Airmass calculation

Craig · August 24, 2019, 12:36am

I use MaximDL as the camera interface.
When executing a sequence, the images recorded by MaximDL are downloaded to Voyager. Does Voyager then calculate the airmass and add it to the FITS header?
The reason I ask is because the airmass value in the FITS header is acting like a step function meaning one or more of the variables used to calculate the airmass are being rounded. This results in the airmass not changing over several minutes and then jumping to the next value.
I tested MaximDL on its own and the airmass value is changing slowly as expected. So I am assuming then that Voyager must be changing it. Ideally, if the FITS header from MaximDL already has the airmass value then it should not be changed. The airmass value calculated by Voyager should only be added if not present in the FITS header from the camera.
Craig

Voyager · August 24, 2019, 5:30am

Airmass calculation is not rounded. Calculation is according paper for professional and is used from research team. I cannot change in Fit sorry. I’ll check the resolution.

All the best

Craig · August 24, 2019, 8:34am

I think one of the variables used to calculate the airmass in voyager is rounded or few digits of precision. My first question though, does Voyager add the airmass value to the FITS header when it writes the image to disk? MaximDL calculates the airmass and puts it in the FITS header. Does Voyager change this?

Craig

Voyager · August 24, 2019, 10:27am

How many digits you need ?
I need to check on the code.

Craig · August 24, 2019, 11:29am

The airmass value should have enough precision to represent a change in airmass over a duration of 1 minute or less.

Craig · August 24, 2019, 11:41am

For example, the calculation of airmass includes the zenith distance, which can also be the altitude of the telescope. Suppose you round the altitude to a degree and it takes 15 minutes for the altitude to change 1 degree. Then even if you calculate the airmass to 12 digits it will not matter because the altitude of the telescope only changes once every 15 minutes. Which means the airmass will only change once every 15 minutes. But, if the altitude is rounded to seconds then every second the airmass will change because the altitude changed. So it is important that all of the variables used to calculate the airmass have enough precision to show the airmass changing over periods of a minute or less.

Using images captured over a declination arc of -3 Hours to +3 Hours and graphing to airmass vs time you should see a curve something like a smooth parabola. But what I saw instead was steps, ie, the airmass was constant for about 20 minutes, then it jumped to a new value for another 20 minutes then jumped again, etc. It was not a smooth arc but instead a series of steps.

Voyager · August 24, 2019, 1:11pm

Dear Craig,

i dont round anything … all the value coming from the mount driver and are used in double variable.

Voyager use the polinomyal formula of Hardie:

