How to focus a spectrometer?

[SteveBz]

10 hours ago, Merlin66 said:

You need to flip the spectrum. We standardise spectra processing with the blue wavelengths to the left.

I always forget this until VSpec tells me.  Probably I should rotate my camera 180 degrees so that it's always that way without having to remember!

[robin_astro]

5 hours ago, SteveBz said:

 

I think all of this is really about where the focal point is and how to get the focal plane lined up with the sensor.  In this case 'high dispersion' means high angle of diffraction such that the adjacent and hypotenuse of the triangle formed by the red ray, the blue ray and the sensor are different lengths.  I thought to fix this with the prism, but really you can never map an arc of a circle onto a straight line.  

 

 

This is only part of the problem (The focal plane is curved so you can only be in focus at one wavelength. You will also find with achromatic refractors that you will not get perfect focus at all wavelengths, particularly at the blue end because of chromatism in the telescope optics. This appears as a fishtail like shape at the blue end).  The main problem is the converging beam. Because the beam converges leaving the telescope you get what is called chromatic coma which means you can never get a perfectly focused spectrum image even using a wedge prism which as you have seen can actually make the problem worse. This gets worse at higher diffraction angles (more lines/mm) and at lower telescope focal ratios (The beam converges at a steeper angle.) This is the reason the Star Analyser has 100l/mm and is not recommended for low focal ratio telescopes.

For a full analysis of the converging beam setup, see Christian Buil's website here

http://www.astrosurf.com/buil/us/spe1/spectro1.htm

 

Cheers

Robin

Edited January 17, 2021 by robin_astro
typo

[robin_astro]

4 hours ago, SteveBz said:

 

 

I think, then, maybe, I just need to buy the SA100 + prism this month and get comfortable with it. 

 

If you do decide to go this way, I would not worry about the wedge prism, at least to start with. It does not make a big difference to the resolution with the SA100 but makes wavelength calibration more difficult. You can see an example of the difference in tip #3 of Christian Buil's useful tips using the Star Analyser here

http://www.astrosurf.com/buil/staranalyser3/userguide.htm

Cheers

Robin

 

[robin_astro]

5 hours ago, SteveBz said:

It's this one:

Diffraction Grating Slide Linear 500 Lines/mm Holographic Physics Spectrum Color | eBay

with a circular wedge prism from Edmund Optics behind it.

Unfortunately most of these gratings tend to be very inefficient. (Spectroscopy spreads out the  light very thinly so we need all the efficiency we can for astro spectroscopy.) A simple test is to look through the grating. You want as much  light as possible to be in one of the first order spectra. Any light in the zero order (the light that goes straight through) or any other order is wasted.  This is what the SA100 looks like (from the user manual)

[Merlin66]

As Robin says...these cheap eBay gratings are very inefficient and really of no use in astronomical spectroscopy.

Invest in a SA100 grating.

 

[SteveBz]

I've taken out the wedge prism and mounted the grating very close to the sensor (inside the nosepiece).  I'd like to have one more test, but I'm beginning to believe it won't work unless the angle is small [sin(x) = x].  Which everything you have said so far leads me to believe is the point of the SA100.  The Christian Buil tip #3 you showed me didn't make much of a case for the prism and I think probably ease of calibration is higher up my list of priorities than marginally sharper lines.

In terms of aligning the spectrum. I'm normally aligned with the N-S axis aligned with the up-down axis of my sensor - it looks right and it makes navigation with RA/Dec buttons easier.  I think your website, Robin, is saying turn it through 90 degrees? Is that right?  Then when it drifts it widens the spectrum rather than blurring the lines.  If I did that I could still rotate the photo on my screen to retain use of the RA/Dec keys. 

I'm probably not so interested in the chemistry of stars, but more in their velocities.  I saw you, Robin, had measured the radial velocity of a comet the other day.  How hard was that?  I've always imagined that objects like 3c273 could be measured easily but that closer object like solar system objects and local stars would be difficult for amateurs.  Ideally, I'd like to be able to measure the relative velocities of binary stars and the stars in globular clusters.  I don't think the later would be possible without a slit, but binaries?  

Thanks for your input guys.

Steve.

Edited January 19, 2021 by SteveBz
Whoops 3c273

[robin_astro]

5 hours ago, SteveBz said:

I'm probably not so interested in the chemistry of stars, but more in their velocities.  I saw you, Robin, had measured the radial velocity of a comet the other day.  How hard was that?  I've always imagined that objects like 3c271 could be measured easily but that closer object like solar system objects and local stars would be difficult for amateurs.  Ideally, I'd like to be able to measure the relative velocities of binary stars and the stars in globular clusters.  I don't think the later would be possible without a slit, but binaries?  

