How to Measure Sky Brightness From Your Images Using ASTAP

[JSBach1801]

Hopefully, Lyle has a dark from the same dataset that I can use to find the pedestal value (mean ADU) so that I can try to have it calculate SQM. I also need his location to make it work (hopefully). I am thinking that it should be a nice quality sky in Utah!

Jim, through this link you will find a dark, flat, and the same light frame as before:
https://drive.google.com/drive/folders/ ... sp=sharing

My exact location was: 41.769001466606, -111.78048724357184

I actually bet my dark sky quality from the particular location I shot at was not great. This is where I was located. Where the little arrow of the dialogue box is pointed, east of the town:

I was shooting back to the west southwest over a Bortle 5/6 city, through the light pollution dome it created. Orion was imaged from about 35 degrees above the horizon down to 20 degrees. It was my first night out, and it was the only spot I could think of to go that I would not be bothered. However, I have since scouted out a new location to the west of the town in the green/blue transition zone on the map. Should have very nice skies from this new location looking north, south, and west.

[chris_g]

@Juno16 , it's ASPS that needs the pixel size, couldn't find anything in ASTAP. My first stop was astrometry.net and it solved a JPG conversion in a couple of seconds. So, it had to be some sort of setting somewhere. It's weird though that ASTAP wouldn't solve it with coordinates of the nebula provided. Any ways, !

Clear Skies,
Chris

[Juno16]

Hopefully, Lyle has a dark from the same dataset that I can use to find the pedestal value (mean ADU) so that I can try to have it calculate SQM. I also need his location to make it work (hopefully). I am thinking that it should be a nice quality sky in Utah!

Jim, through this link you will find a dark, flat, and the same light frame as before:
https://drive.google.com/drive/folders/ ... sp=sharing

My exact location was: 41.769001466606, -111.78048724357184

I actually bet my dark sky quality from the particular location I shot at was not great. This is where I was located. Where the little arrow of the dialogue box is pointed, east of the town:


Capture.JPG



I was shooting back to the west southwest over a Bortle 5/6 city, through the light pollution dome it created. Orion was imaged from about 35 degrees above the horizon down to 20 degrees. It was my first night out, and it was the only spot I could think of to go that I would not be bothered. However, I have since scouted out a new location to the west of the town in the green/blue transition zone on the map. Should have very nice skies from this new location looking north, south, and west.

Hi Lyle,
My apologies. I have failed you!

Everything works fine in ASTAP as far as going through the instructions. I can load your light, dark, and flat into ASTAP, but the results are obviously incorrect and repeatable incorrect.

As you can see, the altitude is WAY off and the SQM is WAY low (high Bortle 7).
I have tried it many times and many different ways, but I keep getting results very similar to this.

On the opposite side, my data is very much what I expected. When I start imaging into the east and above my streetlight, I see reasonable values approaching Bortle 8. When I use a light frame at the meridian, they return SQM values of high Bortle 5 to Bortle 6. Exactly what I thought!

I am just learning ASTAP and I don't know if it is because of the non-FITS format or something I am doing incorrectly.


I found this list of recommended Pre-conditions on the ASTAP web site.

Pre-conditions
1) Image is astrometrical solved for star flux-calibration against the star database magnitudes.
2) The background value is larger then pedestal value or mean dark value. If not expose longer.
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.
5) DSLR/OSC raw images require 2x2 binning. For DSLR images this is done automatically.
6) Most of the image is free of deepsky nebula.
7) The calculated altitude is correct. The altitude will be used for an atmospheric
extinction correction of the star light.
8) The H17 or H18 star database is used containing the Gaia blue magnitudes (400-700 nm) and no filter is used except a UV/IR block filter.

All of the conditions seem ok, except I assume that #3 is unprocessed, #6 Hopefully the image is not too occupied by M42, #7 altitude is definitely incorrect, and #8 you did not use a filter???

Sorry my friend, I tried!

[Juno16]

@Juno16 , it's ASPS that needs the pixel size, couldn't find anything in ASTAP. My first stop was astrometry.net and it solved a JPG conversion in a couple of seconds. So, it had to be some sort of setting somewhere. It's weird though that ASTAP wouldn't solve it with coordinates of the nebula provided. Any ways, !

Clear Skies,
Chris

Hi Chris,
I couldn't find it either. I solved it in ASTAP back on post 17. I had to widen the search range.

I also solved it in NINA (which uses the same copy of ASTAP). I did have to enter the pixel size and fl.

When you get a chance, check out my last post. I could not get reasonable SQM values for Lyle's data in ASTAP, however, my FITS files (my data) seems to produce reasonable and predictable results. Wish I had the answer!

[chris_g]

When you get a chance, check out my last post. I could not get reasonable SQM values for Lyle's data in ASTAP, however, my FITS files (my data) seems to produce reasonable and predictable results. Wish I had the answer!

I pixel dived the image, there is a lot of noise in it, that might be what's throwing it off. It might not work quite as well on non FIT mages. I'll check it out on some of my Canon stuff.

[Juno16]

When you get a chance, check out my last post. I could not get reasonable SQM values for Lyle's data in ASTAP, however, my FITS files (my data) seems to produce reasonable and predictable results. Wish I had the answer!

I pixel dived the image, there is a lot of noise in it, that might be what's throwing it off. It might not work quite as well on non FIT mages. I'll check it out on some of my Canon stuff.

Hey Chris,
Thanks for that. I know that ASTAP requires a certain quality level of light frame.
Just to let you know. My original instructions simply said to load the dark, flat, and light from theFile menu.
After updating the star database files (from H17 and H18), I couldn’t calculate SQM the same way without entering a pedestal value.
However, if you add a light, dark, and flat in the stack menu (the sigma symbol), the load the light again in the main File menu, the SQM calculates using the dark and flat without the pedestal value.
Please let me know how it works out.

[JSBach1801]

Hi Lyle,
My apologies. I have failed you!

...and #8 you did not use a filter???

Sorry my friend, I tried!

Jim, I will never forgive you!

But seriously, you have gone above and beyond trying to get this to work! I deeply appreciate the effort. And regardless of what data we can glean from the raw file, my image of Orion still turned out nice, so I am satisfied.

And yes, no filter.

[bobharmony]

I did a measurement using the tool as Jim described it on some Coma Cluster images I took last month. At 41 degrees early in the evening I got a reading of 17.96. Just after the meridian flip at 66 degrees I'm down to 18.60. Pretty bright skies in any case. Solidly in the Bortle 7-8 range, I guess.

Bob

[Juno16]

I did a measurement using the tool as Jim described it on some Coma Cluster images I took last month. At 41 degrees early in the evening I got a reading of 17.96. Just after the meridian flip at 66 degrees I'm down to 18.60. Pretty bright skies in any case. Solidly in the Bortle 7-8 range, I guess.

Bob

Hi Bob,
Does that result sound reasonable?
I have had issues with my old Nikon data calculating SQM in ASTAP (non-FITS files).
I have had better (more reasonable results) by opening a dark file in AP Lab and getting its mean/median adu value.
Then starting ASTAP, loading a light, solving, then calculating SQM. Enter the mean adu of the dark measured in APLab as the pedestal value and deselecting
The “Apply Dark and Flat checkbox.
Let me know if it works better for you.