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Hi all

Thought I'd try and rekindle my spectroscopy mojo! I've been ploughing through the video tutorials but it all morphs into a blur after a while... Was wondering if there was a workflow/crib sheet I could print out somewhere? I'm thinking of just the data acquisition part in the first instance. Like how do I get the exposure right for a given star plus all the other things I need to think of and do. I get that it's a good idea to go for an A-type star but not sure about how best to do the exposure. Is it better to take an avi or individual fits subs? Should I stack them or let RSpec do that? I'll be using my 80mm APO at f4.8 with a SA-100 and qhy183m, probably bin 2x2 with SharpCap Pro for acquisition. That gives me a dispersion of 11.2 and coverage of 0-30,943A assuming a grating to sensor distance of 43mm (it's hard to measure exactly).

Thanks for any help/advice :)

Louise

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You can get RSpec to do a lot in real time but personally for best results I would first use the real time feature of RSpec using an avi to get the best focus on a brightish main sequence A  star which would also be your reference calibration star and then capture a series of subs of both reference and target using Sharp Cap which you then preprocess off line before passing back to your favourite spectrum processing program which could be RSpec or another program. The subs can be an avi for bright targets, which are then dark subtracted, aligned, selected for sharpness and stacked like planetary imaging to produce a 16 bit fits image or longer exposure full bit depth subs up to how long you can track for with  faint targets, like in  deep sky imaging, The objective is to produce  the best (well, not over, exposed, low noise, sharp) image of the spectrum you can to pass to the spectrum processing program.

Cheers

Robin

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Also binning could give an under sampled image with such a short focal length which can give processing problems with spectra (artifacts) so I would recommend running unbinned 

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3 minutes ago, robin_astro said:

You can get RSpec to do a lot in real time but personally for best results I would first use the real time feature of RSpec using an avi to get the best focus on a brightish main sequence A  star which would also be your reference calibration star and then capture a series of subs of both reference and target using Sharp Cap which you then preprocess off line before passing back to your favourite spectrum processing program which could be RSpec or another program. The subs can be an avi for bright targets, which are then dark subtracted, aligned, selected for sharpness and stacked like planetary imaging to produce a 16 bit fits image or longer exposure full bit depth subs up to how long you can track for with  faint targets, like in  deep sky imaging, The objective is to produce  the best (well, not over, exposed, low noise, sharp) image of the spectrum you can to pass to the spectrum processing program.

Cheers

Robin

Ok, thanks Robin! I was hoping to capture some images separate from RSpec (which I don't have yet) and to keep things simple anyway. I get the impression I have to be careful not to let the star get saturated... Focussing is always difficult here - almost impossible to get sharp focus because of turbulence. I'll just have to try my best! I'm expecting to keep exposure times down to <30sec, obviously depending on the star. I've never done any planetary imaging....

Louise

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1 minute ago, robin_astro said:

Also binning could give an under sampled image with such a short focal length which can give processing problems with spectra (artifacts) so I would recommend running unbinned 

Ok, it's just that unbinned the qhy183m pixels are only 2.4um but I'll try it out when I can - maybe tonight!

Louise

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Focus on the zero order star first then tweak the focus a bit to best see the spectrum features. As an alternative to main sequence A stars (Like Delta Cas for example) stars with a bright H alpha emission line can be easier to focus on, like Gamma Cas or P Cygni. Look for a bright dot in the spectrum towards the red end. As here for Gamma Cas for example

http://www.threehillsobservatory.co.uk/astro/spectra_12.htm

Cheers

Robin

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2 minutes ago, robin_astro said:

Focus on the zero order star first then tweak the focus a bit to best see the spectrum features. As an alternative to main sequence A stars (Like Delta Cas for example) stars with a bright H alpha emission line can be easier to focus on, like Gamma Cas or P Cygni. Look for a bright dot in the spectrum towards the red end. As here for Gamma Cas for example

http://www.threehillsobservatory.co.uk/astro/spectra_12.htm

Cheers

Robin

I'm a bit limited with choice of stars as I have a very restricted view of the sky - can only see to the East and whatever is passing my small window...

I'll just have to see what I can do in SharpCap :)

Louise

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Louise, having focused on the star, as Robin advises, you will need to tweek the focus in (I.e. towards the objective or mirror) due to the geometry of the diffraction. It's  not very much so easy does it.

Regards Andrew 

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1 minute ago, andrew s said:

Louise, having focused on the star, as Robin advises, you will need to tweek the focus in (I.e. towards the objective or mirror) due to the geometry of the diffraction. It's  not very much so easy does it.

Regards Andrew 

Hi Andrew

Thanks. I can  use  the SharpCap FX box  to aid focus on a particular star (via the SA100) but not really sure how to 'tweak' it.... I'll have to download RSpec tomorrow and maybe see how to do it on the live spectrum.  Still too cloudy here at the moment.. It might clear later.  Fingers crossed.

