The Lowspec spectrometer

[ejp1684]

9 minutes ago, Thalestris24 said:

I have a small 45 deg mirror that I made before 🙂 I'll try a finder first and see how I get on - I'd rather do that than hunting around for the zero order. Will post my result 🙂. 

Louise 

The zero order on my Lowspec3 is very close the the zero mark on the micrometer.

Eric.

[Thalestris24]

1 minute ago, ejp1684 said:

The zero order on my Lowspec3 is very close the the zero mark on the micrometer.

Eric.

Oh ok. I'll bear that in mind!

Thanks

Louise

[Paul Gerlach]

8 hours ago, Thalestris24 said:

Ok. Hmm... I'd have to take the grating out to do that - or maybe I could just use a mirror. It's 2:30 am here now so I'm turning in for the night!

Thanks 🙂 

Louise

First remove the camera lens from the LOWSPEC. Place a small guide scope, as Ken suggested, in front of the camera opening that is focused at infinity and look into it while the front-end of the LOWSPEC is facing a light source. Then turn the grating towards the collimator until you see the zero order (the image of the actual slit itself because the grating is now acting as a mirror). Adjust the position of the collimator lens until you see a sharp image. Secure it and place the camera lens back in the LOWSPEC.

Paul

[Thalestris24]

I've set up a mini guide scope and camera. This is the first slit image I took:

Doesn't look good, and maybe overexposed.

This is an image after attempting adjustment:

Better? Adjusted focus and reduced exposure. I have terrible trouble trying to decide on focus or not. Any idea how I might get rid of the ghosting / multiple images? Is it caused by the reflective slit?

Louise

[Thalestris24]

I think this is about the best I can do(after tightening) though it doesn't look right, It seems to 'flare' towards one end - maybe that's to do with the light source - a cfl?

Anyway, after dropping it again earlier, I have try and get the camera attachments back on the body...

Louise

[Thalestris24]

Here is CFL spectrum, taken with the 183M, bin 1, 1s exposure, 20um slit. I think it's a bit sharper than before. There is still some widening / blurriness at the bottom (=top?), as seen above.

Louise

ps I've bought a Nitrogen spectral tube. I just need to put together a power supply to try it. I think they 'strike' at 2kV but I'm unsure what they normally run at. Maybe a fluorescent tube ballast could be used? I think they strike a 1kV pulse but then drop down to below mains voltages. If anyone has any experience of using spectrum tubes and knows how the power circuit works, please let me know!

 

[Paul Gerlach]

6 hours ago, Thalestris24 said:

I think this is about the best I can do(after tightening) though it doesn't look right, It seems to 'flare' towards one end - maybe that's to do with the light source - a cfl?

 

It's better to point at a bright illuminated white wall so the light is more diffuse.
I'm puzzled by the poor image. Did you check the optics after that second drop?
I've made a quick image of order zero (slit image) with the LOWSPEC (3) pointing at an illuminated ceiling. Mind you this is with camera lens in place and ATIK314L (2x2)

Paul

[Thalestris24]

3 minutes ago, Paul Gerlach said:

It's better to point at a bright illuminated white wall so the light is more diffuse.
I'm puzzled by the poor image. Did you check the optics after that second drop?
I've made a quick image of order zero (slit image) with the LOWSPEC (3) pointing at an illuminated ceiling. Mind you this is with camera lens in place and ATIK314L (2x2)

Paul

That's a very clean line! I'm envious... Is that also a 20um slit? How come it doesn't show the slit angled ends? (Clearly visible in mine).  My holder mechanism doesn't always place the slit vertically and in the same/right place. I'll try imaging the zero order as well.

Louise

[Paul Gerlach]

37 minutes ago, Thalestris24 said:

Is that also a 20um slit?

No, that's the 30um slit. The 'old' LOWSPEC 2 has a round opening in the carousel that holds the OVIO slit plate. It appeared to mask off one side of the line. Does yours have that round holes or did you use the adapted version I placed somewhere on this thread?
Check the tension of the spring. Tighten it with the grub screw. The goal is that you can turn it with some force and that it clicks into place. If there still is some play, disassemble the slit holder. Remove the grub screw, spring and steel ball. Try to enlarge the small opening where the steel ball protrudes slightly with a small round file. The aim is to make the ball protrude so much that it completely 'grabs' the notch of the slit carousel.

Did you check the orientation of the collimator and camera lens? The face with the largest curvature should face the grating.

Paul

[Thalestris24]

36 minutes ago, Paul Gerlach said:

No, that's the 30um slit. The 'old' LOWSPEC 2 has a round opening in the carousel that holds the OVIO slit plate. It appeared to mask off one side of the line. Does yours have that round holes or did you use the adapted version I placed somewhere on this thread?
Check the tension of the spring. Tighten it with the grub screw. The goal is that you can turn it with some force and that it clicks into place. If there still is some play, disassemble the slit holder. Remove the grub screw, spring and steel ball. Try to enlarge the small opening where the steel ball protrudes slightly with a small round file. The aim is to make the ball protrude so much that it completely 'grabs' the notch of the slit carousel.

Did you check the orientation of the collimator and camera lens? The face with the largest curvature should face the grating.

Paul

Oh ok - I have the modified holes now. I've tightened/loosened the spring. The main problem is that I can't get a smooth movement. I count that as a small problem since I basically leave it on the 20um slit at the moment. Do you think I might have a problem with the slit illuminator part? I reprinted it to match the modified holes. I'm trying to take an image of the zero order at the moment but can't seem to focus on it - it's 'optically' rather distorted with ghosting even against just my illuminated wall. Something is amiss - I just don't know what. The focusing mechanism is very stiff which doesn't help.

