What does 656.281 nm look like?

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Astrophil
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What does 656.281 nm look like?

Post by Astrophil »

I’ve seen many H alpha images, some taken in the blue wing and some more towards the red, but I still can’t identify the center reliably. Here are five images spanning about 0.04 nm (estimated by temperature change of the etalon) and the center wavelength should be in there somewhere. The temperature is shown in the upper left corner of each image. This SS etalon shows an Op. Temp of 55 on the label and that’s where I usually image. I’m not so sure anymore and would like some experienced opinions.

This was not a scientific test. Seeing was variable, small regions of blue sky between fast moving clouds limited lengths of some videos stacked, some flats were applied, and the processing varied. However, I would suspect the image closest to the center wavelength could be determined.

Thank you in advance.

Phil
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Re: What does 656.281 nm look like?

Post by george9 »

Others are more expert than me, but it looks like you may need to go to higher temperatures and then work your way back. Based on the pattern of spots, 53.5 seems off band, but on the other end, 57.2 seems near the center line.

George


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Re: What does 656.281 nm look like?

Post by Astrophil »

Thanks George, maybe I quit too soon. I couldn't tell until they were processed and I tend to agree with your assessment.

Phil


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Re: What does 656.281 nm look like?

Post by Valery »

There is a simple and correct explanation of such event.

All etalons are qualified when they work in a parallel beam - the only correct way of test.

But in a telescope with telecentric system all rays, except the extreme center, have a tilt. The farther from the center of an aperture, the more the tilt. In your telescope the aperture center does not work at all - due to the central obstruction. Rays tilt results CWL shifts blue. To compensite this CWL shifts an etalon need to be heated higher. And we see that this works exactly as it must do. And the faster your F/D, the more correction will require.

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Re: What does 656.281 nm look like?

Post by Astrophil »

Thanks Valery. That makes sense but will that also broaden the bandwidth? Is this a limitation of this type of telescope in solar imaging?

Phil


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Re: What does 656.281 nm look like?

Post by Valery »

Astrophil wrote: Wed Jan 23, 2019 3:48 am Thanks Valery. That makes sense but will that also broaden the bandwidth? Is this a limitation of this type of telescope in solar imaging?

Phil
Yes, this broadens the bandwidth. The narrower the bandwidth, the more _relative_ broadening. The faster the F/D, the more broadening.

At F/30 your system really works as 0,43 - 0,45A.
I would advice you to work at F/50 and use faster focal reducer. Your system will work at about 0.33A.


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Re: What does 656.281 nm look like?

Post by Astrophil »

Valery,

Good information. I will try a 2.5x Powermate and aperture of 5" for f45. The focal reducer can be relocated to provide a greater reduction. Would it be better to double stack with the Quark Combo?

Phil


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Re: What does 656.281 nm look like?

Post by Valery »

Astrophil wrote: Wed Jan 23, 2019 5:22 am Valery,

Good information. I will try a 2.5x Powermate and aperture of 5" for f45. The focal reducer can be relocated to provide a greater reduction. Would it be better to double stack with the Quark Combo?

Phil
I'd go with PST etalon for DS with Solar Spectrum. PST etalon behavior with tilt change is MUCH more foreseeable. So, you will work faster with better results. Also, PST etalons usually are much more luminous than Quarks. The most important for your DS PST etalon is it's higher luminocity and uniformity. I do use Quark with PST for my DS images and they show contrast I never saw around except professional images of 0.1A class.


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Last edited by Valery on Thu Jan 24, 2019 6:18 am, edited 1 time in total.


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Re: What does 656.281 nm look like?

Post by christian viladrich »

Hello phil,

There are 3 criteria which may help to find the center of the band :
- measure the average intensity of the image => the darkest image is the one on the center of the band ;
- the short dark "little things" coming in bunches are groups of spicules seen from above. They are at the border of the surpergranular cells. If you look carefully at your images, you will see that the lenght of these short black hairs increases from one image to the next. This is because there are the longest when the filter is on the band.

So basically, the last image is the closest to the center of the band. You should try to increase the temperature futher.

Now the last criteria :
- if T is a temperature "on the band", then the images at T + dt and T - dt look the same. If not, it means that T is not on the band.

Allow 10- 15 min stabilisation when changing temperature setpoint in order to be sure the filter is stabililized.

Taken note of the F/D since the CWL changes (a little bit) with the F/D.


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Re: What does 656.281 nm look like?

Post by christian viladrich »

BTW, the Televue X2.5 is not telecentric. It is much better to use a telecentric system.


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Re: What does 656.281 nm look like?

Post by Astrophil »

Christian,

That is extremely helpful, thank you. I will continue increasing the temp and look for improvements. This is much less subjective than just guessing at maximum contrast. I will post additional images for confirmation when testing allows.

Phil


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Re: What does 656.281 nm look like?

Post by Astrophil »

I just finished four more images taken before reading Christian”s criteria for finding the center of the band but think they still might be somewhat informative. The images were taken with increasing etalon temperatures starting where i left off from the previous tests. I’m guessing about 58º might be close? Closer than the temperatures I have been using to image in the past. I welcome your thoughts.

Phil
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Re: What does 656.281 nm look like?

Post by Valery »

Astrophil wrote: Thu Jan 24, 2019 4:07 am I just finished four more images taken before reading Christian”s criteria for finding the center of the band but think they still might be somewhat informative. The images were taken with increasing etalon temperatures starting where i left off from the previous tests. I’m guessing about 58º might be close? Closer than the temperatures I have been using to image in the past. I welcome your thoughts.

Phil

IMHO, 57.5 is the closest to the H-a core. More details here.


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