PHD for Spectroscopy

[Merlin66]

After discussion with Craig Stark re our needs for spectroscopy, he's updated PHD and the new version V1.12 has a "nudge" option which allows you to use Al's reticle and move the target star onto the virtual slit.
Majic happens!!

[sheeny]

:D

Well done Ken and Craig!:thumbsup:

Al.

[higginsdj]

Whats "Al's reticle"? Given that Spectroscopy will be a very manual process I won't be able to use my current setup and will need to acquire and learn new technologies :)

Cheers

[Merlin66]

Al?
Do you want to tell David or will I???

[Merlin66]

David,
Our own Al Sheehan developed this neat little program which provides a moveable screen reticle. This can be placed over the PHD screen to allow the target star to be accurately positioned on the "virtual slit" - an excellent aid for us in spectroscopy.
http://www.iceinspace.com.au/forum/showthread.php?t=21798
The latest version is V2.2 and is also the Y! group files area.

[higginsdj]

Hmmm.... OK, I assume that I can do whatever PHD can do using MaxIm? MaxIm has cross-hairs and I assume all I need do is maneuver the target into the cross-hairs then start guiding?

Cheers

[Merlin66]

Al's reticle should be able to sit on top of any other screen.
I've only tried it with PHD but I'd assume it could also work with Maxim.
The advantage of a moveable reticle is that it can be positioned to exactly show where the slit is and then be used to position the star on the centre of the slit as well as guide.

[higginsdj]

I'm obviously missing something here, but how does Als Reticle know where the slit is? One must assume that it is located through the centre of the camera FOV - yes? Assuming one doesn't care about the orientation angle (?) one only needs to ensure that star is brought to the centre of the image field and then guided on that?

Cheers

[Merlin66]

Hmmmmm not really.
During the set up and calibration you can move the reticle to sit "virtually" exactly where the slit is positioned in your telescopes optical train. For convenience the length of the slit is usually orientated N-S so that any drift just runs the target up the slit and widens the spectrum. With such accurate guide software a PHD etc the star never really moves. remember the "ideal" slit width is only 3-4 pixel wide..
It's nice to be able to "see" the slit by virtue of the reticle and ensure that the target is as central as possible to the slit height. Also, if you want to get the spectra of nebula etc it helps when orientating the slit to get a spectrum of different/ various areas
When you've used it a couple of times you'll get a better appreciation - I love it!

[higginsdj]

OK back to basics - how do I know where in the image the slit is positioned?

Obviously you must know where it is to put the reticle there but I can't see it!

Cheers

[Merlin66]

David,
welcome to the real world of slit spectroscopy!
One advantage the Spectra-L200 has, it the ability to see the zero order image of the slit.
When calibrated, the grating rotation angle should be set to bring this zero order image to the centre (say along the X axis) of the imaging frame; this will give some control over the central wavelength and the spread of the spectrum recorded.
So far so good.
Now to register and position the target onto the slit .... obviously if the guide system (flip mirror/ guide scope etc) is 100% aligned to the main optics and the CCD chips are exactly square to each other, then a star at x=300, Y=300 in the guide camera will be "registered" and mapped to a point x=500, Y=500 on the main image (assuming chips of 600 x600 and 1000 x 1000 pixel respectively)
This is never easy to achieve in practise...
What happens is that the target is position as close to centre as possible in the guide image, "scanned" across the X-axis until a star image appears in the imaging camera ie the star is visible through the slit. Fine tuning then moves this star image onto the fixed edge of the slit. If you have a rear illuminator fitted then this can allow the rear of the slit opening to be seen in the image with the star superimposed on the slit - it can then be eased up and down until it lies central to the height of the slit. These fine position adjustments are made using the position of the star in the imaging camera - now look at the relative position of the star in the guide image - it may, say be sitting at X= 307, Y=284 this then becomes the "guide" star registration point for future targets. If the reticle is positioned at this coordinate then you can double check, by easing the star up and down the slit the inclination of the slit in the guide image and rotate the reticle to suit. You now have a "virtual" slit superimposed on the guide image which shows exactly where the centre of the slit is located and the direction of the slit. this makes is 100% easier to position the subsequent targets exactly on the slit time after time. Remember the slit will only be a few pixels wide, so there's not a lot of room for error.
Hope this helps,
Ken

[higginsdj]

OK, I think I follow but it seems to assume that the guide camera and main camera see the same FOV. In my system (and most other guiding systems) it doesn't. I have 2 different images from 2 differnet cameras each looking at a different set of stars.

So if I understand this, ideally I will centre the target star in my main camera through the slit as best as possible then nudge it around to 'fit the slit' then start guiding (or start guiding early and nudge the guide star to move the main star!) In any case, in my system Als reticle is of no use since it will be different for every target. I must check if MaxIm allows nudging the guide star manually or I may just switch to PHD and get it to do my guiding (or does PDH need the Reticle to do the nudging?)

Cheers

[Merlin66]

I think we're getting lost in the forest.

Ideally the target star MUST be visible somewhere in the FOV of BOTH the guide camera and the imaging camera. The FOV of the imaging camera is restricted by the "slit aperture", so the target needs to be accurately placed within the FOV to be seen.
The "registation" between the two is the issue.
(The problem is a little more complex if you only use an OAG - then you need to work out plate scales and off-sets)

Once the target is on a wide slit then you should select the same target star in the guide FOV to guide on. This can be done in PHD with or without Al's reticle.
The real issue is to be able to do this quickly and repeateably. So if you know WHERE in the guide FOV the target should be placed (X=, Y=) to position the star on the slit, you can use this setting every time.
The use of Al's reticle only makes this more intuitive and easier.
Ken

[higginsdj]

Well there-in lies a big problem........ My setup won't attain such aligned without extensive changes, something I had not foreseen (I was used to seeing 0th order images of the FOV from the SBIG spectrographs.)

Well I'm going to have to wait for a new CCD camera before I can hook up the L200 (my ST-8 has a built in filter wheel) so that will give me time to look at options to remount my guide scope and camera.

Cheers