Louise

[Merlin66]

From my understanding the slit illuminator illuminates the rear of the slit plate and should have no impact on the quality of the zero order image??????

Louise, you could be over exposing the zero image. Looking at your CFL spectral image you can see the top and bottom "edges" are not tight and clear. This could due to uneven illumination ( from the reference lamp positioning) or vignetting within the optical arrangement.

 

[Thalestris24]

Hi Ken

Vignetting could be a problem - I'm not sure. I just took this image of the zero order (20um) from the camera. It shows up the reflection problem very well. Do you think it's something to do with the reflective slit? I've tried masking off both sides of the slit with some card but it doesn't make any difference. So it must be something else...

 

Louise

[Merlin66]

Louise,

Looks like the slit is not exactly aligned with the grating grooves...

The image seems to have been under exposed and stretched - what did the "normal" exposure show?

 You should be able to get an image close to Paul's example.....

 

[Thalestris24]

1 minute ago, Merlin66 said:

Louise,

Looks like the slit is not exactly aligned with the grating grooves...

The image seems to have been under exposed and stretched - what did the "normal" exposure show?

 You should be able to get an image close to Paul's example.....

 

Hiya

It's not stretched but it is bin2. I'll do another with a longer exposure. What does that mean - 'not aligned with the grating grooves'?

Louise

[Thalestris24]

Here is 30s exposure, bin 1 (cropped), 30s exposure

As well as the ghosting/reflection there is an obvios difference between top and bottom. I'm wondering whether the grating is not vertical ie leaning at the top or bottom towards the camera.

Louise 

 

[Merlin66]

If you look carefully at the slit image you see it has a sort of twisted appearance....the bottom section shows it a bit clearer...

This usually is seen when the slit gap doesn't align with the grating grooves. If the slit plate is not 100% registering to bring the slit alignment this could result. Just try a slight rotation of the disk see if it improves.

(Imagine the grating is sitting with the "grooves" in the grating surface sitting vertically.....the the slit gap should also be arranged so it's height is sitting vertical and parallel to the grating grooves...)

EDIT: The last 30 sec exposure shows what looks like dust on the slit ( top three dark areas) the illumination is not "regular" along the slit. Try similar exposures but with a slightly wider slit gap 30 micron or so.

 

 

Edited February 26, 2020 by Merlin66

[Thalestris24]

Just now, Merlin66 said:

If you look carefully at the slit image you see it has a sort of twisted appearance....the bottom section shows it a bit clearer...

This usually is seen when the slit gap doesn't align with the grating grooves. If the slit plate is not 100% registering to bring the slit alignment this could result. Just try a slight rotation of the disk see if it improves.

(Imagine the grating is sitting with the "grooves" in the grating surface sitting vertically.....the the slit gap should also be arranged so it's height is sitting vertical and parallel to the grating grooves...)

 

 

Oh, I see what you mean now. I mentioned I had problems with the slit to Paul earlier.

Louise

[Thalestris24]

I think this one is maybe a bit better. I took the slit assembly out. I noticed that when looking through from the illuminator side that the bottom of the slit was being cut off! I've taken the illuminator out so this image is without it. Bin 1 this time, just the illuminated wall, 15s.

There is still a tendency to cut off the very bottom. It's looking like I'll have to try reprinting the slit holder body... Another thing I noticed from looking through from the illuminator side is that if tilt the assembly such that I'm not looking through the slit itself, I can still see the cfl bulb. Maybe this is what's causing the ghosting? Maybe it's not an issue with a less bright source.

Louise

[Merlin66]

Louise,

Yes looking better!

Still some dust (?) near the top of the slit... 

Could be you should use a diffuser (sheet of paper) between the CFL lamp and the inlet.

I'd run with what you have and work towards getting a star spectrum.

Onwards and Upwards.

 

[Thalestris24]

9 minutes ago, Merlin66 said:

Louise,

Yes looking better!

Still some dust (?) near the top of the slit... 

Could be you should use a diffuser (sheet of paper) between the CFL lamp and the inlet.

I'd run with what you have and work towards getting a star spectrum.

Onwards and Upwards.

 

I set the grating back to normal mode but it looks completely out of focus now. Surely it should still be in focus or close to it?

Louise

[Merlin66]

Louise,

Hmmm, shouldn't be "dramatically" different......

Remember you're using achromat lenses....the focus will vary with wavelength.

That's why you should refocus (using the reference lamp) on an emission line close to your required CWL, to get best FWHM every time.

Edited February 26, 2020 by Merlin66

[Thalestris24]

11 minutes ago, Thalestris24 said:

I set the grating back to normal mode but it looks completely out of focus now. Surely it should still be in focus or close to it?

Louise

Maybe not completely out of focus but made worse by the reflection. I adjusted the focus and used a translucent plastic sheet as a diffuser. I think the focus is still off a bit. 

CWL? 

My focuser on the Lowspec is dreadful... I hate having to use it. 

I'll definitely have to reprint the slit assembly.

I'm tired - going to bed now. Thanks for your help.

Louise

ps Working in my living room I can't keep dust out - can but try and keep it to a minimum. All the optics have dust on it...

 

Edited February 26, 2020 by Thalestris24

[Merlin66]

CWL = central wavelength

Have a good night's sleep!

 

[Fellside]

Hi

I`m right at the beginning of my Lowspec journey.

Can anyone tell me approximately how much material is required to print the Lowspec parts and what is the preferred material to use.

Many thanks

Graham

[saac]

I think I got mine printed all out on one whole real with some left over. I used PLA, I think most are doing that. 

Jim 

Edited March 3, 2020 by saac