The Lowspec spectrometer

[Thalestris24]

4 minutes ago, Merlin66 said:

Louise,

I've tried slit illuminators.... (ASA, p203/4) but to be honest they didn't add any functionality.

The key is it have a reflective slit plate which allows the slit gap to be seen (most of the time) against the sky background, and just position the target star in/on the gap and guide.

AstroArt has a neat slit guiding routine. I also persuaded the guys at PHD2 to include a slit guiding option. This produces a "virtual" slit in the guide screen which can be positioned exactly over the actual slit position, makes life much easier.

With the OVIO reflective slit plate you'll be OK.

 

Oh ok - as it happens, I was in fact just looking at the 'back slit illuminator' image in your book That gives me the impression it might be useful for positioning the target star in the slit but it doesn't reference reflective slits. I'll try without first! I've got most of the Lowspec done and assembled but need to finalise the optics and the cameras. I'm not sure which cameras I'll be using yet. I expect just a qhy5l-IIm for guiding. I was thinking about the qhy183m for imaging but maybe that will be a bit heavy - I'll have to think about it.

Thanks

Louise

[Merlin66]

Louise,

The slit illuminator was used on the early non reflective slits (Surplus Shed) - I used separate beamsplitters mounted in front of the spectrograph to provide a guide image (Using Al's reticle ).

As I said, the availability of reflective slit plates has improved over the years, and is the recommended solution.

The QHY5_I will work pretty well as a guider, the imaging camera...start with what you have, you can check the wavelength coverage and probable R value using the SimSpec spreadsheet. I'm still using the ATiK 314L.

 

 

[Thalestris24]

6 minutes ago, Merlin66 said:

Louise,

The slit illuminator was used on the early non reflective slits (Surplus Shed) - I used separate beamsplitters mounted in front of the spectrograph to provide a guide image (Using Al's reticle ).

As I said, the availability of reflective slit plates has improved over the years, and is the recommended solution.

The QHY5_I will work pretty well as a guider, the imaging camera...start with what you have, you can check the wavelength coverage and probable R value using the SimSpec spreadsheet. I'm still using the ATiK 314L.

 

 

Maybe I'll try the Atik383l+ first (assuming I can otherwise get everything working!). It's a lot less sensitive and more noisy than the qhy183m, and has slightly lower resolving power and spectral resolution, but it will be fine on brighter stars, I think. A problem I do have, though, is in precisely finding targets. I'm used to plate solving. I guess I could use the guide scope with a different (probably new...) camera. It's hard to line up with the imaging scope but I could probably calculate a fixed offset. It's something I'll have to fiddle with!

I take it there's no advantage to using a reducer/flattener for spectroscopy? The scope is 115mm, f6.96 without a reducer.

Louise

[Merlin66]

Louise,

An f6.96 will be a good starting point, no need for reducers/ flatteners.

Unless you intend to work with very faint, obscure stars/ targets I'm sure you'll find a well centred finder will position the target into the reflective slit aperture.

I use a 260mm fl eFinder (with a DMK51) on the C11 @f10 (!!!!) and can easily position the target (I use FireCapture with cross-wire overlay) very near the slit gap.

I use AstroArt or PHD2 for guiding.

Plate solving with the reflective slit plate has two issues - the limited FOV and the secondary images from the internal reflections in the glass slit plate.

If I use Plate solving (not very often) then I use the eFinder FOV.

Ken

 

[Thalestris24]

3 minutes ago, Merlin66 said:

Louise,

An f6.96 will be a good starting point, no need for reducers/ flatteners.

Unless you intend to work with very faint, obscure stars/ targets I'm sure you'll find a well centred finder will position the target into the reflective slit aperture.

I use a 260mm fl eFinder (with a DMK51) on the C11 @f10 (!!!!) and can easily position the target (I use FireCapture with cross-wire overlay) very near the slit gap.

I use AstroArt or PHD2 for guiding.

Plate solving with the reflective slit plate has two issues - the limited FOV and the secondary images from the internal reflections in the glass slit plate.

If I use Plate solving (not very often) then I use the eFinder FOV.

Ken

 

Yeah, as I say, I can platesolve via the guide scope though it might get complicated using 3 cameras at once! The guide scope isn't precisely lined up with the imaging scope and it's bolted in place and can't be easily adjusted. It is nice and rigid though! So to find a particular, individual star will likely be tricky. But where there's a will...

Thanks for the advice

Louise

[ejp1684]

2 hours ago, Merlin66 said:

I also persuaded the guys at PHD2 to include a slit guiding option. This produces a "virtual" slit in the guide screen which can be positioned exactly over the actual slit position, makes life much easier.

I use PHD2 for guiding and find this feature very useful.

Eric.

[drjolo]

Together with one of my colleagues we also assembly two LowSpecs. I am working on medium resolution version with 600lpmm grating, and my friend ( http://www.jgao.pl/ ) is doing low resolution version for supernova program. We already have all elements 3D printed (all fits perfectly, great design) and almost all mechanical and optical parts ready. We have used entry level Ender 3 printers and Impact PLA filament. Impact PLA is PLA family filament, but more like ABS durable and more heat resistant - regular PLA can become soft at 50*C already. 

 

We have little doubts about reflective OVIO slit orientation. In original PDF manual the slit has its front (metal plated) side directed towards the grating. But then one of users noticed that guiding is somehow degraded, so he decided to place OVIO disc oriented with front towards guiding camera (like in my image to the right). But then I suspect it may affect the spectrum quality, because light after passing the slit still needs to travel through OVIO disc glass. 

