Rating transparency and seeing

sharptrack2 (Kevin) · #1 10-12-2015, 10:57 PM

Hi all,

This is probably directed more to the folks in Sydney and more specifically the Central coast.

Before I head off to bed (work tomorrow :-( ) I would like to get some opinions on the quality of the skies tonight (22:00 - 23:00, Thur. 11/12).

I've read the copious amounts of posts and documentation and as there is not a standard scale, I would like to know what the more experienced people thought of the conditions tonight, if they did any observing. This will help me "calibrate" my own judgement of what is OK, good, and excellent.

Thanks in advance,

Good night and happy observing.

glend (Glen) · #2 10-12-2015, 11:31 PM

Well the cloud has rolled back in up here but before it did the seeing was pretty poor, which is understandable given the 85% humidity after all the rain over the past few days. There is a standard if sort that some people use, the Sky Quality Metre - which measures the level of darkness. I have one, and Allan does as well. You could also look at the light pollution map for your area. Easiest is too look at the stars, if the are twinkling the seeing is usually poor - atmospherics at work. This is a big topic.

Eden (Brett) · #3 10-12-2015, 11:48 PM

As Glen correctly pointed out, this is a big topic and a lot of variables come into play. Your actual seeing can be influenced by very local factors such as neighbouring properties (house next door has a hot roof, for example) or local geographical features (hills which influence airflow, lakes and other bodies of water).

I usually check SkippySky to get an approximate idea of what local weather-based seeing conditions will be like and would recommend that others do the same. Anything more specific than that requires measurement from your actual location.

sharptrack2 (Kevin) · #4 11-12-2015, 08:12 AM

guys,

Looks like I have more reading to do.

Quote:

Well the cloud has rolled back in up here but before it did the seeing was pretty poor, which is understandable given the 85% humidity after all the rain over the past few days.

Glen,

Since stars will twinkle just from diffraction due to distance, how do you discern if it is mostly atmospheric causes? Is it simply just cause and effect reasoning?

GrampianStars (Rob) · #5 11-12-2015, 08:14 AM

Quote:

Originally Posted by glend

Well ...... You could also look at the light pollution map for your area. Easiest is too look at the stars, if the are twinkling the seeing is usually poor - atmospherics at work. This is a big topic.

Hi Kevin
Here is the Bortle sky guide

for any area
http://www.bigskyastroclub.org/lp_bortle.html
It should help with identifying your level of darkness where you are

glend (Glen) · #6 11-12-2015, 09:46 AM

Kevin, sorry I responded late last night and didn't have time to find the link. So here is the link to the Light Pollution Map:

http://www.lightpollutionmap.info/#z...yers=B0TFFFFTT

When you open it up, close the central white box which is just informational, and you will see a default map of Europe. Don't be put off by that as it's the starting point. Just click and drag the map overlay over to Australia, then you can zoom in on your area of the Central Coast of NSW. The map provides pretty good street level detail I find, and you can easily see the effect of shopping centres, and other major light sources. There are buttons so that you can go back to previous years data, and it is sad to see how light pollution spreads. I have not worked out a way to save my position and still have to drag it back each time I check it. My local light pollution level is just in the light blue range, but it is getting worse (or closer to Green) each year of the map history. Your area near the top of Brisbane Water is probably close of Orange, or Yellow at best, depending on the exact location of your street, but you can zoom in on it.

Now as to the Sky Quality Metre (SQM) , the details are here:

http://www.unihedron.com/projects/darksky/

The SQM works by calculating the magnitude per square arc sec.

It is a very useful tool for comparing night sky locations, and Allan and I use it to rate the various dark site locations we visit. Typically your looking for SMQ readings well above 21.0 to qualify as a dark site. Bretti, for example, regularly returns readings above 21.7, with Lake Chaffey near Tamworth slightly behind that figure (due to Tamworth Sky Glow on the north-western horizon). Seasonal factors can affect the SQM reading, for example, having the expanse of the Milky Way directly overhead can produce a lower reading due to the relative 'light pollution' from the Milky Way.

Experience with sky assessment can give you a perspective on linking the Bortle Scale with the SQM number but they are separate and the Bortle Scale is somewhat subjective. The advantage of the SQM, in my opinion, is that it is producing a reading based what it is gathering from the sky and thus is objective in nature.

