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philipdo
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question about H-alfa filter
      #5496373 - 10/30/12 03:23 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


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DAVIDG
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Re: question about H-alfa filter new [Re: philipdo]
      #5496506 - 10/30/12 04:59 PM Attachment (45 downloads)

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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philipdo
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Re: question about H-alfa filter new [Re: DAVIDG]
      #5496543 - 10/30/12 05:22 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


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Re: question about H-alfa filter new [Re: philipdo]
      #5496567 - 10/30/12 05:38 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


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philipdo
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Re: question about H-alfa filter new [Re: DAVIDG]
      #5496667 - 10/30/12 06:47 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


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Re: question about H-alfa filter new [Re: philipdo]
      #5496810 - 10/30/12 08:17 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


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nytecam
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Re: question about H-alfa filter new [Re: philipdo]
      #5498898 - 11/01/12 04:07 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 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



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philipdo
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Re: question about H-alfa filter new [Re: nytecam]
      #5498947 - 11/01/12 06:16 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


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Re: question about H-alfa filter new [Re: philipdo]
      #5499095 - 11/01/12 09:49 AM

Quote:

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.net/dgroski/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


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highfnum
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Re: question about H-alfa filter new [Re: DAVIDG]
      #5499740 - 11/01/12 07:00 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


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highfnum
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Re: question about H-alfa filter new [Re: highfnum]
      #5500919 - 11/02/12 12:58 PM

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

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philipdo
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Re: question about H-alfa filter new [Re: highfnum]
      #5500976 - 11/02/12 01:33 PM

Highfnum,

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


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highfnum
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Re: question about H-alfa filter new [Re: philipdo]
      #5501069 - 11/02/12 02:38 PM


83mm doublet


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Re: question about H-alfa filter new [Re: philipdo]
      #5501072 - 11/02/12 02:40 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


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highfnum
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Re: question about H-alfa filter new [Re: DAVIDG]
      #5501086 - 11/02/12 02:51 PM

Doublet works fine for me

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philipdo
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Re: question about H-alfa filter new [Re: highfnum]
      #5501163 - 11/02/12 04:01 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.

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Re: question about H-alfa filter new [Re: philipdo]
      #5501197 - 11/02/12 04:22 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


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philipdo
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Re: question about H-alfa filter new [Re: DAVIDG]
      #5501389 - 11/02/12 07:00 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.


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highfnum
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Re: question about H-alfa filter new [Re: philipdo]
      #5501462 - 11/02/12 07:47 PM

Dave do you know where to get h-beta

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Re: question about H-alfa filter new [Re: philipdo]
      #5502240 - 11/03/12 11:24 AM

Quote:

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


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Re: question about H-alfa filter new [Re: highfnum]
      #5502245 - 11/03/12 11:31 AM

Quote:

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


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philipdo
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Re: question about H-alfa filter new [Re: DAVIDG]
      #5502454 - 11/03/12 01:58 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.


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philipdo
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Re: question about H-alfa filter new [Re: philipdo]
      #5515924 - 11/12/12 12:33 PM Attachment (36 downloads)

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.

Edited by philipdo (11/12/12 12:37 PM)


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philipdo
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Re: question about H-alfa filter new [Re: philipdo]
      #5515934 - 11/12/12 12:38 PM Attachment (17 downloads)

close-up of the internals

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highfnum
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Re: question about H-alfa filter new [Re: philipdo]
      #5515951 - 11/12/12 12:44 PM

cool nioe work

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Re: question about H-alfa filter new [Re: highfnum]
      #5515979 - 11/12/12 01:07 PM

Philip,
Nice work and glad to hear the new filter is working well. I found on my unit that if I polish the occulting cones to a mirrored finish and then blacken just the very edge of the cone that I didn't get any stray reflections.
Does you unit have an adjustable iris in the light path between the first lens which the cones is attached and the second rear relay lens ? If not installing one will also help in removing any stray reflections.
Are you able to offset the eyepiece so you can place the edge of the cone with a prominences visible in the center of the field of the eyepiece ?

- Dave



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philipdo
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Re: question about H-alfa filter new [Re: highfnum]
      #5515980 - 11/12/12 01:07 PM

Thanks Highfnum,

all I did was restore the coronagraph. It was originally built in the early eighties by Belgian/French amateur Dany Cardoen, founder of the Puimichel observatory in France. He quickly turned professional and makes optics for ESA, ESO and other institutions.


