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question about H-alfa filter

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#1 philipdo

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Posted 30 October 2012 - 02:23 PM

Do you guys think that a Baader Ha filter, 1 nm (10 A), dating back from 1992 would still be usable in a coronagraph? The previous owner had not used the coronagraph for a couple of years but he claims that these Baader filters were specially treated against deterioration from age.

I e-mailed Baader about this and they responded that if the inner structure of the filter is still homogeneous, it would be OK. When I look at the sun, I can observe perfectly sharp images of sunspots through the filter, so I assume the inner structure of the filter is still OK, or am I wrong? Any thoughts?

Clear skies

#2 DAVIDG

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Posted 30 October 2012 - 03:59 PM

When one of these filters goes bad, it starts from the outer edge and works toward the center. So if you see any difference in the appearance of the coatings especially toward the edge then the filters has gone bad or is going bad. You have to be careful because a filter this old could have had the coatings fully oxides and look uniform. Here is a picture of a filter that is going bad.
You can purchase a new filter from Omega Optics on Ebay that have narrower bandwidths then the original Baader unit. One with 1.5A bandpass will shows the prominences is much higher contrast against the background sky then the original 10A (1nm) unit.

- Dave

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#3 philipdo

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Posted 30 October 2012 - 04:22 PM

Thanks for the info, Dave. The red side of the filter looks even with no degradation around the edge. On the shiny, reflective side, the edge looks a bit duller than the center. Maybe it is starting to degrade. I have a friend with a spectrometer. I will ask him to measure the exact transmission.

philip

#4 DAVIDG

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Posted 30 October 2012 - 04:38 PM

Philip,
Be sure to have the filter scanned from the UV into the IR to be sure it is still safe. My guess is that the filter was made to transmitt around 657nm or slightly higher, maybe as high as 660nm and you tuned it to the H-alpha line at 656.28nm by tilting it. Usually as the filter ages it drifts toward shorter wavelengths and once it passes the H-alpha frequency you can no longer tune it to see prominences. Ten years is about the average life time for one of these filters.

- Dave

#5 philipdo

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Posted 30 October 2012 - 05:47 PM

Dave,

the spec sheet says 656.4 nm. I did not know you had to tilt the filter in a coronagraph. I'm getting convinced the filter has become useless by it's age. I Googled Omega Optical, but could not find any H-alfa filters, not even on the Bay.

philip

#6 DAVIDG

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Posted 30 October 2012 - 07:17 PM

Philip,
The tuning depends on the center wavelength, and the bandwidth of the filter and also on the Doppler shifts of a prominences. Since the center bandwidth of your filter is 656.4nm with band width of 1nm you should not have to tilt it. One my Criterion coronagraph the filter had a center bandwidth of 660nm so I have to tilt it to bring it on-band so the prominence become visible. On my homemade coronagraph the filter had a center bandwidth of 656.3nm so I didn't have to tilt it.
Here is a link to an Omega H-alpha filter on Ebay that might work for you. Omega filter


- Dave

#7 nytecam

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Posted 01 November 2012 - 03:07 AM

There is some confusing over the term "H-alpha filter" which come in two types - 10A for isolating and recording deepsky H-alpha nebula and <1A for viewing/recording solar phenomena eg prominences and disk detail.

A bandpass of 10A would be too wide to view solar H-alpha phenomena. The H-alpha line itself is <1A wide :o Tuning, by tilting the filter, may bring H-alpha to the 'centre' but the view will still be swamped by the excessive bandpass and be potentially unsafe. Personally I'd never contemplate looking at the sun with a 10A bandpass H-alpha filter even in a Coronagraph [artificially eclipsing the sun's disk] unless the scope guidance system perfectly tracked the sun :o



#8 philipdo

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Posted 01 November 2012 - 05:16 AM

I tend to disagree on this one, Nytecam. I've looked through several coronagraphs in the past, all clearly showing prominences, equipped with 10 A bandpass filters. Using a 0.5 or 0.7 A filter would simply eliminate the need for a coronagraph. They would show the solar disc and the prominences in H-Alpha light, without an occulting cone. I've just ordered a new filter with 1.5 A bandwidth. This should even increase the contrast of the prominences. When the filter arrives, I will report further on the results. I agree on the need for a perfectly tracking mount to keep the sun occulted. When perfectly aligned, most mainstream mounts will do.

Kind regards,

philip

#9 DAVIDG

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Posted 01 November 2012 - 08:49 AM

I tend to disagree on this one, Nytecam. I've looked through several coronagraphs in the past, all clearly showing prominences, equipped with 10 A bandpass filters. Using a 0.5 or 0.7 A filter would simply eliminate the need for a coronagraph. They would show the solar disc and the prominences in H-Alpha light, without an occulting cone. I've just ordered a new filter with 1.5 A bandwidth. This should even increase the contrast of the prominences. When the filter arrives, I will report further on the results. I agree on the need for a perfectly tracking mount to keep the sun occulted. When perfectly aligned, most mainstream mounts will do.

Kind regards,

philip


I agree with Philip, a 10A bandpass H-alpha filter when used with a coronagraph will show prominences in great detail and contrast. I designed an built my own coronagraph using a 10A filter and I taught a class were we made 20 of them in a weekend. http://www.considine...oski/promscope/
Both Baader and Criterion sold commerical units that used 10A filters.
If your optics are designed to be very scatter free and the air is very clear, a simple red filter will allow one to see prominence when used with a coronagraph.
If the filter is designed to block UV and IR, even if the tracking isn't perfect, there is no danger if the Sun comes out from behind the occulting disk. One just see a red image of the Sun without the prominence being visible.

