The Lowspec spectrometer

[Thalestris24]

The lenses arrived from Surplus Shed today - yay! At a glance they look ok - no obvious scratches or marks. Each was individually wrapped and in its own little envelope taped to the packing note and inside a padded envelope. Looking forward to trying them out fairly soon

Louise

[andrew s]

2 minutes ago, Thalestris24 said:

The lenses arrived from Surplus Shed today - yay! At a glance they look ok - no obvious scratches or marks. Each was individually wrapped and in its own little envelope taped to the packing note and inside a padded envelope. Looking forward to trying them out fairly soon

Louise

I am sure you know this but doublets have a right and wrong way round.

Normally the strongest outside curve is the side for the collimated, parallel, light.

Regards Andrew 

[Thalestris24]

11 minutes ago, andrew s said:

I am sure you know this but doublets have a right and wrong way round.

Normally the strongest outside curve is the side for the collimated, parallel, light.

Regards Andrew 

Yes, I know, thanks. It's also pointed out in the assembly guide

Louise

[Thalestris24]

Am making progress with the assembly I've partially put it together with some test components so as not to risk damaging the critical/expensive ones. I bought some craft mirrors the other week. They only cost a couple of quid. Mind you they seem quite good! I bought some 1" square and 1" dia round. I put one of the round ones in the main mirror holder and another one where the guide mirror should go (it's supposed to be 1/2" really). Another one I cut through the back coating with a scalpel and made a diy 'slit' which I put in the slit holder. I've installed a 13mm dia lens, (bought from Surplus Shed) for the guide camera. Had to make a new holder for that. Put my Touptek microscope camera in place of a guide camera. Was able to get an image (focused on the slit itself). I was pleased with that as it showed things were lining up ok. Took this image of the slit with Sharpcap:

Tomorrow I'll try it with a proper guide cam and some sort of scope so that I can get an image from infinity. Maybe I'll be able to connect a 60mm finder or something.

It's coming along, bit by bit!

Louise

 

[Merlin66]

Louise,

Another step along the way......

[Thalestris24]

Just now, Merlin66 said:

Louise,

Another step along the way......

Yeah, I've had some holdups, especially with trying to print the t-threads - 'mare! Also with getting things to fit together properly - that hasn't been straightforward. On the plus side, I've learnt a lot on the way. 

Louise

[Merlin66]

Louise,

A telescope is not necessary....basically what you’re doing is good to check the guiding on the slit...

The next stage is to get the collimator focused on the rear of the slit plate.

softly softly......

[Thalestris24]

5 hours ago, Merlin66 said:

Louise,

A telescope is not necessary....basically what you’re doing is good to check the guiding on the slit...

The next stage is to get the collimator focused on the rear of the slit plate.

softly softly......

Although, normally, the guide cam would be focused on a star, not the slit itself? Yes, checking the collimator light path and focus is coming up next, together with testing the imaging camera, which I assume will also be focussed on infinity? 

I'm wondering how to check the collimator-slit focus? The only way I can think of is via the imaging camera - or am I missing something?

Thanks

Louise

[andrew s]

The guide camera should focus on the slit. You then focus the star on the slit using the telescopes focuser (optimal throught put may be slightly out of focus).

To get the collimator focused on the slit you can use several methods but they amount to observing the slit via the collimator wth a camera or telescope focused at infinity.

Exactly how to do the will depend on the layout and may need a folding mirror or even using the grating.

Regards Andrew 

Edited November 23, 2019 by andrew s

[Thalestris24]

2 minutes ago, andrew s said:

The guide camera should focus on the slit. You then focus the star on the slit using the telescopes focuser (optimal throught put may be slightly out of focus).

To get the collimator focused on the slit you can use several methods but they amount to observing the slit via the collimator wth a camera or telescope focused at infinity.

Exactly how to do the will depend on the layout and may need a folding mirror or even using the grating.

Regards Andrew 

Ah, ok - thanks, Andrew - re the guide camera, I should have figured that out, it's too early ha ha. Need second coffee  

Louise

[andrew s]

8 minutes ago, Thalestris24 said:

Ah, ok - thanks, Andrew - re the guide camera, I should have figured that out, it's too early ha ha. Need second coffee  

Louise

Just having my first. I was just about to add it is best if the telescope or camera has a ( much) longer focal length than the collimator as this reduces the error in setting the collimator. I uses on old f10 guide scope with a reticule eyepiece . Others e.g. C Buil have used a DSLR with telephoto lens.

 

Regards Andrew 

Edited November 23, 2019 by andrew s

[Merlin66]

Louise,

Andrew beat me to it!

 

[Thalestris24]

48 minutes ago, andrew s said:

Just having my first. I was just about to add it is best if the telescope or camera that has a ( much) longer focal length than the collimator as this reduces the error in setting the collimator. I uses on old f10 guide scope with a reticule eyepiece . Others e.g. C Buil have used a DSLR with telephoto lens.