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philipdo
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Re: question about H-alfa filter new [Re: DAVIDG]
      #5515987 - 11/12/12 01:12 PM

Thanks Dave,

blackening the edges of the cones seems like a good idea, especially since I already blackened other surfaces (lens edges, lens mountings...). I have already added an adjustable iris diaphragm. There is no provision for offsetting the eyepieces. I'm tinkering about the construction of a simple excenter to take care of this.


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nytecam
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Re: question about H-alfa filter new [Re: philipdo]
      #5517243 - 11/13/12 03:39 AM

Thanks Phil and Dave - I bow to your superior knowledge and experience. I was being over cautious on coronagraph optics

I visited the late Horace Dall's rooftop workshop many decades ago with all his wonderful gear including his Promscope with solar occulting disk and wideband H-alpha filter but didn't view through it as it was cloudy

David/Phil - could you clarify - is a 'Coronagraph' for viewing the solar corona [with a coronal line filter under exceptionally clear skies] or does the term extend to the chromosphere and H-alpha phenomena


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Re: question about H-alfa filter new [Re: nytecam]
      #5517518 - 11/13/12 09:13 AM

Nytecam,
One of the references I used when building my coronagraph/ promscope was an article by Horace Dall. The optics in a Promscope and Coronagraph are the same. The design was invented by Lyot. The problem with viewing the corona is that it requires very clear and scatter free skies hence the need to observe on of a top of mountain or from a ballon, even with a narrow band filter. When you add a narrow band H-alpha interference filter, one can now observe prominence will less then ideal sky conditions because the Lyot optics greatly reduce the scatter and the filter does the rest and also the fact that the prominence are very bright at the H-alpha wavelength. Under very favorable conditions one can observer prominences using a coronagraph with just a simple red glass filter.

- Dave


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philipdo
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Re: question about H-alfa filter new [Re: DAVIDG]
      #5517870 - 11/13/12 12:47 PM Attachment (12 downloads)

@Nitecam
As Dave said. Perhaps with one exception: it is easier to see the corona during a total eclipse, since we are in a rather large shadow cone, with no diffraction or scattering at all. Even the best constructed coronagraph barely covers the solar disk, causing small amounts of light scattering. The corona is hard to see because the chromosphere (with prominences) has an intensity about 1,000 times less than the the surface of the sun (photosphere). The corona has an intensity of 1,000 less than the chromosphere...

@Dave
I solved the excenter problem, not by building an eccentric mechanism, but by making an eccentric eyepiece. When I turn it around in the eyepiece holder, I am able to scan the entire limb of the sun with a magnification of about 130.


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Re: question about H-alfa filter new [Re: philipdo]
      #5517951 - 11/13/12 01:26 PM

Philip,
I was going to sugguest something very similar. Great job on your eyepiece and on the restoration of your coronagraph.
One of my farther modifications to my unit will some day be to add a filter wheel so I can observe the prominences in different wavelengths like the H-Beta, CaK and Helium D lines. The problem is finding the filters of narrower enough bandpass for those wavelengths.
I'm restoring an orginal Hale spectrohelioscope, exactly like what is pictured in ATM-1 so I hope to have it working soon so I can observe the Sun in any wavelength I wish.

All the Best,
- Dave


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philipdo
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Re: question about H-alfa filter new [Re: DAVIDG]
      #5517988 - 11/13/12 01:48 PM

Dave,

how did you find such a fine instrument as a Hale spectrohelioscope? Any pictures?


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highfnum
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Re: question about H-alfa filter new [Re: philipdo]
      #5518181 - 11/13/12 03:46 PM

"but by making an eccentric eyepiece"
oh so clever - I gota make one of those!

philipo - yopu gotta get a baloon and get uo to about 60K feet

Edited by highfnum (11/13/12 03:49 PM)


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philipdo
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Re: question about H-alfa filter new [Re: highfnum]
      #5518288 - 11/13/12 04:52 PM

Highfnum,

60K feet...Do you think the scope would qualify as airplane hand luggage ? For the eccentric eyepiece I used a singlet lens. There is no need for a wide field, since the eyepiece lens is almost nested against the inner wall of the barrel, thus already restricting the field of view.


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highfnum
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Re: question about H-alfa filter new [Re: philipdo]
      #5518511 - 11/13/12 06:55 PM

"I used a singlet lens." what dia and F.L in mm plz

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philipdo
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Re: question about H-alfa filter new [Re: highfnum]
      #5519263 - 11/14/12 09:46 AM

appr. 8 mm diameter and 10 mm FL

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Re: question about H-alfa filter new [Re: philipdo]
      #5519302 - 11/14/12 10:20 AM

One of the ways I was thinking of making eccentric adapter would be to use a 2" focuser and then make a 2" diameter plug that fits in the focuser and drill an 1.25" hole off center Then one could use any eyepiece you wish and rotate the plug with the eyepiece to view and area around the limb of the Sun.

