Lhires III fine tuning

[Ken Harrison]

Andy,
Hmmm
It seems that the telescope image (as seen in the guide camera) is still not in focus with the slit plate.
What happens if you leave the collimator focused on the neons and then re-focus the telescope to give the narrowest spectral band?
What does the guide image look like???

[AndyWilson]

Hi Ken,

That is actually what I did in my last post. I focused the spectrograph collimator to give nice sharp neon lines. I then did not touch the collimator again, taking the 2 sets of images with the only difference being the telescope focus.

I am really baffled. It may be some I am doing wrong but I've tried carefully adjusting everything I can think of. This is why I wondered if it might be the main camera distance as this is fixed in the Lhires III by the connector. I might also take out the doublet holder as I see another user had a problem there.

Thanks
Andy

[Franck Houpert]

Hello,

(sorry for my English, I use Google translator)

I have experienced this problem.

The grating is forced due to the expansion of the support.

It occurs when cold.

1 / If the grating is glued to its support, Shelyak contact that knows the problem.
For me, Shelyak changed grating and its support.

Malge exchange grating and support, I had a small problem

2 / if your grating is held by both sides as here
http://www.astrosurf.com/thizy/lhires3/ ... uille1.jpg
I removed the two holding pieces of the grating.
Scotch double-sided: grating - support
I handed the two parts but are no longer in contact with the grating.
It's just for safety.

Franck

(la traduction semble mauvaise, si une bonne âme passe par ici. Le problème vient d'une contrainte du réseau par son support. Ceci est dû à la dilatation des matériaux.
J'ai galéré pendant presque deux ans avec ce problème)

[AndyWilson]

Hi Franck,

Thanks for your post, which is very interesting. Here is a photo of my 2400l/mm grating, and it does appear as though the grating may be under pressure at the edges. I'm not too bothered, as long as I can find the problem and fix it.

2400 l/mm grating

2400_Grating.jpg (30.51 KiB) Viewed 10188 times

I've emailed Shelyak to see what they think.

Thanks,
Andy

[Franck Houpert]

the grating is maintained only by the double sided tape over its entire rear surface.

me too, 2400l/mm

[Robin Leadbeater]

Hi Andy,

I suspect astigmatism. This means best focus for the narrowest spectrum vertically ie you star spectrum) does not correspond with the focus with narrowest spectrum lines horizontally (ie your neon spectrum). You could check this by taking a sky spectrum or perhaps a G/K star say with many lines. The sharpest spectrum will probably correspond to the sharpest neon (and star in focus on the slit), not the narrowest star spectrum.

I have seen this a couple of times. Check that your grating is not pinched, which can bend it slightly in one direction. The clamps should just clamp the grating, just enough to stop it sliding in the holder. (Actually, thinking about it, this is odd. I thought the gratings were permanently fixed these days, unlike my original ones where the gratings could be changed in their holders) Also check that the collimator lens is not tilted in its holder (Do the neon lines move significantly back and forth when you turn the collimator for example ?)

It is also important to space the camera correctly (ideally within a couple of mm) otherwise the beam will not be collimated perfectly at the grating and you will not get pefectly sharp lines (see my posts earlier in the thread)

Cheers
Robin

[Robin Leadbeater]

Actually a little astigmatism is not a complete disaster and is a characteristic of some designs (Check out the new Starlight Xpress spectrograph if you want to see what severe astigmatism looks like !) including the Littrow where the return beam cannot be exactly on the same axis as the incoming beam, though it is clearly excessive in this case. Where astigmatism is present, the best procedure is to focus the neon lines to be as sharp as possible and focus the star on the slit to maximise throughput. Having a wide spectrum because of astigmatism is not ideal though as you have to include more sky background and camera noise when you bin the spectrum

Robin

[Mike Potter]

I've been knocking around with this problem ever since acquiring the LHiRes. I focused the slit using a 300mm telephoto lens and DSLR, shooting through the grating position (the grating had been removed). The telephoto/camera had previously been focused to infinity, and then the auto-focus turned off. Once I had tuned the focus for that configuration by rotating the doublet I re-assembled and took NeAr reference spectra. I had started with the camera set to the backfocus suggested by Shelyak. But when looking at the NeAr lines I had to rotate the doublet a bit more than 1.5 rotations which indicated a positioning error for the main camera of around 3mm. After making a 3mm spacer the slit (as seen by the telephoto) and calibration lines imaged after re-installing the grating are both in focus within less than 1/10 turn of the doublet. All fine and dandy - until trying to guide. In order to get a decently narrow spectral trace of a star I need to de-focus the star, as seen in the guider, by enough to make the star's image a donut around 6 pixels in radius (14.8 micron pixels). Basically the guide camera needed to be moved out a couple of millimeters - which got the star in focus but made the slit so far out of focus it can only barely be seen. So I adopted a compromise position that has the slit slightly out of focus and the star a very small donut. The guider is happy, the spectral traces are in focus. I also agree - this is MUCH worse in cold weather. I've found that it takes about 1/2 rotation of the doublet to compensate for a difference of 50 degrees F (about dT = 28C). But because the guide camera is not affected by that re-focusing it is put out-of-focus with respect to the spetrograph itself. No idea how to compensate for that.

I've also noticed that both viewing the slit via the DSLR/telephoto, and focusing on the NeAr calibration lines there are two "best" foci, separated by 1/2 turn of the doublet. One position results in narrower NeAr lines but broad "wings", the other position a fatter (about 1 pixel larger FWHM) lines but higher throughput and no wings. So I go for the higher throughput and no wings. I've also used the 19-micron slit with hole to measure how much the image moves around when rotating the doublet. In my case it can be as much as 80-90 pixels (9-microns each). I reduced that greatly by rotating the doublet lens in it's cell until that effect was at a minimum. There is still some "wandering" as the doublet is turned but it's much smaller than before. Some of that is perhaps due to lens wedge and imperfections in the lens "cell", and some of it is due to the fact the lens is a bit smaller than the cell, so it can be a bit off-center.

Other perhaps relevant info: C14 @f/11, SBIG ST8xme main camera binned 1x2, SBIG remote guide head binned 2x2 for guiding. And a lot less hair.

Mike Potter
Baltimore, MD
http://www.beverlyhillsastronomer.org

[AndyWilson]

Thanks Robin, Franck and Mike,

I've not got to the bottom of the problem yet, but I think the best candidate is pinching of the grating.

I have tried to loosen the grating holder for my 2400 l/mm grating, but no matter how hard I try I cannot loosen the screws and I risk damaging something.

I did swap to my 600 l/mm grating and I got the following result. Though not perfect it is a whole lot better than with the 2400 l/mm grating. I'm not sure if the extra lines per mm of the 2400 grating would exacerbate any astigmatism or back focus issue, or whether this indicates it is a problem specific to my 2400 l/mm grating.

Spectrum with 600 l/mm grating.

600 l/mm grating

Thanks,
Andy

[Ken Harrison]

Andy,
I assume that image was obtained with the target star "focused" on the entrance slit and the neon lines in focus??
If so, it looks much better.....