The El cheapo DIY SM1-compatible optical slit

Ten months ago when I first read about the ramanpi, I immediately bought a small collection of some of the things on fl@tc@t’s bill of materials. In hindsight I should probably have done a little more research before shopping.¹

For example I bought a 10µm optical slit. Theoretically this is a very good choice of slit width, because the pixel width of the TCD1304 is 8µm.²

This is of course in a perfectly well aligned spectrograph. Mine won’t be perfect, so I’ll go for more light throughput. However I’m not about to pay another 100€ or so for a new slit. The price pr m² for that kind of thing is terrible.

Instead I set out to make one myself, I hope it will be good enough. Anyway, enough talk. Here is my El cheapo DIY SM1-compatible slit:

DSC02003

The El cheapo DIY SM1-compatible slit next to a SMA fiber adapter.

The El cheapo slit consists of a 3D printed structure fitted in a Thorlabs SM1RR retaining ring. Four small neodymium magnets (Ø=2mm, h=1mm) are positioned under the two razor edges comprising the actual slit.

More pictures:

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From the top left: an SM1RR retaining ring, a SMA fiber adapter, the El cheapo slit exposing its magnets and the rather expensive 10µm mounted slit.

In this last picture the El cheapo slit is screwed into the SM1 lens tube holding the collimator lens (an achromatic doublet):

DSC02002I’ve not yet verified the slit width I’m able to achieve, I will do this next week when I get close to a microscope, but it is creating a nice diffraction line.

Oh and of course: here is the openscad file for the DIY 3D printed optical slit.

Update: I’ve measured the attainable slit widths to be as high as 0.13mm and as low as 25µm. These figures are very approximative.

The important thing is that the razors are easily quite accurately aligned. The human eye is surprisingly good at evaluating parallellity.

¹ I’ve already substituted the cheap ebay laser with a proper JDSU µgreen laser. I’ve also moved away from the Czerny Turner spectrograph, and so I’m left with a few spherical mirrors that I haven’t found a new use for.

² Sometime ago, somewhere I don’t remember, I saw a calculation showing the optimal slit width vs CCD pixel width.

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2 thoughts on “The El cheapo DIY SM1-compatible optical slit

  1. Morten says:

    Hvis man ellers kan gå ud fra, at barberblade er rette på æggen, kan man skubbe dem sammen om to stykker enkelt tråd (litze) fra en ledning, hvor man kan komme til at måle tråden med en mikrometerskrue inden. ihvertfald kan det give en mere ensartet montage, hvis det bliver nødvendigt at sætte spalten op igen. En anden mulighed er, at lime en møtrik i hver ende, vinkelret på det ene blad og et anlæg på det andet. En skrue har ret fint defineret gevindstigning, M3 = 0.5mm, M4= 0.7mm, M5=0.8mm og M6 =1mm, og man kan så justere sig til parallel

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    • Det er et glimrende forslag, og det ville helt sikkert klare problemet med reproducerbarheden.

      Jeg har justeret barberbladene så godt jeg kunne indtil jeg synes de lå parallelt og så kigget på det under mikroskopet med 10x og 20x forstørrelse, og der ser det forbløffende pænt og parallelt ud. Barberbladene ser meget rette ud, selv ved stor forstørrelse.

      Til at evaluere spaltebredden har jeg monteret det i et forstørrelsesapparat (Durst M600) og målt på projektionen og regnet mig frem til spaltebredden ud fra forstørrelsesgraden (jeg kunne ikke finde et okular med gitterlinjer til mikroskopet).

      Jeg kunne godt finde på at bruge din løsning med møtrikkerne i et senere projekt – et almindeligt UV-vis spektrometer. Lige i Ramanspektrometerets tilfælde er det nødvendig at have så lille afstand som muligt mellem lyslederadapteren og spalten, så der bliver det upraktisk med møtrikkerne.

      Igen, tak for gitteret, jeg glæder mig til at tage det i brug.

      Like

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