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Transparency and Limiting Magnitude

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#1 Eddgie

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Posted 19 January 2020 - 11:18 AM

 A very nice observing session last night at Mansfield Dam about 15 miles west of downtown Austin, Tx.

 

We have been either moony or cloudy for about two weeks but a strong front came in from the northwest last night and these fronts are dry and (by Austin Tx standards) cold so it was time to take my new (to me) Orion 8" f/4 and my new (to me) Az Mount Pro out to Mansfield Dam for a nice couple of hour session.

 

First, I wrote up the Az-Pro and thought that my level was off, then decided the level was not off, but last night I could see that once loaded, there was some deflection as the scope moved around the Az axis.   I guess a second counterweight is in order.  Aside from having to do some "Align" functions, the mount worked fantastically well!  It is so quiet that it is difficult to hear over the ambient noise of the deep suburbs, so I caught myself looking at my phone to see if the mount had acutally moved on shorter slews.   It is uncanny.  Also, except for turning it on and doing my alignment stars (once again, way off on Az at the one alignment star, but very close on altitude), I did the entire session using the Sky Safari Pro.  I can report that with gloves on, the screen (Galaxy S9) is not that responsive, but it was enough that I continued to use it for the entire session. After alignment, not once did I use the handset.  

 

Also, the wind was quite strong and except for the concessional hard gust, the mount was dead steady.  I mean rock solid.  I would feel this big gust of wind and see the field bounce maybe a quarter of a degree, then bounce back.  I love this mount.    

 

The 8" worked great!  I used it at f/4 and with the filter wheel, it was quick and easy to make comparisons.  The true field is a bit narrow at about 1.3 degrees but most small nebula like Orion fit comfortable into the field of view. The scope was an aboslute joy to use though, and I got it at a super great price, so the Matrix was looking out for me.

 

All of this typing and I am just now getting to the meat of the post, which is my observations with transparency. When we get these dry fronts with strong winds, these fronts do an amazing job of scouring the dust and air pollution (which are real uncredited culprits in light pollution in that without these things, light would not be reflected back towards from the sky).

 

My typical central Austin back yard SQM-L reading is typically between 18 and 17.5, so not NYC or LA levels of light pollution but pretty bad. On the Bortle Scale it would range from 8 to maybe 8.5.

 

At Mansfield dam, a typical reading in the summer would be maybe 18.7 so Bortle Scale would be "Bright Suburban" Bortle 6. Now 15 years ago, the dam was probably transitional to Bortle 4 but I did not have an SQL meter back then so I could not rate it other than to say that visually, the Milky Way while not lustrous with detail, but easily visible.   10 Years ago, there would have been 10 telescopes out at Mansfield Dam on a clear night, and last night, I was completely alone.    

 

Anyway, the first thing I did when I got to the dam was pull out my SQL meter and shoot the sky, and in the direction of Orion, I got a pretty fantastic 19.52!  Now I know that this does not seem all that dark, but for Mansfield Dam, that means it is going to be a really good night! Bortle rating on this is "Suburban" and if this is what life in the suburbs is really like, I would say that with Night Vision, going to very dark skies would not be nearly as appealing as it is for people form Bortle 8 skies.  But let me say what this meant to observing.

 

Again, the sky was dry. It was clearly a bit darker than typical, but it was the dry air that I was excited about.  My first subject, even before I got the telescope out, was done with my new Mod 3 binocular with thin film P43 MX-11769 tubes and a pair of H-a filters, a 7nm and a 6nm.

 

Now under my typical skies, I Barnard's Loop is visible and while it isn't really "dim," it is attenuated enough by atmospheric absorption and particulates that to see it, I have to pan around. Some of this is due to the band shift of the filters and when looking at it, it is easy to think that this is the primary culprit, and clearly it is a factor, but it is more noticeable when the signal is interfered with and attenuated.  Above the loop, the profile of the fish can be difficult to discern even when it is in the center of the field. Mostly what one sees is the leading edge of the fishes face and it seems to fade pretty aggressively as you move back to the side of the fish.

