The Lowspec spectrometer

[Thalestris24]

Just now, Merlin66 said:

Louise,

PM sent with details

ken

 

Got it, thanks

Give me a few mins - it's on another PC

 

[Merlin66]

Louise,

Don't bother!

I found the answer was to open the image (in SGL) then save.

It is full size!

I assume the laser matches...

Ken

 

[Thalestris24]

2 minutes ago, Merlin66 said:

Louise,

Don't bother!

I found the answer was to open the image (in SGL) then save.

It is full size!

I assume the laser matches...

Ken

 

Oh ok - Was just about to send... Let me know if image is good enough

Thanks

Louise

[Merlin66]

https://commons.wikimedia.org/wiki/File:Fluorescent_lighting_spectrum_peaks_labelled.gif

Shows a Hg line around 4046.6 A.....

Working on it...

 

[Thalestris24]

26 minutes ago, Merlin66 said:

https://commons.wikimedia.org/wiki/File:Fluorescent_lighting_spectrum_peaks_labelled.gif

Shows a Hg line around 4046.6 A.....

Working on it...

 

Very kind of you  

It picks up the broad spectrum of my nominal 365nm UV torch as a bright, wide, well, sort of a rectangle! I think that it is brightest between 360 and 375nm. I'll have a search for my green laser tomorrow. It's here somewhere... I could also try one of the neon bulbs I bought - needs a little soldering to put a resistor in series . Not particularly bright but I think they have a characteristic spectrum at the red end.

Louise

[Thalestris24]

Haven't found the green laser pointer but found my Kensington wireless presenter with red laser pointer. I don't have any spec for the wavelength but it's pretty bright so I think it's 635nm. Here's a messy spectrum - I think you can tell pretty much where the line is.

Again it's slanted a bit - weird! I've noticed the lasers catch on the tip of the guide mirror holder. I'll have to try and do something about that. Off to bed now!

Louise 

[Merlin66]

Louise,

Thinking about the ghosting...it could be typical of internal reflections within the glass slit plate. These are normally seen by the guide imager rather than in the spectral image.

Could you double check the slit plate is correctly installed with the reflective surface sitting towards the collimating lens

[Thalestris24]

2 minutes ago, Merlin66 said:

Louise,

Thinking about the ghosting...it could be typical of internal reflections within the glass slit plate. These are normally seen by the guide imager rather than in the spectral image.

Could you double check the slit plate is correctly installed with the reflective surface sitting towards the collimating lens

Hi again

Yes the shinier side is towards the collimator. The other side with the reverse numerals is also very shiny though. Obviously the lasers and the 75W cfl are very bright and there is the problem I mentioned with the light catching and scattering from the tip of the guide mirror older. I'll have a look at that tomorrow and try and get a better alignment of the slit on the guide mirror + guide cam.

Cheers

Louise

[Merlin66]

Louise,

OK...

Looks like you were observing the 2nd order spectrum! The dispersion, using your two lasers are reference gives 1.49 A/pixel rather than the 1st order 2.88 A/pixel.

This could account for the ghosting - these could be fainter lines out in the 1st order red showing into the 2nd order.

You need to re-set the micrometer to rotate the grating back to find the bright zero order of the slit, then just a small adjustment to bring the brighter 1st order spectrum into view.

Ken

 

 

Edited December 16, 2019 by Merlin66
image added

[Merlin66]

19 minutes ago, Thalestris24 said:

reverse numerals is also very shiny

Louise,

When viewed from the front i.e. the telescope side these numerals will appear normal. Sounds like it's in the wrong way.....

Ken

 

[Thalestris24]

7 hours ago, Merlin66 said:

Louise,

When viewed from the front i.e. the telescope side these numerals will appear normal. Sounds like it's in the wrong way.....

Ken

 

Tut!

 

[Thalestris24]

Well, I've reversed the slit but I don't think it's made much/any difference. It's hardly exposed on the grating side anyway. With the slit illuminator part in place there is only a large slit-sized aperture visible from the grating side. Also, the edge slit size markings now no longer make any sense...

I've taken a sequence of images whilst turning the micrometer. From 0 deg i.e. plane to the camera lens (from the first sight of spectral features) up to what seems to be the max - about 20 deg. (a to k): 

First (a)

 

Last (k)

 The spectrum obviously shifts to the left with the clockwise micrometer rotation. I can't see a zeroth order? a-f seems to cover the first order? It's still slanted - I've no idea why!

Louise

[andrew s]

@Thalestris24 the slant is most likely due to a tilt of the diffraction grating out of the plane perpendicular to the optic axis. Try adjusting it.

Regards Andrew 

Edited December 16, 2019 by andrew s

[Thalestris24]

3 minutes ago, andrew s said:

@Thalestris24 the slant is most likely due to a tilt of the diffraction grating out of the plane perpendicular to the optic axis. Try adjusting it.

Regards Andrew 

Yeah, I was thinking it must be to do with the grating itself. I've tried reseating it and held it tight to the holder whilst tightening the grub screws. Will see if that helps - fingers crossed!

Cheers

Louise

[Merlin66]

Louise,

Hmm I was assuming the slit plate was similar to the original Custom design they produced for me... do the numbers show correctly from the front - in the guide camera?

The zero order will be further to the left - micrometer anti-clockwise rotation.

As Andrew says check the grating grooves align with the slit gap.

Ken

 

[Thalestris24]

17 minutes ago, Merlin66 said:

Louise,

Hmm I was assuming the slit plate was similar to the original Custom design they produced for me... do the numbers show correctly from the front - in the guide camera?

The zero order will be further to the left - micrometer anti-clockwise rotation.

As Andrew says check the grating grooves align with the slit gap.

