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How much sky to you gain by elevating the RA Axis

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#1 BarrySimon615

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Posted 15 July 2013 - 10:19 AM

Possibly like a number of other club observatories, particularly those in the eastern United States or in other "non-desert" areas which are heavily wooded, encroaching trees can be an issue.

At our club observatory located about 50 to 60 miles north of New Orleans, we cleared some land on a lease back in 1994/95 to create our observing field. At that time we could easily see Polaris from where we typically set up. Our west probably had a horizon down to about 7 to 10 degrees, our south to about 15 degrees, and our east to about 18 to 20 degrees. We have had trees (mostly pine) topped before and we have even removed some trees. About 12 or so years ago we even created an "Omega Notch" so that we could see Omega Centaurus in the spring. (Max height from our location for Omega is about 12.2 degrees above the southern horizon.) Today, our horizon to the north is probably 27 degrees above the true horizon, toward the west we can probably only see down to about 20 degrees above true, maybe 25 degrees to the south and 30 degrees to the east. Our sky window is getting smaller.

The "Omega Experiment" showed just how much cutting a lower opening exposed us to light pollution to our south. (On the best of nights our limiting magnitude is about 6.1 magnitude at the zenith, but readings at the horizon fall off and that opening was noticeably brighter than the rest of the sky.

In addressing a new member's query about why we didn't or when will we cut/trim trees, I got to thinking about just how much of a difference a higher mounted tube assembly (think refractor) might make in allowing you to see some tree skimming objects when you have a situation like ours. (the cleared part of our property is about 1 acre in size) Think about it, if the RA axis of a refractor (or schmidt-cass or Mak) sits about 2 feet above the RA axis of a newtonian reflector or the altitude axis of dobsonian reflector, how much of a difference does that make? Without considering angles or distance it would make a two foot difference if say Polaris was at tallest tip of a pine tree in a reflector, and above that in a refractor. The pine tree would have to have another season or two of growth to effect the view of Polaris in the same way, if viewing is done thru a refractor. The height advantage of the refractor on a taller mount becomes more of a factor on smaller observing fields. Something to ponder.

Here is a photo of our field.

Barry Simon

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#2 George N

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Posted 15 July 2013 - 10:42 AM

A number of years ago there was an article in the NY Times about a 16-year old with rich parents who gave him a Meade 16-inch SCT for his birthday. They faced the same tree problem at their ‘vacation home’ in NY’s Catskill Mountains. Mom, an architect, designed an elevated tower observatory to get the scope above the tree line. Four posts, a central pier, with wraparound staircase to a trap door, clamshell observatory. Scope completely isolated from the structure. It not only opened up all of the sky to the scope, and provided them a great new scenic view of the mountains, it greatly improved the seeing by getting the scope above the ground-level boundary layer of hot air. They were quite surprised at how much better the scope performed.

#3 BarrySimon615

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Posted 15 July 2013 - 11:02 AM

George, how high does a ground-level boundary layer of hot air typically extend? I imagine that variables like altitude, season and the nature of the ground all play a role.

Your comments got me doing a bit of research and I found a PDF file that is interesting. I think it gives some detail to what you are talking about (ground level boundary layer). Here it is: http://www.envirocom...lution/Ch.3.PDF

Perhaps someone else can reinterpret this and rephrase it in a meaningful way for everyday utilization in selecting observing sites.

Barry Simon

#4 Jon Isaacs

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Posted 15 July 2013 - 12:49 PM

The height advantage of the refractor on a taller mount becomes more of a factor on smaller observing fields. Something to ponder.



If one is stuck in the trees... it probably makes some difference, I know it helps in my backyard... But out this way, observing sites can have a certain lack of trees...

Jon

#5 Eddgie

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Posted 15 July 2013 - 02:54 PM

It is a basic geometry problem but you didn't want the math, so the answer is that if the trees are not right on top of you, raising the mount won't make any difference.

Also this. Even if you could see lower in the sky (and you won't see enough at 2 feet to make a difference) this won't matter with respesct to the light pollution.

The light pollution in the vicinity of the target is not going to be affected by the height of the mount. If the VLM is 5, then you can put the mount on top of one of those threes, and if a star sits in an area where the VLM is 5, then it will still be 5. It is the glow of the sky around the target that lowers its contrast.

The only way you will fix that problem is to move to the other side of New Orleans.

#6 GlennLeDrew

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Posted 15 July 2013 - 03:53 PM

If we assume a distance to the tree line of 100 feet, a 2 foot gain in height results in an angular gain of a massive 1.15 degrees!

Sarcasm aside, typical variations in mount height count for little, unless the obstructor is quite nearby.

#7 BarrySimon615

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Posted 15 July 2013 - 09:02 PM

If we assume a distance to the tree line of 100 feet, a 2 foot gain in height results in an angular gain of a massive 1.15 degrees!

Sarcasm aside, typical variations in mount height count for little, unless the obstructor is quite nearby.


