spectral images for smile and/or tilt correction correction

[jnp]

Hi Everyone,

Can anyone send me some raw fits spectral images that need smile and/or tilt

correction removed, so I can test some software.

Many thanks

John

[the rev]

I can send you some tomorrow. Do you need the instrument response dat as well?

Kate

[jnp]

Hi Kate,

Thanks for helping.

If you can provide the FITS images before and after smile/tilt correction for comparison, that would be even better.

John

[Jack Martin]

John,

What do you mean by smile correction?

Regards,

Jack

[jnp]

Hi Jack,

If you are using a slit-less sa100 for your spectra you probably wont have encountered the smile or tilt problem. This is an aberration that only affects slit spectrometers. The absorption (or emission) lines can exhibit a curve and or a tilt effect. This is a separate issue with aligning the spectra horizontally (thanks for sending me your sa100 images Kate, you were not to know about smile/tilt).

I have provided a link to an example of smile in bitmap format.

http://dl.dropbox.co...c8bit_smile.png

I am still looking for some examples in FITS format.

many thanks

John

[robin_astro]

We should probably clarify the terminology

Tilt is when the spectrum dispersion direction is not horizontal. You will potentially encounter this in any spectrum incuding thes taken with the star analsyer though ideally the camera should be set so there is not tilt. Note all tilt correction algorithms introduce artifacts to some extent so are best avoided by making the spectrum level in the frame

Slant is when the spectrum lines are not vertical. Often seen in extended objects (including calibration lamps and the night sky pollution) taken with slitless spectrographs. This should be corrected for even when the spectrum might be quite narrow and the slant not obvious eg stellar spectra otherwise resolution will suffer. You will also come across this if you use a fixed mount with the Star Analsyer and allow the star to trail the spectrum

Smile is when the line is curved (a symptom of Littrow spectrographs) This rarely needs to be corrected for stellar spectra as it is not normally severe enough to reduce resolution. The exception is if there are background lines from light pollution. These need to be straightened if background removal is to work properly

The key to all this is to make sure the calibration lamp and star spectrum are treated in exactly the same way otherwise you will lose registration.

Cheers

Robin

[Jack Martin]

Robin,

Thanks for the explanation.

Regards,

Jack

[Jack Martin]

John,

Can't upload a FITS file.

Jack

[Jack Martin]

John,

You can adjust the tilt (distortion) in photoshop.

Jack

[jnp]

Thanks for the elaboration Robin. Bang on again, although I have seem 'smile' in images taken with a DIY classical spectrograph.

I didn't account for slant due to a trailing/moving object as we tend to mostly capture stars on a RA tracking mount, (superb capture of 2012 DA14)

The terminology is non-intuitive, I have also seen "skew" mentioned in addition to slant and tilt. Would be nice is there was a single diagram with an example of each and what correction was required?

Jack - I don't have photo shop, and i'm not sure it will tilt a FITS image. I will PM you my address if you have a FITS example?

many thanks

John

[robin_astro]

Hi John

Thanks for the elaboration Robin. Bang on again, although I have seem 'smile' in images taken with a DIY classical spectrograph.

If you see smile in a classical spectrograph then it is probably misaligned (The smile in the Littrow arrangement comes from the inevitable misalignment due to the return beam not being able to follow exactly the same path as the incoming beam. Alternatively it could be an aberration in the lenses used eg barrel or pincushion distortion

The terminology is non-intuitive, I have also seen "skew" mentioned in addition to slant and tilt. Would be nice is there was a single diagram with an example of each and what correction was required?

Not sure where the terminology originates but I learned it from Christian Buil where the terminology goes back at least 10 years in IRIS eg here

http://www.astrosurf.com/buil/iris/new380/new380_us.htm

http://www.astrosurf.com/buil/iris/new383/new383_us.htm

Though there was even confusion here as in the command list he refers to SLANT as correcting a tilted spectrum

http://www.astrosurf.com/buil/iris/nav_pane/CommandsFrame.html

Cheers

Robin