What is the correct orientation of reflective OVIO disc?

Edited November 12, 2019 by drjolo

[ejp1684]

The mirrored surface should face the grating, not the guiding camera. Better to get the best specta and sacrifice some quality in guiding.

Eric.

[Merlin66]

Looking good, well done!

Definitely the reflective surface is at the rear, towards the grating.

Any internal reflections then affect the guiding not the important spectral image.

Ken

 

Edited November 12, 2019 by Merlin66

[Thalestris24]

Hmm... Paul Gerlach's design and assembly has it with the shiny side facing the guide mirror - isn't having the reflective side being seen by the guide camera how the reflective slit principle works? Now there is after the slit an aperture in the plastic which contains the slit illumination part. I'm wondering whether that might interfere?

Louise

 

[Merlin66]

Louise,

Definitely mirror face towards the grating.

The guider can seen the reflection in the rear surface, no problems!

I don't know what Paul did with the illuminator.......

[Thalestris24]

10 minutes ago, Merlin66 said:

Louise,

Definitely mirror face towards the grating.

The guider can seen the reflection in the rear surface, no problems!

I don't know what Paul did with the illuminator.......

I believe you ha ha. Here is what my slit holder looks like without the slit fitted. I am a bit suspect of the back illumination part as it has a slit-shaped hole but the position of it seems offset a bit (maybe it lines up - hard to tell without putting the slit in). Anyway, probably best to leave it out altogether if not using the illuminator option?

Louise

ps I was thinking of spraying the inside with matt black paint as there are some shiny reflective bits in the black plastic?

Edited November 12, 2019 by Thalestris24

[Thalestris24]

Actually, I just took my slit out and unwrapped it. As far as I can tell both sides are identical, both equally shiny. So it doesn't look like it will make any difference anyway.

Louise

[ejp1684]

The illuminated slit works well for me 'as is'. In fact, this addition has drastically reduced the time it takes me to line up a star on the slit.

Eric.

[Thalestris24]

1 minute ago, ejp1684 said:

The illuminated slit works well for me 'as is'. In fact, this addition has drastically reduced the time it takes me to line up a star on the slit.

Eric.

I guess you switch it off once you've lined up? What do you use for the illumination? Does the white slit back illuminator piece line up ok?

Thanks!

Louise

[ejp1684]

I already had an eyepiece with an illuminated reticle. The illumination part fitted straight into the base of Lowspec. Paul has an equivalent part listed on page two of his pdf, from Baader, I think. 365astronomy sell one which might fit, but don't quote me on that!

I just switch it on to align the guide slit on PHD2 with the illuminated slit and then switch it off again. The white slit on the back was quite small to print and had a few blobs on it which I filed away. Works well.

Eric.

[Thalestris24]

17 minutes ago, ejp1684 said:

I already had an eyepiece with an illuminated reticle. The illumination part fitted straight into the base of Lowspec. Paul has an equivalent part listed on page two of his pdf, from Baader, I think. 365astronomy sell one which might fit, but don't quote me on that!

I just switch it on to align the guide slit on PHD2 with the illuminated slit and then switch it off again. The white slit on the back was quite small to print and had a few blobs on it which I filed away. Works well.

Eric.

In an earlier post I reckoned a simple adjustable led would do the job but Ken thought illumination unnecessary - I'll try both . Also, I'm wondering if Ovio may have recently changed the design of their circular slit to make both sides identical ie both shiny? Were previous ones only shiny on one side? (I noticed there was a mark on one side of mine which is a bit annoying though won't affect the slits.)

Louise

[Gina]

This is amazing stuff - I have enough trouble with ordinary imaging!!!

[Thalestris24]

13 minutes ago, Gina said:

This is amazing stuff - I have enough trouble with ordinary imaging!!!

I think it's the dark side of the dark side!  

Louise

[Merlin66]

Louise,

Look carefully at the slit plate.. it is only reflective on one side....

The numbers look "correct" when viewed from the front (non reflective side)...

The attached image shows the original OVIO slitplate I designed for the Spectra-L200.

 

[Thalestris24]

17 minutes ago, Merlin66 said:

Louise,

Look carefully at the slit plate.. it is only reflective on one side....

The numbers look "correct" when viewed from the front (non reflective side)...

The attached image shows the original OVIO slitplate I designed for the Spectra-L200.

 

I don't get it - mine is like a mirror on both sides?

       

Am I missing something?

Louise

 

[Merlin66]

Louise,

If you look at the plate at a glancing angle (it's only 1.5mm thick) you should see the difference....

[Thalestris24]

5 minutes ago, Merlin66 said:

Louise,

If you look at the plate at a glancing angle (it's only 1.5mm thick) you should see the difference....

Hmm… maybe a subtle difference but it's hard to tell under room light (my eyesight is poor away from bright daylight ). Both sides still seem reflective to a degree. So the idea is to have the front side, with the numbers correct, pointing towards the grating?

Louise

[Merlin66]

Louise,

No. The front side faces the incoming beam (and the guider).

The rear surface which has the engraved slits sits towards the grating - there's no glass thickness between the slit and the grating.

 

[Thalestris24]

2 minutes ago, Merlin66 said:

Louise,

No. The front side faces the incoming beam (and the guider).

The rear surface which has the engraved slits sits towards the grating - there's no glass thickness between the slit and the grating.

 

Oh dear, I'm confused! Isn't the front side the shiny side? I'll have to look closer with a magnifying glass...

Thanks

Louise