 

Last night!  Wow!  The best view of this area I have had since my last trip to really dark skies!  The entire loop was fully visible without panning around and the amount of structure was considerable.  Barnard's Loop is rich is structural detail and even at 1x, it continues to astound me with just the fact that it exists at all, and to see it in such favorable conditions is one of my all time favorite things to do.   And the fish!  Under transparent skies, the profile is hard and distinct, and you can follow the body out several degrees across the sky. This is such a remarkable feature in the sky and if it were swimming anywhere else in the sky other than around Barnard's Loop or the California Nebula, it would totally dominate your attention. Flame was rich and the Horse Head was practically bright, and even at 1x, the notch was super easy to locate.

 

Now I have seen these things from Bortle 2 skies, so it is not a surprise to me how utterly beautiful they are, but the point of the post is to say what can be done on a good clear night even from the suburbs. When the sky is dry and dust free, with NV, and SQM is even 19.5, these objects really come alive.  

 

California Nebula was vast.  This is another giant. Most long exposure images simply fail to capture the full extension.  On a typical night from my back yard, I see what the imagers usually show, which is the bright namesake structural element, but even in 19.5 dry skies, this thing goes far past what these pictures tend to show.  Here, the H-a overlay in Sky Safari Pro comes far closer to the truth, showing it as more like 4.5 degrees vs the 2.5 degree typical catalog size.  This thing is a giant Comma in the sky and when the sky is even moderately dark but very dry, it blazes to near its full size (though it is bigger under very dark and dry skies, here it is seen almost as big as it can be seen. Perhaps 5.5 degrees under very dark and dry skies).  There was much more, but I did actually use the telescope.

 

The big big surprise in the telescope was M1, the Crab Nebula.  From my home skies, M1 is very disappointing. It is simply pale and inconspicious.  The 12" f/4.9 struggles to show it as more than a hazy patch, and from the back yard, the 6" f/2.8 shows it brighter, but so small that it is not very appealing.    Under these 19.6 skies (I shot this one with the SQM-L meter to get a local reading on it) in the 8", even at f/4, it was bright! I mean even unfiltered, it was absolutely easy to see.  Now at this scale, it does not reveal fine detail, but since it was so bright, I turned to the unfiltered Barlow in my filter wheel and was pleased to see that it was still very bright (scope is probably working at f/6 with the filter wheel) and some of the structure in the outline became a bit more pronounced, though still there was not much internal structure but it was easy to see that it was not featureless.  It looked kind of mottled.  This was a giant surprise though because this was perhaps my best view ever of the the Crab. 

 

I should mention that for these observations I was using my newest Mod 3 with higher PCR and gain than my other tubes, and while the EBI is a bit higher than my other tubes (.5 vs .1 and .2) on cold nights like this, EBI is improved (it is hard to improve on .1 so that tube performs about the same regardless of temps).  I do not think the tube made the difference here, I think it was the conditions.  The humidity was probably in the 20s, and that is what was at work here (it is 28% as I write this).

 

Rosette.  Stunning.   Does not fit in the field, and the scale for dust lanes is kind of small, but still I was pleased to see how much structure was visible.  Always visible even from my yard, but under dry skies, the level of detail really kicks up.

 

Galaxies were pretty good too but this scope is too small to show galaxies that well. M82 was bright and the inclusion was dead easy to see and very pronounced but even from my home on a decent night, these features are easy to see. (I did Barlow it with the filter wheel and this went form about 30x to perhaps 45x and this did indeed make a nice difference, but it is better in the 12" at f/4.9 simply because it is even bigger).

 

I did though poke in on the Whirlpool and while it is typically difficult from my back yard in any scope, here, even at f/4, it was easy to see the core and there was some very faint spiral structure, but really faint. 

 

On M31, the dust lane was visible at 30x but only as a small slightly darker trough across in front of the nucleus.  Still, this is pretty good for this location.

 

I did a lot of off the beaten path clusters (Be, Tr, others) but these do not get the same gain as nebula do. Still, the Sky Safari and WiFi control make it so easy to find these that these are high priorities on my observing list.  They could be difficult to find using conventional scopes and I was too lazy to put in all of the coordinates before, but the Az Mount Pro and WiFi, along with Sky Safari Pro make is sooooooo easy that now I am on a mission to see all of them!

