SQM (sky background) measurement using an image and the ASTAP program

[han59]

The free ASTAP program version ß0.9.480 has a new experimental option to measure the sky background accurately in magnitudes per square arc second . This is the same value as reported by portable Unihedron SQM meters.

As soon an image is solved, the program can calculate the link between measured star flux and the star magnitude from the database. This flux/magnitude relation can also be used to measure the sky background value. The achieve the highest accuracy a pedestal values can be entered which is the same as the mean value of a dark/bias image.

Below the first result of a field test this evening. Unfortunately the Moon was shining so it doesn't go further then SQM 18.4 and I had to avoid some passing by clouds but the result is good and accurate. See the graph below. Camera used was the ASI1600MM in bin 2x2. Exposure time adapted from 0.5 second to 30 seconds. No filter.

Operation is simple. Load an image, hit the solve button, then select the fully automatic SQM measuring tool.

This is a new feature. Feedback is appreciated.

Han

Menu:

 

Comparison between a Unihedron SQM-L meter and ASTAP

 

 

 

[skybadger]

Just to check- are you using the unihedron to calibrate ASTAP or the way I would expect, plate solving to calibrate the unihedron  ? 

[han59]

The ASTAP measurment doesn't require an external calibration. After solving you can measure the "star flux/(2.51^magnitude)" ratio using the star database and this value will be constant.  Outliers  and saturated stars will be ignored. With the "star flux/(2.51^magnitude)" ratio you can precisely express the background signal in  magnitude per square arc second.

So this measurement doesn't need calibration. It is calibrated each time against a star database. Dirty optics or aging has no influence as with the Unihedron meter.

As long the CCD or CMOS sensor is linear it will work. Maybe the biggest problem is the signal reduction at the corners of the image but that can be compensated by applying a flat.  But probably not required due to averaging.  Above measurement was done without applying a flat.

Han

 

Edited January 23, 2021 by han59

[Paul M]

Thanks Han, although SQM isn't something I concern myself with presently, I am a huge fan of ASTAP which I use stand alone and from within APT. It's also become my preferred solving and stacking program.

I've also recently connected HNSKY to APT as the planetarium option. As much as I love Stellarium it can hang when connected to APT, HNSKY seems to coordinate very sleekly with APT and is a very slimline application. Rather like KStars is to Ekos.

I'm sure I'll find a use for the SQM feature!

[han59]

Just realizing that altitude and transparency plays a role. Measurements where done at zenith. At lower altitudes the stars are fainter so probably it is better to talk about relative SQM value. So maybe it is a coincidence that the values are so close.

Quote

Extinction becomes significant when altitudes are lower than about 45 degrees. At sea level, zenith extinction is 0.28 magnitudes, and at an altitude of 45degrees it’s 0.40 magnitudes

 

 

Edited January 23, 2021 by han59

[skybadger]

10 hours ago, han59 said:

The ASTAP measurment doesn't require an external calibration. After solving you can measure the "star flux/(2.51^magnitude)" ratio using the star database and this value will be constant.  Outliers  and saturated stars will be ignored. With the "star flux/(2.51^magnitude)" ratio you can precisely express the background signal in  magnitude per square arc second.

So this measurement doesn't need calibration. It is calibrated each time against a star database. Dirty optics or aging has no influence as with the Unihedron meter.

As long the CCD or CMOS sensor is linear it will work. Maybe the biggest problem is the signal reduction at the corners of the image but that can be compensated by applying a flat.  But probably not required due to averaging.  Above measurement was done without applying a flat.

Han

 

Agrees the stellar photometric way is the gold standard.it's just the way I read the post. 

I'm struggling to understand why the measurement is dependent on altitude,  your signals are relative between star and background at any altitude. Perhaps you are saying the sky is brighter further from zenith ?

[han59]

The reference are the stars.  Like the Sun they are fainter near the horizon due to extinction. So the ASTAP SQM value will be lower at  lower altitude/elevation because the stars are fainter as at zenith and therefore the sky background will be seen as brighter. 

