IC4593 strange pattern on my Alpy600... (solved !)

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Re: IC4593 strange pattern...

Postby Robin Leadbeater » Tue Jun 13, 2017 8:08 pm

Also check that the core of the lines showing the doubling are not saturated.

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: IC4593 strange pattern...

Postby Robin Leadbeater » Wed Jun 14, 2017 8:43 am

Some other things to check:

Is the target image smaller than the slit and the moving in the slit during guiding? This is unlikely with a pn but you can check this by comparing the width of the line with the width of the lamp line. If the variations in the line profile are inside the lamp line width, then this is the problem. Try defocusing the telescope slightly so the target fills the slit.

Is the spectrograph stable (compare the lamp lines at the start and end of the target exposure)

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: IC4593 strange pattern...

Postby Daniel Dejean » Thu Jun 15, 2017 7:49 am

Hello Robin,
Many thanks for your attention. Your comment is a great help "If the variations in the line profile are inside the lamp line width, then this is the problem". This is quite easy to check (see the picture below). Apparently this is the case... As a new comer in spectro this is a first time I am getting a recommendation to defocus... very encouraging step !!!... :-) It make sense in this specific case. Will try it next time.
Thank again.
Cheers
Daniel
Attachments
IC4593 vs calibration.jpg
to compar the NP line (blue) with calibration lamp (red) / raw images around 4861.
IC4593 vs calibration.jpg (49.64 KiB) Viewed 271 times
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Re: IC4593 strange pattern...

Postby Robin Leadbeater » Thu Jun 15, 2017 10:50 am

Hello Daniel,

I think it is quite unusual to find the star image significantly smaller than the 23um slit with a 200mm aperture. Your telescope collimation, focus and seeing would need to be very good. (What focal ratio is your telescope?)
I would also recommend re-checking your spectrograph focus. The lamp lines look a bit wide to me (about 18A FWHM instead of the expected ~12A). When the spectrograph is in good focus the FWHM of the lamp lines should be similar to the slit width (23um or ~5pixels for your ATK428)

Here is a lamp line profile (in pixels) for my setup (ALPY600 ATK428 calibration lamp module)

lamp_ALPY600_ATK428_THO.png
lamp_ALPY600_ATK428_THO.png (8.97 KiB) Viewed 261 times


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: IC4593 strange pattern...

Postby Daniel Dejean » Thu Jun 15, 2017 5:41 pm

Hello Robin,
You are right and I am not expecting a super performance from my telescope a very basic SkyWatcher D200mm / F1000mm on EQ6 mount...
Let's look on the calibration FWHM on my ATIK428EX camera (pixels : 4.54 microns). My resultts are near 5pixels but not as good as your 4.7px.
All processing is generaly done with ISIS but here graphic and FWHM computation are done with Rspec.
Résults :
- at 733px (4764A) : 5.5px FWHM
- at 1172px (5852A) : 5.3px FWHM
- at 1482px (6599A) : 4.9px FWHM

I still have room for improvement to reach your 4.7px result. I understand it may be the cause of such strange pattern.
Thank again for your help. You are providing an excellent reference point for progrees.
Cheers,
Daniel
Attachments
calibration FWHM.jpg
FWHM on calibration lines (pixels) Alpy600/ATIK428
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Re: IC4593 strange pattern on my Alpy600...

Postby Daniel Dejean » Sun Jun 18, 2017 7:42 am

Hello Robin,
Coming back on your question : why I am sure it is not a real effect ? I should admit that, by lack of experience, I had a doubt at the beginning and this the reason why I had to ask "experts" for their advise.However I am now convince it is kind of artefact related to my specific setup. Image received from Michel Verlinden also with an Alpy600 shows clean lines around Hbeta but a also a strange pattern on Halpha (at this frequency I have no problem on my side). After multiple exchanges on that forum (thank for your own inputs) the most probable cause appears to be related to a slight defocus of the spectrograph (line calibration around Hbeta at 5.2 fwhm instead of 4.7 for your benchmark. I will continue to investigate using the present setup on bright/large NP as M57 and a bright Be with strong emission line at Hbeta (Sheliak could be an interesting target in the same area).
Cheers,
Daniel
PS : by the way, it is my understanding you are also using an ATIK428EX. I am very happy with that camera, same type of pixels than the ATIK460EX but smaller CCD (just the minimum required) and less expensive…
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Re: IC4593 strange pattern on my Alpy600...

Postby Francois Teyssier » Sun Jun 18, 2017 8:21 am

The typical expension velocity of PN is tycally few tens of km/s. So the lines are not resolved with a low res spectrograph.
and many lines should be affected, not only H beta region. This not the case.
A spectrum of IC4593 can be seen in[url] http://www.underoakobservatory.com/High ... C4593.html
[/url]

There are two possibilities:
1. Focus of the scope: defocused star products a donnut.
2. Focus of the spectro. 2.1. Check the profiles of the Ne/Ar lines 2.2. Always verify the focalisation. When I used LISA, I checked the green line ~5400 visually and with the visualisation tool of ISIS (line PFS in "Image") or MaximDL

Looking at the raw image of the spectrum I think this is the focus of the scope. The lines should be narrower.

All the best,

François
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Re: IC4593 strange pattern on my Alpy600...

Postby Daniel Dejean » Mon Jun 19, 2017 12:36 pm

Hello François,
Many thanks for your very valuable input.
In a trial to get refernce data that source (oakobsevatory) has been also identified. However the defects I am talking about are very smalls compared to the defintion of that picture. An overall overview of our spectra looks very similar at this scale. Regarding the process to check the Ne/Ar lines you will find above my present result It looks acceptable for my point of view but I may be totaly wrong as we are missing detailed benchmark. Robin had a very usefull input : fwhm goal around 5 pixels for a CCD with 4.54 microns pixels size. In the Hbeta region I am getting an Fwhm of 5.3 and 4.9 arond Halpha (Robin is at 4.7). This 10pct difference may explain that quality defect but I am not sure it is the only cause. Also it should point ot again that we got the same type of defect on 2 very differents telescope : T60 (less visible) and a Newton 200mm, using the same Alpy equipment. It may be a small but complex issue on which I apologise to generate so many exchanges on that forum.
Next step for me will be to improve the Alpy focus to reach Robin's benchmark. Trial will also be done to improve the telescope focusing method. However I am not sure how this will impact the line profile as previous exchanges tend to indicate we are not far for expected level. Here also it is hard to find documented numeric results for reference (expected emission line width or spectrum width for specific setup).
Thank again for your help.
Cheers,
Daniel
Attachments
IC4593 overall view .jpg
our result vs reference : oakobservatory
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Re: IC4593 strange pattern on my Alpy600...

Postby Daniel Dejean » Thu Jun 22, 2017 12:09 pm

Hello All,
I alm happy to close that subject with an experiment to confirm Robin's assumption : the source of the issue was a slight defocalisation of the Alpy spectrograph, probably during transportation on our way back from Pic de Midi to Toulouse... After focus ajustement the FWHM of calibration line in pixels went down from 5.2 to 4.4 and the small defects on emmision lines has now disappear (see graph below on a zoom to Hbeta - OIII 5007 region). FWHM in angstroms is now at 9.9 for the OIII 4959 line.
Thanks to Robin and others for recommendation to correct that issue.
Enjoy the sky !
Daniel
Attachments
ic4593 after focus adjustment v1s.jpg
ic 4593 spectrum before and after focus adjusment of Alpy focus. Great improvement !
ic4593 after focus adjustment v1s.jpg (104.3 KiB) Viewed 186 times
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