Louise

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6 minutes ago, Thalestris24 said:

Hi Andrew

Thanks. I can  use  the SharpCap FX box  to aid focus on a particular star (via the SA100) but not really sure how to 'tweak' it.... I'll have to download RSpec tomorrow and maybe see how to do it on the live spectrum.  Still too cloudy here at the moment.. It might clear later.  Fingers crossed.

Louise

With Rspec it gas a feature to do it in real time. Other than that it's trial and error. Inspecting images as you go.

Regards Andrew 

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1 minute ago, andrew s said:

With Rspec it gas a feature to do it in real time. Other than that it's trial and error. Inspecting images as you go.

Regards Andrew 

Yeah, as I say, I'll to download it and have a go. It can't need much of a tweak at such a short focal length (384mm). I might switch over to my 115mm frac which has an Ascom focuser, so easier to adjust remotely. 

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5 minutes ago, Thalestris24 said:

Yeah, as I say, I'll to download it and have a go. It can't need much of a tweak at such a short focal length (384mm). I might switch over to my 115mm frac which has an Ascom focuser, so easier to adjust remotely. 

ps thinking about it, trying to tweak the focus is probably a waste of time since it's never steady here in the first place. I'll probably have to just try and see what I can get...

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Louise,

As has been said, you'll definitely get better results when you focus on the spectral image rather than the zero image. You should see the absorption features gain contrast and some clarity.

Start with short exposures and don't worry about the zero order image being over exposed.

Position the zero order towards the left hand side and the bright (1st order) spectrum close to the centre of your chip.

Practise, practise......

 

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39 minutes ago, Merlin66 said:

Louise,

As has been said, you'll definitely get better results when you focus on the spectral image rather than the zero image. You should see the absorption features gain contrast and some clarity.

Start with short exposures and don't worry about the zero order image being over exposed.

Position the zero order towards the left hand side and the bright (1st order) spectrum close to the centre of your chip.

Practise, practise......

 

Yeah, I'll need the RSpec for that. As I mentioned, though, it's very difficult to get sharp focus on anything here :(

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Louise,

You don't need RSpec... you'll see the difference in images using your normal capture software.

I use AstroArt for acquisition and BASS Project for processing.

 

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2 hours ago, Merlin66 said:

Louise,

You don't need RSpec... you'll see the difference in images using your normal capture software.

I use AstroArt for acquisition and BASS Project for processing.

 

Ah, I thought you meant as per the video tutorial where the guy adjusts the focus in RSpec whilst capturing an avi. Anyway, I've had a go. I think some of the ones earlier in the evening were maybe not so well focused - looked more like a smear... I'll try and download the RSpec sometime this week and have a look at what I've managed to get tonight. I was quite surprised - and pleased! - that I could still platesolve even with the SA100 in place. I'll post an update when I've looked at the data :)

Louise 

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I eventually arrived at this spectrum of 66 Gem (Castor). The RSpec software was ok-ish but it didn't always do what I expected it to and it's never clear how to save your work. There's no back button! Very annoying at times! According to the tutorials I should have been able to left click and draw a box around a section of the spectrum but I wasn't able to do that :( . The y-scaling tended to go a bit crazy on me... 

Anyway, here are the spectra:

1. Pre instrument response normalisation:

66Gem_Castor_calib_5_4A.JPG.c3072d271fcb4edd0d3758e320bae349.JPG

 

 

2. Post Instrument Response:

66Gem_Castor_calib_5_4A_InstResp.JPG.e03e185241b3a4fef8d88fe4f9138e4c.JPG

I used a stacked image - 61 x 2s. Maybe I'll have another go via the individual images - if I can get in the mood! Castor is a spectral type A1V (The Stellarium details say 'A1.5IV'). However, there are no actual A1 types in the RSpec reference library so I wasn't sure about the calibration. I got the Balmer lines to line up with the features. My H-Beta line was to the right of the main peak rather than to the left as in the Vega example. I would have imaged Vega if only my scope could see it! Vega is a type A1V - not sure if the slight difference in spectral types accounts for the differences? Though maybe the star in the tutorial video wasn't actually Vega?!! I didn't think at the time but Castor is a double star. However, it's probably of no consequence as Castor itself is so bright.

Anyways, any constructive criticism gratefully received :)

Louise 

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It has been interesting reading the comments here. I have the spectroscopy bug and just waiting for a clear night. Looks like a great first effort Louise, hope I can achieve something similar.

 

John

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17 minutes ago, Hughsie said:

It has been interesting reading the comments here. I have the spectroscopy bug and just waiting for a clear night. Looks like a great first effort Louise, hope I can achieve something similar.