 

I usually wrap up with doing some more low power work after the scope and mount is back in the car, and this time, I did my close at 3x.   Andromeda was spectacular.  Once again, improved over typical dam conditions and much better than home, with maybe 3 degrees of extension.  The companions are easy even at 3x.

 

M45 Is brilliant at 3x.  Such a jewel.  Again, stellar targets or not so much better even when the sky is dry but I love to see some of these objects. 

 

Before going, I put in the H-a fitlers and scanned.   The surprise here was the Monkey Head Nebula. I had observed it in the 8" and it was very nice, but what surprised me was how brilliant it was in the 3x view.  I think the gain on these thin film tubes must be pretty high though and my eyes seem to respond really well to the P43.  I bought the thin film binocular because I like the way it does with stars, but Monkey Head seemed profoundly bright last night.

 

The Heart and Soul showed amazing structure.  It is typically easy to see even from my home on a typical night, but under these very dry skies, there was a lot of structure that I cannot normally see from the Mansfield. At this point I was cold though but I wish I had started the night with 3x.   At 3x, the Rosette/Cone complex was all connected by nebula.   How amazing.  California Nebula though was astounding. At 1x, you don't get much structure, but at 3x, it really comes alive.  I wish I had brought my 180ED Nikon lens for these two.  3x Is wonderful, but here, I think 6x would have been pretty titanic. 

 

Last view of the night was at home when I got out of the car.  I had left the H-a filters on the binocular and when I got out of the car, I shot SQL for my home location.   The SQL was 18.5 and this is pretty good for my home, but the transparency was not as good.  When I looked at Barnard's Loop, it was almost fully seen without sweeping, but the Anglefish really suffered.  This is I thing primarly due to the fact that there is more smog and dust over the city.  Mansfield is not all that much darker, but it almost always has better transparency.

 

Last night was a rare night for me.  These frontal systems almost always provide some of the best nebula observing I do.  Even at my dark sky locations, dust is often a serious issue and humidity piled on top of that can really attenuate nebula.    The difference that transparency makes is in my book, about 85 miles, which is about how much further I have to drive to get to Bortle 3 skies. 

 

A long read for you I am sure, but if you made it this far, thanks for reading.  I know for many more norther observers, humidity can be very low in the winter and even in suburbs, this kind of result might be more common, but for me, it was a treat. 

 

Have a most excellent day! 

 

 


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#2 ShaulaB

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Posted 19 January 2020 - 11:41 AM

At almost 11am it is 16 degrees F where I live. What a treat to read that someone is having a good observing run! Thanks Eddgie.


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#3 Eddgie

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Posted 19 January 2020 - 11:56 AM

At almost 11am it is 16 degrees F where I live. What a treat to read that someone is having a good observing run! Thanks Eddgie.

Yeah, that is pretty brutal.  I has not been 16 degrees in Austin since I lived here.

 

It was probably in the high 30s out at the dam last night when I wrapped up, but I did not expect the wind chill to be so bad. I had a heavy jacket with a hood and a stocking cap under that, so my core and head were warm, but I had light tennis shoes and light gloves and my hands and feet got really cold.  My bad for not wearing some thick socks and boots.  I could have stayed out another hour or so.  

 

16 Though, I don't think I would be outside for more than a few minutes at a time. 



#4 Eddgie

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Posted 19 January 2020 - 12:33 PM

This is the screen capture from Sky Safari Pro.  Sky Safari Pro has an H-a overlay in the Milky Way settings. For image intensified astronomy I leave this on all the time.  The reason is that often, imagers only capture things up to their threshold of pain and patience, but since I started using image intensifiers, I have found that almost all of the showcase objects are iceberg tips and that when viewed in H-a with an image intensifier, the boundaries typically understate the full extension and sometimes quite dramatically.

 

The circles represent the 2.4 degree field of the 6" and the .8 degree field of the 12".  The 8" field is not shown, but it would be 1.3 degrees with no reducer.

 

thumbnail_1579454019314.jpg

 

As can be seen, the chart shows an outline that does not quite fit into the 2.4 degree circle but when actually using the 6" at f/2.8, even from the city, it generally fills this circle but it does not appear bright and that is just an artifact from the fact that it entirely fills the field of view, so with no dark sky for comparison, it is "hard to see."  I mean you see it.  You are looking right at it!  But becuase it over-runs the field, there is no contrast to highlight it form the background. 

