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Seeing gage for EAA?

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#1 SanjeevJoshi

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Posted 05 March 2021 - 10:29 AM

I am interested in making up a seeing gage for EAA that will help me decide if it is worth setting up based on what I am trying to do.   Too often the forecast lets me down.  F2 or slower?  Galaxies or nebulas?

 

so concept is this.   Some kind of a finder scope.   Connected to a camera like 462 MC ideally, small and compact or I get out a bigger one worst case.   Finder to cam to my mini PC.  Some simple fixed stand.

 

Point it directly overhead, take a quick measure and see if worth it at all to set up.

 

What do you all think?

 

the only component I don’t have is a finder scope / small scope that has sufficient quality for FWM measurement.    One option perhaps is to get a small refractor that could do double duty over time for observing??

 

 



#2 jerahian

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Posted 05 March 2021 - 12:43 PM

I am interested in making up a seeing gage for EAA that will help me decide if it is worth setting up based on what I am trying to do.   Too often the forecast lets me down.  F2 or slower?  Galaxies or nebulas?

 

so concept is this.   Some kind of a finder scope.   Connected to a camera like 462 MC ideally, small and compact or I get out a bigger one worst case.   Finder to cam to my mini PC.  Some simple fixed stand.

 

Point it directly overhead, take a quick measure and see if worth it at all to set up.

 

What do you all think?

 

the only component I don’t have is a finder scope / small scope that has sufficient quality for FWM measurement.    One option perhaps is to get a small refractor that could do double duty over time for observing??

 

That would work.  You should look at using MetaGuide for seeing monitoring.  It's main use is for guiding and collimation, but it also allows you to "measure size of the star spot as it moves around over each 2-second interval, with live plots."

 

https://smallstarspo...1853_1810275633

 

CS, Ara



#3 SanjeevJoshi

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Posted 05 March 2021 - 12:48 PM

Thank you.   What is the smallest scope you think could work for this?   Yes I have used metaguide, nice app.   Presumably needs to have enough FoV to do it quickly, and support a small camera backfocus.



#4 jerahian

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Posted 05 March 2021 - 01:21 PM

Thank you.   What is the smallest scope you think could work for this?   Yes I have used metaguide, nice app.   Presumably needs to have enough FoV to do it quickly, and support a small camera backfocus.

Well, it would be the combination of both the scope focal length and the camera pixel size you would be using.  Those two values would give you the resolution of your imaging train, in arcseconds per pixel.  This is often referred to as your pixel scale, as well.  The formula is:

 

pixel_scale = ( pixel_size / focal_length )  x  206.265

 

For average seeing, you would want your pixel_scale to be around 1" / px.

 

So, since your ASI462MC has 2.9 um pixel size, this would calculate your ideal scope focal length to be ~ 600mm.  If you have better seeing where you live, you could go even longer.  Anything shorter may cause you to undersample and thus not measure your FWHM effectively enough.

 

-Ara


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#5 SanjeevJoshi

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Posted 05 March 2021 - 01:27 PM

thank you.  i will post a picture of my contraption when i have it ready :)


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#6 GaryShaw

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Posted 05 March 2021 - 05:52 PM

I am interested in making up a seeing gage for EAA that will help me decide if it is worth setting up based on what I am trying to do.   Too often the forecast lets me down.  F2 or slower?  Galaxies or nebulas?

 

so concept is this.   Some kind of a finder scope.   Connected to a camera like 462 MC ideally, small and compact or I get out a bigger one worst case.   Finder to cam to my mini PC.  Some simple fixed stand.

 

Point it directly overhead, take a quick measure and see if worth it at all to set up.

 

What do you all think?

 

the only component I don’t have is a finder scope / small scope that has sufficient quality for FWM measurement.    One option perhaps is to get a small refractor that could do double duty over time for observing??

Why not just purchase a simple SQM meter? I use it to check the sky every clear night. 



#7 SanjeevJoshi

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Posted 05 March 2021 - 07:52 PM

Why not just purchase a simple SQM meter? I use it to check the sky every clear night. 

 

Cool, thank you.  I looked it up, I cannot tell how to use it for seeing (vs. sky brightness in general).  How do you use it to separate below average, average, and better than average seeing?



