LOWSPEC - 3D Printed but Calibration Question

[Rockmover]

I just printed a LOWSPEC Spectrometer, and am installing the optics now.  However, I have NO idea how to go about focusing the mirrors and calibrating it.  I am very new to spectrometry and have only played with a SA-100 so far.

I have made a video on the 3D printing aspect (below).  I intend to do two more to help others out, but I would prefer to not give bad advice so any help would be so appreciated.  I assume first is to set it up on a scope and get the pick-off mirror aligned and the guide camera focused and positioned correctly (with a star focused on the OVIO slit ring).  

I have an artificial star (the pinhole flashlight model) thing I used years ago for collimation I was going to use.  Next I was planning on maybe shining a laser down the scope to rough align the mirrors.  But from there I'm lost. Clearly the main reflection mirror is next and should point to the collimation optics.  But where should the collimator lens be positioned (back, middle, front of the slot)?  How does this effect the image?  Then how to align the reflective grating?  There is a set screw to adjust the vertical plane (on the front of the base inside, and on the rear on the outside of the case). 

Again, I am new, and maybe I am missing a manual someone already wrote on this?  If not I am happy to do so for the next users because this seems like an awesome, and fun, project!

Thanks!!!

Edited November 25, 2019 by Rockmover

[Rockmover]

Also, here is my unit assembled, without the optics. Looks good so far for $35 of PLA, $10 for the micrometer, $30 for the illuminator, and about $15 of bolts and hardware.

Edited November 25, 2019 by Rockmover

[Merlin66]

Rockmover,

You're doing well.

I believe Paul's assembly instructions show how to set and align the optics (?) 

Once installed you can do most of the preliminary testing on the bench. You can add the guide camera and get it focused on the slit plate.

I usually check by using a laser collimator - fitted to the centre of the inlet port and a paper target centre spotted on the camera port.

The beam should go through and by moving the micrometer, you should be able to bring the bright spot into the image port and onto the centre mark.

Set up a fluoro lamp, and point the inlet port to the lamp. This will give you a nice spectrum which can be used to initially calibrate the instrument.

Ken

 

[Rockmover]

Paul has fantastic instructions on the assembly, but I don't see anything on setting/adjusting the mirrors.  Unless I am missing a second manual somewhere? 

Maybe I'm overthinking things and it will become obvious once I start using it.  I was planning on using the laser beam to adjust the main reflecting mirror, and then the angle of the diffraction grating. Is that also how the front/back tilt is set (ie the vertical)?  Also, do I just start with the collimating lens in the middle?  If so why have a slot for it and not make it fixed?  What does the position of that do?

 

[Merlin66]

Things will become a bit more obvious when assembled.

I think the positioning is based on using the nominated optics, so there should be little variance.

The collimator should focus on the rear of the slit plate.

The grating should be vertical, and the rotation free and consistent. Rotating the grating, as you will see, brings the different target central wavelengths to the imaging port.

Ken

 

[Thalestris24]

Hi Rockmover

Just to butt in I'm just assembling mine also and also faffing around with lenses. The set up is as defined by the design of a classical reflection grating slit spectrometer (with reflective slit). I 3D printed an item to hold a small 1" craft mirror at 45 deg. I put it in place of the grating mechanism and the centre of the mirror is in line with the centres of the lenses and mirror. I can look down and check that the collimator lens can focus on the back of the slit. I'll also use it to check the camera lens focal plane.

It's a surprisingly useful little tool! As I say I've been trying different lenses which have required me to print new lens holders. Hopefully, tomorrow I'll be able to check the imaging camera focus. At the moment, I only have the guide lens in place. I've not touched the grating at all yet (very delicate!) and will probably put that in last of all. In the meantime I have a dummy grating (another piece of 3d printing with a mirror on it). I haven't put the actual slit in yet either but again made a practice one by scratching the back of another craft mirror with a scalpel. It's good enough to test with. I don't exactly know for sure how to set up the grating but Christian Buil has some useful info here and here. He includes calculations for the grating angles of incidence and angles of refraction (alpha and beta). I think basically you do your own calculations for whatever components and scope you have. That should let you approximately set the angle of the grating and you can do fine adjustments using the micrometer with everything in situ. Oh, there is a handy Excel spreadsheet here to make some of the calculations easier although Paul Gerlach (the Lowspec designer) has obviously already fixed some things in the design.

Hope that's helpful - I'm no expert, just finding my way

Louise

[Rockmover]

Thanks Louise, and now the movement makes sense.  Ie the collimation len should focus the light on the grating (I actually didn't realize that). 

The idea of temporary using a mirror in place of the grating is also a great idea.  That allows everything to be approximately positioned before installing the sensitive grating, and allows a first time user the ability to get a feel for the adjustments before installation of that.

[Thalestris24]

2 minutes ago, Rockmover said:

Thanks Louise, and now the movement makes sense.  Ie the collimation len should focus the light on the grating (I actually didn't realize that). 

The idea of temporary using a mirror in place of the grating is also a great idea.  That allows everything to be approximately positioned before installing the sensitive grating, and allows a first time user the ability to get a feel for the adjustments before installation of that.

