Second Results Alpy 600

[EdWiley]

I had my second session with the Alpy. Comments and questions. I thought I thought I had good focus on the slit at the bench. But at the scope I found that when I focused the spectrum the star image in the guider was out of focus. I made the adjustment at the scope. The assumption I made (correct??) is that if the star image is in focus the slit is in focus.

I was working with very bright targets and it was hard to see the change in magnitude when the star moved into the slit. This, I think, is related to question 1. The fix was using the fine grid in PhD.

Below are two image of Betelgeuse. Note that the first image has a dark bar through the middle.

Question 1: Is this bar an indication that I am slightly off the slit? (Perhaps interactions of light and the slit from bring slightly off?)
Question 2: Is the image with the bar useful for analysis?

More general question 3: Does the optical path leading to the guide camera distort the star image?

The reason I ask this is that last time I collimated the results were good. But the off-focus image through the guide camera is off center ("doughnut hole" displaced from the center).

Thanks for any and all comments.

Ed

[Robin Leadbeater]

Hi Ed,

Assuming these are very short exposures with no drift along the slit, your images show the star image is out of focus on the slit. The two lines in the top image are the top and bottom of the out of focus donut. The bottom image is possibly nearer focus, though it could be that you are just looking at the edge of the donut. When in focus you should have a narrow line the same width as the diameter of an in focus star

Your guide camera focus may have shifted on the transfer to the telescope. You can check the slit image on the scope in daylight with a diffuser over the scope if you need to limit the light, in twilight or using the light from the flat or calibration lamp depending on the exposure range of your guide camera.

I dont try to focus bright stars in the guider image. Instead I slew to a nearby fainter star which I can expose correctly and then slew back to the bright target.

The image of the star should not be significantly distorted in the guide image when centralised in the field close to the slit but small collimation errors are not a problem. Check that the ALPY module is pushed fully home against the stop in the outer housing before tightening the 6 thumb screws. This ensures the mirror slit is in the correct position under the guide camera.

HTH
Robin

[EdWiley]

Thanks for the fast feedback, Robin. I will follow your advice. The attached image of Alnitak was taken after I attempted to focus the star by moving the guiding camera in until the star looked focused while keeping the spectrum focused. (If it is focused, this is...) So my questions are:

(1) If the slit is focused on the bench, should the star also be focused on the scope? Or, if the star is not focused at the scope, when I bring the star into focus is the slit still focused?

(2) Should I be worried that the spectrum is so wide? Seeing was not that good (2.5/5), it rarely is in eastern Kansas.

(3) What does the dark horizontal streak in the middle indicate? Still out of focus? Not centered? Normal on bright targets? Inquiring minds want to know.

Cheers, Ed

[Robin Leadbeater]

Hi Ed,

I would say from the width of the spectrum that the star was still quite a long way from focus at the slit and if you could have seen the slit in the guide camera at the time, I suspect it would have appeared out of focus, even when the star was in focus. (You could test this by moving the telescope focus until the spectrum image is at its narrowest. At that point the star will be in focus at the slit.) Compare with the attached raw image from my Alpy for example (C11 ~f6 ~3 arcsec seeing)

Remember we are looking in a mirror in the guide camera image, not a ground glass screen like in an SLR viewfinder for example. This means that if the slit is not in focus in the guide camera it is quite possible for the star to be in focus in the guide camera camera but actually be out of focus at the slit, (giving the wide spectrum.) The way to be sure the star is in focus at the slit is to focus the guide camera on the slit first. Then if the star is in focus in the guide camera it will also be in focus at the slit. From that point you can then if you want fine tune to maximise the light in the spectrum reaching the imaging camera.

Note that the focus of the spectrum lines however is completely independent of what is happening ahead of the slit (as you can see from the nice sharp lines you are still getting even with the star out of focus) so you don't need to worry about the quality of your spectrum resolution even if the star is not focused, just that you will be potentially losing a lot of light if the star is not in focus at the slit.

Cheers
Robin

[EdWiley]

Hi Robin:

Many thanks for your patient tutoring. Your analysis makes complete sense. I think I understand enough now to make another "education" run the next clear night. I am saving all the mistakes as diagnostic tools. I find it valuable to know what images look like when they are not optimal since I tend to learn by my mistakes.

