What wavelength error is acceptable with an Alpy?

[John Coffin]

Method

Spectra of Zeta Cas and Z And were collected, the latter was exposed for 240s to avoid saturating the H alpha emission line and then a further set of exposures for 600s were taken in order to show more detail in the continuum and other emission lines.  Towards the end of the series taken for 600s the telescope reached the meridian and the lamp image for that set was not taken until after the meridian flip. On the other occasions the lamp image was taken immediately after the end of a capture sequence.

Results

The spectra were processed in ISIS in three different ways.

Balmer lines only

This couldn’t calibrate Zeta Cas so was not suitable for Z And either

Mixed Balmer and lamp lines calibration

Each time the lamp lines for the particular target were used.

Zeta Cas               H alpha blue shift 1 A, accurate from H beta onwards towards the UV end.

Z And (600s)       H alpha red shift 2 A, gradually increasing so at H delta the red shift was 3.4 A.

Z And (240s)       H alpha blue shift 6 A, reducing so at H delta the blue shift was 4.0 A.

In all targets using the mixed method the wavelength errors become more shifted towards the red at shorter wavelengths.

Lamp lines only calibration

Zeta Cas               H alpha blue shift 1 A, increasing so at H delta the blue shift was 4.5 A.

Z And (600s)       H alpha red shift 0.7, accurate calibration from H beta to H delta. H epsilon was blue                        shifted by 1.5 A.

Z And (240s)       H alpha blue shift 6.0 A, increasing so that at H delta the blue shift was 7.7 A.

So in all the targets calibrated by the lamp lines alone, the wavelength error shifts towards the blue at shorter wavelengths. 

I will be very interested to see the result of your Filly Dot experiment. I may have to throw away my calibration module. (I’ll keep it for the flat fields actually).

Cheers, John

[Robin Leadbeater]

Hi John,

Your  wavelength error trend using just the lamp lines seems similar to mine qualitatively eg post #37.  Your variable offset as shown at H alpha is different to what I see though. My calibration is consistently good at H alpha.

I now have some data for the Filly lamp (I used the “cross” lamp which is just Argon)  There does appear to be  something subtly different. I will report back when I have figured out what is going on.

Cheers

Robin

[Dr Andrew Smith]

The only thing I can think of is the the dispersion direction of the prism and grating are not aligned in such a way that the path differences with a fully illuminated entrance pupil and a partially illuminated one results in more dispersion from the prism in the fully illuminated case.

Clutching at straws!

Regards Andrew

[Robin Leadbeater]

The errors are within the FWHM of the slit image so I wonder if it could be something distorting the line profile eg :-

Coma from the ALPY camera lens distorting the line shapes more at the edges, which will depend on the f ratio of the incident light and appear as a variation in dispersion.

On stars, some sort of  chromatic aberration in the telescope optics producing a wavelength dependent asymmetry in the psf of the star which alters the distribution of light across the slit and hence the line profile.

Cheers

Robin

[Robin Leadbeater]

Hi John,

Don’t throw out your calibration module just yet !

There is a just detectable systematic difference between my ALPY internal lamp and the Argon “Filly Cross” lamp mounted at the edge of the telescope aperture in line with the slit but it is small (Calibrating the Filly lamp with the ALPY lamp gives +1A error around 7000A, 0 error around 5000A and -0.7 around 4100A).  

Cheers

Robin

[Robin Leadbeater]

Having said that, comparing the line profiles of both the narrow lined star I used and the internal lamp, they have the same FWHM within 5% and there is no sign of asymmetry in line profiles across the spectrum in either case (dominated by the slit width) so it looks like this is not the cause.

Robin

[Dr Andrew Smith]

Hi Robin could you try the filly lamp at 90 deg to the correct position to see if the off axis lines mirror the behaviour of the star.

Regards Andrew 

[John Coffin]

Dear fellow Alpy users, please see my post #41

Everything In my Alpy is screwed down tight as far as I can tell. Yet I am getting these large disparities in the calibration of H alpha.

It suggests something does change when the instrument moves from one star to another, i thought of checking the Relco bulb isn’t loose, presumably the grism  is unlikely to move as it’s held in place by a spring.

The best calibration using the lamp lines occurred when there was a meridian flip between the target and the acquisition of the lamp lines.

[Robin Leadbeater]

Hi John,

If something is moving in the spectrograph, you should see this in the lamp spectrum ie calibrating the lamp spectrum using an earlier lamp spectrum should show an error.  The spring in the core module is there to hold the collimator lens in position, To hold the grism in position, the allen head grub screws should have been retightened after adjusting to make the lines perpendicular to the dispersion direction.

Cheers

Robin  

[John Coffin]

I’ll do as you say. Thanks.

John

[John Coffin]

The best fit for Z And was when the only frame taken after the meridian flip was used, and it was calibrated using the mixed Balmer and lamp method. The Balmer lines were found from Pi And (also taken after the flip).

The lamp was the one taken for Z And, after the flip. No heliocentric or corrections for radial velocity were made.

The errors were 

H alpha                 -0.4

H beta                   +0.65

H gamma             +1.2

H delta                  +1.0

 H epsilon             -1.0

When the same frame was calibrated using its lamp lines only and not the Balmer lines from Pi And

Errors are:

H alpha                 -0.4

H beta                   -1.45

H gamma             -1.8

H delta                  -2.5

H epsilon             -3.5

So for me it seems the mixed method gave the best results 

[John Coffin]

I took your advice and compared the lamp images from the evening where I had calibration problems. There was a shift of minus 3 A between the first and second lamp image. The meridian flip was between the first and second images. Thereafter the lamp image shifted plus 0.4 A and stayed there.

I thought I had checked everything was tightened, but today I’ve discovered the two Allen head bolts holding the slit in place had worked loose. No idea how! 

 I don’t think this explains everything though. The star spectra have a different amount of shift at different wavelengths, while the lamp spectra have the same shift all the way along. I can only think that the different light paths of the lamp and the stars are at the root of the problem as you have been saying.  

[Robin Leadbeater]

Hi John,

OK so comparing with the errors you had at the start of the thread it looks like you now have a good calibration across the range, including at H alpha where ISIS only uses  lamp lines.  We still do not understand why the lamp gives increasing errors towards the blue end but I think we should probably accept this as a win for now !

Cheers

Robin

[Author]

Posts