 

John

Ta for the encouragement! It's actually not my first go - I did do a bit a couple of years ago but got a bit frustrated with it and also didn't have the time. It's quite easy and quick to acquire spectra once you have good focus etc. But the processing is a little long winded. Having said that, like many things, I expect it will get quicker and easier the more I do. I have a number of stars to process, Castor was just the first. I acquired images of mostly A-type stars but also did some others. Good luck if you have a go.

Louise

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Posted (edited)

Hi Louise

Nice result.  Rather  noisy though, particularly for a 60x2 sec stack on such a bright target. The noise looks like it may be pixel artifacts which are sometimes seen with short focal length systems where the star size is very small. (a pixel to pixel sawtooth)   Did you have to rotate the spectrum to get it horizontal?  Rotation can introduce this sort of effect so taking care to get the spectrum horizontal helps

The difference between A0v and A2v is very small but if you want to be spot on you could average the two Pickles references to produce an A1v (Not worth it though!).  Vega is usually considered to be A0v but precise spectral classification is an inexact science. You can see the various classifications in the literature for many stars in Brian Skiff's huge catalogue here.

http://vizier.u-strasbg.fr/viz-bin/VizieR?-source=B/mk

The position of the maximum in the raw profile depends on the spectral response of grating plus the particular camera so is not consistent between different setups even for the same star.

Cheers

Robin

 

Edited by robin_astro
typo

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8 minutes ago, robin_astro said:

Hi Louise

Nice result.  Rather  noisy though, particularly for a 60x2 sec stack on such a bright target. The noise looks like it may be pixel artifacts which are sometimes seen with short focal length systems where the star size is very small. (a pixel to pixel sawtooth)   Did you have to rotate the spectrum to get it horizontal?  Rotation can introduce this sort of effect so taking care to get the spectrum horizontal helps

The difference between A0v and A2v is very small but if you want to be spot on you could average the two Pickles references to produce an A1v (Not worth it though!).  Vega is usually considered to be A0v but precise spectral classification is an inexact science. You can see the various classifications in the literature for many stars in Brian Skiff's huge catalogue here.

http://vizier.u-strasbg.fr/viz-bin/VizieR?-source=B/mk

The position of the maximum in the raw profile depends on the spectral response of grating plus the particular camera so is not consistent between different setups even for the same star.

Cheers

Robin

 

Yeah, I thought it seemed noisy too. And, yes, I did have to rotate the image - by 209 deg! I suppose all the stars I did will be like that then :( Oh well. At the moment  the SA100 is mounted in a TS filter changer but the grating is too deep to allow removing the drawyer, so I'll have to disassemble in order to rotate it to the right position. That's a bit annoying... I might transfer the SA100 to the 115mm APO + Atik 383l+ That might give better results but the guiding won't be so good ant the Atik is rather slow. But at least it has a filter wheel - hope there's  no problems fitting the SA100 viaa a 2" adapter. If that doesn't work I'll probably give up on it again!

Thanks

Louise

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Another thought Re Noise....

Are you sure you're working with the brighter 1st order spectrum?

Ken

 

 

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4 minutes ago, Merlin66 said:

Another thought Re Noise....

Are you sure you're working with the brighter 1st order spectrum?

Ken

 

 

Hi

Pretty sure - I don't think there was anything else nearby and wouldn't expect to see anything else with only 2s exposures. I'll have a look at some of the stars I did longer exposures with when I get the chance. Dentist appt. tomorrow then I have a load of homework to do.

Louise

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Louise, I know it doesn't necessarily help you, but I just wanted to say that I share your pain. I said this in another thread. I was very interested in spectroscopy and my wife even bought me a SA100 diffraction grating for Christmas last year. I have RSpec as well to use it. I've only used it a couple of times though. It's honestly not that intuitive and I don't have enough time to sit down and play with it which means I'm less likely to bother learning it. I too have been through all the tutorial videos and it all runs together for me too. Which is fine if I'm at home and post processing because I can watch them over and over again. However, I can never remember what I need to do when I'm out in the field if I want to use it for live viewing and sharing with people in a public setting.

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23 minutes ago, Thalestris24 said:

Hi

Pretty sure - I don't think there was anything else nearby and wouldn't expect to see anything else with only 2s exposures. I'll have a look at some of the stars I did longer exposures with when I get the chance. Dentist appt. tomorrow then I have a load of homework to do.

Louise

This is the image I used:
66Gem_Stack_16bits_61frames_122s.jpg.b1f545180c2bd756e33483747ada9a72.jpg

 

This is the fainter image from the opposite side:

66Gem_Stack_16bits_61frames_122s_2.jpg.448d4a9167f0c11439ca1101b7f3ca45.jpg

You can barely see this one!

Louise

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