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Relationship Between SQM, Elevation and Resolution

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#1 Peter10

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Posted 13 June 2020 - 01:23 AM

I am looking to build an observatory in West Virginia and am in the process of identifying a site.  Since I am performing this search remotely using a computer, it is difficult to assess the impact of various factors on image quality.  I will be performing mostly deep sky astrophotography at a focal length of approximately 1200mm.  My question is how to determine the dominant drivers that will impact image resolution. Particularly, how do you trade off SQM readings and elevation.  Two of the sites I have been looking at have the following numbers.

 

Site 1:

SQM 21.65 mag/arc-sec^2

Elevation:  1022 meters

Brightness: 0.236 mcd/m^2

Bortle: Class 3

 

Site 2:

SQM 21.90 mag/arc-sec^2

Elevation:  267 meters

Brightness: 0.189 mcd/m^2

Bortle: Class 2

 

Site 2 has a higher SQM reading, however; it is also at a lower elevation.  Considering that I will be imaging through more atmosphere at site 2, does the SQM reading really tell the whole picture?  Is there generally an easy way to sort this sort of thing out?

 

Peter

 

 



#2 Peter10

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Posted 13 June 2020 - 01:28 AM

I should point out that I am aware that SQM (measure of sky brightness) is not fundamental to resolution. I should have stated "overall image quality" as my goal!

 

Peter



#3 Astro-Master

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Posted 13 June 2020 - 01:45 AM

A site with good seeing is more important than a few extra tenths of a percent on the SQM, IMHO.


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#4 TOMDEY

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Posted 13 June 2020 - 02:47 AM

Both matter; the research you are doing is good. Being that they are all within ~striking distance~ of each other... and your home? At this point other considerations take over. >>>

 

>likelihood of ~development~ in the areas. Expressways, housing, congested flight paths. Those things that could degrade the skies over years or decades.

>weather and climate statistics

>building codes, permits, requirements, etc.

>availability of electricity, cable, water, gas, sewer, roads, well water depth, drainage

>police, hospitals, stores

>availability of land and cost

 

That is, once you actually decide to do it... those other considerations can often be overwhelming. When I decided to build, I actually asked the realtor to research "remote depressed dark rural, with no signs of development, within a 50 mile radius of where I work" ... the opposite of what most buyers are looking for. The realtor absolutely refused to believe that was what I wanted, and kept showing me quasi-rural properties too close, too bright, too developing... When I finally found the best place, she suddenly appeared, wanting her half of the finder's fee.

 

Paramount: When you think you've found the best spot... go there, in the middle of a few good nights... and observe!    Tom


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#5 Peter10

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Posted 13 June 2020 - 03:11 AM

Great input Tom and Astro-Master.  I like your description "remote depressed dark rural, with no signs of development".  On my quest, I started out using the "light pollution map" to isolate regions of the state where the sky was dark. I then used this information to search for lots and existing properties within these zones.  The one challenge is sorting out the nominal atmospheric water vapor content to gauge typical sky transparency.  Not sure how to view this data historically.  The "consideration" you posted are very helpful!

 

Peter



#6 Jon Isaacs

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Posted 13 June 2020 - 04:28 AM

The seeing depends on local conditions so it's difficult to predict.  I observe at two sites, one at 120 meters, one at 1100 meters. The seeing is nearly always better an the lower elevation because it's near the ocean.

 

Prevailing winds, ridges, geography, these all effect the seeing.

 

Tom's recommendation of going there and spending a few nights is a good one.

 

Jon



#7 LDW47

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Posted 13 June 2020 - 08:00 AM

The seeing depends on local conditions so it's difficult to predict.  I observe at two sites, one at 120 meters, one at 1100 meters. The seeing is nearly always better an the lower elevation because it's near the ocean.

 

Prevailing winds, ridges, geography, these all effect the seeing.

 

Tom's recommendation of going there and spending a few nights is a good one.

 

Jon

I agree, how can you make an educated decision without being there for a while, preferably several times under different conditions, to actually test the sites out ? Your wallet depends on it, lol !  Clear clear skiys !



#8 LDW47

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Posted 13 June 2020 - 08:02 AM

Any project like this will take, maybe extensive, leg work !  Clear lengthy skize !



#9 TOMDEY

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Posted 13 June 2020 - 10:41 AM

Any project like this will take, maybe extensive, leg work !  Clear lengthy skize !

When I decided on my rural location... I drove a total of 2000 miles, meandering all over the region surrounding my work place, within a radius of 60 miles. There was something ~other~ wrong with nearly all properties that looked good on paper. My final choice was where we still are and enjoying, now, 40 years later!