As to my comment on star twinkling, this is a quick look sort of baseline. Atmospherics (winds aloft, differing density movements, etc) produce the observed affect. Ulrich's recent trip around Australia demonstrated this well, as his posts back generally reported a very stable atmosphere - based on his visual assessment of the twinkle. Full details are here:

https://en.wikipedia.org/wiki/Twinkling

Hope all of this helps in some way. The links are worth reading.

BTW, the highest SQM reading that I have seen from my backyard observatory is 20.92, which is pretty good and certainly better than any of the major cities. However, there is a big difference between 20.92 and Bretti's 21.77 and that's why some of us continue to make that trip as often as we can.

sharptrack2 (Kevin) · #7 11-12-2015, 10:50 AM

No problem Glen, I have been staying up late all week long for one reason or another and it was weighing heavily on me as well.

5 am wake up for work gets tough after a couple of "after midnight" observing sessions.

It's too bad my brain is past its prime now a days, seems like every question opens up a whole new avenue of learning, with "days" of reading.

But I do appreciate all the reference material and explanations. I secretly hope, as I get more experience, I will develop a "intuitive" ability to gauge all of this, before my eyes give out from all the reading

. I've bookmarked the links and will start reading.

I am surprised and somewhat disappointed as I thought last night was quite good, from my perspective. I easily found and tracked a couple of dimmer targets, that I have not been able to see up to now. But then maybe I'm just getting better at finding them...

glend (Glen) · #8 11-12-2015, 11:13 AM

Kevin as I said last night the cloud moved back in here just about when you posted but it had been pretty good prior to that - good enough that i was considering imaging. If your using your binos to judge the magnitude on the Bortle scale at least you get abit more 'reach' than the naked eye. If you have the scope out you can easily 'see' the atmospheric conditions by selecting a star and defocus (outward usually) to see the effects of the atmo on your view - if It's wobbles all over the circle your seeing will be atmo limited, if it is rock solid and you can see the collimation rings clearly then you have good seeing, relatively speaking.

sharptrack2 (Kevin) · #9 11-12-2015, 11:59 AM

Thanks Glen,

Yes the clouds started to limit my viewing as well which is why I quit when I did.

Quote:

If you have the scope out you can easily 'see' the atmospheric conditions by selecting a star and defocus (outward usually) to see the effects of the atmo on your view - if It's wobbles all over the circle your seeing will be atmo limited, if it is rock solid and you can see the collimation rings clearly then you have good seeing, relatively speaking.

This actually brings up another question I had, and it still relates to viewing, but is more a collimation question. :-)

When I de-focus either one of my scopes, the image I get is no where near textbook. It has always been a full light circle, with multi-colored scintering inside. On the newt I can see the spider as well. I have concluded that this was due to atmospheric reasons, mostly heat related. Am I correct in the understanding that this is probably more a combinative effect of the atmosphere in general, including light pollution, and that I would never see the collimation circles until I have a much better sky to look through?

glend (Glen) · #10 11-12-2015, 12:12 PM

Kevin, Goggle 'star test collimation' and just look at the images it returns. Some of the images show the effects of atmo turbulence, and there are good seeing examples as well.

sharptrack2 (Kevin) · #11 11-12-2015, 03:15 PM

Thanks Glen,

I have looked through most if not all of the first 15 returns of a search like that. The closest that comes to what I'm seeing is the Youtube video, https://www.youtube.com/watch?v=WjjpaNxUfy8.

I have not been able to recreate the concentric rings that so many examples say should be there, and as I focus through and past focus in the opposite direction, the diffraction spikes are not symmetrical.

Without going into too much detail and migrating this thread towards collimation, I can say with 100% confidence that I have a collimated scope (using a cheshire and laser). If I can get enough clear sky tonight after the southerly arrives, I'll try to take a photo or video (try out my new cheap chinese made universal camera adapter

) Doubtful about the weather, but I'll try.

I'm suspecting a poorly finished mirror or I have introduced micro-scratches when I cleaned the mirror before I knew how to clean the mirror.

But that doesn't explain everything I'm seeing.

And all of this is just in the Newtonian. I haven't yet worked long enough with the 8" SCT to know if anything is unusual there. The previous owner used it regularly so I am reasonably sure it should be fine.