Ken

 

Hi Ken

I can't see anything as big as the numbers in the guide cam - the fov is very small, can only see just either side of the slit. I've lined up the slit in the guide cam so it runs diagonally. I've not been able to see anything I could identify as a zero order - just what is in the sequence above. Could it be the design doesn't allow for it? I'll have another look when I can.

Louise

ps I just watched the grating whilst I turned the micrometer anti-clockwise and noticed that it may be that the grating isn't going back far enough because the spring is a little big and stops pulling the grating back

Edited December 16, 2019 by Thalestris24

[Merlin66]

Louise,

Looks like there's an issue with your grating rotation.

Image #4 shows the typical flouro spectrum but reversed...

You need to either flip the images or rotate the camera by 180 deg to get blue to the left of the spectral image.

When flipped and slant corrected it calibrates nicely

 

[Merlin66]

No ghosts in the machine!!!

 

[Thalestris24]

3 minutes ago, Merlin66 said:

Louise,

Looks like there's an issue with your grating rotation.

Image #4 shows the typical flouro spectrum but reversed...

You need to either flip the images or rotate the camera by 180 deg to get blue to the left of the spectral image.

When flipped and slant corrected it calibrates nicely

 

Yes, I did say it could be 'the wrong way around' - it's just as it comes out of SharpCap so could be flipped. I'm waiting for a T2 360 rotator so that I can adjust the camera easily.

So, my image 'd' is correct but flipped?

Thanks

Louise

[Merlin66]

Thinking about it.....

I think your seeing the - 1st order (minus 1st order).... as you move the micrometer clockwise you're moving down towards the blue....you should keep this clockwise adjustment going further and you'll find the zero order, keep going and you'll eventually find the blue region of the 1st blazed order come into view.

As you increase the micrometer reading you should be moving towards the red end of the spectrum.

Yes, the spectrum generally looks good, it will be even better when you find the 1st order.

You're almost there......

 

[Thalestris24]

7 minutes ago, Merlin66 said:

Thinking about it.....

I think your seeing the - 1st order (minus 1st order).... as you move the micrometer clockwise you're moving down towards the blue....you should keep this clockwise adjustment going further and you'll find the zero order, keep going and you'll eventually find the blue region of the 1st blazed order come into view.

As you increase the micrometer reading you should be moving towards the red end of the spectrum.

Yes, the spectrum generally looks good, it will be even better when you find the 1st order.

You're almost there......

 

I feel like I'm working in the dark - metaphorically speaking. For some reason my spectrum height seems to have suddenly collapsed to only about 10 or 20 pixels high... No idea why that is... It is getting very frustrating!

Thanks

Louise

[saac]

On 09/12/2019 at 22:16, Merlin66 said:

Jim,

The separate lenses of the doublets..... you'll need to space them. Three small strips of sellotape equally spaced around the circumference would work.

The SimSpec spreadsheet with give you a good approximation of wavelength coverage with your camera, as well as the resolution.

 

 

SimSpec V4.3_Paul3D.xls 78.5 kB · 3 downloads

Ken, thanks for that. I ordered the lenses in haste as they were a good bit cheaper on Surplus Shed compared to ThorLab or EO. I'll give it a go and see what happens. Thanks for the SimSpec spreadsheet as well, something to play with over the Christmas holidays. 

Jim 

[Thalestris24]

30 minutes ago, saac said:

Ken, thanks for that. I ordered the lenses in haste as they were a good bit cheaper on Surplus Shed compared to ThorLab or EO. I'll give it a go and see what happens. Thanks for the SimSpec spreadsheet as well, something to play with over the Christmas holidays. 

Jim 

I got mine from Surplus Shed too - they are fine! I wasn't going to pay Thorlabs prices!

Louise

[saac]

32 minutes ago, Thalestris24 said:

I got mine from Surplus Shed too - they are fine! I wasn't going to pay Thorlabs prices!

Louise

Louise I think I was a little too casual with the specifications when I placed the order , a few look too large  - oops  .   I'm looking to have all the internals printed off by the weekend coming and then I will play around with fitting the optics to see exactly what I have have. I'm also still waiting for the micrometer head it's on the slow boat from China   I've yet to place an order for the mirrors but I have a couple of first surface ones from previous projects again they may or may not fit .  Until then I'm enjoying reading the progress you are making  

Jim 

Edited December 16, 2019 by saac

[Thalestris24]

2 minutes ago, saac said:

Louise I think I was a little too casual with the specifications when I placed the order , a few look too large  - oops  .   I'm looking to have all the internals printed off by the weekend coming and then I will play around with fitting the optics to see exactly what I have have. I'm also still waiting for the micrometer head it's on the slow boat from China   I've yet to place an order for the mirrors but I have a couple of first surface ones from previous projects again they may or may not fit .  Until then I'm enjoying reading the progress you are making  

Jim 

Hi Jim

Yeah, you have to be careful what you order... I got the mirrors from Thorlabs - I only fitted them the other day! I got some round and square 1" craft mirrors of Ebay - very cheap! I used them in place of the proper ones until I was ready. The Thorlabs ones are a little expensive for what they are but not that much in absolute terms - plus they fit perfectly. I've got so far but am struggling a bit with the finer points. I don't think mine will be stable enough for astro-spectrometry. Getting and maintaining focus is proving very difficult. I'm sometimes getting some odd results but don't understand why.

Here's one I just did of the CFL bulb:

I think that's the zeroth order on the left and I've flipped it so it should fit Ken's calibration ok. It's all on a slant - not sure why. The spectral lines are slanted too - presumably to do with the seating of the grating. I've tried to adjust it but to no avail. I'm getting old and tired and impatient! I'm losing my grip - keep dropping things, and my cataracts make it hard to see what I'm doing! My back's been killing me too. I think 2020 will be my last year of doing astro things .

Louise