Attempting an SA100 spectrum on 2Lyr and 4Lyr/5Lyr

[vlaiv]

1 minute ago, robin_astro said:

You can get a rough response by this method. In fact this is essentially what happens with a slit spectrograph when you take a flat, all explained in my document. At the end of the day though it is not particularly useful as you still have to include the effect of the atmosphere which is done by taking a standard star. Professionals do tend to separate the effect of the instrument and of the atmosphere using a standard instrument response and then correct for the atmosphere separately by taking telluric standards on the night but they rely on much more stable atmospheric conditions. In general most amateurs take a nearby reference star on the night which includes all effects in one go.

Cheers

Robin

Is there any way to separate SA100/200 part of response and camera response / optics response?

I guess that it can be done if one has sensor with known curve available - that way optics + SA can be extracted.

 

[SteveBz]

18 minutes ago, robin_astro said:

As I said earlier what you are measuring is not just the instrument response. (This is a common misunderstanding which is why I emphasise this in the document on my website). It also includes the effect of the atmosphere which is significant and changes from night to night and with height above the horizon (air mass). Although you can get a rough result by using a response from a previous night, provided the difference in air mass is not too great, for the most accurate results it is a good idea to take a response on the night at similar air mass, particularly for targets low in the sky where the air mass is high. See this example by Christian Buil of  the effect of air mass on the spectrum

http://www.astrosurf.com/buil/atmosphere/transmission1.png

From his page explaining the effect of the atmosphere

http://www.astrosurf.com/buil/atmosphere/transmission.htm

Francois Teyssier has made a nice  spreadsheet to find suitable bright stars with known spectra near your target. There is a link to it in my document on flux calibration

Hi Robin,

I was just being lazy with my vocabulary.  I meant all the stuff (and I hope that is what I did - I followed your instructions to the letter).

But I think the answer to your question, is - like building up a dark library - that you need a set of compensating corrections for each physical situation eg one for every 10 degrees of Az, maybe.

Steve.

[robin_astro]

1 minute ago, vlaiv said:

Is there any way to separate SA100/200 part of response and camera response / optics response?

I guess that it can be done if one has sensor with known curve available - that way optics + SA can be extracted.

 

It is difficult without specialise lab calibrated equipment but why would you want to ?

Robin

[SteveBz]

6 minutes ago, robin_astro said:

You can get a rough response by this method. In fact this is essentially what happens with a slit spectrograph when you take a flat, all explained in my document. At the end of the day though it is not particularly useful as you still have to include the effect of the atmosphere which is done by taking a standard star. Professionals do tend to separate the effect of the instrument and of the atmosphere using a standard instrument response and then correct for the atmosphere separately by taking telluric standards on the night but they rely on much more stable atmospheric conditions. In general most amateurs take a nearby reference star on the night which includes all effects in one go.

Cheers

Robin

My issue, really, is that it's very manual and, for me at least, very error-prone.  I was wondering how I could use the effect of averages to reduce the error.

[SteveBz]

9 minutes ago, robin_astro said:

You can get a rough response by this method. In fact this is essentially what happens with a slit spectrograph when you take a flat, all explained in my document. At the end of the day though it is not particularly useful as you still have to include the effect of the atmosphere which is done by taking a standard star. Professionals do tend to separate the effect of the instrument and of the atmosphere using a standard instrument response and then correct for the atmosphere separately by taking telluric standards on the night but they rely on much more stable atmospheric conditions. In general most amateurs take a nearby reference star on the night which includes all effects in one go.

Cheers

Robin

My issue, really, is that it's very manual and, for me at least, very error-prone.  I was wondering how I could use the effect of averages to reduce the error.

[robin_astro]

2 minutes ago, SteveBz said:

that you need a set of compensating corrections for each physical situation eg one for every 10 degrees of Az, maybe.

Still not good enough I am afraid as atmospheric conditions can be different from night to night even at the air mass. You still have to make a measurement of this on the night for best accuracy. If you are familiar with photometry it is the same as doing differential photometry

Robin

[SteveBz]

9 minutes ago, robin_astro said:

You can get a rough response by this method. In fact this is essentially what happens with a slit spectrograph when you take a flat, all explained in my document. At the end of the day though it is not particularly useful as you still have to include the effect of the atmosphere which is done by taking a standard star. Professionals do tend to separate the effect of the instrument and of the atmosphere using a standard instrument response and then correct for the atmosphere separately by taking telluric standards on the night but they rely on much more stable atmospheric conditions. In general most amateurs take a nearby reference star on the night which includes all effects in one go.

Cheers

Robin

My issue, really, is that it's very manual and, for me at least, very error-prone.  I was wondering how I could use the effect of averages to reduce the error.

[robin_astro]

Just now, SteveBz said:

My issue, really, is that it's very manual and, for me at least, very error-prone.  I was wondering how I could use the effect of averages to reduce the error.

