First Results

[EdWiley]

My first real session with the Alpy 600. Results for Betelgeuse, 10 second exposures calibrated, F/4 Schmidt-Newtonian. Resolution is only 284. The Alnitak spectrum (same parameters) is a bit better, resolution = 315. Self analysis: The Bad: (1) Focus is off, I must modify spacers so that I have some focus travel. Right now I am up against the base of the focuer with a 35mm focus extender. Time to dust off the lathe and modify the extender to give me more room. The Good: as bad as the focus and subsequent resolution, the profile of Beletgeuse matches fairly well with the profile from isis_data. For Alnitak the profile seems to match on the continuum but badly misses in the far blue. I don't know how to interpret that as I am very much a novice with these kinds of data.

My assumption in this analysis is that low resolution is directly related to focus quality. If this is not true, pleas let me know.

Greeting from Kansas,
Ed

[Ken Harrison]

Off to a running start there!
Which camera are you using?
You should be able to obtain and apply an instrument response which would improve the shape of the spectral profile.
Well done.
Onwards and Upwards.

[EdWiley]

Thanks Ken, your comments are encouraging. Camera is an Atik 314L+. The guide camera is an ST402ME, but I think I might switch to something lighter.

When I first posted I completely forgot that some of the differences not due to resolution were due to camera response. I will be working with that aspect today as I travel up the learning curve.

Ed

[EdWiley]

This morning I reanalyzed and incorporated camera response based on a cropped Alnitak response. Results very encouraging, with resolution jumping to r = 477. Profiles attached. I would still like feedback on spectrum focus if anyone has time to look.

Ed

[Robin Leadbeater]

Hi Ed,

Congratulations on first light.

How are you measuring resolution in the spectrum? Your spectrograph focus looks pretty good to me (The ALPY optics are corrected to give good focus in the UV-red region rather than into the IR so when tuning spectrograph focus, concentrate on getting the lines at the blue end sharp and all will be well elsewhere) I have the calibration module which makes tuning for focus easier but the daylight sky can be used, concentrating on the sharpness of the lines in the Ca H K doublet region. I have not noticed any significant change with temperature so all this is done on the bench and with ALPY is a one off (or at least infrequent) procedure.

Cheers
Robin

[Robin Leadbeater]

For comparison attached are a couple of full size screen shots of raw spectrum images and binned spectra for moonlight and the calibration lamp from a few nights ago. I have measured the FWHM of a lamp line in the blue which gave 2.9 pixels or 18.8 um which is as expected for a well focused spectrum (I am using a special 18um spectrophotometric slit with an 18 um slit rather than the standard 23um)

Cheers
Robin

[EdWiley]

Hi Robin:

Thanks for the feedback, glad to know I am close on focus. The resolution reported is from the ISIS analysis, I assumed this was valid but do not know enough yet to know if this is the standard way of measuring resolution.

The idea of using the sky or moon spectrum is great. I don't understand the calibration remark. I also have the calibration unit but I'm not sure exactly how the calibration unit figures in to the focus routine, can you provide details? For example, if the calibration spectral lines are sharply focused will the star spectrum also be sharply focused? If so then that makes it easy.

BTW, I think I now have the right spacing, no more bumping against the focuser base. Weather looks good this weekend, so I am hoping to share more results soon.

Cheers,

Ed

[Ken Harrison]

Measuring the FWHM of the reference lamp lines at a known wavelength will give you the R value.
You can check this against the ISIS result....
If the calibration lamp lines are in tight focus ie minimum FWHM then the spectroscope/ camera/ target spectrum will also be well focused on the slit, and give the best spectral resolution.
The tight star images seen in the guide camera FOV verify the focus of the telescope onto the front of the slit.
Two stage process - focus the camera on the rear of the slit (FWHM ref lamp) and the telescope target on the front of the slit (tight star image in guider)
HTH

[Robin Leadbeater]

Hi Ken,

A couple of caveats:

When using lamp lines to measure resolution the beam should illuminate the spectrograph in a similar way to the starlight. For example illuminating the Alpy with a lamp on the bench without a telescope or aperture stop will give a somewhat low resolution while using a single point source lamp at the telescope aperture will give a somewhat high resolution. (The latter I know to my cost when one time I tried focussing my LHIRES using the lines from a filly lamp at the telescope aperture. Because the focal ratio is effectively so high in this configuration, adjusting the spectrograph focus makes no difference, the lines always look sharp - it took me a while to figure out what was going on

Focussing the star in the guider image gets you close to optimum throughput, particularly with the new mirror slits compared with the old polished jaw slits but you can often squeeze out a bit more throughput by then using the techniques I described in my earlier post. See also Christian's page on this here.
http://www.astrosurf.com/aras/slit/method.htm

Cheers
Robin

[Robin Leadbeater]

Hi Ed,

Thanks for the feedback, glad to know I am close on focus. The resolution reported is from the ISIS analysis, I assumed this was valid but do not know enough yet to know if this is the standard way of measuring resolution.

I don't know how ISIS estimates the resolution either. I dont rely on it.

The idea of using the sky or moon spectrum is great. I don't understand the calibration remark. I also have the calibration unit but I'm not sure exactly how the calibration unit figures in to the focus routine, can you provide details? For example, if the calibration spectral lines are sharply focused will the star spectrum also be sharply focused? If so then that makes it easy.

The calibration unit lines can indeed be used to focus the spectrograph and estimate the resolution (The beam from calibration unit is designed to mimic an ~f5 beam from the telescope. (see also Ken's post for more details and my comments)

Cheers
Robin