Some tests with double stack etalons (mica & FS) and the C8 EdgeHD

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christian viladrich
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Some tests with double stack etalons (mica & FS) and the C8 EdgeHD

Post by christian viladrich » Sat Feb 01, 2020 8:21 pm

Dear All,

Here are some tests I made last August with the C8 EdgeHD at F/35 and a double stack combinaison consisting in a DayStar PE 0.6 A and a Coronado SMN35 0.5A.
As a reminder, the Coronado SMN35 0.5A is a fused silica etalon made at the begining of Coronado when David Lunt was experimenting different options for the manufacturing of etalons. Too bad there were only a handfull made.
The advantage is the excellent transmission (compared to mica-spaced etalons) and larger refractive index (compared to air-spaced etalons).
It makes it possible to double stack the PE 0.6 A and the SMN 0.5A, while keeping a reasonnable exposure time.
As a comparison, the contrast of the 0.6A + 0.5 A combination is much higher than the SolarSpectrum 0.3 A.
No need to say that the tuning of the center wavelength of both etalons is a bit tricky. I am still probably a little bit away from the center of Ha here.

Here is an image of the North Pole :
Image

Somewhere on the disk :
Image

Somewhere else :
Image

I tried an animation over a 43 min period, with 1 frame /min. It is a very shaky. One image/30 s would certainly have been better. Still, it shows that even when there is nothing much on the Sun, it is still very active in Ha.

Clear skies !
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Re: Some tests with double stack etalons (mica & FS) and the C8 EdgeHD

Post by MalVeauX » Sat Feb 01, 2020 8:40 pm

Really nice Christian,

I wonder though, why the double limb persists with the two etalons?

Do you think this could be done with one etalon and a reasonably narrow blocking filter (1A to 3A)?

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Re: Some tests with double stack etalons (mica & FS) and the C8 EdgeHD

Post by christian viladrich » Sat Feb 01, 2020 8:49 pm

This is an interesting question Marty.

Indeed, the double limb effect depends on the rejection of the photospheric light and also on the resolution of image.

The double limb effect is less visible when the resolution is lower (the contrast of the photopheric limb is lost). There are some very interesting images taken by professionals (and large telescopes) where the double limb effect is still visible while using narrow bandpass.
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Re: Some tests with double stack etalons (mica & FS) and the C8 EdgeHD

Post by MalVeauX » Sat Feb 01, 2020 8:56 pm

Thanks Christian, the resolution you have there is truly impressive. It's why I wondered, versus how tricky it is to tune two etalons together in such a configuration to begin with, under such resolution. Must have been incredible seeing! Thanks for sharing!

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Re: Some tests with double stack etalons (mica & FS) and the C8 EdgeHD

Post by Astrophil » Sat Feb 01, 2020 10:13 pm

Interesting test and superb images, Christian.

Phil

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Re: Some tests with double stack etalons (mica & FS) and the C8 EdgeHD

Post by Bob Yoesle » Sun Feb 02, 2020 6:24 am

Hi Christian,

Very nice images! ;-)

I'm thinking one or both of the filters is slightly off-band, and hence the double limb is still present. Alternatively, even at f35 you may not be getting the theoretical 0.5A and 0.6A FWHM of either filter, especially if your telecentric optics are not optimized for the C8 Edge HD.

Frankly, I suspect some professional astronomers wouldn't necessarily be that concerned or perhaps astute about the dialing in of their filters to notice they aren't exactly on-band either, or that their equipment might not be properly optimized. At these narrow bandpasses, even minor mis-adjustments might affect tuning, etc.