#8 Rickster

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Posted 05 March 2021 - 08:56 PM

I second the use of an auto guiding program.  I haven't used Metaguide, but I do know that PHD2 works well for judging seeing conditions.  It has real time displays that give you a very good idea of what seeing is doing.  And, as crazy as it may seem, you don't need a high resolution rig to make it work.  PHD2 measures the centroid of the selected star, down to a small fraction of an arc second.    I have been able to guide at less than 0.5 arcsec RMS using a 50MM finder scope cobbled to a 290mini.  It is focal reduced to 142mm focal length using a $6 focal reducer that generates halos.  The theoretical arc sec/px is about 4.  But the FWHM is no better than 15 arc sec.  So you don't need to worry much about arc sec/px if using PHD2.  The same probably goes for Metaguide.


Edited by Rickster, 05 March 2021 - 08:57 PM.


#9 SanjeevJoshi

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Posted 05 March 2021 - 09:47 PM

I agree, i am trying to figure out what combo of guide scope or refractor and camera can be used for this in a simple way.    And by simple I mean, i carry it out on a stand, pointed up, take a few quick measurements with PHD2 or metaguide, and make my plans.   I dont want to be selecting stars, etc.  Hoping to use a field level calculation if I can avoid pointing (eg. using multi star FHWM in Sharpcap).

 

From what i can see, a refractor type design with 600 ish focal length could do the job well.

 

If i use a small guide scope, it likely will not work without pointing.

 

The sky meter is an interesting idea if Gary is able to correlate it to seeing from his experience.


Edited by SanjeevJoshi, 05 March 2021 - 09:48 PM.


#10 SanjeevJoshi

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Posted 05 March 2021 - 09:57 PM

As food for thought, something like the TPO ultrawide with a 2X barlow maybe?  That will get the focal length to about 360 mm, can work with my ZWO 183 MC Pro well enough I think.

 

https://optcorp.com/...hotography-lens

 

that is, unless Gary's sky meter idea can work, will wait to see what he says.



#11 dcweaver

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Posted 05 March 2021 - 11:06 PM

This is a timely topic since I am also struggling with a way to determine seeing conditions.  My confusion is a bit more fundamental.  SharpCap or Phd2 provide FWHM values, but they seem to vary with the particular star being measured.  Sirius, or other bright stars for example, give FWHM values that are typically greater than 20.  I Have tried dialing the exposure back to get the histogram in better shape, but that still gives mixed results.  Smaller, dimmer stars, seem to produce values closer to FWHM that I would expect for the current atmospheric conditions I am seeing, e.g. 6-8.

 

Is there a recommended star magnitude, exposure setting, or histogram shape that is recommended for FWHM measurement.



#12 GaryShaw

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Posted 05 March 2021 - 11:15 PM

Cool, thank you.  I looked it up, I cannot tell how to use it for seeing (vs. sky brightness in general).  How do you use it to separate below average, average, and better than average seeing?

Pretty quickly you develop a sense for what’s typical for your site. Then when the reading is lower I often then have a rationale for going with a good book and a beverage inside. When it’s higher, I know it’s time for a long night to take advantage of the conditions. You are right though Sanjeev, it is really just for measuring darkness and not so much seeing. Sorry I was off point on that. 
Gary 



#13 SanjeevJoshi

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Posted 06 March 2021 - 02:01 AM

No worries Gary, i will get one any way and learn with it.

 

DW -- On an unrelated topic, I have been playing with the semi-automatic focus functionality in Sharpcap Pro.

 

I think that the multi-star FHWM measurement, especially average out over a handful of cycles is likely good enough.

 

So i am thinking of doing two things.  Not 100% sure but heading in this direction.

 

Get a sky meter and develop some learnings from it with every observing night -- thanks Gary.

 

Put a simple measurement system together and see what i can learn from it.   Perhaps thru a combination of the sky meter and taking measurement with the simple system over some sessions, I can learn more about this.



#14 Noah4x4

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Posted 06 March 2021 - 02:31 AM

I have been very disappointed with APPs like "Clear Outside". However, I have found "Good to Stargaze" much more reliable at predicting fickle UK conditions. To be fair, I leave my scope fully assembled so I can be up and running inside five minutes, but it encourages undue optimism!