 

Regards Andrew 

Oh, ok. I don't have anything F10. F7 is the longest I have and that would be the scope (sans reducer) I was planning to use with the spectrometer. Presumably it's also possible to tweak the collimator in use, to get the best image, or is that not advisable? There isn't a lot of movement of the collimator possible - maybe 5 or 6mm max. Measuring with a ruler, I'd say the collimator-slit distance is approx. 119 -123mm max and obviously fixed to those limits by the physical layout, though the fl of the collimator is specified as 125mm in the design. As it happens, the candidate lenses I ordered from Surplus Shed are 120 and 121mm fl so, hopefully, should fit in that range ok. The lenses I took from the old finders have a fl of 125mm (with some depth of field) but their diameter might be too big to fit. I'll just have to see how things work in situ.

Thanks

Louise

Edited November 23, 2019 by Thalestris24

[andrew s]

It's focal length that is important, longer than the collimator if possible. Nothing to sweat over. It's best to set it as well as you can to get parallel light onto the grating. 

Then you focus the spectra onto the ccd with the camera lens.

Regards Andrew 

[Thalestris24]

1 minute ago, andrew s said:

It's focal length that is important, longer than the collimator if possible. Nothing to sweat over. It's best to set it as well as you can to get parallel light onto the grating. 

Then you focus the spectra onto the ccd with the camera lens.

Regards Andrew 

Ok, thanks. I'm struggling a bit today as it's cloudy and dark - can't see . I remembered I have a F6.3, 500mm Mirror lens with the option of a 2x teleconverter so could use that as a 'scope' - if I can get the backfocus right. After fiddling a bit, I can just about see a daytime image on the slit at F6.3, so that's a start. I'll put the proper guide cam in next.

Louise

[Thalestris24]

Update time! I had a bit of a struggle with the imaging camera focus mechanism the other day. It went quite happily one way but refused to go the other. So I had to undo it all and fiddle with it. Eventually got it going ok though it never seems quite as smooth as you might hope for!

Anyway, I have been fiddling with the collimator lens and choosing a different one to what I'd planned. The ex-finder 125mm is a really good lens but is a tad too big. Instead, I'm now using a Surplus Shed lens which has a fl of 121mm and Dia of 26mm. I had to make another holder for it but it fits better in the light path. I decided I just couldn't rely on my poor eyesight to decide whether or not it was in focus. So I set up a web cam (MS LifeCam Cinema) to look down on the 45 deg mirror and through the lens back to the slit. I still couldn't decide on focus with certainty! The lens seems to have a reasonable depth of field and the precise positioning didn't seem to make a lot of difference. I did find that if you move back towards the main mirror too much then it encroaches on the light path. Anyway, it's currently positioned about midway along its 'groove'. I took some pics:

This is the dummy slit backlit (from a torch at an angle). The slit illuminator has been removed so that the slit position is fairly clear:

And this a Thorlabs post-it on a piece of card and in place of the slit:

Moving the card backwards and forwards in the slit holder space didn't seem to make much difference to the focus. Perhaps with the grating and other proper components in place, the focus point might be more or less better at slightly different positions of the lens. As you can see on the 'Thorlabs' pic, the edge of the lens is slightly encroaching from the south east.

Anyway, I'm going to try attaching the camera next. I'll try and use the 45 deg mirror again to look back at the camera sensor and see if the lowspec focus mechanism can adjust the camera lens to be in focus.

Louise

[Thalestris24]

Hi Ken @Merlin66

Useful info in your last post on Rockmover's thread Paul changed his design slightly - he made the camera fl = 100mm and dia 30mm so fr = 3.3. However, I currently have one which is fl=87mm and dia=26mm (fr=3.34). It should allow attaching my Atik without too big an extension. My collimator lens is fl=121mm. So with a slit gap of 20 um it would appear as 14.4um. With my Atik383l+ that means it should cover 2.7 pix when focused. I suppose that means 3 pix in practice? When I've tried to test focus the collimator it's proved quite difficult since illuminating my diy practice slit from the 'scope' side tends to generate diffraction though I did get some ok images via Sharpcap. However, I'll be testing it more properly with all the right components in place - hopefully within the next week. Um, I have been assuming that, on its own, the imaging camera is focused on infinity/a distant object? Hope that's correct... I was using a second test mirror to see outside through a window with a Touptek camera in place of the proper imaging camera. I was able to get an image of distant objects ok. I'll repeat with the Atik attached next. I understand that an attached scope will focus it's image (normally of a star) on the slit? The photons which exit from the slit are collimated by the collimating lens which transmits the beam to the grating and the spectrum is then (hopefully!) seen by the imaging camera. Hope I've got that right!

Thanks

Louise

[Merlin66]

Louise,

Yes, you've got the main points.... the telescope focuses the star image onto the front of the slit plate (and seen by the guide camera), the light goes through the slit and is made parallel by the collimator... the beam hits the grating, the light is dispersed into a spectral image which is then focused by the imaging lens onto the CCD chip.

