Sky brightness measurement

[Ronald]

Just wondering how long does it take sharpcap 3.2 to do a sky brightness measurement with the brain function when I tried it seems to just go on and with no result, i did a trial test with a lens cap and got results in two minutes. My camera is a zwo 224 the sensor analysis was fine thank you in advance

[admin]

Hi,

It rather depends on how dark your sky is – SharpCap is trying to increase the gain and exposure until it can clearly detect the brightness of the sky in the image above the black background that you expect from dark frames. If you've covered your lens and there is no light landing on the sensor then this could take a very very long time! If you're lucky enough to observe from a very dark site then it could still require exposures of one minute or more to be able to make this measurement.

Cheers, Robin

[cbbella]

Hi Robin, when doing the sky brightness measurement should I do the measurement for each filter I use like h-alpha, Oxygen and sulphur. I ran the background test with my OIII filter in place and it started doing various exposures of 40sec, 2mins, 8mins and then 17mins at which point i cancelled the test. I'm in a bortle 8 sky area

[admin]

Hi,

yes, in theory you should measure with each filter in place, as the background light levels will vary with each filter due to the amount of light it cuts out.

It sounds like your background is very dark for it to try exposures of >10 min, in which case, it may be quicker/easier to use our sky brightness estimation tool (http://tools.sharpcap.co.uk) to estimate the sky brightness that the camera will see for each filter, then enter those values into the brain.

cheers,

Robin

[cbbella]

Okay I'll give that a try Robin

Thanks

[azcubs76]

Hi Robin,
I've been using the Brain/Smart Histo extensively and it has surprised me with recommendations to reduce both gain and exposure times and I am getting great results! Thank you so much for that feature.

I am wondering that since you have an online calculator for estimating "Sky Brightness" in e/pixel/s is there an equation to go the other way and calculate SQM in mag/acrsec based on measured Sky Brightness? I've started logging the Sky Brightness measurement by digging it out of the logs as shown below but it would be great to have that logged automatically in a more accessible place like in the Sharpcap capture by date folders. This allows me to compare my own conditions night to night but the calculation to mag/arcsec would allow users to compare conditions to each other.

Smart Histogram Brain shown
Finished ZWO Exposure of 4000ms, gotFrame = True
Starting ZWO Exposure of 4000ms
Adjusting Exposure and Gain
Setting gain to 500
Setting black level/offset to 24
Finished ZWO Exposure of 4000ms, gotFrame = True
Switching ZWO camera into video mode, exposure is now 1000ms
Waiting for image to settle...
Background extraction gives area {X=1104,Y=2256,Width=276,Height=188}, channel medians (LRGB) 7256, 6512, 8704, 4960, using 8704
Frame darkest area median Green channel ADU : 8704, Real ADU : 2176, electrons : 20.63754
FWD is 184.7702, exposure is currently 1.000s, setting target of capturing 25 electrons
Adjusting exposure from 1000ms to 1386.47943139026ms
Adjusting exposure to 1.4s
Waiting for image to settle...
Background extraction gives area {X=1104,Y=2256,Width=276,Height=188}, channel medians (LRGB) 9500, 8464, 11504, 6304, using 11504
Frame darkest area median Green channel ADU : 11504, Real ADU : 2876, electrons : 28.72403
FWD is 184.7702, exposure is currently 1.386s, setting target of capturing 25 electrons
Sky background measurement complete : 20.7172468055174

[admin]

Hi,

Good to hear that the results are helping you get good images

Probably the easiest way to find the SQM from the e/pix/s is to work backwards in the tools.sharpcap.co.uk page by adjusting the sky brightness until you get a photon rate that matches the observations. You can reverse the calculation if you want, I will include my notes below on how I calculate the e/pix/s from other factors. Currently SharpCap doesn't do this as it requires knowledge of the focal ratio of the instrument, which SharpCap doesn't keep track of.

cheers,

Robin

Code: Select all

// https://web.archive.org/web/20181217054033/http://www.astro.umd.edu:80/~ssm/ASTR620/mags.html
// 1Jy = 1.51E7 photons/s/m2/(dlambda/lambda)
// for mag 0, B (440nm) => 4260Jy, V(550nm) => 3640 Jy, R(640nm) => 3080Jy. Use V as typical of visual
// total photon flux for given dlambda/lambda then 3640*10^(-0.4M)*1.51E+7*dlambda/lambda
// 5.4964E+10*10^(-0.4*M)*dlambda/lambda
// focal length l, diameter d, pixel size mu (mu in microns, others in meters)
// rad/pixel = mu / (10^6*l) 
// arcsec/pixel = mu * 3600 * 180 / (10^6*l*PI)
// area/pixel (arcsec squared) = mu^2 * 3600^2 * 180^2 / (10^12 * l^2 * PI^2)
// collecting area = PI * d^2 / 4
// photons collected per sec/arcsec^2 = PI * d^2 * 5.4964E+10*10^(-0.4*M)*dlambda/lambda / 4
// photons collected per pixel = (PI*d^2/4) * (3600^2*mu^2*180^2) / (10^12 * l^2 * PI^2) * 5.4964E+10 * 10 ^ (-0.4*m) * dlambda/lambda
// simplify... 1.8366E+9 * d^2 * mu^2 / l^2 * 10 ^ (-0.4m) * dlambda/lambda
// l/d = focal ratio, for mag 21.7 (mag/arcsec2), get 3.837 * mu^2 / f^2 * dlambda/lambda

[Ardsley Astro]

Hi,
Quick question about measuring the sky brightness. Would it be better to measure sky brightness at the zenith rather than on an object.

My thinking is, Given where I live you can physically see the sky glow brighter nearer the horizon and to maximize imaging time I will sometimes start imaging when the object is quite low.

In theory, measuring near the zenith will give longer sub exposures than what would be required when near the horizon but will compensate for the darker skies as the object passes the meridian.

My thought is if you measure on an object that is near the horizon it will give shorter exposure times than would be required for when the scope nears the zenith

Regards

Anthony

[admin]

Hi Anthony,

that's a good point and it depends on a few factors....

The first is how long you are planning to image that target for - it probably has to be more than 1 hour for it to be worth worrying about. Beyond that, take care if the moon is around as moving to the zenith then may not actually give you the darkest background you are intending to image in. Finally, there is also a consideration of changing conditions - if transparency improves, the whole sky may get darker, if it worsens, the opposite...

Measuring near the target gives the right value 'now', measuring elsewhere is taking things into your own control and being responsible for thinking about things like the above...

I must admit that I use the tool now and then - not for every target - you quickly get a feel for settings that work for your sky conditions and start there.

cheers,

Robin

[Adariano Palenga]

in the page "Sky Background Calculator" (https://tools.sharpcap.co.uk/) I can't select the filter type.
Is there a my mistake or is there a workaround?
regards,
Adriano