#15 barbarosa

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Posted 06 March 2021 - 05:05 AM

Go outside and look. Seriously you can assess seeing better than you might think.

 

Check the near real time sat imagery, the cloud cover maps, the winds aloft at windy.com. This is in my opinion the best all in one weather site on the web. Even the free version has more tools than you can tryout in a single session.

 

Use the weather app in Windows 10 for a slightly different look at cloud cover and radar. The sat imagery is very good and the graphical cloudy forecast is also pretty good for showing you what is headed your way.

 

 If you get all excited about FWHM (I want 2 but settle for 3 or 4) point your scope and measure it with SharpCap 's tools, or your guiding software.  But hey that means you are already setup. 

 

SQM? We ain't got no SQM. We don't need no SQM. I don't have to show you and stinkin' SQM!

 

Its EAA for goodness sake.


Edited by barbarosa, 06 March 2021 - 05:38 AM.


#16 alphatripleplus

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Posted 06 March 2021 - 07:22 AM

I usually rely on the Astrospheric website for a local estimate of seeing conditions at my location. For EAA, I will usually take below average or better and go out. If it is windy, I usually don't bother. I find that Astrospheric's seeing forecasts are more reliable than estimates of cloud cover.


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#17 dcweaver

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Posted 06 March 2021 - 10:10 AM

Funny you mention the calibrated "eyeball" approach.  It does work remarkably well.  The other night Sirius was flickering like a candle and a quick FWHM measurement confirmed lousy seeing.

 

The interest in measuring FWHM is more of a long term thing.  I am interested in gravitational lensing and want to assess whether it's worth spending money on equipment that has a chance of splitting 1 arcsecond doubles, triples, and quadruples.  I live in the same general area West of Awahnee and was trying to quantify seeing conditions to see if they are ever good enough, and frequent enough, to warrant the cost of equipment that can resolve small, closely spaced, objects.


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#18 Rickster

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Posted 06 March 2021 - 12:07 PM

I played around with this concept last night.  Here are some random thoughts.

 

Its nice to have a good idea of what to expect well before the sun goes down.  Astrospheric and a winds aloft site work best for me.  I typically use aviation weather website for winds aloft because, as a pilot, I am used to it.  But I think most people will find windy.com to be better.

 

An initial assessment using binoculars is very telling.  Look at a bright star low in the sky (so that you are looking through as much air as possible).  If it is blinking, or looks like a fireworks display, its not going to be good.

 

A quick to set up meter is going to be a challenge.  It will probably be as much trouble as doing an initial assessment with your main rig.  If it is to work, I am thinking that the target should be Pollux or one of the dimmer stars near Pollux.   It is always in the same location.  Any other area of the sky is going to need tracking.  With Pollux, one could set up a pier (e.g. fence post) with a stationary cradle for the scope.  And using the same star/area makes night to night comparisons meaningful.  Also, Pollux is bright enough to be seen during twilight. And Pollux is a double star.


Edited by Rickster, 06 March 2021 - 12:10 PM.

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#19 SanjeevJoshi

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Posted 06 March 2021 - 02:29 PM

Excellent guidance Rickster!  I should have added in my original post that I generally find a couple hours if I am lucky on a typical night.   Often need to get to bed by 10 for early morning calls, so anything that avoids a waste of set up (and take down) is a blessing.

 

Plus its just curiosity.  Seeing is so important but we dont necessarily have a way to measure it.  Thats what got me thinking.

 

I am going to try out the combination of sky meter and binoculars along with FHWM measurements in my EAA set up to start developing a sense.   I think the sky meter part of it is easy to do, and at some level the sky glow / sky brightness change for the same observing location should imply a change in transperency at a minimum.



#20 hornjs

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Posted 06 March 2021 - 02:47 PM

I have relied on Astrospheric up until last night.  Its usually pretty accurate for poor conditions and when not to setup.  Last night I was blue and dark blue in all the important categories...seeing, transparency, cloud cover.  So I got everything setup in preparation for a stellar night, but after  nightfall the sky just seemed hazy AF.  Star brightness was bad and just couldn't see much at all.  So came inside and checked cleardarksky which had white and grey in the ECWF cloud model.  So did very little as a result and packed up.  I'm gonna check the two over the next couple nights because they are divergent in their forecast we will see.  




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