Re - your post and the one above....as I mentioned before, (math aside), height matters little when the tree line (or other obstruction) is far away. However in the case of closer trees, it matters more, shorten the distance and the effect of a few feet difference in telescope light path height can matter a lot. In the case of our club observing site, a 1.15 degree difference would be a benefit. Could mean the difference between seeing Omega Centaurus and not seeing it. Looking at the picture I posted I will say that the significant trees to the north, behind our dome are approximately 60 feet away to the trunks. These will be topped or removed in the near future.

At an observatory that one of our members manages, he has calculated that the telescope inside should be raised, the pier is too short. Given the lower boundary of the slit opening, raising the pier one foot will give us an additional 7 degrees of sky below the current cut-off. That is significant!

Barry Simon

#8 Jon Isaacs

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Posted 16 July 2013 - 04:41 AM

At an observatory that one of our members manages, he has calculated that the telescope inside should be raised, the pier is too short. Given the lower boundary of the slit opening, raising the pier one foot will give us an additional 7 degrees of sky below the current cut-off. That is significant!



Height has significance when viewing over nearby houses and trees, fences and other obstacles. Of course, being a bit higher doesn't help the localized poor seeing such objects typically cause.

In situations like the one depicted in the photo below, mount height would hardly matter in seeing over the obstacles. When wind is an issue, tall mounts and long scopes are undesirable.

Jon

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#9 Maverick199

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Posted 16 July 2013 - 11:43 AM

Why not just build a concrete platform with steps and move the observatory up. :question:

#10 mikey cee

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Posted 16 July 2013 - 12:15 PM

With my home ROR observatory I've always taken the stance you live with what you have. I've developed the "let it come to you" approach. I'm spoiled rotten with my big refractor sitting on the west side of the pier. I can easily see things rise in the east and follow them into the SW sky. But to see further west I'd have to roll the scope thru the meridian to the east side and use the highest level in my observing chair or use the step ladder. True by doing this I'm losing more time in the west sky but new stuff is constantly rising in the east. It's nice to be able to stretch your viewing horizons like nomadic amateurs do but when you have a permanent set up with obstacles you just have to learn that you can't have it all. Like I'll never see Venus or Mercury at night sky and that's OK because there's nothing I can do about it. Just like "losing" objects sooner in the west sky. Sooner or later it's going to happen and I've learned to accept that loss. ;) Mike

#11 Jon Isaacs

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Posted 16 July 2013 - 12:38 PM

With my home ROR observatory I've always taken the stance you live with what you have. I've developed the "let it come to you" approach. I'm spoiled rotten with my big refractor sitting on the west side of the pier. I can easily see things rise in the east and follow them into the SW sky.



With a more compact scope, a more easily moved scope, one can just move it about as needed. I can't see Mercury or Venus in the evening sky from my backyard so I setup in the front driveway.

Jon

#12 BarrySimon615

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Posted 17 July 2013 - 07:51 AM

Why not just build a concrete platform with steps and move the observatory up. :question:


The pines are growing at the rate of 12" to 18" per year. Do you suggest that we raise our observatory every year by that amount? :shocked: :shocked: :shocked:

Barry Simon

#13 Eddgie

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Posted 17 July 2013 - 08:36 AM

Better to notch the trees. If the trees are growing, are you going to raise the telescope 2 feet every year?

Ignore the light pollution from New Orleans. It will have zero on the overhead sky if you notch the trees. Zero, nada, zip.

If the light bothers your night vision, just put a screen up close to you that blocks the gap.

Raising the mount will not improve the view of low sky objects any better than the notch would.

As I mentioned, you sound convinced that raising the scope is your solution though, so why are we debating this? You could have done it by now for all the talking about it.

Then you will have your answer.

#14 BarrySimon615

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Posted 17 July 2013 - 11:21 AM

My original query was somewhat "tongue in cheek". Having said that, a higher mount does help is some circumstances like in clearing the lower threshold of an observatory dome or roll off roof wall if initial measurements were "in error".

I have been at this for over 40 years and I am not a naïve initiate to observing and what and what you cannot do to mitigate light pollution and just how much altitude you have to have to begin to really improve seeing.

While some of your advice is good -
"If the light bothers your night vision, just put a screen up close to you that blocks the gap."

Some of it is, in certain circumstances" not correct -
"Raising the mount will not improve the view of low sky objects any better than the notch would."

Many of us chase open areas of sky from home. That may mean, in some circumstances, setting up on a concrete or asphalt driveway or a patio deck. Radiating heat can effect the view and this would be a magnified effect with lower objects. Magnified as well with scopes that have a mirror closer to the hot/warm pavement (dobsonian) and less so with a refractor that may have the objective 7 to 10 feet above the pavement.

As hinted at before, I was just pulling the chain a little bit for some of our dobsonian friends.

Barry Simon

#15 Jon Isaacs

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Posted 17 July 2013 - 01:37 PM

As hinted at before, I was just pulling the chain a little bit for some of our dobsonian friends.

Barry Simon



I am most surprised... :lol:

Of course down your way, the highest point in the entire state is only 525 feet above sea level... Around here that's not even a hill.. Some of my spots provide views that below what should be the horizon... not a lot but I calculate a degree or so...

I don't mind a little kidding when I already have it so good...

