LHiResIII - Ne-calibration lines

[Erik Bryssinck]

Hi all,

I have a certain problem with the calibration of the LHiResIII and internal lamp. I am not sure if this is normal behavior. So below my experiences.

First explain my setup: I use a C11 on f/10, with LHiResIII and 2400 grating, with a Trius H814 mono CCD in 2x2 binning. The slit size of the LHiResIII is set to 35 µm. The LHiResIII has serial number 1K-501 and is from Jan. 2018.

1. I notice that focusing with the internal focuser is not easy, when I opt to have the maximum light intensity at the peak, I notice that the resolution is less good. It is better to have the peak as narrow as possible on the image, this does not always correspond to the maximum light intensity of the peak.

2. I also notice that the temperature has a big influence. If I observe in the summer, I easily reach R> 15000 . In winter this becomes difficult and I barely reach about 13000 ... 13400. I always do a good focus on the ne lines. There is also nothing changed in the setup . Does this have to do with the lamp (temperaturinflucence), which I doubt or another cause ?

3. The ne lines look pretty wide to me. (elephant foot)

4. The resolution is different for each Ne-line, ranging from the first (approx. 6506 AngStr.) of about R=13400, next (approx. 6532 AngStr.) R=13100 and last (approx. 6598 Angstr.) R=11200 at a slit of 35 µm.

I've been browsing the forum for a while and I notice that the doublet was once modified by Shelyak. I don't know if my LHiResIII already has the improved doublet on board (see above purchase date from Jan.2018.). Or can I check this myself ?

Can I improve anything here ? All tips are welcome !

kind regards,
Erik

[etienne bertrand]

Hello Erick,

In my case i use Atik314L+ in binning 1x1 and i think the focus of the doublet it's important for each star, it's a chromatic doublet so if you dont make the focus at each star the focus aren't good and the resolution wasn't better.
With 35µm slit i have a resolution with best focus near 15 000 but if i dont touch the focus and if i change the angle ofobservation the resolution down.
Put photo for illustrate, but i think all is ok with your lhires.
Maybe it's different with your CCD ? You have 3.69µm x 3388 pixels and it's more tall captor ?

(I'm maybe so interested for the futur for a apo-chromatic doublet, it's more confortable but cost, really interest ?)

Regards.

[Benjamin Mauclaire]

Hi Erik,

I confirm what Etienne said.
Lhires3 isn't a stable device as it suffers from mechanical deformations during night, almost coming from different positions used when pointing different targets.
Res. power varies from ~13000 to ~15000 with 35 um slit.
If you have a backyard observatory, here are some advices:
- A good star is to set lens doublet at temperature closed to mean night one. You could do it one in summer and in winter too;
- You can tighten all the screws. Once a year is ok;
- Use 6 screws to fix the man camera.

Benji

[Franck Houpert]

Hi Erik,

Except the camera (QSI Kaf 1600), I have the same configuration as you.

At first, I had a big problem with focusing. Impossible to make a correct spectrum, a real disaster. On my Lhires (# 194 - 2009), the grating (2400) was stuck on its support. By negative temperature, the grating was subjected to too much constraint. Shelyak changed the support and the network for the current system.

With the current system, y also notice a difference between summer and winter but less important. 15000 - 14300. NE6598

Personally, I think that the difference in resolution, between summer and winter, simply comes from the grating . It does not have the same performance depending on the temperature. It is a feeling, I have no evidence of what I claim. Shelyak could give us the technical characteristics of the network.

I motorized the focusing of the doublet.
http://www.youtube.com/watch?v=9ZuRXrL3Q4s
http://www.youtube.com/watch?v=Dbnf_TkwoYw

http://www.spectro-aras.com/forum/viewt ... it=arduino

Focusing between two targets is very easy. In addition, I have a small application that calculates the resolution directly for me. I no longer look at the image of the neon but the result.

<<
I've been browsing the forum for a while and I notice that the doublet was once modified by Shelyak. I do not know if my LHiResIII already Has the doublet Improved on board (see above purchase dates from Jan.2018.). Or can I check this myself?
>>
Do you have a link that shows the change ?

