Alpy600 Focus and slit orientation

[EdWiley]

Many thanks to Christian for answering the slit orientation question posed earlier. I have the Alpy on the bench and just finished what I hope is correct orientation and focus. I would appreciate comments on the results for both focus and orientation.

Thanks, Ed

[Robin Leadbeater]

Hi Ed,

Getting the spectrum horizontal is more important than getting the lines vertical. Rotating spectra can give artifacts. This adjustment (rotating the camera) can either be done on a star or a hole aperture in place of the slit. (If you have the photometric slit, the line where the slit changes width can also be used as a guide) Unfortunately this correction is quite tricky to do on the ALPY without disturbing the focus. I have added a separate rotatable adapter between ALPY and camera (An ATIK 314L) to make this easier. It also allows the camera to be changed over between setups but it only works if you have enough room.

It does not matter too much if your lines are not quite vertical when the spectrum is horizontal. (They will also have a small curve as you can see) as this can be corrected by ISIS but if the slant of the lines is large or you find it annoying, you can adjust it by rotating the grism relative to the slit (an internal adjustment.) Mine had moved in transit so I had to adjust it. It was straightforward. These are the instructions I used (I think you will be amazed how small the grism is I know I was )

simply losen the three screws around the the
grism (headless), on the core body.
Be careful : there is a spring inside the core - not highly compressed,
but it can be surprising.
Rotate the grism when looking through the spectroscope (with naked eye)
: you will see that you can precisely tune the slant angle.
And tighten back the screws when the position is correct (in the same
time, push on the grism holder, to ensure to put the spring under pressure).

Cheers
Robin

[Robin Leadbeater]

Here are the before and after adjustment images on my ALPY (daylight spectra, spectrograph on the bench, click on the embedded image to view before and after spectra) Note how the dark lines from dust on the slit are exactly horizontal in each case (Dust specs can be useful to align the spectrum horizontal )

Cheers
Robin

[EdWiley]

Excellent, Robin. When I asked Jim Edlin about this he suggested rotating the camera. Possible with a Lisa (apparently), not with the Aply. I have to add an extension tube to my image train to reach focus, so I think I have plenty of room for a rotatable adapter. I will contact you off forum to see what you use.

Ed

[EdWiley]

Thanks to Robin for the tip on rotating the camera. I now have a rotator and can produce a horizontal spectrum. I have two questions.

(1) Slit focus with the guide camera. Does the attached image look good for focusing the slit? (It is a bit of an angle.)
(2) Slit focus versus spectrum focus: If the star is focused in the guide camera (using a mask), will the spectrum also be critically focused? Or do I need to do some additional focusing by paying closer attention to the Ha or Ca lines of my reference star?

No images from last night. I have some but are not satisfied. Perhaps tonight if the weather continues to be good.

Many thanks for all the help I have received.

Ed Wiley

[Robin Leadbeater]

Hi Ed,

(1) Slit focus with the guide camera. Does the attached image look good for focusing the slit? (It is a bit of an angle.)

That looks fine to me. The angle does not matter. I actually run the slit vertically in my guide camera as I run with the slit in the Dec direction so it seems more natural.

The orientation of the whole spectrograph relative to the sky is an interesting subject. Some run the slit in the RA direction and in the days before autoguiding this was the preferred option as it meant the PE moved the star up and down the slit, not on and off it. What works best depends on your mount and guider. (Or to really show off and provided your guide program can cope with odd angles, you can even orientate it at the paralactic angle to minimise atmospheric dispersion effects)

(2) Slit focus versus spectrum focus: If the star is focused in the guide camera (using a mask), will the spectrum also be critically focused? Or do I need to do some additional focusing by paying closer attention to the Ha or Ca lines of my reference star?

The focus of the spectrum is independent of the focus of the star on the slit as the spectrum is effectively a multitude of images of the slit.
It is important to focus the star on the slit though to get as small star image as possible to maximise throughput.

First make sure the image of your slit (at the point where you are going to place the star) is exactly in focus in the guide camera. Then with the star image just off the slit, focus the scope to give the tightest star image. This will get you close to optimum but if you want to get the maximum out of the starlight there are various other tricks, for example

With the star on the slit, readjust the focus to minimise the overspill (reduce the exposure to the minimum to best see this effect)

Watch the spectrum image, focussing to minimise the vertical thickness of the spectrum (useful for minimising any chromatic aberration effects in the scope too which "fishtail" the spectrum, particularly at the blue end. This is my favourite technique with ALPY when using the C11 with a focal reducer. I usually tune for most even width of spectrum over the full range)

Ultimately tune for maximum counts in the spectrum.


Cheers
Robin

[EdWiley]

Thanks, Robin, excellent feedback. BTW, the reason the illumination looks so strange is that I managed to invert the pickoff mirror. Naturally this produced some strange effects at the scope until I figured out the problem.

Ed