Garden observations limitations

[bomberbaz]

Ok I know as do most seasoned observers that light pollution has a huge impact on nearly all objects bar lunar and some planetary. I no longer even bother with any local deep sky observing due to light pollution and even narrowband viewing is not particularly rewarding and not really worth the effort. 

Also I have lost count of the number of occasions I could have had impromptu sessions in the back garden when forecast and weather haven't agreed or just a couple of hours of clear weather have graced the evening. It is such occasions I think I need to be taking more advantage of.

So I think I will encourage myself to target more planetary viewing using the 100ED and also think I will join in the 60mm frac double challenge that I have been loosely following on this forum. 

But what I want to know is, what is the guide / formula etc that provides the following:

"magnitude = aperture - bortle less local light pollution" (if indeed such exists)  so I can segment as it were my viewing list(s). 

No doubt similar queries have been thrown out before, and tbh feel I should know better myself but it is  always good to get a fresh take on such things so any help will be gratefully received.

Tia

steve

 

 

 

[Stu]

There are a few calculators around which factor in LP, aperture and a number of other parameters. Mel a Bartels does a lot on this on his site.

https://www.cruxis.com/scope/limitingmagnitude.htm

https://www.bbastrodesigns.com/ObjectContrastCalculator.htm

[bomberbaz]

12 minutes ago, Stu said:

There are a few calculators around which factor in LP, aperture and a number of other parameters. Mel a Bartels does a lot on this on his site.

https://www.cruxis.com/scope/limitingmagnitude.htm

https://www.bbastrodesigns.com/ObjectContrastCalculator.htm

Thanks Stu,  I shall have a play later on my pc

[Mr Spock]

I've just tried that site. For both the 12" and 4" it gives two magnitudes fainter than I can actually see. I suspect it doesn't take into account the type of light pollution. My sky is grey from LED streetlights. 

[pipnina]

I think the biggest impact for me besides light pollution is atmosphere quality.

I have observed from my back garden and seen M51 easily, I've observed another time and it's been gone. The city lights hadn't changed but the atmosphere sure did!

I've been to my dark site too, last year and I almost couldn't HELP but see M31! I was just stood there and it seemed to jump at my eyeballs from my peripheral vision, and in my dob I could see texture and a hard gap left by the innermost dust bank! I've seen on a year before that at the dark site, the outer winter milky way going through orion where the whole thing was like looking through glass. It had incredible texture and was so bright! I spied the flame quite easily in my dob that night. I also got lucky once with the sky quality and flew around the leo area of the sky. I felt like I could point my dob anywhere and see clusters of galaxies!

Then, in contrast to that there's my most recent session. Murky and opaque. There weren't any clouds but andromeda wasn't visible to my eye alone. The milky way core was visible but seemed dull. I could spy a few friends like the veil but they were less pronounced than I've seen before. Andromeda was a faint blob.

For reference my home is bortle 5 and my dark site is bortle 4, so both are far and away from either extreme end of darkness and city flood lamp. However I am on the coast so haze might not be as big an issue for people further inland?

This country has the potential to give us great views but I feel we need to win a small lottery to get them! I often dream of moving to a spanish mountain so I can get dark and transparent skies AND almost no cloud cover! Sadly I am not sure how I would afford such a lifestyle and astronomy equipment at the same time haha.

[Paz]

In my back garden the issues include high walls/fences and houses in most directions, local direct light pollution, nearby town/city light pollution, and heat from the local houses and roads on the estate where I live. However there is a strip of sky I can see from the back garden and the fact that I can't observe anything low means I observe more at higher altitudes with the benefits that brings. 

[IB20]

My local conditions I.E. light pollution, surrounding structure heat loss etc, are quite poor until probably ~1am. Just so happens my 8” dob doesn’t really perform that well until a lot of the surrounding heat dissipates either.

Due to my current circumstances I don’t really get the chance to stay up that late to observe, so have 3” & 4” fracs that are getting a lot of use. My viewing is primarily solar system objects, doubles or open clusters and has been for about 18 months I can’t remember the last time I looked at a galaxy. I do still try the odd planetary nebula and globular but they aren’t quite the same as they were when I could stand in my garden at 2am with the big light bucket.
 

