Hartridge spectrometer with optical cable at input

[bojan]

Today I was working on a setup that could be OK for taking stellar spectra with telescope.
Hartridge spectrometer is coupled to light source with optical cable.
To minimize the light losses, I used old eyepiece from broken theodolite to obtain parallel beam ~ø6mm, which was then directed to slit via cylindrical lens (it is part of spectrometer).
Now I am working on bracket that will keep the fiber in focal point of my C11.

[bojan]

For coupling optical fiber with scope, I will probably use adapter (thread is M42x1) that I made long time ago - it was meant to be used for laser beam communication transceiver, but I never had opportunity to finish this project.
Mounted in the centre is beam splitter (50%) which may be useful for aiming and focusing.. I will modify the assembly such that it will be removable to maximize light coupling when taking spectra.

[Merlin66]

Very interesting...
Look forward to seeing your results.
Ken

[bojan]

Long time ago I also made flip mirror box... I used it with my MTO1100 as finder/eyepiece (similar arrangement as Questar).

It will be ideal for this project, very little needs to be modified.

[bojan]

As for camera...
I grabbed 2 Connectix 4000 Pro today on ebay.
It is easy to do long exposure mod, see here (https://nightskyinfocus.com/diyprojects/logitech-40003000-long-exposure-modification-sc1/).

[bojan]

Telescope end optical cable interface, adjustable cable gland used to accomodate Broadcom POF Simplex Fibre Optic Connector (https://au.rs-online.com/web/p/fibre-optic-connectors/7511469?ef_id=EAIaIQobChMI_8Wf44W55 gIVySMrCh2CGQLMEAQYAyABEgIN4_D_BwE: G:s&s_kwcid=AL!8733!3!99325693714!!!g!4 41403653319!&cm_mmc=AU-PLA-DS3A-_-google-_-PLA_AU_EN_Connectors-_-Rf_And_Coaxial_Connectors-_-PRODUCT_GROUP&matchtype=&aud-386146035031:pla-441403653319&gclid=EAIaIQobChMI_8Wf44W55gIVySMrC h2CGQLMEAQYAyABEgIN4_D_BwE&gclsrc=aw.ds).
I am almost ready for the first light..... when Melbourne weather permits.
But first I need to modify one of those 2 Logitech 4000 Pro cameras that I picked up from ebay (for $10 both!)

[bojan]

Sort of ready.... on workbench.

[bojan]

... and at scope...

[bojan]

The first attempt last night to acquire stellar spectrum (Achernar) was not successful, un-moded camera is not sensitive enough for this application.
But the whole contraption should be OK on the mount, there was not much to adjust to acually see Acrenar's light through fiber.
So, the next step is to modify webcam for long exposures, or to use one of my Canons (currently I do not have adapter).

[Merlin66]

Bojan,
What sized fibre are you using? 50 micron?
Can you test in on the bench using an artificial star? I use an old Foucault tester lamp with a Hubble artificial star disk, and fluoro lamp.

[bojan]

It is 1mm plastic fiber.
50u would be too difficult to focus on, at least for the start.
Rough bench test with spiral lamp looks OK.


I was planning to assemble test jig (artificial star) with small LED and some optics (from lasers) to mimic f/10 beam from C11, to test the whole back end (including graticule for focus and aim at target.

[Merlin66]

OK.
I assume there's a slit in the Hartridge onto which the fibre optric will sit?

[bojan]

There is adjustable slit....
My setup goes like this:
Round fiber Ø1mm -> condenser lens (old eyepiece, to form paralell from divergent beam) -> cylindrical lens (part of spectrometer) -> slit.
There are lots of losses in this setup.
I tried to get hold of stacked fiber bundle, O to I... see here (https://www.azom.com/article.aspx?ArticleID=13370) for example.. this would reduce losses and possibly allow eliminating the slit. Still waiting for reply from supplier.



This one (https://www.ebay.com.au/itm/100x-LED-Fiber-Optic-Light-0-75mm-Beautiful-Fairy-Optical-Fiber-Light-for-Party/283673064368?hash=item420c3b73b0:g: xZsAAOSwkfRdypL6) is too thick... If I could find a bundle made of 50um fibers, I would try to use that.

[bojan]

Last time I tried this contraption, it was VERY hard to find and place the star at the input of the optical fibre..

So in order to align the mirror box (which goes to telescope), I made the artificial star with small red laser ($1 on ebay).
I removed original colimator lens, and replaced it with 45mm FL objective from old SMENA-8 camera.

Such optical system produces F~10 conical beam, visible in eyepiece when mirror is placed in the optical path.
Because the mirror box was made pretty precisely by a friend of mine decades ago, only a small mirror adjustment was necessary to bring the "star" image to the centre of cross hair (glass reticle from crappy "military" Russian plastic binoculars).


I will try this on some bright star on first occasion.

[Merlin66]

Bojan,
If you use a guide camera.....you can make use of Al's reticle to present a "virtual slit/ alignment cross" which can be set to the physical slit/ fibre input.

[bojan]

Yep..
At the moment I do not use guide scope, and I have to bring the star image to fiber entrance (Ø 1mm) somehow..

This contraption is just the tool to align everything before attempt on real star...

[bojan]

Test with fluoro lamp.
Dispersion is pretty good, 0.373 nm/px.
Resolution is 1~2 nm....