Cheers, Ed

[Terry Bohlsen]

Ed
I don't have an Alpy but a LISA so there is a difference but I will tell you how I achieve focus.
I first focus the main camera on the slit. This is done by illuminating the slit with any light and taking pics with the camera and focussing to make the image as narrow as possible. This is to get the focus roughly correct.
I then illuminate the slit with a neon light and choose a bright line near at the shorter end of the spectrum. I then select a small cross section of the line and start to take subframe focussing images of this line with ~1sec exposures. I can then measure the FWHM of the line in the X axis and change the focus to get the best FWHM. I use CCDSoft to do this but I hope that most image capture software will have some way of measuring this. When I have the best focus I lock it down.
The next step is to focus the guide camera. I illuminate the slit from above and take images with the guide camera. The slit appears as a dark line. I then fiddle with the guide camera until I have the slit as narrow and sharp as possible. I also make the slit run vertical N/S as I prefer to guide in RA. This solves the problem of backlash that I have in the dec axis and it is much easier to guide in RA to keep a star on the slit. I also like to orient my guide camera so that North is up as I find it easier to identify my target star this way.
Once I have focussed the guide camera on the slit and made it accurately vertical I don't touch the focus again. I have found the focus to be extremely stable and only needs to be changed if I move the slit in it's holder for some reason.
Once you have focussed the guide camera accurately on the slit you can then aim at a star. I choose a bright star and place a Bahtinov mask on my scope. I position the star near the slit and then focus the telescope to bring the mask into accurate focus. You can use any technique you like to focus the star but you must do this focussing with the telescope and not touch the focus of the guide camera or imaging camera.
If you have accurately focussed the guide camera on the slit then the image in the guide camera of a star at best focus should be as accurate as you can get.
Start to guide either on a bright star on the slit or a field star somewhere else and then take a spectra image with the imaging camera.
You should get an image somewhat like this one
http://i566.photobucket.com/albums/ss10 ... grcopy.jpg

Hopefully this helps.
Cheers
Terry

[EdWiley]

Hi Terry:

Many thanks for the comments, most useful. Between your comments and Robin's I think I am getting a handle on my problems. (1) I need to go back and make sure that the camera is well focused on the slit. (2) The I need to adjust the guide camera. Like you, I am a firm believer in using a Bahtinov mask at the scope. The Alpy is off the scope and on the bench, so its back to the beginning with careful attention to the camera/slit focus before I go through the guide camera/slit focus. The Alpy instructions are excellent, I just have to pay very careful attention to the quality of the images as I go through the process. On the other hand, even my poor images are giving me a chance to learn ISIS, so they have proven useful for learning to process results.

Cheers,
Ed

[Olivier Thizy]

Ed,


others have already replied with plenty of details. I would just summarize with the steps I am doing for telescope focusing whose goal is to maximize light flux (which is independant of the spectrograph focus whose goal is to maximize spectral resolution).

1/ Focus the guiding camera so that the slit appear in focus on the guiding image

2/ point a medium star (too bright and exposure is too short; too faint and exposure is too long; I'm looking for an exposure time around 10sec to get a reasonably bright spectrum)

3/ focus telescope to get the star approximatly sharp (minimize FWHM on the star shape, but a rough focus is OK at this point)

4/ set autoguiding ON that star (target position is on the slit)

5/ acquire a spectrum, measure the flux (I use max of intensity but measuring integrated flux per column would be better actually), adjust telescope focus to maximize the flux.
I also sometimes adjust the target position on the slit by +1/-1 or even +0.5/-0.5

6/ I usually do not do it but at this point, telescope is focused and you can move the guiding camera to get a focused star in case you want to use the star size to redo focus later on - but this is something I do not do.

7/ then I move to target and start acquisition session. Depending on where in the sky my telescope points to, I notice my autoguiding doesn't react the same so I regularly adjust the target position for the guiding within +/-0.5 pixels. With the Alpy, it is fairly stable and I think it is more an issue with the mount itself, specially with the NEQ6 mount; it is much less of a problem with the Losmandy Titan mount!


I hope this helps,

Cordialement,
Olivier Thizy
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