You need the right kit for the magnitude of the velocity you are measuring. 

The Star Analyser can measure the high velocities (redshifts) of some galaxies eg

http://www.threehillsobservatory.co.uk/astro/spectra_21.htm

and the expansion of supernovae (bottom of page)

http://www.threehillsobservatory.co.uk/astro/spectra_6.htm

but to measure orbital velocities you need a slit spectrograph with higher resolution (with good technique and a stable slit spectrograph  a velocity precision is ~1/10 of the resolution is relatively straightforward  eg at a resolution of 5A ~20km/s  and at 0.5A resolution 2km/s)

examples are:-

David Boyd "observing with a Lisa spectrograph" measuring binary star radial velocities to ~6km/s, slide 43 on

https://www.britastro.org/downloads/15701

My measurement of velocities to 1km/s precision  in the dusty eclipsing disc during the eclipse of epsilon Aurigae using a LHIRES spectrograph

https://britastro.org/node/19640

and velocities due to pulsations in Deneb to 0.5km/s in my talk "pushing the limits of commercial spectrographs" using a LHIRES spectrograph

https://britastro.org/node/19378

and measuring exoplanets to a few metres/second  precision  using a stable high resolution fibre fed echelle spectrograph as here by Christian Buil  

http://www.astrosurf.com/buil/exoplanet2/51peg.htm

 

Cheers

Robin

 

Edited January 18, 2021 by robin_astro
clarification

[robin_astro]

5 minutes ago, robin_astro said:

but to measure orbital velocities you need a slit spectrograph with higher resolution

There are also some  nice examples on here using the 3D printed Lowspec spectrograph at high resolution eg

 

[robin_astro]

5 hours ago, SteveBz said:

I saw you, Robin, had measured the radial velocity of a comet the other day.  How hard was that?

This was done using a LHIRES slit spectrograph at ~0.5A resolution

https://britastro.org/observations/observation.php?id=20200710_225300_18b30a0a785a7ab3

Cheers

Robin

[robin_astro]

5 hours ago, SteveBz said:

relative velocities of ... the stars in globular clusters

This is probably beyond the range of amateur equipment because of the trade off between resolution and limiting magnitude. ie you can either measure high velocities of faint objects  like this high redshift QSO with my modified ALPY 200 (resolution ~45A)

or low velocities of bright objects eg the pulsations of Deneb at ~0.3A resolution using a LHIRES III 

Edited January 18, 2021 by robin_astro

[SteveBz]

22 hours ago, robin_astro said:

This is probably beyond the range of amateur equipment because of the trade off between resolution and limiting magnitude. ie you can either measure high velocities of faint objects  like this high redshift QSO with my modified ALPY 200 (resolution ~45A)

Do you have a view on measuring rotation curves of galaxies?

I don't see myself going all out for the LHIRES III.  My equipment is not flash enough for that.  I'd have to upgrade my mount and scope and everything.  If I can get interesting information with the Alpy or Dados, that seems like the limit of my travel, at least for the foreseeable future.  At some stage in the future I'd like to get a 250mm RC on an EQ6, but that's still a way away.

Still waiting for a break in the clouds.

Thanks

Steve.

[robin_astro]

10 hours ago, SteveBz said:

Do you have a view on measuring rotation curves of galaxies?

Yes it is possible for amateurs to detect galaxy rotation but unless you have access to a large telescope, only on a few of the brightest galaxies. Some examples

http://www.astrosurf.com/buil/forum/ngc7331_poster.png

http://www.spectro-aras.com/forum/viewtopic.php?f=6&t=1682

http://www.spectro-aras.com/forum/viewtopic.php?f=6&t=2232

http://www.spectro-aras.com/forum/viewtopic.php?f=6&t=2420

http://www.spectro-aras.com/forum/viewtopic.php?f=6&t=2618

 

Robin

Edited January 19, 2021 by robin_astro
typo

[robin_astro]

9 hours ago, SteveBz said:

I don't see myself going all out for the LHIRES III

it really depends where your interests lie. The LHIRES III cannot do everything. It is good for high/medium resolution  on bright objects but does not work so well at low resolution. The ALPY works better at low resolution and can go much fainter  (and the spectra with my even lower resolution modified ALPY 200 are some of of the faintest recorded by an amateur)

Cheers

Robin

[robin_astro]

10 hours ago, SteveBz said:

My equipment is not flash enough for that.  I'd have to upgrade my mount and scope and everything.  If I can get interesting information with the Alpy or Dados, that seems like the limit of my travel, at least for the foreseeable future. 