- Dave


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Re: question about H-alfa filter new [Re: philipdo]
      #5519322 - 11/14/12 10:38 AM

Quote:

Dave,

how did you find such a fine instrument as a Hale spectrohelioscope? Any pictures?




The spectrohelioscope was installed in the Cook Observatory not to far from me. Here is a link to a PDF that shows the instruments installed at that observatory including the Hale spectrohelioscope that I'm restoring. It was then given to the University of Penn and after many years they no longer wanted it. Some of parts are missing. My good friend Matt Considine located it and the University of Penn asked he if would like it. Matt and I retrived it on one of hottest days of the years, a few year back from the 5th floor of the Physic building. Matt has donated it to Springfield Telescope Makers were it will be installed at Stellafane. Here is a link to a YouTube video that Matt gave about it and the plans to locate at Stellafane. http://www.youtube.com/watch?v=XrfdLIPSC34
I'm in the process of restoring it. It is about 80% done. Matt and I gave a talk this Summer for the Workshop at the Hartness House for the Antique Telescope Society on the progress so far in the restortation. Here is a link to a couple of pictures of the workshop http://www.flickr.com/photos/30623046@N08/7828173638/in/photostream/

- Dave


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philipdo
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Re: question about H-alfa filter new [Re: DAVIDG]
      #5519507 - 11/14/12 12:58 PM

I'm impressed Dave. What a project !

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highfnum
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Re: question about H-alfa filter new [Re: philipdo]
      #5521812 - 11/15/12 07:48 PM

is it true a spectrohelioscope can get down to .2A and even .1A resolution?

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Re: question about H-alfa filter new [Re: highfnum]
      #5522481 - 11/16/12 09:05 AM

Quote:

is it true a spectrohelioscope can get down to .2A and even .1A resolution?



Yes, it is a true spectrohelioscope and it is the exact same designed as published in Hale's article in ATM-1. It was commerically made by Howell and Sherburne. Here is a link to their sales booklet http://authors.library.caltech.edu/29194/1/The_New_Hale_Spectrohelioscope.pdf
I'm replacing the original 600 lines/inch grating with modern 1200 lpi one blazed at 550nm to improve the resolution. It's missing the Anderson prism assembly which I'm in the process of reproducing. I have the replacement prisms themselves.

- Dave


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highfnum
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Re: question about H-alfa filter new [Re: DAVIDG]
      #5522505 - 11/16/12 09:30 AM

link came up dead
i gotta get up to stellafane to see this thing once its working


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Re: question about H-alfa filter new [Re: highfnum]
      #5522514 - 11/16/12 09:37 AM

I built a 2400 line per mm spectroscope
a while back
took this shot

http://www.cloudynights.com/photopost/showphoto.php?photo=17725

1) will 2400lines per mm work for such a device
2) why are some lines "thicker" then others - H-a line is thicker than H-b line


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DAVIDG
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Re: question about H-alfa filter new [Re: highfnum]
      #5522522 - 11/16/12 09:44 AM

Quote:

link came up dead
i gotta get up to stellafane to see this thing once its working




Try the link again, it just worked for me. The server might have been down.

- Dave


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Re: question about H-alfa filter new [Re: highfnum]
      #5522544 - 11/16/12 09:57 AM

Quote:

I built a 2400 line per mm spectroscope
a while back
took this shot

http://www.cloudynights.com/photopost/showphoto.php?photo=17725

1) will 2400lines per mm work for such a device
2) why are some lines "thicker" then others - H-a line is thicker than H-b line




Yes a 2400 lines per mm grating will work well. The width of the lines is a function of the electron transistion in the atoms and also the Heisenburg Uncertainty Principle.

- Dave


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highfnum
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Loc: NE USA
Re: question about H-alfa filter new [Re: DAVIDG]
      #5522977 - 11/16/12 02:12 PM

do you guys know about Fredrick Veio work with spectrhelioscope's?

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DAVIDG
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Re: question about H-alfa filter new [Re: highfnum]
      #5523033 - 11/16/12 02:58 PM

Quote:

do you guys know about Fredrick Veio work with spectrohelioscope's?




Yes, very much so, there are references from me posted on his site and on the Yahoo spectrohelioscope group.

- Dave


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