 

>depressed rural

>superior darkness

>superior elevation

>superior seeing

>sparsely populated and decreasing over time

>nearby, surrounding State Parks ~forever wild~

>surrounding farms, lakes, and forests

>wind developers chased out

>lakefront developers chased out

>no mosquitoes (due to elevation)

>expressway far enough away

>hospital @ 10 miles... small local annex of big teaching hospital

>decent local schools @ 6 miles

>local town fire, police, ambulance

>relaxed minimalist rural building codes, big observatories not an issue

>supportive local government... not ~pushy busy-body~

>supportive neighbors

 

The list goes on. Worked out well!    Tom

 

here's short video showing the observatory just 1000 feet from the house, and one of the many local park lakes >>>

 

https://www.youtube....h?v=9DUMPgsY0dw

 

and an image of the observatory from the other side of the valley >>>

 

~click on~ >>>

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#10 Astro-Master

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Posted 13 June 2020 - 11:06 AM

When I decided on my rural location... I drove a total of 2000 miles, meandering all over the region surrounding my work place, within a radius of 60 miles. There was something ~other~ wrong with nearly all properties that looked good on paper. My final choice was where we still are and enjoying, now, 40 years later!

 

>depressed rural

>superior darkness

>superior elevation

>superior seeing

>sparsely populated and decreasing over time

>nearby, surrounding State Parks ~forever wild~

>surrounding farms, lakes, and forests

>wind developers chased out

>lakefront developers chased out

>no mosquitoes (due to elevation)

>expressway far enough away

>hospital @ 10 miles... small local annex of big teaching hospital

>decent local schools @ 6 miles

>local town fire, police, ambulance

>relaxed minimalist rural building codes, big observatories not an issue

>supportive local government... not ~pushy busy-body~

>supportive neighbors

 

The list goes on. Worked out well!    Tom

 

here's short video showing the observatory just 1000 feet from the house, and one of the many local park lakes >>>

 

https://www.youtube....h?v=9DUMPgsY0dw

 

and an image of the observatory from the other side of the valley >>>

 

~click on~ >>>

Tom,

 

It looks like you did a good job researching the best spot for your home and observatory.  Just how dark are you're skies with a SQM.  I usually get a SQM reading of 21:40 to 21:65 in the desert 75miles from home, but each year it get a little brighter.

 

Bruce



#11 TOMDEY

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Posted 13 June 2020 - 11:12 AM

Tom,

 

It looks like you did a good job researching the best spot for your home and observatory.  Just how dark are you're skies with a SQM.  I usually get a SQM reading of 21:40 to 21:65 in the desert 75miles from home, but each year it get a little brighter.

 

Bruce

Hi, Bruce!

 

My SQM here measures between 21.3 and 21.5, which is good for New York, and has not increased with time. The seeing is usually decent and frequently superior, which I think is related to the local terrain and vegetation.    Tom



#12 Peter10

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Posted 13 June 2020 - 02:44 PM

Tom:

 

Great job with the observatory!  With any luck, I will slog through the issues associated with finding the ideal location for my dome.  I think the next step for me is to spend some time at the candidate observatory locations.

 

Peter



#13 LDW47

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Posted 13 June 2020 - 04:25 PM

When I decided on my rural location... I drove a total of 2000 miles, meandering all over the region surrounding my work place, within a radius of 60 miles. There was something ~other~ wrong with nearly all properties that looked good on paper. My final choice was where we still are and enjoying, now, 40 years later!

 

>depressed rural

>superior darkness

>superior elevation

>superior seeing

>sparsely populated and decreasing over time

>nearby, surrounding State Parks ~forever wild~

>surrounding farms, lakes, and forests

>wind developers chased out

>lakefront developers chased out

>no mosquitoes (due to elevation)

>expressway far enough away

>hospital @ 10 miles... small local annex of big teaching hospital

>decent local schools @ 6 miles

>local town fire, police, ambulance

>relaxed minimalist rural building codes, big observatories not an issue

>supportive local government... not ~pushy busy-body~

>supportive neighbors

 

The list goes on. Worked out well!    Tom

 

here's short video showing the observatory just 1000 feet from the house, and one of the many local park lakes >>>

 

https://www.youtube....h?v=9DUMPgsY0dw

 

and an image of the observatory from the other side of the valley >>>

 

~click on~ >>>

Lucky guy but you worked for it so you earned it !  Clear black skiys !




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