But now, if I were in the mosquito trapping business, I might be a little irritated because there is a definite lack of mosquitoes out this way and I would be facing possible bankruptcy. :)

Jon

#16 BarrySimon615

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Posted 17 July 2013 - 03:39 PM

Of course down your way, the highest point in the entire state is only 525 feet above sea level... Around here that's not even a hill..

Jon


Yes, but our coastal skies are quite steady. Exceptional planetary seeing is the rule, not the exception. As far as the mosquitoes go....they are our friends. We attach little cleaning pads to their feet and they keep the dew off of our objectives.

Kidding aside, we do appreciate higher altitude. In the first week of September I am going to my sister's getaway about 10 miles south of and 1300 feet above Gunnison, CO. We will be observing from 8960 feet. So I think we will have the height we need without getting into the "Goofy Zone". At least goofy beyond what is normal for me.

Anyone in the area care to join us for some observing some evening? We will have a TEC 140 and a pair of Miyauchi 20x100 binoculars.

From here we head to Monument Valley where we will be staying in the new hotel on the Navaho Reservation "The View", which is in Monument Valley. Our room has an open balcony on the top floor and we are doing this at "new moon" so hopefully those high western skies will not let us down.

Barry Simon

#17 Jon Isaacs

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Posted 18 July 2013 - 06:28 AM

Yes, but our coastal skies are quite steady. Exceptional planetary seeing is the rule, not the exception. As far as the mosquitoes go....they are our friends. We attach little cleaning pads to their feet and they keep the dew off of our objectives.

Kidding aside, we do appreciate higher altitude. In the first week of September I am going to my sister's getaway about 10 miles south of and 1300 feet above Gunnison, CO. We will be observing from 8960 feet. So I think we will have the height we need without getting into the "Goofy Zone". At least goofy beyond what is normal for me.



Barry:

One advantage of San Diego is that one can enjoy the pleasures of stable seeing along the coast while an hour's drive takes one into the mountains for reasonably dark skies and elevations over 6000 feet. In the winter, the desert offers warmer conditions beyond the rain and snow shadows of the mountains.

Jon

#18 Maverick199

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Posted 18 July 2013 - 07:06 AM

Why not just build a concrete platform with steps and move the observatory up. :question:


The pines are growing at the rate of 12" to 18" per year. Do you suggest that we raise our observatory every year by that amount? :shocked: :shocked: :shocked:

Barry Simon


I assumed they have grown to their full height. :)

#19 Eddgie

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Posted 18 July 2013 - 02:10 PM

So basically we responded with answers to help you with the decision only to find that we were basically wasting out time.

Thanks for that.

#20 BarrySimon615

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Posted 18 July 2013 - 09:55 PM

So basically we responded with answers to help you with the decision only to find that we were basically wasting out time.

Thanks for that.


Look at my first post - I narrated a story, I did not ask a question. The one thing phrased as a question was rhetorical and I answered it myself in the next sentence. You chose to answer a question I did not ask.

Later someone else mentioned ground level boundary layers and I did ask more about that. I was curious about that as I am sure others may be. Based on that I have done additional research, but once again, I was not looking for answers to my first post, I just ended with saying the story I narrated was something to ponder.

If you feel you wasted your time.....sorry about that, but I don't see how that could be considered my fault.

Barry Simon

#21 KWB

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Posted 18 July 2013 - 10:01 PM

Now that this is out of the way,let's get back to the topic of the thread,not the criticism of one another. Please.

#22 Starhawk

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Posted 18 July 2013 - 10:10 PM

Taking a look at this, the boundary layer is approximately at the treetops.

If you could be on the bunkhouse roof, it would help!

-Rich

George, how high does a ground-level boundary layer of hot air typically extend? I imagine that variables like altitude, season and the nature of the ground all play a role.

Your comments got me doing a bit of research and I found a PDF file that is interesting. I think it gives some detail to what you are talking about (ground level boundary layer). Here it is: http://www.envirocom...lution/Ch.3.PDF

Perhaps someone else can reinterpret this and rephrase it in a meaningful way for everyday utilization in selecting observing sites.

Barry Simon



#23 Starhawk

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Posted 18 July 2013 - 10:16 PM

What you really need to do is have more than one event at Percy Quinn each year. Those skies were nice.

-Rich

#24 BarrySimon615

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Posted 18 July 2013 - 11:19 PM

What you really need to do is have more than one event at Percy Quinn each year. Those skies were nice.

-Rich


Rich, Percy Quin (State Park) will always be remembered fondly. We held the Deep South Regional StarGaze there for 22 years (1983 thru 2004). However due to increasing lights and a smaller and smaller overhead window, we moved in 2005.

Our current site, the Feliciana Retreat Center, has much better skies. On good nights our limiting magnitude is about 6.5 to 6.6 which is pretty darn good for east of the Mississippi River. Ammenities are nicer too - including motel type rooms, great meals and a very accommodating staff.

We hold the regular Deep South Regional StarGaze in mid October to early November. Approximately 6 months later we do what we call the "Deep South Spring Scrimmage" which is held, naturally in the spring (typically late April to early May). It has fewer people, typically about 20, as is more laid back.

Anybody recognize the winner of one of our door prizes?

Barry Simon

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