The difference between zenith and 45 degrees altitude/elevation is maybe 0.12 magnitude but at zenith there there is still an extinction of typical 0.28 magnitude (depending in humidity and  observer  altitude) . I didn't measure that difference for unknown reasons, but it will be interesting to see if the values from the Unihedron meter and ASTAP start to differ at lower altitude/elevation. So I'm very interested in other test reports. I also know that some Unihedron meter show a different value

[han59]

For the record: Today I looked more in detail to atmospheric extinction. The likely reason that the Unihedron and ASTAP reported values are very simular is the fact that the Unihedron calibration is about 0.35 magnitudes corrected. This is equal to the extinction in the zenith:

http://www.lightpollution.it/download/sqmreport.pdf

Quote

The different zero point is likely due to the fact that the relation has been obtained with outside-the-atmosphere magnitudes V’ and B’. As an example, assuming an extinction of 0.35 mag in V and 0.15 mag in B and replacing V’=V-0.35 and B’=B-0.15, where V and B are the apparent magnitude below the atmosphere, we obtain SQM−V= 0.2(B−V)−0.31 in agreement with my previous results. The zero point of the current SQM calibration made by Unihedron gives SQM≈V’ for stars with B’=V’, above the atmosphere, and consequently the below-the-atmosphere SQM-V correction factor for the alpha Lyrae spectrum come out different from zero. Fig.18 shows that for alpha Lyr is SQM-V=-0.35 mag arcsec−2.

 

[Jonk]

A thought - when SGP for example uses ASTAP to platesolve, can the SQM value get passed to the software to be recorded in the FITS header of the next few subs?

Or, pass it to the weather panel to be displayed in place of a hardware measurement every image or something? The software could start the next exposure, and then plate solve the previous image / update, or will that not work for a 30 minute sub?

Also is it a luminance image that's required, does it not work for broadband or narrowband subs, in which case I'll shut up!

[han59]

Measurement of the SQM is accessible via the command line so SGP could read it.  In principle you could also get a good indication via the image background value if you keep the exposure the same, so I don't think it is essential to monitor it all the time.  It is more to get an absolute reference value for a site. 

It will work  even with H-alpha filter as long the image background is increased b y the sky background. So at my site, SQM=20.4 best and 200 seconds exposure it works. The value differ slightly  compared with broad band subs. 

30 minutes sub, could work but the image should be solvable and stars of magnitude 18 are not saturated. Saturated stars will be ignored. 

Han

 

[ONIKKINEN]

@han59 Im trying to use the SQM measurement tool in ASTAP but it reports the altitude wrong, any idea whats wrong? The time, latitude and longitude are all correct or at least very close to correct.

Edit: Just updated the software and it now reports everything correctly.

Edited December 8, 2021 by ONIKKINEN
Updated and its fixed

[han59]

Yes in a previous version there was a bug in the altitude calculation. If I remember well it was for RAW files only .  RAW files are now automatically binned 2x2 for the measurement .

Han

 

[han59]

Below the results of a comparison test between the Unihedron L meter and ASTAP with an ASI1600 camera on April 2, 2022 . It was a clear evening/night.
The SQM was measured at zenith and at 30 degrees above the horizon. Measurements were made during twilight until it was astronomically dark. At the highest SQM values, the Unihedron meter indicates approximately 0.3 to 0.4 magnitudes higher. I suspect this is because the telescope sees the entire spectrum while the Unihedron is most sensitive in the green part of the spectrum. The camera then sees more light pollution. The spectral response of the Unihedron is comparable to that of the human eye.

A comparison test with a green/Johnson-V filter in front of the camera may be able to provide the answer.

Han

 

 

 

[han59]

After some thought it looks like using the V17 Johnson -V magnitudes as a reference for SQM measurements is the wrong thing to do. My camera is sensitive over a wider spectral range and therefore sees more light pollution than the Unihedron SQM-L meter. Especially in the blue area the Unihedron less sensitive. With the H17 or H18 database containing the Gaia blue magnitude (BP) as reference the SQM values reported by ASTAP values match better with the Unihedron values. See new charts.

I'm interested in reports of other users with an Unihedron meter.

 

 

The bandpass of my Baader UV/IR blocking filter matches very nicely with the Gaia blue bandpass.

