More LHiRes focus questions

[Mike Potter]

I figure this must be the greatest repository of knowledge regarding the LHiRes III.

I've never been able to get both the star and slit in focus at the same time in the guider. As it is currently (mal)adjusted, I have the slit nicely focused. However, if the star is in focus I get the double spectral traces in the main camera (I use an SCT so understand this is due to the "doughnut" shape of the out-of-focus star image). And when the spectral trace in the main camera is best focused the image in the guider is a doughnut, which makes it really hard to guide. I'm guessing the problem is that the guider fold-back mirror needs to be moved either in or out to solve the problem? If so, does anyone have guidance as to which direction I need to move the mirror? The star comes to focus with the guide camera set about 1mm farther out from the body of the spectrometer as compared to where it is if the slit is in focus.

Mike Potter
Baltimore, MD

[Andrew Smith]

Mike, I am not an expert but as no one else has replied I will try to help. I did have a LHIRES III once and found this link of help. http://www.astrospectroscopy.eu/Einstei ... rung_e.htm. You need to have the star image in focus with the slit image (it may for optimal transmission be very slightly out of focus). You also need the slit to be at the focus of the doublet done by adjusting the doublet position. Then the dispersed slit images need to be in focus by adjusting camera backfocus. Only then can you fine tune the on camera image by moving the doublet.

The link indicates how to do this. Google does a fair translation.

Regards Andrew

[Paul Luckas]

Mike,

Moving the guide mirror in and out affects only the position of the slit 'up' or 'down' in the guide view - and if anything will affect both the slit and guide star.

Andrew's link is your best bet. Focus the collimator to the slit with the grating removed using the 'finder scope set to infinity' method, then with grating installed and without touching the collimator, set the camera position for best focus. This can be tricky without a means to adjust camera back focus (e.g., helical focuser), but in the first instance at least it will serve as a test to see if your slit to collimator and collimator to sensor distances are equal (which they should be).

FWIW I recently discovered my sensor to collimator distance was slightly too long (by 3mm) which required some machining of my camera adapter.

Paul

[Robin Leadbeater]

The only time I have had an issue with the focus of spectrum and star on the slit being significantly different was when I inadvertently overtightened the grating in the holder (one of the original versions where the user could fit different gratings) the resulting curved grating produced severe astigmatism in the spectrum which meant the spectrum was out of focus in the vertical direction when the neon lines were in focus in the horizontal direction and star was in focus on the slit. Perhaps a tilted doublet might also produce a similar effect. The focus of slit and star were also measureably different with my original metal slit (probably a slight chamfer on the edge of the reflective surface near the slit) but I dont see that with the new mirror slits. The effect was not enough to produce a double image though.

Robin

[Mike Potter]

Thanks Andrew, Paul, and Robin:

I was able to get everything nicely focused - finally! I've been around this block a couple of times previously. I used my DSLR and a 300mm telephoto to focus on the slit as viewed from the grating position. Each time I pre-focused the lens at infinity (either the moon or some distant clouds if during the day), then turned off the auto-focus and was careful not to touch the lens's focus ring. I took a series of photos at different collimator settings and measured the FWHM of the slit's image in an image processing program. The two previous attempts had included using the standard Shelyak adapter for my ST8 camera (which I later learned was about 1.3 mm too long) and the second attempt with a home-made adapter which is a lot more robust than the Shelyak adapter and which should have set the main camera at the correct back-focus distance. But this time remeasuring the home-made adapter (basically the SBIG 2-inch nosepiece with a flat spacer cut to fit under the main camera flange on the LHiRes) I found I'd made a big mistake measuring its distance - about 3mm off! So this time, after getting best focus for the slit using the DSLR and 300mm lens I found that the Ne lines in the main camera were also exactly in focus without having to re-adjust the collimator doublet. Tonight I had the first opportunity to actually try it out on some stars and it appears to be perfectly focused! So thanks again for your comments - they forced me to re-visit adjustments I'd done poorly in the past.

Mike Potter
Beverly Hills Observatory

[Valerie Desnoux]

Hi Mike

Even if you perform the focus looking at the star (assuming you did the guiding camera correctly focused on the slit) , I always recommend to tweak looking at the spectrum itself. I always do it at the beggining of the observing session. I iterate to make sure I have both a thin spectrum and the highest intensity. Even if my star is a little bit unfocused.

Valerie

[Mike Potter]

Thanks Valerie:

That is, in fact, what I do. At the beginning of the night I do the best I can to focus the Neon lines using the doublet. Since it is now colder I've found that the star is, again, slightly out of focus in the guide camera for best focus in the main camera. But it is now close enough I can live with it. I try not to move the guide camera and keep it so that the slit is in sharp focus.

Clear Aether!
Mike Potter