A good quick check that your response is sensible is to apply it back to your reference star and make sure the result matches that from the library (again in my document)

Robin

[SteveBz]

Just now, robin_astro said:

Still not good enough I am afraid as atmospheric conditions can be different from night to night even at the air mass. You still have to make a measurement of this on the night for best accuracy. If you are familiar with photometry it is the same as doing differential photometry

Robin

So do you still do this each night for each target, or set of targets?  And manually draw the response correction line?

[SteveBz]

Just now, robin_astro said:

A good quick check that your response is sensible is to apply it back to your reference star and make sure the result matches that from the library (again in my document)

Robin

I did, and it was sort of close but not perfect.

[vlaiv]

1 minute ago, robin_astro said:

It is difficult without specialise lab calibrated equipment but why would you want to ?

Robin

I want to be able to measure camera response. It has to do with imaging and color reproduction.

That way I can take any light source and produce accurate spectrum of it - that is one thing. Other thing is that I can produce QE response curve of sensor.

Without knowing camera and SA response - I can measure response curves of filters - and that is one part that I'd like to do (simply do with / without filter and divide). But I would also like to measure sensor response curve. This dates back to when I discovered that Christian measured ASI1600 QE and it was significantly different to published value.

I have no idea which one, if either is correct.

In any case - these are all low resolution applications without need for high accuracy.

[robin_astro]

Just now, SteveBz said:

I did, and it was sort of close but not perfect.

It does need practise and for amateurs +-10% error in the shape of the continuum is about as close as you are likely to get (If you want to hone your skills try a few stars with known spectra to see how close you can get as I did in the examples on my website)  At the end of the say though it is the features in the spectrum not the exact shape of the continuum which is most  important as they contain most of the astrophysical information so don't get too hung up about response correcting, particularly with the Star Analyser  which I developed to give a relatively gentle introduction to the subject and is mainly best for targets which show clear strong features. 

for a good example while it is bright enough I recommend looking at the recurrent nova RS Oph if it is high enough for you. It is in a rare outburst (every 15-20 years)  and has and interesting spectrum for the star analyser. This is what it looked like back in 2006 with the Star Analyser

http://www.threehillsobservatory.co.uk/astro/spectra_24.htm

and this is what it looks like currently with my ALPY 600 spectrograph, from the thread covering this outburst

https://stargazerslounge.com/topic/381251-eruption-of-the-recurrent-nova-rs-oph/?do=findComment&comment=4136409

Cheers

Robin

 

[robin_astro]

14 minutes ago, vlaiv said:

I want to be able to measure camera response. It has to do with imaging and color reproduction.

Yes camera response is tough to measure as you need both a calibrated light source and spectrograph or a reference sensor to compare with.  If this is scientific astronomical imaging though the atmosphere will also have an affect so you could use a similar process to that used in spectroscopy/photometry , matching the colour of a known star, typically making G2v stars in the image white

Robin

[robin_astro]

31 minutes ago, robin_astro said:

 

for a good example while it is bright enough I recommend looking at the recurrent nova RS Oph if it is high enough for you. It is in a rare outburst (every 15-20 years)  and has and interesting spectrum for the star analyser.

 

Another good one currently for the Star Analyser  is Nova Cas 2021. It exploded back in March but unlike most novae it refuses to fade and has been bouncing around mag 7 ever since.  This is the AAVSO light curve

and an ALPY 600 spectrum I took a couple of weeks back, here in the BAA spectroscopy database

https://britastro.org/specdb/data_graph.php?obs_id=10399

There is a long thread on it  here

 

Cheers

Robin

Edited September 9, 2021 by robin_astro

[vlaiv]

3 hours ago, robin_astro said:

matching the colour of a known star, typically making G2v stars in the image white

Yes, that is the last step - removing influence of atmosphere, however G2V star is not really white - it is this color:

here is white for reference:

 

[vlaiv]

Sorry to hijack the thread, I'll try not to do it any more

Just need to ask one question @robin_astro

If I do the following, can I extract SA response:

- take transparency sheet for over head projector (printable one) and measure its response (divide artificial star spectrum with / without front aperture cover made from transparency sheet)

- print full aperture grating with laser printer

- get spectrum of artificial star using full aperture grating and correct for its response that I got in step 1 (which leaves optics + camera response and source spectrum)

- get spectrum with SA and use above to extract only SA response

What I'm trying to ask is will adding the grating change spectral response of transparency sheet for overhead projector?

Or similar question - if I 3d print front aperture grid (assuming I can print that fine) - will "air gaps" have response of 1? (does grid affect response in any way - or is it just optical properties of material in gaps?).

[robin_astro]

16 hours ago, vlaiv said:

Or similar question - if I 3d print front aperture grid (assuming I can print that fine) - will "air gaps" have response of 1? (does grid affect response in any way - or is it just optical properties of material in gaps?).