In my experience, two ~ 0.7A FWHM SM90 filters have enough resolution to show little if any hint of parasitic continuum leakage/double limb:

Coronado DS BF30 181458.jpg
Coronado DS BF30 181458.jpg (916.78 KiB) Viewed 456 times

I also note the tell-tail hi-res "spiky" appearance of the dark mottles on your images of the chromospheric disc, similar to the image below + 0.5 A off-band:

Bray & Loughhead.jpg
Bray & Loughhead.jpg (397.26 KiB) Viewed 456 times

Note also the dark narrow band lying right above the photospheric limb in your first image. This is very similar to that noted by Bray and Loughhead for the - 0.75A image in Plate 2.8 (a) below:

Bray & Loughhead2.jpg
Bray & Loughhead2.jpg (356.44 KiB) Viewed 450 times

Bray and Loughhead, The Solar Chromosphere, 1974.
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Re: Some tests with double stack etalons (mica & FS) and the C8 EdgeHD

Post by marktownley » Sun Feb 02, 2020 7:38 am

Excellent image Christian, I agree with Bob, you're slightly in the red - not that this detracts from your images, I think they look great. Any chance of a few pickies of the Coronado etalon, not come across one before.

Useful info too thanks Bob.
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Re: Some tests with double stack etalons (mica & FS) and the C8 EdgeHD

Post by MAURITS » Sun Feb 02, 2020 10:02 am

Unbelievable beautiful images Christian.
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Re: Some tests with double stack etalons (mica & FS) and the C8 EdgeHD

Post by MapleRidge » Sun Feb 02, 2020 1:05 pm

Christian...

The images you posted through the C8 Edge look amazing...be they a tad off band or not ;)

The additional images and comments in the thread are very informative too.

Brian
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Re: Some tests with double stack etalons (mica & FS) and the C8 EdgeHD

Post by christian viladrich » Sun Feb 02, 2020 7:15 pm

Hi guys,

Thanks for your interesting comments.

Here are some examples of solar limb images taken with large professional telescopes.

The first one is from the VTT, with a double-stacked etalons in collimated beam.The FWHM is 0.043 A (no mistake ...):
Image
Source : K.G Puschmann et al, The new Göttingen Fabry-Pérot spectrometer for two-dimensional observation of the Sun. A&S 451, 2006.

There is still some hint of the solar limb at the line center.

The CRISP double-stack etalon (on the STT) has less parasitic light, even if the FWHM is "only" 0.06 A (telecentric beam):
Image

The limb is very faintly visible at + / 0.6 A from the center line, and more readily visible at +/- 0.8A.

BTW, at F/35 and in a telecentric beam, the mica-spaced 0.6A has an effective FWHM of 0.65 A, while the effective FWHM of the 0.5 fused silica about 0.6A :
Image

At the end of the day, I'll have some more tests to do when the Sun is back and highher to actually tune this thing on the center line ;)
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Re: Some tests with double stack etalons (mica & FS) and the C8 EdgeHD

Post by eroel » Mon Feb 03, 2020 2:59 am

Christian:
Super images and information.
Thanks for sharing.
Eric.

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Re: Some tests with double stack etalons (mica & FS) and the C8 EdgeHD

Post by Valery » Mon Feb 03, 2020 3:23 am

Nice images, Christian!

BTW Can you, please, show us this Lunt etalon?


Thanks,

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Re: Some tests with double stack etalons (mica & FS) and the C8 EdgeHD

Post by Bob Yoesle » Mon Feb 03, 2020 4:18 am

Very nice references Christian :-)

Bray and Loughhead noted the photospheric limb begins to manifest itself at +/- 0.65A off the center line, which seems to be essentially confirmed and refined by the higher resolution SST CRISP double stack to be +/- 0.6A. They also noted - and the CRISP images clearly show - that the "general chromosphere extends above the average spicules... Thus the popular view that spicules rise above the chromosphere is incorrect, except insofar as the macrospicules are concerned." Therefore, it is wholly inaccurate to describe the double limb of an on-band image as the "spicule layer."

The lower resolution VTT "line center" image indeed seems to be at odds with the other two sources in showing the narrow band at the photosphere limb...?