To get the best out of any slit spectrograph you are going to need a mount which can  guide sufficiently well to keep the star on the slit for long exposures using the spectrograph guide camera. The telescope has to be reasonably well matched to the spectrograph too, for example the ALPY works best at ~f5 but the DADOS and LHIRES need an f10 scope 

Edited January 19, 2021 by robin_astro

[robin_astro]

3 minutes ago, robin_astro said:

To get the best out of any slit spectrograph you are going to need a mount which can  guide sufficiently well to keep the star on the slit for long exposures 

If you start with the Star Analyser though you don't need to guide, just expose for as long as your mount can track for and align and stack multiple exposures

Edited January 19, 2021 by robin_astro

[SteveBz]

9 hours ago, robin_astro said:

Yes it is possible for amateurs to detect galaxy rotation but unless you have access to a large telescope, only on a few of the brightest galaxies. Some examples

http://www.astrosurf.com/buil/forum/ngc7331_poster.png

http://www.spectro-aras.com/forum/viewtopic.php?f=6&t=1682

http://www.spectro-aras.com/forum/viewtopic.php?f=6&t=2232

http://www.spectro-aras.com/forum/viewtopic.php?f=6&t=2420

http://www.spectro-aras.com/forum/viewtopic.php?f=6&t=2618

 

Robin

Hi Robin,

I love these!  They're great.  Really well done Christian and friends.

Steve.

[SteveBz]

9 hours ago, robin_astro said:

it really depends where your interests lie. The LHIRES III cannot do everything. It is good for high/medium resolution  on bright objects but does not work so well at low resolution. The ALPY works better at low resolution and can go much fainter  (and the spectra with my even lower resolution modified ALPY 200 are some of of the faintest recorded by an amateur)

Longer term, the Alpy sounds OK and might work with my f/5, cashflow permitting.  I don't have an f/10 or > £3k for the LHIRES III, sadly.

9 hours ago, robin_astro said:

If you start with the Star Analyser though you don't need to guide, just expose for as long as your mount can track for and align and stack multiple exposures

I'll probably buy this when the clouds clear.  I just want to see what the cheapo-cheapo grating can do.  It's satisfying to see what you can get out of kit for minimal investment.

9 hours ago, robin_astro said:

To get the best out of any slit spectrograph you are going to need a mount which can  guide sufficiently well to keep the star on the slit for long exposures using the spectrograph guide camera. The telescope has to be reasonably well matched to the spectrograph too, for example the ALPY works best at ~f5 but the DADOS and LHIRES need an f10 scope 

So, although I have a SkyWatcher EQ5 with horrendous backlash (I nearly typed whiplash, and it wouldn't have been far wrong), I've belt-modded the Dec axis with massive improvements and installed OAG, and I probably don't need to worry so much about the RA axis (I'll probably do it anyway at some stage).

Tx

Steve.

[SteveBz]

Hi Guys,

All the following calculations are performed with the rspec calculator, here:

https://www.rspec-astro.com/calculator/

In the scenario that I buy the SA100 without prism for use on the small refractor mounted to the main Celestron, I get the following calculated minimum cosmic distance I could measure:

If I mount the SA100 on the INSIDE of the nosepiece, near to the sensor, dispersion = 13 A per pixel.  Ie 1 pixel = 13 A = 1.3 nm /550nm x 300,000 km/s (ie c) / 70 km/s/mpc = 10 mpc

So I could measure the recession of a galaxy about 30 million light years away (eg NGC 7331).  But NGC 7331 is quite large and probably would need a slit.

I could double the resolution by moving the SA100 to the other end of the nosepiece.  That would be equivalent to an object 15 million light years away, but fainter (too faint according to the calculator).  And I could double again with the SA200 8 million light years away.

I could switch guiding to the refractor, which might be fun - it would be like a guide-scope.  In this scenario I could use the C8-N as the imaging scope, but amazingly the calculations come out the same.  I guess they would all be a bit brighter.

In all these scenarios, the object is large and either far away or faint.  So I think I need to use the SA100 for practice before I think about moving to the Alpy.

Measuring radial velocities seems quite a hard problem to solve with a limited budget.

Galaxies typically rotate at 200+ km/sec, binary stars maybe 70 km/sec and the limit in the faintest configuration above would be maybe 170 km/sec.

I'm rambling.  That's mainly because it's raining hard and I feel a bit frustrated that I can't get out and test!!!😞

Have a good day people and clear skies.

Regards

Steve.

 

 

[robin_astro]

2 hours ago, SteveBz said:

So I could measure the recession of a galaxy about 30 million light years away (eg NGC 7331).  But NGC 7331 is quite large and probably would need a slit.