At this stage bench testing is the way to go...a bright desk lamp (or fluoro) shining into a paper diffuser on the entrance should give enough light to work with at the slit.

The "test" imaging system should be focused on infinity and not re-adjusted during the test. If the collimator is correctly positioned, the slit gap will be in focus.

You may be able to use your dummy grating (mirror on 3D block) to align and focus the collimator and confirm the final imaging arrangement. 

Which program will you use to obtain your spectral image? It should be able to analyse the image (using the measurement/ profile tools) and give you a FWHM result - mine (AstroArt) shows the result down to 0.01 pixel.

Ken

 

Edited November 27, 2019 by Merlin66

[Thalestris24]

8 minutes ago, Merlin66 said:

Louise,

Yes, you've got the main points.... the telescope focuses the star image onto the front of the slit plate (and seen by the guide camera), the light goes through the slit and is made parallel by the collimator... the beam hits the grating, the light is dispersed into a spectral image which is then focused by the imaging lens onto the CCD chip.

At this stage bench testing is the way to go...a bright desk lamp (or fluoro) shining into a paper diffuser on the entrance should give enough light to work with at the slit.

You may be able to use your dummy grating (mirror on 3D block) to align and focus the collimator and confirm the imaging arrangement. 

Which program will you use to obtain your spectral image? It should be able to analyse the image (using the measurement/ profile tools) and give you a FWHM result - mine (AstroArt) shows the result down to 0.01 pixel.

Ken

 

Hiya

I'd not really thought about specialist imaging software - I kind of assumed I'd be able to do it with SharpCap as I did with the Star Analyser? Is Christian's IRIS a possible? Possibly AstroArt, especially as you recommend it , but I wouldn't want to fork out for something unless I already know all the kit is working ok. I don't get the best guiding here, not helped by having to do my imaging through an open window... So I might put everything together only to find my results are rather poor .

Thanks

Louise

[Merlin66]

Louise,

Don't know how ShapCap handles darks, alignment and stacking. I use an ATik 314L with the Spectra-L200.

Buil's ISIS, Valerie's VSpec and John's BASS Project are all suitable for the spectral processing - taking the raw spectral image to a 1D profile.

Edit: Misread... you mention IRIS not ISIS. Yes, this can work, but it's now getting a bit dated.....

Edited November 27, 2019 by Merlin66

[Merlin66]

Louise,

I should also have added that spectral exposures can be very long - to build up the signal and improve the SNR. I use 20 x 240s min subs (!) hence the benefit of a cooled mono camera.

I don't know if SharpCap can handle cooled cameras and long exposures.

 

[Thalestris24]

Just now, Merlin66 said:

Louise,

Don't know how ShapCap handles darks, alignment and stacking. I use an ATik 314L with the Spectra-L200.

Buil's ISIS, Valerie's VSpec and John's BASS Project are all suitable for the spectral processing - taking the raw spectral image to a 1D profile.

 

Oh, ISIS not IRIS! I have some image processing software called IRIS... SharpCap pro does live stacking with optional darks, flats and auto alignment (or can just guide with PHD2). I can't see any way to take flats, though. Not sure about image measurement. 

Louise

[Thalestris24]

Just now, Merlin66 said:

Louise,

I should also have added that spectral exposures can be very long - to build up the signal and improve the SNR. I use 20 x 240s min subs (!) hence the benefit of a cooled mono camera.

I don't know if SharpCap can handle cooled cameras and long exposures.

 

Yes, SharpCap can control cooling and do long exposures ok. However, the length of my subs is generally limited by the city skyglow . With the Atik383l+ I can probably do 360s with a normal sub but then I'd be using a lp filter or Ha filter. Not sure how long I'll be able to get in pure luminance. That looks a nice Eta Carinae spectrum! That is about mag 6? So quite bright and the Atik314 has quite a high qe. My Atik383l+ isn't so sensitive so presumably would need longer exposures. My snr would be a lot poorer. I can only see to the East and not much higher than alt=35 deg. Nothing involving astro imaging is simple or straightforward for me!

Louise

[Merlin66]

Louise,

The good news is light pollution doesn't existing in spectroscopy!!

Valerie does her work from the centre of Paris (!!) Christian works from his balcony near Toulouse.....

We can remove the unwanted sky glow signal - sodium, mercury lights etc. very easily during processing. LED lights.....well that is more difficult.

Ken

 

[Thalestris24]

1 minute ago, Merlin66 said:

Louise,

The good news is light pollution doesn't existing in spectroscopy!!

Valerie does her work from the centre of Paris (!!) Christian works from his balcony near Toulouse.....

We can remove the unwanted sky glow signal - sodium, mercury lights etc. very easily during processing. LED lights.....well that is more difficult.

Ken

 

Well, that sounds encouraging! There's a wide mix of Glasgow lighting types contributing to the skyglow. Even with Ha I'm limited. I usually don't bother trying to do anything until the nearby all-weather pitch switches of it floodlights after 10pm. I'll just have to see what I can do.

Louise