Here's a link to an IOM FS SMn series filter - and interesting related discussion:

https://www.cloudynights.com/topic/5970 ... try8244890
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Re: Some tests with double stack etalons (mica & FS) and the C8 EdgeHD

Post by george9 » Mon Feb 03, 2020 4:18 am

Those are awesome images, Christian! The annotation says 0.4 arcsec seeing. If you commonly get that kind of daytime seeing, I wonder what a C14 would show.

I agree that with those two filters double stacked and centered, you should eliminate the double limb. If not, then something else is going on. Like leakage on a side spike through the blocking filter. If the space between spikes (FSR?) on one filter is narrower than the blocking filter is made for, then high transmission on that one could get through the high shoulders of the other enough to cause a double limb. Not sure what the spacing is for the Coronado SMN35.

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Re: Some tests with double stack etalons (mica & FS) and the C8 EdgeHD

Post by Montana » Mon Feb 03, 2020 12:19 pm

These are superb images Christian, I am really looking forward to seeing more from this pair in the future. Lovely crisp images too :hamster: :bow

Alexandra


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Re: Some tests with double stack etalons (mica & FS) and the C8 EdgeHD

Post by george9 » Wed Feb 05, 2020 12:31 pm

george9 wrote:
Mon Feb 03, 2020 4:18 am
I agree that with those two filters double stacked and centered, you should eliminate the double limb. If not, then something else is going on. Like leakage on a side spike through the blocking filter. If the space between spikes (FSR?) on one filter is narrower than the blocking filter is made for, then high transmission on that one could get through the high shoulders of the other enough to cause a double limb. Not sure what the spacing is for the Coronado SMN35.
Oops. Never mind about this idea. Going through some old correspondence, I realize that the SMN35 is a complete unit, like a DayStar or a SolarSpectrum, so the correct blocking filter is integrated into it. The owners were very happy with it.

George

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Re: Some tests with double stack etalons (mica & FS) and the C8 EdgeHD

Post by Martin_S » Wed Feb 05, 2020 1:12 pm

Great Images Christian, actually the whole thread is amazing I really appreciate all the technical information, many thanks for all this information Christian and Bob.

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Re: Some tests with double stack etalons (mica & FS) and the C8 EdgeHD

Post by christian viladrich » Mon Feb 10, 2020 8:03 pm

Thanks to all :-)

Here are some pictures of the SMN35 :
Image
- on the left, the fused silica etalon in its oven,
- on the center, the ITF filter (from Beloptik),
- on the right, the BF with 2 A FWHM on a very astute rotating tilt system.

When stacked on the DayStar 0.6A, I use only the fused silica etalon part (left side).

Georges, the FSR of the etalon is 10 A. Accordingly, the spikes of transmission are blocked by the DayStar etalon and its BF (well, I guess so ...).

In this view, we can see how transparent is the fused silica etalon. It is difficult to make the difference between the window plates and their reflections.
Image

The tricky thing is that the temperature setting is through a very small screw that is difficult to access, and without any reference position ...
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Re: Some tests with double stack etalons (mica & FS) and the C8 EdgeHD

Post by george9 » Tue Feb 11, 2020 5:34 am

Christian, yes my original blocking filter (for an ASP60) was also 2A (Prom15T), not because it needed 2A but I think because they simply repurposed their old prom filter. Nevertheless, if after tuning you still see a double limb, I would just try the 2A blocking filter and see if it eliminates it. (I am assuming that each one individually looks about right.)

Of course the image looks great even with the limb.

George

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Re: Some tests with double stack etalons (mica & FS) and the C8 EdgeHD

Post by Valery » Wed Feb 12, 2020 1:50 am

Christian,

I believe you can do the following - use only the secondary etalon with it's BF and Trimmer and in combination with a telecentric you do use. Adjust the the image to the best contrast (or better for the lowest brightness with histogram as a reference). Then install the secondary etalon behind the main DayStar etalon and adjust the DayStar for the maximal brightness in the DS combination. You should be really very close to the best match of the two etalons CWLs.

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