Hi

The collimation lens is focused on the slit and passes a collimated beam to the grating. It's just that at the position of the grating you can see, via the mirror tool, the back of the slit in focus. I don't find it easy to do cos I wear glasses and can't always tell when something is in focus! I've found it easier to do it with something in place of the slit but at the same position/distance. I'm not convinced a lens with a fl of 125mm is able to focus on the back of the slit but it might do. I've made a new lens holder to take a 26mm dia lens of focal length 121mm and with a shorter 'foot' so I can move it more in its slot. Hopefully, I'll be able to properly check it out tomorrow. It's 2:30 am now and I'm off to bed!

Louise

[Rockmover]

Last night I tried aligning the guide camera, but the angle was way off and I couldn't adjust the pick-off mirror enough.  Eventually I cracked the slider trying to get it aliened.  I re-printed another piece, but looking at the part in CAD the pick-off angle is 30 degrees.  Shouldn't this be 45?  Why 30?  I am going to re-draw this at 45, unless I am missing something?  Surely if it is supposed to be 45 other users must be having this same problem to?

Edited November 26, 2019 by Rockmover

[Thalestris24]

16 minutes ago, Rockmover said:

Last night I tried aligning the guide camera, but the angle was way off and I couldn't adjust the pick-off mirror enough.  Eventually I cracked the slider trying to get it aliened.  I re-printed another piece, but looking at the part in CAD the pick-off angle is 30 degrees.  Shouldn't this be 45?  Why 30?  I am going to re-draw this at 45, unless I am missing something?  Surely if it is supposed to be 45 other must be having this problem to?

Yes, should be at 45 deg. Here are my dimensions:

I just fixed a 1" mirror to the angled part with a small drop of glue. The angle actually came out at 44.94 deg, I think, but it's fine!

Louise

[Rockmover]

Thank you!  I thought I was missing something, and it was driving me crazy! 

I am going to just redraw this part with the correct dimensions this morning and reprint it since I have that little mirror here.  Seems like kind of a significant mistake though, I am surprised no one else has caught it.

 

[Thalestris24]

3 minutes ago, Rockmover said:

  Seems like kind of a significant mistake though, I am surprised no one else has caught it.

 

? Isn't it your design?

ps you can probably round up the above dimensions e.g. 36.98 = 37, 17.96 = 18, and 18.98 = 19  

Edited November 26, 2019 by Thalestris24

[Rockmover]

Surly not my design, Paul Gerlach gets all the credit!!  I'm just a newbee in spectrometry and stumbled across this project a couple weeks ago!  Bought the optics and started printing! In no way do I want to take away any of Pauls hard work, but I am certainly more than happy to help make this project better for others to enjoy !!!  

Edited November 26, 2019 by Rockmover

[Thalestris24]

9 minutes ago, Rockmover said:

Surly not my design, Paul Gerlach gets all the credit!!  I'm just a newbee in spectrometry and stumbled across this project a couple weeks ago!  Bought the optics and started printing! In no way do I want to take away any of Pauls hard work, but I am certainly more than happy to help make this project better for others to enjoy !!!  

Oh, I was only referring to the 45 deg mirror holder.....

[Rockmover]

LOL!    But yes, I have it done and am throwing it in CURA now.

 

Edited November 26, 2019 by Rockmover

[Thalestris24]

1 minute ago, Rockmover said:

LOL!    But yes, I have it done and am throwing it in CURA now.

 

That looks rather like the guide mirror holder?

[Rockmover]

Yes...that what I have been asking about!

"Last night I tried aligning the guide camera....."

[Rockmover]

Guide mirror centered and at 45 degrees.

 

Edited November 26, 2019 by Rockmover

[Thalestris24]

Just now, Rockmover said:

Mirror Centered.

Oh, sorry, I was confused - thought you were referring to my 45 deg mirror tool! That's what I gave you the dimensions for above! The guide mirror holder should print out fine with no changes.

Apologies for my confusion!

Louise

[Rockmover]

LMAO....

So why is it at 30 degrees, and not 45?  is it not a right angle or is there some optics stuff there I don't understand?

 

[Thalestris24]

10 minutes ago, Rockmover said:

LMAO....

So why is it at 30 degrees, and not 45?  is it not a right angle or is there some optics stuff there I don't understand?

 

Because it's reflecting light from the slit to the guide camera - via the guide lens

 

Edited November 26, 2019 by Thalestris24

[Rockmover]

Ahh...so the slit has to be in place for it to work correctly?  I just set a piece of paper in there, focused on a flashlight across the room to get a rough focus on where the slit will be, and then moved the scope a little to then focus that same light source on the guide camera.  Seems this is a flawed approach?  

[Rockmover]

OMG, I had assumed (incorrectly) that like a off axis guider, light is picked off the incoming light and reflected into the guide lens. This is clearly not the case, and I had the mirror holder in 180 degrees backwards!

A picture is worth a thousand words.  To be honest if this image was in the manual it would have helped me a lot!!  

Step 17 shows it, I just 100% had my head wrapped around a normal off axis guider setup.  

Edited November 26, 2019 by Rockmover

[Thalestris24]

Well I used a diy slit - 

https://stargazerslounge.com/topic/343342-the-lowspec-spectrometer/?do=findComment&comment=3746287

 

Edited November 26, 2019 by Thalestris24

[Rockmover]

So is the light "reflected" off the slit, or how does it bounce back to the pickup mirror?  It seems it would all just pass though? I am still waiting to get my slit ring from France (it's set to be delivered next Monday). Perhaps once I get it, it will make more sense.  Or is the surrounding slit surface a mirror and its that light that is bounced back to the guide mirror?