Gaia and Johnson bandpass:

[symmetal]

Hi Han,

I like the SQM feature. 😊 Last week when there were several clear nights in a row I took a Unihedron SQM meter reading around midnight and it reported 21.20 as far as I can remember. Not sure of the last figure. I've just checked some 60s L subs of M101 I was imaging around that time, near the zenith, and using a dark and flat your program's SQM measurement was 21.21 at 71 deg, which is very impressive. I checked some subs around 03:00 the same night and your program measures 21.89 at an altitude of 78 deg. I didn't think it got that dark here but could be wrong.

Next time I'm imaging, (may be a while as next week doesn't look good) I'll note down the Unihedron readings at various times and compare it to subs at the same times and let you know. 🙂

Alan

[han59]

Hi Alan,

Thanks for the feedback. You have set a high record with your HFD measurement. You must be located at a superb site. More reports are welcome.

There is a different behavior between ASTAP and Unihedron I haven’t discussed.  At very good sites the background light caused by stars stars will reduce the Unihedron SQM measured value.  So if you point the Unihedron to the Milky Way it will report a lower SQM then when you point outside the Milky Way. ASTAP will be less effected because it looks in principle to the background and not to stars.

An other difference is for the case there is less atmospheric transparency/more extinction. Then ASTAP will report a lower SQM because the starlight is fainter. The opposite effect will occur if your on a mountain will less air above you. The ASTAP SQM will go up because the stars are a little brighter.

Since the detected star flux is the reference and of interest for the astronomer, these effects are bonus.

Han

[symmetal]

56 minutes ago, han59 said:

At very good sites the background light caused by stars stars will reduce the Unihedron SQM measured value.

I've wondered about that, that as the sky gets darker the stars themselves become the SQM reading limiting factor. The Unihedron has a wide acceptance angle so will always have stars in view. It will be interesting to see what actual sky background SQM values you program will reveal.

Your graphs show ASTAP reading slightly lower than Unihedron at 20.00 SQM so I assume if the readings are correct at some darker sky level ASTAP will start to read higher than the Unihedron.

Alan

[han59]

The Unihedron FAQ reports this:
 

Quote

 

The northern Milky Way contributes about 0.10 mpsas under 21.5 mpsas (moonless) skies.

The southern Milky Way might be as big an effect as 0.85 mpsas where it goes near-overhead.

 

Where mpsas is magnitudes per square arc second.

ASTAP will measure the sky glow by finding the background peak in the image histogram. So detectable stars will be ignored but stars hidden in de background noise could contribute to the measured background value. This Milky Way effect will effect ASTAP less then the Unihedron but how much is unknown.

 

[DavidSAstro]

Hi all and thanks for this conversation above and it sounds like ASTAP will assist me in assessing a new dark sky site our society is about to establish and I wanted to leverage the thinkings and learnings here. 
 

The site we are looking at is reported at 21.99 from 2015 data and in a visit in two weeks time I wish to validate that and ASTAP measurements appears a good way to go. We will have 5 scopes on site with differing focal lengths and qualities ( Tak , Tele Vue, SCTs etc). The discussion above leads me to understand that ASTAP basically compares star light measurements against background measurements and then against the ASTAP DB for normalisation to produce its measurement. 
 

I would still imagine moon glow would be an issue ( we are going when 4%) and better measurement will happen at zenith.  Focal length wouldn’t be an issue due to the comparative nature of the calculation and the normalisation.

Is there anything I’m missing or are there any additional particular points that people would recommend in our considerations / planning?

David

[han59]

Hi David,

Reads like an interesting experiment. The Moon will increase the sky background level,  but that is depending on air humidity and dust in the sky.  The elevation of the mount can be retrieved from the typical FITS header. ASTAP will try to compensate for telescope elevation by calculating the airmass and loss of star light.

It would be nice to link the Unihedron SQM measurements to the images to compare the results later. Keep me informed on the results

 

Note ASTAP v 2023-2-26 was a bad one for SQM measurment but it was only 2 days available. Older and new versions should work fine.

Han

 

[mgutierrez]

Han, thanks for such a nice feature.

