Possible bug in ISIS 5.9.6

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Re: Possible bug in ISIS 5.9.6

Postby Paolo Berardi » Thu May 16, 2019 2:26 pm

Thanks Robin!

Paolo
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Re: Possible bug in ISIS 5.9.6

Postby David Boyd » Thu May 16, 2019 7:49 pm

Robin said:
"...if I enter a 1/planck 2750K curve in the instrument response box instead of leaving it blank when calculating the instrument response, ISIS v5.9.6 behaves the same as ISIS before 5.9.4. "

This works and you get a response profile that looks the same as before and you can smooth it in the usual way. The problem comes when you then use that response profile to calculate the reference star spectrum. The result contains the inverse planck function so you need to multiply the reference star spectrum by the planck function to get the correct result. In my case the level of the reference star spectrum was then too high, so I had to normalise the spectrum before I could compare it to the library profile as a check. This is starting to get messy.

If you don't do what Robin says and just initially process the reference star with a blank instrument response box, the resulting response profile is extremely sharply peaked (at least it is in my case) towards the blue end. This means the level of the response profile is now 10x greater at the blue end than at the red end. You can generate the correct reference star spectrum using this response profile but it surely isn't good to have such a large range in the response profile. I imagine that was the reason Christian incorporated the planck function in the analysis in the first place.

I hope this makes sense. From a user's point of view, it isn't clear to me what is the best way forward.

David
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Re: Possible bug in ISIS 5.9.6

Postby Robin Leadbeater » Thu May 16, 2019 10:57 pm

Hi David,

David Boyd wrote:Robin said:
"...if I enter a 1/planck 2750K curve in the instrument response box instead of leaving it blank when calculating the instrument response, ISIS v5.9.6 behaves the same as ISIS before 5.9.4. "

This works and you get a response profile that looks the same as before and you can smooth it in the usual way. The problem comes when you then use that response profile to calculate the reference star spectrum. The result contains the inverse planck function so you need to multiply the reference star spectrum by the planck function to get the correct result.


You are right. I did not think that through. :oops: You get the advantage of the flatter 5.9.3 shape instrument response but that is not what 5.9.6 is expecting when you use it to reduce any spectra so you have to first multiply the response by the 2750K curve before using it in 5.9.6

I think the pros first remove the large scale dispersion direction variations in the flat (using a low order spline) before applying the flat correction. Provided the same "flattened" flat is used for both reference and target everything then works ok. I am staying with 5.9.3 for now

Cheers
Robin
LHIRES III #29 ATIK314 ALPY 600/200 ATIK428 Star Analyser 100/200 C11 EQ6
http://www.threehillsobservatory.co.uk
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Re: Possible bug in ISIS 5.9.6

Postby David Boyd » Fri May 17, 2019 8:59 am

I see a practical problem as we are planning a training course on using ISIS here in the UK in August and we will be asking attendees to download ISIS onto their laptops ahead of the course.

As far as I can see only 5.9.6 is now available to download from Christian's website. It will not be easy to explain to new users how to deal with these issues.

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Re: Possible bug in ISIS 5.9.6

Postby Christian Buil » Sat May 18, 2019 3:47 pm

About my methods concerning computation of instrumental response and use of the Planck, see here: http://www.astrosurf.com/buil/instrument_response/

(evolution cycle of ISIS is a normal situation).

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Re: Possible bug in ISIS 5.9.6

Postby Robin Leadbeater » Sun May 19, 2019 10:18 am

Hello Christian,

How is the 2D spatial content in the flat corrected for with this method? (For example dust on the camera which will affect the profile and sky background subtraction depending on the binning zones)

Cheers
Robin
LHIRES III #29 ATIK314 ALPY 600/200 ATIK428 Star Analyser 100/200 C11 EQ6
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