Franck

[James Foster]

To: Erik,

RE:"1. I notice that focusing with the internal focuser is not easy, when I opt to have the maximum light intensity at the peak, I notice that the resolution is less good. It is better to have the peak as narrow as possible on the image, this does not always correspond to the maximum light intensity of the peak.
2. I also notice that the temperature has a big influence. If I observe in the summer, I easily reach R> 15000 . In winter this becomes difficult and I barely reach about 13000 ... 13400. I always do a good focus on the ne lines. There is also nothing changed in the setup . Does this have to do with the lamp (temperaturinflucence), which I doubt or another cause ?
3. The ne lines look pretty wide to me. (elephant foot)
4. The resolution is different for each Ne-line, ranging from the first (approx. 6506 AngStr.) of about R=13400, next (approx. 6532 AngStr.) R=13100 and last (approx. 6598 Angstr.) R=11200 at a slit of 35 µm."

Regarding 1, I follow the Shelyak/C.Buil instructions and adjust the housing of the grating to be as aligned as possible to the LhiresIII internal optics, condenser lens, and CCD. Once the grating is aligned with these components, focused condenser lens, aligned and spaced CCD, you should be able to get "sharp" Ne-Ar calibration lamp lines. Remember, its best to get a good solar spectrum with sharp lines on your CCD image (at whatever wavelength (Ha, Hb, or CaK lines) before fine tuning the condenser lens to get sharp Ne-Ar lines from your calibration lamp. I also use the 35 micron width slit.

Regarding 2. as others have noted, this will occur based on the temp ranges you encounter throughout the night and season to season temperature ranges. For a given wavelength observed, I don't change the condenser lens focus, but check my telescope's focus during a long (8 hrs) observing run. Again, as others have noted, the condenser lens might have to be changed btw shooting in 30 deg C weather and 0 deg C weather, summer and winter.

Regarding 3. See my typical Ha, Hb, & CaK neon calibration images below. Are they as bad or worse than yours?
Ha

Hb

CaK


Regarding 4. The resolution R of the spectra for a giving grating, will go down with decreasing wavelength by the common R = λ / Δλ formula. For Ha my R usually is 14K, Hb(4860A) R is 8K, and Cak(3900A) has R about 5K. I find that I usually do not have to change the condenser lens focus btw the Ha(6564A) and Hb(4860A) wavelengths; If there is any difference, it would amount to less than 1/10th a turn btw Ha & Hb. Below Hb, say going from Hb to Cak (4860A -> 3950A) I turn my condenser lens 1 full rotation CCW to achive crisp Ne-Ar lines seen in the Cak image above. Note that the chromatism of the condenser lens increases with decreasing wavelengths; its almost impossible to get all Ne-Ar calibration lines focused in the CaK or bluer regions.

James

[Robin Leadbeater]

Hi Erik,

Is your camera at the correct distance? This is critical for collimation of the beam at the grating and should be within 2mm of the correct distance
http://www.astrosurf.com/buil/lhires_eval/optique.htm

Cheers
Robin

[Robin Leadbeater]

Is your camera at the correct distance?

The nominal distance of the sensor is shown here (54.85mm)
astrosurf.com/thizy/lhires3/d_Bague-T%20Montage%20CCD-3%20-%20Feuille.jpg
but can be checked using the technique on Christian Buil's page

Robin

[Robin Leadbeater]

Also if you are seeing "elephants foot" lines, take a look at this thread on the VdS forum.
https://forum.vdsastro.de/viewtopic.php?f=2&t=5609
This was finally solved in April 2019 with a replacement doublet. I am not sure when the spectrograph was originally made but the original thread dates from April 2018
https://forum.vdsastro.de/viewtopic.php?f=2&t=5182

Cheers
Robin

[Robin Leadbeater]

This is my LHIRES lamp line profile with a 35um slit (PSF FWHM = 4.9*6.54=32um)

LHIRES_1200_35um_slit_6.png (232.02 KiB) Viewed 5582 times

Cheers
Robin

[Robin Leadbeater]

The LHIRES is not a very stable instrument (both temperature and flexure can significantly affect it). It also shows significant chromatism so the focus must be adjusted with the spectrograph mounted on the telescope, at operating temperature and at the wavelength to be measured. The focus can be quickly checked using the PSF function in ISIS and adjusted if neccesary for minimum FWHM (measure it with the camera unbinned)

Robin