fainter than M13.

That's in reach in very dark skies of, say, 21.5-22.0 with 300mm, but probably not at 20.8.



#17 Redbetter

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Posted 01 October 2021 - 04:40 PM

Last night, backyard, 300mm reflector, SQM-L 20.8, transparency 3.5/5, seeing 2/5.  The seeing gave very soft focus at 375X.

 

I was working in western Aquarius and I noticed that M72 was nearby.  After twenty years of visual DSO observing I have not yet seen all the Messier objects and this was one of the unseen, so I had to look.  The GC was obvious at 45X but would not resolve at any magnification, I credit the poor seeing.  The GC's extent could be seen as a glow and size estimation was relatively straightforward.  At all magnifications it showed as "grainy".  I could tell it was a GC but couldn't resolve any stars.  Finally, in desperation, I went to 375X magnification which provides me with a .8mm exit pupil.  At this mag, even though it was soft, I saw about six stars flashing in and out.  My conclusion, you need both transparency and seeing to get the most out of a globular.

 

My transparency was very good last night, I was observing low surface brightness MGC galaxies in the area but failed to see the Aquarius dwarf.  I must admit the AQU dwarf may have been impossible for my aperture, but never let that stop you.

The main limitation for low surface brightness objects is sky brightness.  The Aquarius dwarf is at around 23.6 MPSAS with little in the way of central brightening.  In 20.8 MPSAS sky the sky itself is 2.8 magnitude brighter than the galaxy.  This is doable if the total integrated magnitude is sufficient, but the dwarf is at about 13.9 mag.  With a size of ~2x1 arc minutes, you would need to be able to detect a 13.9 mag star defocused and bloated to about 1.5' arc second, a tall order. 

 

In ~21.3+ MPSAS sky you would have more of a sporting chance.  I sketched it back in 2016 using the 20" in dark sky, referring to it as "very subtle."  While the sky was quite dark overhead that night (21.4 to 21.6 MPSAS overhead, it was not so dark in the direction of the dwarf, which I happened to check with my SQM-L and measured at 21.14 MPSAS right beside the sketch.


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