 

In the 3x lens from the city though, the extension is probably over 4 degrees on a decent night and a bit more on a very clear night. 

 

Last night though, because it was decently dark but super dry and clear, the full extension to the ESE was very similar to what is shown in the overlay.  The extension to ESE seems narrower and brighter than the overlay shows, with a thin, long wispy "tail".

 

This image shows the approximate extension I was able to see last night.  Now this picture shows it with more detail than I could see at 3x, but this picture comes pretty close to showing the extension that was visible last night.  Even on a not so clear night, the tail is a prominent feature, but last night, I think a bit more extension in the real time view than shown here.  It is a very big nebula. 

 

https://pixels.com/f...ld-wittich.html

 

And a big shout out to Reinhold for having a high pain threshold and a lot of patience.  He captured more extension that is typically captured by many imagers.  It is bigger in NV, but past this and it is very diffuse so there is nebula there, but no structure.


Edited by Eddgie, 19 January 2020 - 12:35 PM.

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#5 chemisted

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Posted 20 January 2020 - 03:52 PM

As chance would have it, for the first time in weeks I had a very clear and moonless (but cold and WINDY) night last night and decided to use my Micro in a slightly different way.  I attached it to the Nikon TC-14A telecompressor which was then attached to my Nikkor 105 mm f/1.8 lens.  I used the Astronomik 12 nm filter screwed into the Nikon to C-mount adapter as I believe I first learned from Eddgie.  The final setup gives ~150 mm focal length at f/2.5 and the views were glorious.  Unfortunately I would start out thinking the wind was not quite so bad (temperatures were in the 30s) and then a big gust would come up and chase me inside for awhile to recover.  I probably did this a dozen times.  Anyway, the California Nebula was the best I have ever seen it.  My FOV was just under 7 degrees and the nebula was definitely more than 5 and quite like the image that is posted above.  I did this all just hand holding it since the lens arrangement is light enough for that to be done steadily.  I am quite taken with how well this worked.  The nebulae were all very bright and inviting with plenty of detail at this image scale and I certainly will be doing more of this in the future.


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#6 Eddgie

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Posted 20 January 2020 - 10:01 PM

Great report!  You said "telecompressor" but I take it you meant "tele-extender" and that is a great way to get some scale.

 

Interestingly, your conditions, though colder than mine, were similar in that it sounds like cold front my have passed with dry air behind it.  Maybe not,  but it sounded similar. Those conditions though almost always give some of the best nebula views for me. 

 

Yeah, California Nebula is one big nebula. I never ceases to amaze me how big some of these things are. I mean not only their angular size, but in absolute size.  Catalogs list the distance a 1,000 to 1500 light years and we see it as more than 5 degrees in angular size (though catalogs list it as 2.5 degrees, and we can easily see that it is longer than this).  

 

Wiki says it is 60 light years across! Our five light hour across solar system would be tiny fraction of a tiny fraction of one pixel in and image of the California nebula.

 

What a remarkable thing to be able to see. 


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#7 chemisted

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Posted 21 January 2020 - 06:51 AM

Good catch.  Nikon calls these little gems "teleconverters" and I clearly misremembered that.  I bought all my Nikon system lenses decades ago when they were languishing in the used section of the then-common photography shops.  I had developed a fetish for really fast glass and, boy, has that turned out to be a benefit with these intensifiers.  I have kept nearly all of that film-based equipment for unknown reasons and now I know why!


Edited by chemisted, 21 January 2020 - 06:53 AM.

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#8 Eddgie

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Posted 21 January 2020 - 07:39 AM

  I have kept nearly all of that film-based equipment for unknown reasons and now I know why!

The Matrix



#9 GeezerGazer

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Posted 22 January 2020 - 02:44 AM

Really nice report Ed... enjoyed reading it. 

 

I don't think I've had conditions good enough to see or image the full extent of the California Neb like Reinhold's.  I have imaged it with different optics but mainly with a Nikon 300 with 3.3° FoV... and it does fill that FoV but is clipped at each end from band shift.  I'll have to try it at 150-180mm to see if I can grab a larger extent.  Thanks for that. 




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