- Dave

#10 highfnum

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Posted 01 November 2012 - 06:00 PM

I built a David's corona graph with some minor mods
It's a thing of beauty works like a charm
In my case I use with a 83mm lens
When conditions are good prom detail is better than 60 mm dedicated solar scope

#11 highfnum

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Posted 02 November 2012 - 11:58 AM

Would be nice to go in a balloon to lets say 60000 feet and try corona graph

#12 philipdo

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Posted 02 November 2012 - 12:33 PM

Highfnum,

What type of objective do you use, a single lens or an achromatic doublet?

#13 highfnum

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Posted 02 November 2012 - 01:38 PM


83mm doublet

#14 DAVIDG

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Posted 02 November 2012 - 01:40 PM

In terms of the least amount of scatter, a singlet is better then a doublet. This why a true coronagraph used for viewing the solar corona uses a singlet for the objective. Optically, the issue with a singlet is that with spherical curves it can not be fully corrected for both spherical aberration and coma at the same type. The Sun is 1/2 degree in diameter so it presents a large image and one is viewing the edge of the disk which needs to be kept center so it is eclipsed by the occulting disk so off axis coma is a concern in degrading the image quality. What is done is the singlet is designed to be fully corrected for coma and then one of the surfaces is aspherized to correct for spherical. In a doublet design, both coma and spherical aberration can be corrected for using spherical curves. This why many commerical H-alpha that are operating using monochrome light uses an achromat doublet as the objective. It is not for the color correction but for the fact that coma and spherical aberration or fully corrected for using spherical curves on the surfaces of the elements. It is also cheaper to produce a two element objective with spherical curves then a singlet with an aspheric curve.

- Dave

#15 highfnum

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Posted 02 November 2012 - 01:51 PM

Doublet works fine for me

#16 philipdo

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Posted 02 November 2012 - 03:01 PM

I think I can't go wrong with my 70/1000 mm Lichtenknecker doublet. In the meantime I have ordered a new H-alpha filter from Omega to replace the Baader filter that went out of tune due to its age. Hope it gets here soon.

#17 DAVIDG

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Posted 02 November 2012 - 03:22 PM

For H-alpha work with prominences a doublet will work well. A singlet would be useful if you wanted to try to observe the actual corona, in which case one needs the absolute least amount of scatter in the optics and in the atmosphere.
By the way you can observe prominences in other wavelengths beside H-alpha, like the H-beta and/or Calcium K lines. So if you can find filters for those wavelengths, it might make for some interesting observations as well.

Philip,
Do you have occulting disks of different sizes for the changing diameter of the Sun thru out the year ?


- Dave

#18 philipdo

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Posted 02 November 2012 - 06:00 PM

Dave,

the coronagraph came with four occulting cones of different diameters. I've seen that some amateurs work with only one smaller cone to deflect the sunlight, fitted with different diameter disks. No disks here but different cones instead.

#19 highfnum

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Posted 02 November 2012 - 06:47 PM

Dave do you know where to get h-beta

#20 DAVIDG

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Posted 03 November 2012 - 10:24 AM

Dave,

the coronagraph came with four occulting cones of different diameters. I've seen that some amateurs work with only one smaller cone to deflect the sunlight, fitted with different diameter disks. No disks here but different cones instead.


Hi Philip,
As you know the cones are sized for the changing diameter of Sun through out the year (Summer Winter, Spring and Fall) with the Winter one being the largest. They are sized for the solar image that your telescope produces so if you decided to use the coronagraph with different focal length objective you need to make new cones.
Criterion made a 100mm coronagraph that had some very nice features. One "trick" they did was to use only one occulting cone and installed a barlow lens in front of it. The barlow was adjustable in it's position, which allowed the magnifaction of it to be easily changed. So by adjusting it, you could match the size of the solar image to the occulting cone exactly no matter what day of the year it was and you never had to change the occulting cone.
Could you post some pictures of your unit ? I've never seen your model before.

All the Best,
- Dave

#21 DAVIDG

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Posted 03 November 2012 - 10:31 AM

Dave do you know where to get h-beta


John,
I haven't found a H-beta that would be narrower enough. All the 'DeepSky' type have a bandpass of 10nm (100A) or more but you might try looking for a filter designed for an Argon laser which is 488nm vs 486nm for the H-beta. When you tilt the filter it will shift toward shorter wavelengths and transmitt the H-beta wavelength. Here is a listing on Ebay from Omega optics for a narrow band 488nm filter with 1nm bandwidth. You'll need to be sure that it doesn't transmitt UV and IR or add addition filter to make safe but with coronagraph it might work.
Omega Filter

All the Best,
- Dave

#22 philipdo

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Posted 03 November 2012 - 12:58 PM

Dave,

I like the idea with the barlow. I will post some pictures later on, when I get the new filter. It's an old homemade scope, made by Dany Cardoen of Puimichel, France. Probably from his early days in France, somewhere around 1983.

#23 philipdo

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Posted 12 November 2012 - 12:33 PM

Last week I received my Ha-filter 0,15 nm / 1,5 A from Omega Optical. Guess what, it has been rainy and overcast since. A few short sunny spells allowed me to quickly test the new filter. I liked what I saw: beautiful, contrasty prominences all around the solar limb, a beautiful arch... I was bothered however by some kind of internal reflection, probably due to the occulting cone that did not completely cover the sun's disc. With the appropriate cone, the reflection was gone. I've added some pictures of the coronagraph, hatch open, mounted to my Orion Atlas by means of a makeshift saddle.

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#24 philipdo

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Posted 12 November 2012 - 12:38 PM

close-up of the internals

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#25 highfnum

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Posted 12 November 2012 - 12:44 PM

cool nioe work






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