I have a question. Point #2 requirement states that background value must be higher than pedestal; if not, expose longer. The provided light must be uncalibrated, right? So, given a particular pedestal; how is possible that the corresponding light could have a lower background even with very short exposures since that light has already the pedestal?

For example. Let's say my pedestal is 400, as measured from the corresponding dark. The corresponding light should have always a background value higher than 400, since the pedestal is included in the light. Am I missing something?

m

[han59]

I will phrase it differently.  It should be measurable higher then the pedestal. Assume your pedestal value is 1000 ± noise. For a short exposure the sky background introduces maybe 1 extra resulting in 1001 ± noise. This  could be difficult to measure. The pedestal could drift or the sky background signal could drown in the noise or being significant less then the  flat correction . A sky background addition of 100 will likely be measurable resulting in 1100 ± noise.

So for short exposures the SQM can be measured but could become unreliable. 

Calibration is best for accuracy.  You can apply the routine on calibrated lights and enter for the pedestal 0.  Or  provide darks (and flats & flat darks) in corresponding tabs for automatic calibration .  Else type in the pedestal value manually based on a dark. How better the routine can measure the sky glow influence the more accurate the SQM value will be.

Han

 

 

[mgutierrez]

18 minutes ago, han59 said:

I will phrase it differently.  It should be measurable higher then the pedestal. Assume your pedestal value is 1000 ± noise. For a short exposure the sky background introduces maybe 1 extra resulting in 1001 ± noise. This  could be difficult to measure. The pedestal could drift or the sky background signal could drown in the noise or being significant less then the  flat correction . A sky background addition of 100 will likely be measurable resulting in 1100 ± noise.

So for short exposures the SQM can be measured but could become unreliable. 

Calibration is best for accuracy.  You can apply the routine on calibrated lights and enter for the pedestal 0.  Or  provide darks (and flats & flat darks) in corresponding tabs for automatic calibration .  Else type in the pedestal value manually based on a dark. How better the routine can measure the sky glow influence the more accurate the SQM value will be.

Han

 

 

Thanks for the reply Han. That's much clearer for me. Regards

[han59]

Is this understandable?

Pre-conditions
1) Image is astrometrical solved. (for flux-calibration against the star database)
2) The image background value has measurable increased above the pedestal value or mean dark value.
    If not expose longer. This increase is caused by the sky glow.
3) Apply on single unprocessed raw images only.
4) Providing dark image(s) in tab darks (ctrl+A) or entering a pedestal value  (mean value of a dark)
     increases the accuracy. If possible provide also a flat(s) in tab flats. Calibrated images are also fine.
5) DSLR/OSC raw images require 2x2 binning. For DSLR images this is done automatically.
6) No very large bright nebula is visible. Most of the image shall contain empty sky with stars.
7) The calculated altitude is correct. The altitude will be used for an atmospheric
    extinction correction of the star light. The altitude is calculated based on time, latitude,
    longitude. Note that poor transparency will result in lower values compared with
    handheld meters.

 

Han

[mgutierrez]

13 minutes ago, han59 said:

Is this understandable?

Pre-conditions
1) Image is astrometrical solved. (for flux-calibration against the star database)
2) The image background value has measurable increased above the pedestal value or mean dark value.
    If not expose longer. This increase is caused by the sky glow.
3) Apply on single unprocessed raw images only.
4) Providing dark image(s) in tab darks (ctrl+A) or entering a pedestal value  (mean value of a dark)
     increases the accuracy. If possible provide also a flat(s) in tab flats. Calibrated images are also fine.
5) DSLR/OSC raw images require 2x2 binning. For DSLR images this is done automatically.
6) No very large bright nebula is visible. Most of the image shall contain empty sky with stars.
7) The calculated altitude is correct. The altitude will be used for an atmospheric
    extinction correction of the star light. The altitude is calculated based on time, latitude,
    longitude. Note that poor transparency will result in lower values compared with
    handheld meters.

 

Han

For me, it is.

But for those who wondered the same as me. maybe for point #2, is it a good hint to include an example as you did in the previous post? (ie. to get a 1100 of sky background given a 1000 dn of pedestal) Maybe astap could for example warns if the sky background is under pedestal+10% ? Or any other hint for the user to know that the image is correctly exposed.