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Anyone Know How Maximdl displays OBS time and AIRMASS for Image Stack?

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#1 GaryShaw

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Posted 20 February 2024 - 06:17 PM

Hi

 

I'm stacking a bunch of images to increase the SNR of some faint photometric targets and need to understand how Maximdl handles the Time of Observation and Airmass at the time of Observation.

 

Example, say I have a stack of 10 - 90 sec images that were taken in sequence with no delay between exposures.  So, over that 900 seconds of image capturing, the Airmass changed a bit as well as the time. Does Maximdl know to calculate the midpoint of all the exposures as well as the average Airmass during that 900 second period and place those values into the FITS Header for each final, stacked image?

 

Couldn't find this out from the manual so your experience with Maximdl would be a big help.

Thank you!

Gary

 



#2 rutherfordt

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Posted 21 February 2024 - 10:14 AM

Gary,

 

I just did a small stack of 8 images and it looks like the stacked image has the same the time and airmass values as the first image in the stack, which is not what I would have expected (I would have expected them to be from somewhere in the middle).

 

This was with Maximdl 5.07.  Checking with Maximdl 6.11 gives the same result-- if you have a later version to might want to check on it since its possible they have changed it in some way (I have the current version but don't have it installed yet so can't check).

 

Tom



#3 Ed Wiley

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Posted 21 February 2024 - 10:15 AM

Gary,

Why not do an experiment. Stack some images and consult the stack's FITS header and see if you can figure it out? I have never tried this, so I'm shooting in the dark. I stack directly in VPhot or do a time series in VPhot with binning.

Ed



#4 GaryShaw

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Posted 21 February 2024 - 12:45 PM

Gary,

 

I just did a small stack of 8 images and it looks like the stacked image has the same the time and airmass values as the first image in the stack, which is not what I would have expected (I would have expected them to be from somewhere in the middle).

 

This was with Maximdl 5.07.  Checking with Maximdl 6.11 gives the same result-- if you have a later version to might want to check on it since its possible they have changed it in some way (I have the current version but don't have it installed yet so can't check).

 

Tom

Tom:

I just did the same test with the same outcome using Maximdl 6.29.

 

I'm a bit surprised at this as well. I'm using 15 stacked images to get adequate SNR for analysis, reporting and low error. When Maximdl reports the stacked observation based on the start time of first image, the time for the observation is almost 9 minutes earlier than the midpoint of the overall observation. This could create confusion with comparisons of the magnitudes arrived at by others. Maybe its all within the error and doesn't really create problems.

thanks

Gary

Gary



#5 GaryShaw

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Posted 21 February 2024 - 04:09 PM

Gary,

Why not do an experiment. Stack some images and consult the stack's FITS header and see if you can figure it out? I have never tried this, so I'm shooting in the dark. I stack directly in VPhot or do a time series in VPhot with binning.

Ed

Thanks Ed, I did try this experiment as mentioned above. Does it puzzle you that Maximdl displays the same Airmass and 'time of exposure' data in the stacked image (no matter how many images are in the stack) that it does in the first image of the stack?

Gary



#6 ColinH

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Posted 21 February 2024 - 04:35 PM

The best place to get support for a Diffraction product is over on their Official Tech Support Website:  https://forum.diffractionlimited.com

Since you could be stacking images made on many nights over the course of the year, there is no reasonable way to compute an answer that would apply in all cases.



#7 pvdv

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Posted 21 February 2024 - 05:07 PM

I don't have a Maxim licence anymore (well, I guess I still have but it is an extremely old one) but the way some other stackers handle it on stacks is to have

DATE-OBS referring to the start of the observation
DATE-END referring to the end of the observation

There is very little actual standardization, except for the format, in FITS headers. IMHO, it is always better to double check manually.

 


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#8 robin_astro

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Posted 21 February 2024 - 07:36 PM

Unfortunately the FITS standard is somewhat ambiguous on this but DATE_OBS would normally be the time of the start of the observation. 

 

"The value of DATE-OBS shall be assumed

to refer to the start of an observation, unless another interpretation is clearly explained in the comment field."

 

from the fits standard 4.4.2.2

https://fits.gsfc.na...tandard30aa.pdf

 

Cheers

Robin


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#9 GaryShaw

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Posted 22 February 2024 - 09:06 AM

The best place to get support for a Diffraction product is over on their Official Tech Support Website:  https://forum.diffractionlimited.com

Since you could be stacking images made on many nights over the course of the year, there is no reasonable way to compute an answer that would apply in all cases.

Hi

Wouldn't you think though Kevin, that they could add a line to the Fits Header that would provide the midpoint date and time of the series of images? If the images were spread over days or weeks, that unnecessary data could just be ignored. For the case where it’s a relatively short photometric series, the midpoint time data would be useful and available. 
Gary 




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