[John]

Another variable to add to the formula is target altitude. Atmospheric extinction takes it's toll on brightness. There are formulae that help calculate this but they seem quite complex, to my brain at least 🙄

Atmospheric Extinction and Refraction – Asterism.org

[bomberbaz]

3 hours ago, John said:

Another variable to add to the formula is target altitude. Atmospheric extinction takes it's toll on brightness. There are formulae that help calculate this but they seem quite complex, to my brain at least 🙄

Atmospheric Extinction and Refraction – Asterism.org

Thanks John and @Stu for these. I agree entirely with the altitude being a strong consideration but also and as mentioned by @Mr Spock, localized lp. 

Stu's cruxis.com calculator asks for either brightest star at zenith or sqm at zenith. Now this is great if you have a fairly even sky in all directions, but if like me you have really bad light pollution in one direction but reasonable skies in the other your going to run into issues as you hunt out you object, more so as you drop down in altitude where I think once you get below 45 degrees it gets increasingly problematic. 

So either I stick to one half of my sky or take multiple sqm readings and in doing so add another layer of complexity before even looking at John's atmospheric calculator!

There really is no easy answer to this I am afraid. 

Cheers all 

Steve

[Stu]

1 hour ago, bomberbaz said:

Thanks John and @Stu for these. I agree entirely with the altitude being a strong consideration but also and as mentioned by @Mr Spock, localized lp. 

Stu's cruxis.com calculator asks for either brightest star at zenith or sqm at zenith. Now this is great if you have a fairly even sky in all directions, but if like me you have really bad light pollution in one direction but reasonable skies in the other your going to run into issues as you hunt out you object, more so as you drop down in altitude where I think once you get below 45 degrees it gets increasingly problematic. 

So either I stick to one half of my sky or take multiple sqm readings and in doing so add another layer of complexity before even looking at John's atmospheric calculator!

There really is no easy answer to this I am afraid. 

Cheers all 

Steve

The calculator has an entry for atmospheric extinction which should allow for that.

[bomberbaz]

2 minutes ago, Stu said:

The calculator has an entry for atmospheric extinction which should allow for that.

So it does, my bad missing it. 

Cheers stu

[Deadlake]

2 hours ago, bomberbaz said:

So either I stick to one half of my sky or take multiple sqm readings and in doing so add another layer of complexity before even looking at John's atmospheric calculator!

SQM will also vary over the night, it get's complicated.

[Stu]

There’s another calculator here:

https://www.bbastrodesigns.com/NewtDesigner.html

More info…

https://www.rocketmime.com/astronomy/Telescope/SurfaceBrightness.html

[Stu]

Of course, none if this stuff is exact, just gives an indication of what might be visible or not.

[bomberbaz]

42 minutes ago, Stu said:

There’s another calculator here:

https://www.bbastrodesigns.com/NewtDesigner.html

More info…

https://www.rocketmime.com/astronomy/Telescope/SurfaceBrightness.html

 

42 minutes ago, Stu said:

Of course, none if this stuff is exact, just gives an indication of what might be visible or not.

And as often happens I have opened an astro Pandora's box 🤣

[John]

My back garden (where I do most of my observing from) sounds a bit like yours Steve - "it depends which direction you look" 🙄

I got Triton tonight with my ED120 and that is listed at magnitude 13.5. It was not that high in the sky so may have been "extincted" (not a proper word !) to mag 13.7 perhaps. 

Right on the very limit for the 120mm scope and not readily held visible with direct vision.  

 

Edited October 23, 2023 by John

[bomberbaz]

On 22/10/2023 at 12:56, Stu said:

There are a few calculators around which factor in LP, aperture and a number of other parameters. Mel a Bartels does a lot on this on his site.

https://www.cruxis.com/scope/limitingmagnitude.htm

https://www.bbastrodesigns.com/ObjectContrastCalculator.htm

I can confirm old age is well and truly set in with me. I was going to save the cruxis link to my favourites but when I tried my browser informed me I already had it saved. 

Make way for me an the nursing home 🙄😂