All the best
LO

Voyager · August 24, 2019, 1:27pm

2019/08/24 15.26.00 347 - CRITICAL - [Telescopio 2019/08/24 15.26.01 398 - CRITICAL - [Telescopio 2019/08/24 15.26.02 683 - CRITICAL - [Telescopio 2019/08/24 15.26.03 734 - CRITICAL - [Telescopio 2019/08/24 15.26.04 826 - CRITICAL - [Telescopio 2019/08/24 15.26.05 663 - CRITICAL - [Telescopio 2019/08/24 15.26.06 994 - CRITICAL - [Telescopio 2019/08/24 15.26.08 063 - CRITICAL - [Telescopio 2019/08/24 15.26.09 153 - CRITICAL - [Telescopio 2019/08/24 15.26.10 207 - CRITICAL - [Telescopio 2019/08/24 15.26.11 231 - CRITICAL - [Telescopio 2019/08/24 15.26.12 244 - CRITICAL - [Telescopio 2019/08/24 15.26.13 297 - CRITICAL - [Telescopio 2019/08/24 15.26.14 672 - CRITICAL - [Telescopio 2019/08/24 15.26.15 499 - CRITICAL - [Telescopio 2019/08/24 15.26.16 813 - CRITICAL - [Telescopio 2019/08/24 15.26.17 938 - CRITICAL - [Telescopio 2019/08/24 15.26.18 753 - CRITICAL - [Telescopio 2019/08/24 15.26.20 084 - CRITICAL - [Telescopio 2019/08/24 15.26.20 936 - CRITICAL - [Telescopio 2019/08/24 15.26.22 055 - CRITICAL - [Telescopio 2019/08/24 15.26.23 361 - CRITICAL - [Telescopio 2019/08/24 15.26.24 672 - CRITICAL - [Telescopio 2019/08/24 15.26.25 553 - CRITICAL - [Telescopio 2019/08/24 15.26.26 687 - CRITICAL - [Telescopio 2019/08/24 15.26.27 814 - CRITICAL - [Telescopio 2019/08/24 15.26.29 163 - CRITICAL - [Telescopio 2019/08/24 15.26.30 505 - CRITICAL - [Telescopio 2019/08/24 15.26.31 684 - CRITICAL - [Telescopio 2019/08/24 15.26.32 766 - CRITICAL - [Telescopio 2019/08/24 15.26.33 774 - CRITICAL - [Telescopio 2019/08/24 15.26.35 257 - CRITICAL - [Telescopio 2019/08/24 15.26.36 066 - CRITICAL - [Telescopio 2019/08/24 15.26.37 155 - CRITICAL - [Telescopio 2019/08/24 15.26.38 485 - CRITICAL - [Telescopio 2019/08/24 15.26.39 592 - CRITICAL - [Telescopio 2019/08/24 15.26.40 682 - CRITICAL - [Telescopio 2019/08/24 15.26.41 728 - CRITICAL - [Telescopio 2019/08/24 15.26.42 642 - CRITICAL - [Telescopio 2019/08/24 15.26.44 040 - CRITICAL - [Telescopio 2019/08/24 15.26.45 216 - CRITICAL - [Telescopio ] - [PushFunction ] - AIRMASS = 1,22146633345995
] - [PushFunction ] - AIRMASS = 1,2214663309344
] - [PushFunction ] - AIRMASS = 1,22146632697432
] - [PushFunction ] - AIRMASS = 1,221466324566
] - [PushFunction ] - AIRMASS = 1,22146632086335
] - [PushFunction ] - AIRMASS = 1,22146631783299
] - [PushFunction ] - AIRMASS = 1,22146631435084
] - [PushFunction ] - AIRMASS = 1,22146631178462
] - [PushFunction ] - AIRMASS = 1,22146630795034
] - [PushFunction ] - AIRMASS = 1,22146630564366
] - [PushFunction ] - AIRMASS = 1,2214663018476
] - [PushFunction ] - AIRMASS = 1,22146629865401
] - [PushFunction ] - AIRMASS = 1,2214662961582
] - [PushFunction ] - AIRMASS = 1,2214662923984
] - [PushFunction ] - AIRMASS = 1,22146628894889
] - [PushFunction ] - AIRMASS = 1,2214662855912
] - [PushFunction ] - AIRMASS = 1,22146628279869
] - [PushFunction ] - AIRMASS = 1,22146627942643
] - [PushFunction ] - AIRMASS = 1,22146627584822
] - [PushFunction ] - AIRMASS = 1,22146627360544
] - [PushFunction ] - AIRMASS = 1,22146626971834
] - [PushFunction ] - AIRMASS = 1,22146626617054
] - [PushFunction ] - AIRMASS = 1,22146626306112
] - [PushFunction ] - AIRMASS = 1,22146626025549
] - [PushFunction ] - AIRMASS = 1,2214662568373
] - [PushFunction ] - AIRMASS = 1,22146625347543
] - [PushFunction ] - AIRMASS = 1,22146624981777
] - [PushFunction ] - AIRMASS = 1,22146624618573
] - [PushFunction ] - AIRMASS = 1,22146624276079
] - [PushFunction ] - AIRMASS = 1,22146623882083
] - [PushFunction ] - AIRMASS = 1,22146623578466
] - [PushFunction ] - AIRMASS = 1,22146623163214
] - [PushFunction ] - AIRMASS = 1,22146622939931
] - [PushFunction ] - AIRMASS = 1,22146622617826
] - [PushFunction ] - AIRMASS = 1,22146622255233
] - [PushFunction ] - AIRMASS = 1,22146621988022
] - [PushFunction ] - AIRMASS = 1,22146621566361
] - [PushFunction ] - AIRMASS = 1,2214662126896
] - [PushFunction ] - AIRMASS = 1,22146620975231
] - [PushFunction ] - AIRMASS = 1,22146620598937
] - [PushFunction ] - AIRMASS = 1,22146620339918

Voyager · August 24, 2019, 1:29pm

This is the report each 1s (nearly) of the airmass calculation from Voyager … how you can see change each time.
According math definition of formula : "which gives usable results for zenith angles of up to perhaps 85°"

If this not usefull for you please tell me which formula is ok for your proposal.

All the best
LO

Craig · August 24, 2019, 6:26pm

Hi Leo,
I ran the following test (with observatory roof closed, so you can do this during the day):

Startup Voyager and connect to Telescope drive and Camera
Slew telescope to star HIP 24460 (-2HA, -30 DEC)
Create a sequence to take 4, 1 second images with 2 minute delay between each image
Run sequence
I then used MaximDL to view each image as it was recorded. Here is the airmass values:
1.1182299
1.1182299
1.1182299
1.1182299
so MaximDL FITS viewer shows no change in the airmass over 8 minutes
I then used FITS Viewer in SkyX to look at each image recorded on disk by voyager and the airmass is the same.

Next, I disconnected Voyager from the camera.
Using MaximDL I then took 3, 1 second images with a 2 minute delay between each image.
The airmass is now:
1.1048268
1.1022475
1.0997398
So airmass is now changing. This was confirmed in SkyX by looking at each image FITS header.

When Voyager is connected to MaximDL the airmass does not change. Can you try this with your telescope.

Voyager · August 24, 2019, 8:50pm

If you think the formula is wrong please provide yourself the formula.
If you want that Voyager doesnt touch AIRMASS just tell me and i put a flag to remove.

All the best
LO

Craig · August 24, 2019, 10:09pm

Let’s start with a flag to disable calculating airmass and not changing the FITS header provided by MaximDL.
Also, have an option to specify the Airmass FITS keyword, that way we could compare the two values and see if there is a problem. The default could be AIRMASS which would make the new version of voyager compatible with all previous versions. But in my case I could change that to AMASS and then the FITS header would have AIRMASS provided by MaximDL AND AMASS provided by voyager. I can then compare the two to see if there is a difference.
We could even make this a test version of the software for me to try before making it a general release.
But regardless, I think there should be a flag for “Calculate Airmass”.
Craig

Voyager · August 26, 2019, 12:34pm

Ok Craig, I’ll do in the next Daily Build

Craig · August 26, 2019, 10:47pm

Thanks Leo. I’ll let you know how it works out.

Craig