LOWSPEC - 3D Printed but Calibration Question

[Thalestris24]

5 minutes ago, Rockmover said:

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?

Yes, it's a reflective slit though, as I understand it, the shiny/mirror side points in the other direction. The side with the numerals in mirror image is the one that points to the scope.

ps as you see I made a slit from one of the round craft mirrors I bought. Used the non shiny side for the reflective side.

Edited November 26, 2019 by Thalestris24

[Rockmover]

I found the "missing" 15 degrees.  And I now it makes sense that the surface is polished and the light is all reflected back except what passes through the slit.

Edited November 26, 2019 by Rockmover

[Thalestris24]

I don't recognise that diagram - where is it from? It's the non-shiny side that's used. I found it a bit confusing and counter-intuitive too. However, having tried it out I believe it will work ok. The guide camera is focused on the slit. The telescope is also focused on the slit. The guide camera can then see the star and can guide on it. I think it might be quite challenging to get a particular star lined up with the slit.

[Rockmover]

That is the OVIO guide platform.  Everything makes sense now, I now just need to wait for the OVIO slit ring to arrive from JEULIN....or make one as you did.

[Thalestris24]

6 minutes ago, Rockmover said:

That is the OVIO guide platform.  Everything makes sense now, I now just need to wait for the OVIO slit ring to arrive from JEULIN....or make one as you did.

Ah yes, I recognise it now - not used to viewing it like that! I just took a 1" round mirror and cut a short line in the backing with a scalpel. Then just taped it in place. Not good enough for a proper slit (though maybe with a new blade and a bit of practice...)

[Rockmover]

Yes, this is all making MUCH more sense now. I am going to quickly 3D print a square holder (same size as the grating) with a spot for the guide mirror.  Then I can just put that in and align the collimater position. 

This is a really fun project, and I know its evolving, but I'm surprised no one has made a nice YouTube video as a guide for building and setting this thing up.  Hopefully, if my unit is successful I can make that (PART 2) for others so they don't have the struggles I am having.

[Thalestris24]

10 minutes ago, Rockmover said:

Yes, this is all making MUCH more sense now. I am going to quickly 3D print a square holder (same size as the grating) with a spot for the guide mirror.  Then I can just put that in and align the collimater position. 

This is a really fun project, and I know its evolving, but I'm surprised no one has made a nice YouTube video as a guide for building and setting this thing up.  Hopefully, if my unit is successful I can make that (PART 2) for others so they don't have the struggles I am having.

The struggle is half the fun! If it was made too easy it wouldn't be so interesting. Overcoming hurdles is how you learn about something

 

[Merlin66]

Glad you got things sorted out....

Time to talk serious.....

The instrument you're building is very sophisticated, containing many optical surfaces and requires some dedication and rigour to complete, test and eventually use.

The object of the build is to end up with a capable, usable spectrograph. To achieve success all the optics must be clean, positioned and aligned correctly.

Re slit focus - The slit gap, a few microns when viewed through the system should be measured. The FWHM of the slit gap will confirm the focus.

If you use the collimating lens plus a long focus lens/ scope to view the slit gap then you also need a camera. The ratio of the focal lengths of the viewing lens and collimating lens can give some magnification factor, where the slit gap appears greater/ less than it really is. The slit gap, say 20 micron when used with this system - assume the collimating and viewing lens are the same focal length with a camera which has 4 micron pixels, will appear as a 5 pixel wide image.

I use AstroArt to do all my imaging, and it has (like many other programs) a "profile" option. You can use this to analyse your image of the slit - to measure the FWHM. The collimating lens should be adjusted to give the minimum FWHM slit image. 

In the LowSpec design the imaging/ collimator lens have the ratio of 80/ 125, this gives a ratio of x 0.64 - this means the slit gap will appear smaller than actual (say 20 x 0.64 = 12.8 micron, or less than three pixel in the above example!)

If the collimator is not correctly positioned, then the exit beam to the grating will not be fully collimated and parallel which can compromise the performance of the grating, and the instrument.

 

 

Edited November 26, 2019 by Merlin66

[robin_astro]

5 hours ago, Thalestris24 said:

 It's the non-shiny side that's used. I found it a bit confusing and counter-intuitive too. However, having tried it out I believe it will work ok. 

Yes it is important with glass mirror slits that the reflective coated surface is towards the spectrograph (ie they are back coated like a domestic mirror, not front coated like usual astronomical mirrors.) If you use them the other way round with the reflective surface towards the telescope, some of the light that passes through the slit gets reflected off the back off the internal glass surface, then off the internal side  of the reflective surface and back into the spectrograph  producing a ghost spectrum. (The right way round, you do get a faint ghost image in the guider but this is not a problem)

Robin

[Rockmover]

Ok, Im definitively stuck now and can use some help!  I have all the optics installed in the LowSpec. 

I believe I have the guide scope correctly installed and focused.  Basically I set up a halogen lamp, poked a hole in a box, and initially used that for a starting point.   At first i put a white sticker in place of the "slit" and focused on that.  However, when I focused the guide camera, the slit was not focused at all.  So (and maybe incorrectly) I focused the guide camera on the slit and then adjusted the telescope focus. But this doesn't seem right to me?

As you can see in the image below with the barcode, the slit seems focused, as does the label in the image.  Does this mean the image is properly focused at the slit?

Next, if I use an artificial star, (5 stars all uncovered) I get a huge amount of ghosting, and cant seem to focus on the whole image.  If I focus on one star, then another is unfocused....

Moving on to the main camera, If I put the artificial star in the slit, I cant find any spectrum pattern at all (and I have tried focusing everywhere). 

If I just throw a really bright light in front of the scope, I do see the slit and some bands (last image)?  Is this the spectrum, or just junk?  Is this haw I should find focus of the main camera???

 

I have no idea where to go next....so thanks very much in advance!

 

[Rockmover]

Also, here is part 2 of my video series...I had thought I was doing well....

 

[Merlin66]

Rocky,

Don’t panic!

1. The focus on the slit plate looks promising. Narrow the slit width for the final tests.

2. Use a Fluoro lamp to illuminate the slit gap.

3. Re- Focus the imaging camera to give the narrowest slit image ( check the FWHM)

4. Use the micrometer to view the extent of the Spectral image. You should see, moving from zero order, the blue flouro line, the green line, then the yellow double and onto the red.  

You’re getting very close.

ken

 

[Thalestris24]

Hiya

You're a bit ahead of me. What imaging camera are you using? When you attach your imaging camera, double check that the distance between the plane of the camera lens and the sensor plane of the imaging camera is close (within a couple of mm) to the fl of the camera lens.

Louise

ps just watched your video above - very good! It's probably better (and easier) to do initial testing without the scope attached. The reason for using a fluorescent bulb is because they emit light at particular wavelengths from blue  to green to yellow to red- lookup on google Whereas a halogen bulb emits a continuous spectrum.

Edited December 8, 2019 by Thalestris24

[Merlin66]

Louise/ Rocky,

Yes it's a 100% easier to do the initial testing on the bench with a Fluoro Lamp. 

The attached profile shows the typical Fluoro spectrum. (From Buil's website)