Abnormal slant in spectrum

[Ken Harrison]

I first noticed this a few weeks ago and though it was due to the target star being close to the top of the slit gap and the spectrum sitting high in the CCD FOV.
These have been corrected but I still see the issue/ concern/ problem.
I'm using the Genesis 4" f5 Petzal, a 29micron slit, the Spectra_L200 with1200 l/mm grating. (Doesn't seem to matter which grating - the slant is seen with 300 l/mm) and the ATik314L (1 x 1 binning)
The imaging camera is well focused on the neon lamp lines, and the PHD guiding seems OK...

When the spectrum is slightly extended 10-15pixel high, you start to see an abnormal "slant" roughly 15 degrees (!!) this doesn't correspond to the ref lamp slant etc.
We found a reference to a similar problem:
http://wsdiscovery.free.fr/astronomie/s ... index.html

Obviously when the spectrum height is reduced, it's no longer obvious.....
I attach a typical ref lamp image and a recent V1369 Cen image to show the effect.

Can anyone (Christian?) comment/ provide feedback/ additional info?
Any assistance would be welcomed!!

[Robin Leadbeater]

Hi Ken,

Yes I recall seeing examples of this effect seen with the LHIRES. Is it all the way across in the same direction or just at the edges of the field? If just at the edges it can be due to the doublet with a wide field and slow optics. eg see this page on Christian's site
http://www.astrosurf.com/buil/lhires_eval/optique.htm

I believe correct collimation has a bearing on this too. There is a post somewhere on here (I think by Martin Dubbs) investigating these effects.

Cheers
Robin

[Robin Leadbeater]

Here is an example of the effect (in Martin Dubb's Echelle spectrograph)
http://www.spectro-aras.com/forum/viewt ... 683&p=2775

Robin

[Ken Harrison]

Hmmm, certainly some similarities there....
I'll do some more testing at focus/ inside and outside telescope focus - see if the slant changes direction.
I don't want to change the focus position of the collimator, other than for spectral chromatic focus...
Using a f5 scope probably isn't helping.
If the root cause is aberrations from the collimator then I'm not sure were to go next.

On a similar but different topic: has any fitted a tilt mechanism/adaptor to the imaging camera? Most of the literature I read indicates the camera is tilted to improve the spectral chromatic focus.

[Olivier Thizy]

Ken,


there is no plan to tilt the camera on the Lhires III.


But this is not something new, we have done it for the LISA in the near-IR mode; the near IR kit is:
http://www.shelyak.com/produit.php?ref= ... =12&lang=2

It does include a special mechanical camera adapter that tilt the camera.


Cordialement,
Olivier Thizy
Vous ne verrez plus des étoiles comme avant !
http://www.shelyak.com/en/