The recession velocity of NGC7331 (800km/s = 17A at H alpha) is too probably low  to measure with a Star Analyser in any case. For high precision measurements you need a slit spectrograph where you have  fixed wavelength reference points from a calibration lamp

For slitless spectrographs like the Star Analyser you need a point source but there are active galaxies (Seyferts, QSO) which appear point like and have high enough redshifts to be measured eg

http://www.threehillsobservatory.co.uk/astro/spectra_3.htm

http://www.threehillsobservatory.co.uk/astro/spectra_21.htm

Instead of the small achromatic refractor I  recommend using the C8-N  which is the perfect choice for the Star Analyser 100 (and ALPY). You will need a mono astro camera though for faint objects like these. The DSLR will not be sensitive enough.  In general colour camera are not a good choice for spectroscopy for many reasons

Cheers

Robin

Edited January 20, 2021 by robin_astro
clarification

[robin_astro]

20 minutes ago, robin_astro said:

The recession velocity of NGC7331 (800km/s = 17A at H alpha) is too probably low  to measure with a Star Analyser in any case. For high precision measurements you need a slit spectrograph where you have  fixed wavelength reference points from a calibration lamp

because of the limitations of accurate wavelength calibration with the slitless Star Analyser I would estimate the smallest doppler shift you could reliably detect with the Star Analyser is probably around 1% or 3000km/s based on the measurements of redshifts I have made using it

Robin

[SteveBz]

Just now, robin_astro said:

because of the limitations of accurate wavelength calibration with the slitless Star Analyser I would estimate the smallest doppler shift you could reliably detect with the Star Analyser is probably around 1% or 3000km/s based on the measurements of redshifts I have made using it

Robin

Oh goodness, I was thinking maybe 700 km/s.  Based on 13 A dispersion, but of course I should have used 656 nm for Ha, not 550 nm (green).

The SA200 is very close in cost.  Should I go for that?

Steve

[robin_astro]

3 hours ago, SteveBz said:

Oh goodness, I was thinking maybe 700 km/s.  Based on 13 A dispersion, but of course I should have used 656 nm for Ha, not 550 nm (green).

The SA200 is very close in cost.  Should I go for that?

Remember that you are not measuring a narrow well defined line  1 pixel wide.  What you are trying to do with the Star Analyser  is compare the distance between two  poorly defined fuzzy blobs in the target and reference spectra (The zero order image and the image of the star at H alpha) They will be several pixels wide, dancing about due to the seeing and distorted by the change in focus and the aberrations caused by the converging beam configuration. See the images in the examples I gave above. Measuring this to better than 1% precision is a challenge.  With a slit spectrograph the situation is much better as the slit (not  the seeing, focus and other aberrations), define  the spectral line which is reproduceable so you can measure the position of the (centroid) of the line to a fraction of a pixel. You can measure the wavelength to high precision by comparing with the spectrum of a calibration lamp with well defined lines at known wavelengths. 

Go for the SA100, not the SA200. It works better in the converging beam configuration, particularly with fast telescope like your f5 Newtonian (smaller dispersion angle, less field curvature, less chromatic coma.)  The SA200 is for if you cannot mount the grating far enough away to get the dispersion you want eg if you have a close coupled filter wheel. See my website for more details

http://www.threehillsobservatory.co.uk/astro/spectroscopy_16.htm

The Star Analyser is not designed for making high precision observations. It is for  learning the basics by observing objects with bold features qualitatively to reveal the astrophysics going on before possibly investing in much more expensive equipment. Observations like these I posted near the top of the thread for example

If you experiment with one you will learn first hand the fundamentals of astronomical spectroscopy and understand  the issues I am describing much better than me trying to explain them here.

Cheers

Robin

Edited January 20, 2021 by robin_astro

[SteveBz]

2 hours ago, robin_astro said:

What you are trying to do with the Star Analyser  is compare the distance between two  poorly defined fuzzy blobs in the target and reference spectra (The zero order image and the image of the star at H alpha)

That's probably the statement that's got me furthest.

Roll on clear skies.  Maybe Saturday.

Tx.

Steve.

[SteveBz]

So I got out last night to get some spectra of Bet-Cas.  Here's the .jpg & .NEF (Nikon Raw) file. I can just about see some bands visually, but when I convert to .FIT and open in vspec, it's saturated.  Not sure what's causing that.

You can see that the right-hand side it quite well focused and the left-hand side goes into a fishtail as you said.  Looking visually, so can see about three bands spaced about right in the blue end of the spectrum for H-Beta, H-Gamma and H-Delta, but I can't process that.

The other thing is that DSS stacking doesn't seem to work when the stars are so out of focus because the FWHM is so poor.

Your thoughts about what I should do next would be appreciated.

(So to be clear, this is my DIY spectrum with $5 film off ebay - no pound sign on my keyboard)

Kind regards

Steve.

FOCU_20210123_222253.NEF

[SteveBz]