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Characterising the transmission of your system

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#1 Lucullus

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Posted 19 January 2021 - 03:05 PM

For new professional telescopes there often are characterisations in papers showing various throughput and transmission specifications.

e.g. http://www.cadc-ccda...cs/filtold.html

 

cfhtlsugriznew_en.gif

Has any semi-professional or advanced amateur ever done something like this and characterised his system? How is this done at all?

 

 

 

 

 



#2 TOMDEY

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Posted 19 January 2021 - 04:51 PM

We of course routinely did that sort of metrology at work, aerospace optics. One generally predicts, and later measures the telescope (sans filters) implicit in your attached "mirror" and "optics" curves up there. One would then factor in the separately-characterized detector and various filters, etc. and also the anticipated environmentals. And, once on orbit, the system would get further cals on such things as known stars, the sun, and controlled artificial calibration targets on the moon or earth. Stuff like that is routine, if you actually want to use your instrument as an absolute "quantitative spatial radiometer" which is fancy-speak for a camera that takes really pretty pictures of the cosmos! Many imagers also have "upon demand internal / on-board" radiometric calibration sources. I'm on a couple of patents of such things... and used to ~calibrate the calibrators~.

 

I set up measurements on some hobby telescopes and components. Couple of those shown here >>>    Tom

Attached Thumbnails

  • 50 75 Tom's Solar Simulator.jpg
  • 51 80Tom's Eyepiece Throughout Measurement Bench.jpg
  • 52 80 Tom's Star Diagonal Throughput Measurement Bench.jpg

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#3 Lucullus

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Posted 22 January 2021 - 06:17 PM

How do you analyse a 30" reflector?

#4 TOMDEY

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Posted 22 January 2021 - 09:21 PM

If it's a Newtonian, just the product of the coating reflecivities times the transmission of the Coma Corrector.    Tom



#5 Lucullus

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Posted 22 January 2021 - 10:26 PM

Yes, but do you just look on the computer screen, take the ruler and multiply the rough values of some low-resolution jpg of the coating or optical window/filter supplier? How is it done with an OTA that doesn't fit on your optical bench above if you want your data as detailed and high-quality as in the first post image with peaks and valleys on the 5nm scale?

Edited by Lucullus, 23 January 2021 - 01:01 AM.


#6 robin_astro

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Posted 23 January 2021 - 09:20 AM

It is routinely done in spectroscopy as part of the data reduction using standard stars as the reference, The various instrument components are generally lumped together rather than being separated out but the atmospheric absorption can be separated from the instrument response by  measuring the star at different elevations as here for example by Christian Buil.

http://www.astrosurf...ransmission.htm

The effect of filters, optical windows etc can be measured by introducing them into the optical chain and dividing the filtered  by the unfiltered spectrum. 

 

Cheers

Robin


Edited by robin_astro, 23 January 2021 - 09:27 AM.


#7 robin_astro

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Posted 23 January 2021 - 09:23 AM

See also here for methods from Christian Buil to characterise the response of the instrument

http://www.astrosurf...nt_response_us/

 

Cheers

Robin


Edited by robin_astro, 23 January 2021 - 09:27 AM.


#8 robin_astro

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Posted 23 January 2021 - 09:36 AM

The response of telescope optical coatings in particular is of interest to amateur spectroscopists who are often working outside the normal wavelength range of telescopes designed for visual use. Here are some examples of measurements made using a spectrograph on various telescopes

http://www.spectro-a...php?f=45&t=2277

 

Cheers

Robin



#9 robin_astro

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Posted 23 January 2021 - 10:10 AM

Similarly the response of filters can be easily measured with the spectrograph on the bench and a broad band light source

http://www.astrosurf...ters/curves.htm

These again are by Christian Buil but there are currently some similar measurements recently posted on here using a home built spectrograph

https://www.cloudyni...aph/?p=10821423

 

Cheers

Robin




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