Help with Solar Spectrum etalon

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DavidP
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Help with Solar Spectrum etalon

Post by DavidP »

So I have a new SS RG 18mm aperture 3A filter. As of now, I have it on a 102mm refractor at f/32 using a Baader 4x telecentric. I believe I have the spacings correct with the telecentric. I have been trying to use a .5x reducer ( just an inexpensive GSO). With this, I'm getting terrible vignetting .
2021-02-07-1536_6-U-L_lapl3_ap4597resized.jpg
2021-02-07-1536_6-U-L_lapl3_ap4597resized.jpg (15.5 KiB) Viewed 1410 times
At first, I thought maybe it was a result of using a 1.25" reducer. but a 2" produced the exact same results. So why the vignetting?
Another question I have is where the focal point produced by the telecentric should be placed when a focal reducer is used. At the reducer, or at the sensor? I tried both, but it made no difference in the vignetting. Strange that the vignetting is not concentric to the FOV. On one side the non-vignetted image continues off the side of the image.

So here is an image at f/32 no reducer. It is way oversampled, not good seeing, and no processing, but it shows unevenness which I am supposing is from part of the image being off band. Opinions?
2021-02-07-1604_1-U-L_lapl4_ap8359resized.jpg
2021-02-07-1604_1-U-L_lapl4_ap8359resized.jpg (9.36 KiB) Viewed 1410 times
Last edited by DavidP on Mon Feb 08, 2021 2:12 am, edited 4 times in total.


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Re: Help with Solar Spectrum etalon

Post by EGRAY_OBSERVATORY »

David, Can you attach the/an image as a JPEG/.jpg please as would allow us to comment further...

TIFF not accepted on SolarChat.

Thanks
Terry


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Re: Help with Solar Spectrum etalon

Post by marktownley »

What camera David?

Vigentting is normal. I get it with my 9.1Mp Flir and also the IMX174 Flir when I use the 0.7x Baader solar telecompressor.

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Re: Help with Solar Spectrum etalon

Post by christian viladrich »

Hello,
What camera are you using ?
With a 18 mm free aperture, you get about a 18 x 0.5 = 9 mm field on the camera sensor with the x0.5 reducer. On top of that, the reducer introducies vigneting.


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Re: Help with Solar Spectrum etalon

Post by DavidP »

Ok. So this is normal.
Does one just crop the unvignetted part of the image with ROI during capture?
I've tried both a IMX183 and a IMX253 which has a 1.1" sensor and 3.45 um pixels.
What concerns me is the fact that the image with the reducer does not vignette symmetrically. I don't know if you can see from the image, but the unvignetted part trails off on side of the sensor. The vignetting is in the shape of a "C" rather than an "O". Could this issue be related to why the image without the reducer has one side that seems off band?
My image train seems pretty stiff.


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Re: Help with Solar Spectrum etalon

Post by marktownley »

I found the 0.5x reducers to be junk tbh, hence me using the baader solar telecompressor.

Like Christian says, if the chip is bigger than the 'light cone' any part of the chip not illuminated will be dark.

Are you tilting the camera? This may account for the asymmetry.

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Re: Help with Solar Spectrum etalon

Post by christian viladrich »

I would say :
- remove the reducer and don't use any (I have one to sell ...)
- use bining 2x2 during acquisition. The pixels on these cameras (2.4 micron and 3.45 micronsà are way too small for this setup (f/32).
BTW, an ASI174 would be a good match.
When done, we could see if there is still an issue or not with uniformity of field.


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Re: Help with Solar Spectrum etalon

Post by DavidP »

Good advice. Thanks.
I do plan to buy a IMX174 camera. Trying to decide whether the increased cost over of a Basler over an ASI is justified. I figured my 3.45 micron camera would do well at f/16.
I’ll try binning, and post some images


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Re: Help with Solar Spectrum etalon

Post by marktownley »

DavidP wrote: Mon Feb 08, 2021 10:46 pm Trying to decide whether the increased cost over of a Basler over an ASI is justified.
You pay for what you get - There is much less noise with the Basler.


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Re: Help with Solar Spectrum etalon

Post by Ljungmann »

Hi David!
The best way to test your system is to look at the Sun visually. You will see if your system is uniform. And it will show you if you have tilt problems in the system. A 0.3A filter is extremely demanding if you don't have control of the f32 all the way to the eyepiece/camera.


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Re: Help with Solar Spectrum etalon

Post by DavidP »

Thanks Lars. That’s good advice.
I just need some clear weather. The vignetting with the reducer made things kind of confusing. I’ll try it at f/32 visually. So much to consider when assessing issues. Oven temp, the 4x telecentric, correct spacing, rigid optical train, seeing...Taking the camera and software out of the equation, is a good start

Mark. I generally agree.


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Re: Help with Solar Spectrum etalon

Post by Ljungmann »

Hi David
Forgot to say congratulations with your new filter. I have the same filter from SS and a Quantum 0.6A. The SS filter is an extremely good filter. My filter is on band right out of box. I use it with a Baader 8" Triband and a 3x Baader telecentric and a 0.4x Baader reducer a Takahashi 1.25" prism diagonal and a 32 or 40 mm TeleVue plossl.


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