An interesting question. Does a theoretical perfect unblazed transmission grating have a flat response?  I dont know and have not yet been able to find  the answer on line.

One  potential way of measuring the response of a transmission grating without a calibrated light source could be to use 2 transmission gratings at 90 deg. The first disperses the light into a horizontal spectrum and the second then disperses this spectrum vertically. The resulting diagonal spectrum divided by the horizontal spectrum should give the grating response.

Robin

[vlaiv]

3 minutes ago, robin_astro said:

One  potential way of measuring the response of a transmission grating without a calibrated light source could be to use 2 transmission gratings at 90 deg. The first disperses the light into a horizontal spectrum and the second then disperses this spectrum vertically. The resulting diagonal spectrum divided by the horizontal spectrum should give the grating response.

Trying to wrap my head around this as it sounds like rather interesting proposition to measure SA - regardless of response of front mounted grating (laser printed or 3d printed).

I think I understand.

Resulting spectrum would look something like this:

Then I measure diagonal and divide with vertical as

vertical is : source * optics * first grating

while diagonal is : source * optics * first grating * second grating

Do I have this right? If this is correct - then excellent - knowing source and SA allows me to measure sensor response later on.

[vlaiv]

@robin_astro

Follow up question with regards to above - do we know for sure that Zero Order image is unaffected by grating response?

Above can only work if zero order image has same spectrum as the source (only attenuated) - otherwise it won't.

 

[robin_astro]

18 minutes ago, vlaiv said:

Follow up question with regards to above - do we know for sure that Zero Order image is unaffected by grating response?

Above can only work if zero order image has same spectrum as the source (only attenuated) - otherwise it won't.

The light in zero order is not the same as the original source. It is the undiffracted light so the spectral content is the inverse of the combined grating responses but that does not matter. All we are using the first grating for is to produce a horizontally dispersed light source which is then modified by the second grating. The light in the  diagonal spectrum only comes from this dispersed light source. The ratio of the light at a given wavelength in the diagonal spectrum to that in the horizontal spectrum then gives the efficiency of the second grating at that wavelength. (It probably should not be used to give an absolute efficiency measurement but the variation in efficiency with wavelength should be correct.

EDIT (change horizontal for vertical to match your diagram)

Robin

Edited September 10, 2021 by robin_astro

[vlaiv]

2 minutes ago, robin_astro said:

The light in zero order is not the same as the original source. It is the undiffracted light so the spectral content is the inverse of the grating response but that does not matter. All we are using the first grating for is to produce a horizontally dispersed light source which is then modified by the second grating. The light in the  diagonal spectrum only comes from this dispersed light source. The ratio of the light at a given wavelength in the diagonal spectrum to that in the horizontal spectrum then gives the efficiency of the second grating at that wavelength. (It probably should not be used to give an absolute efficiency measurement but the variation in efficiency with wavelength should be correct.

Robin

Yes, but think for a moment what happens:

If zero order image is not the same as original source - and first dispersion is in effect producing number of "sources" for second dispersion - we don't have recording of those "sources" - only zero orders of those "sources".

 

[robin_astro]

3 minutes ago, vlaiv said:

If zero order image is not the same as original source - and first dispersion is in effect producing number of "sources" for second dispersion - we don't have recording of those "sources" - only zero orders of those "sources".

You first measure the horizontal spectrum without  the second grating in place

[vlaiv]

1 minute ago, robin_astro said:

You first measure the horizontal spectrum without  the second grating in place

Brilliant - did not think of that

 

[robin_astro]

You would need to watch out for flat field effects as the two measured spectra will be in different places in the field. (Best avoided with slitless spectra by placing the target and reference spectra in the same position)  You could perhaps test for this by taking an additional spectrum with the 1st grating at a diagonal (adjusting the dispersion to match that of the combined spectrum) and comparing the shape with that of the horizontal spectrum

[vlaiv]

4 minutes ago, robin_astro said:

You would need to watch out for flat field effects as the two measured spectra will be in different places in the field. (Best avoided with slitless spectra by placing the target and reference spectra in the same position)  You could perhaps test for this by taking an additional spectrum with the 1st grating at a diagonal (adjusting the dispersion to match that of the combined spectrum) and comparing the shape with that of the horizontal spectrum

Yes, that will be interesting to do with the setup I'm planning to use.

I'll probably use F/13 4" maksutov for this and artificial star at close range. First grating spectrum will be significantly shorter (maybe as short as 100-200px). I'll try placing it on several locations and under several directions to see if there is significant difference in measured spectrum.

I'll probably go with 0.5mm front aperture printed grating (with 600dpi that is 25.4 / 600 = 0.0423333mm or 11.8 points per line - I can print with 10-11 points per line) . That is 2000 lines per meter and angle will be

sin(theta) = theta = 300nm / 0.0005m = 0.0006 radians = 123.76"

At 0.6"/px that is about 200px