Strange A spectra

[Andrew Smith]

I have been taking some LISA spectra to learn the ropes and last night I imaged two A stars one after the other for about 1 hour each. I was practicing with fainter magnitudes. I attach an image which is the sum of the sum of the spectra I got as they look starnge to me. They show absorption lines that slope in different directions and have shifts in the UV line positions. However, the air glow remains aligned. When I procesed them using HIP1493 as the reference and HIP2040 as the target the UV shift showed up so initially I thought I had done something wrong.

I would like to know if I have a failure of technique (e.g.I could have tried to put the two stars at the same place on the slit), an issue with the LISA or whatever the cause maybe.

Thanks for an comments and ideas.

Andrew

[Christian Buil]

I suspect a possible large position difference of the star along the slit axis. You confirm ?
The focus (internal objecive lens) is the same for the two spectra ?
The phenomena is similar to a large optical distorsion - not usual.

Christian B

[Andrew Smith]

Christian - The focus (telescope or camera) was not changed from one image to the next. The location on the slit was different by 35x4.54micron pixels i.e. 0.15mm and both were within about 1 -2mm of the centre of the CCD. The reported R>1100 with the 23micron slit which is being well sampled by the 4.45micron pixels. The issue is in all the spectra I took and does not appear to be due to the changing position angle of the stars.

What I find strange is that the air glow does not seem to show any distortion, for example the faint line near H gamma is not distorted but the two H gamma lines are. Similarly the Ne spectra I took between the star spectra look normal.
Could it be that the light glow more uniformly illuminates the slit than the star? Could you propose any tests I could do?

I will try the following:
check to see how close to the recommended distance the CCD is from the back metal of the spectrograph,
image the 19 micron hole and see how clean it's images are,
check the collimator lens is not obviously misaligned.

Thanks Andrew

I just checked the spectrum alignment it is at 2.5mm to the right rather than 2.716 could this make a difference?

[Rainer Ehlert]

Hi Andrew,

That is exactly what I get when the star is not focused correctly on the Slit.

Did you focus the main lens first and then the slit image in your guider cam ? You can easily focus with daylight your main lens of the LISA.

From my experience it has nothing to do where the H-alpha line is positioned as you write.

[Andrew Smith]

Hi Andrew,

That is exactly what I get when the star is not focused correctly on the Slit.

Did you focus the main lens first and then the slit image in your guider cam ? You can easily focus with daylight your main lens of the LISA.

From my experience it has nothing to do where the H-alpha line is positioned as you write.

Rainer - Thanks for your input. I assume you are refering to focusing the telescope (main lens) on the slit. If so I can't do it during the day as my observatory obscures all distant terestial objects. It was designed that way due to local light polution. However, I will try different focua positions to see if that illiminates the problem.

Regards Andrew

[Rainer Ehlert]

Hi Andrew,

You do this focusing without any telescope in front of it. There is a good explanation how to do it in the LISA instruction manual. I assume you have read it

Here http://thizy.free.fr/shelyak/Lisa/DC001 ... -%20En.pdf page 14 section. 5.2.2

[Andrew Smith]

Rainer - I see you are refering to the LISA camera lens - yes I have already focused it onto my CCD. Regards Andrew

[Andrew Smith]

To anyone following this the problem seems to have been that I did not have the CCD close enough to the position specified in the LISA manual. Having set it as close as I can to that value the spectra are now as I would have expected. Thanks Andrew