Coronado BF15 spectrographic testing.

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Merlin66
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Coronado BF15 spectrographic testing.

Post by Merlin66 » Fri Feb 23, 2018 5:43 am

Following from previous discussions and a request from Valery, I pulled my Coronado BF15 apart and used the Spectra-L200 with various gratings to measure the transmission curves for the elements.

ITF Filter - a Coronado original (not the Maier replacement)
Interestingly the transmission curve is not symmetrical nor centred on the Ha wavelength.....
BF15 ITF filter transmission.png
BF15 ITF filter transmission.png (27.99 KiB) Viewed 918 times
BF15 Mirror
Tested at 45 degree orientation. This shows a UV cut-off at 4000A and some transmission resonance bands in the visual but is basically a hot NIR mirror peaking at around 6800A and cutting off around 8000A.
BF15 Mirror transmission.png
BF15 Mirror transmission.png (28.29 KiB) Viewed 918 times
BF15 Blocking filter
This was a surprise! The beautiful Gaussian transmission curve sits just above Ha, and measures a FWHM of only 6.5A
BF15 Blocking filter.png
BF15 Blocking filter.png (23.77 KiB) Viewed 918 times


I have the raw data available...I'll try to normalise these profiles to show absolute transmissions....just need a bit more free time.
Questions/ comments???
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Re: Coronado BF15 spectrographic testing.

Post by Montana » Sun Feb 25, 2018 6:43 pm

Very nice work :)
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Re: Coronado BF15 spectrographic testing.

Post by marktownley » Sun Feb 25, 2018 9:04 pm

Interesting information Ken, and useful to know.
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Re: Coronado BF15 spectrographic testing.

Post by Merlin66 » Sun Feb 25, 2018 9:23 pm

Thanks.
If the ITF and mirror coatings were better "tuned" to the H alpha you'd get a much brighter image.....it would be interesting to do the same testing on a Lunt BF.
"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before
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"Astronomical Spectroscopy for Amateurs" and
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Re: Coronado BF15 spectrographic testing.

Post by Valery » Tue Feb 27, 2018 8:44 am

Hi Ken,

Thanks a lot for letting us know your findings. Very interesting and very useful to know the nature of the BF Coronado.

I can tell you a small statistic of blokers.

1. My BF10 is the brigthest BF I ever met. And very contrasty vs other BFs if testing with the most contrast etalon I got.
2. The BF10 owned by a friend of mine is the second luminous wise. A tad lower contrast vs best BF10.
3. Lunt B1200 diagonal and B1200 straight through are equal and both are about as 1,5x less luminous than my BF10.
4. Lunt B1800 owned by a friend of mine is the darkest BF I ever saw. 3x darker than my best BF10.
5. BF10 with rusted ITF - 2x darker than my BF10.

Too bad the manufacturers makes BF so inconsistently.
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Re: Coronado BF15 spectrographic testing.

Post by christian viladrich » Mon Mar 05, 2018 6:54 pm

Thanks a lot Ken. This is very interesting.
It would be nice to have some measurements of other filters to check variability.
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Re: Coronado BF15 spectrographic testing.

Post by Merlin66 » Tue Mar 06, 2018 12:44 am

Yes, I agree.
Unfortunately being in Oz limits the accessibility to other filters for testing.

If any one has a ol' school spectroscope (Mark?) fitting a SA100, SA200 or Rainbow Optics 200 transmission grating would give the basic filter curves.......
"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before
https://groups.yahoo.com/neo/groups/ast ... scopy/info
"Astronomical Spectroscopy for Amateurs" and
"Imaging Sunlight - using a digital spectroheliograph" - Springer

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Re: Coronado BF15 spectrographic testing.

Post by JohnW » Tue Mar 06, 2018 3:06 am

I have always found the B1200 on my Lunt 100 scope much less bright than the views thru my older Coronado SMII60 which had a BF10. Does this mean that if I brought a BF10 and put it into my Lunt 100 I would likely get brighter views ? Thanks, John W.

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Re: Coronado BF15 spectrographic testing.

Post by Merlin66 » Tue Mar 06, 2018 3:24 am

Hmmmm,
Not necessarily. I found when I looked through a Lunt 60, B1200 fitted to the SM60/ ED80, the view was much brighter than my usual BF15!!!
The problem was a rusted ITF - when the replacement Maier filter was used - majic! the view was everything as bright as the Lunt.
"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before
https://groups.yahoo.com/neo/groups/ast ... scopy/info
"Astronomical Spectroscopy for Amateurs" and
"Imaging Sunlight - using a digital spectroheliograph" - Springer

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