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self made coronograph

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#26 DAVIDG

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Posted 03 February 2013 - 04:51 PM

Alex,
Be sure you check the off axis coma of your objective design. The Sun is 0.5 degree in diameter and since your viewing from the limb outward when the Sun is centered, coma will be an issue.
A double convex singlet can be designed to be "bent" to be coma free and then you aspherize either the front or the back to correct it for spherical aberration.
I chose to use an achromat, because it was both corrected for coma and spherical aberration across the field of view I needed.

- Dave

#27 neo

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Posted 03 February 2013 - 05:23 PM

Well, I set a 1.5 degree field and the off axis coma is well within the Airy disk.

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#28 neo

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Posted 05 February 2013 - 05:16 PM

The past few days I've tryed to fiddle a bit with the numbers (read 'stay up till 2am' :lol: ) in OSLO, with my little knowledge of serious optical design, to see how a coronagraph would look like with Phellipe's prescription.
Till now all I can say he's right on the money ;)

I've used the same 4" f/15 lens as shown above and added the field lens and the reimaging doublet (two of them).
I didn't use the diaphragm (its place is very easy to spot) partially becouse I didn't know how to introduse it in OSLO and secondly becouse I wanted to see what is the aberration response of the whole system. And the diaphragm doesn't quite form any image just stops the stray light from the edge of the main objective (maybe I'm wrong here :question: , I'm not sure).
The first impretion was of great amazement. How can one build such a delicat instrument and be abble to see anything just with off the shelf lenses? The whole graphs were all over the places. :confused:

So after struggling with numbers and compromises I've found out that Phillipe was so right in a number of places.
Fisrt of all the main lens, a PCX lens with a mild asphere on the convex side I've seen it before, it works.
Now the field lens; it also needs aspherization, a pretty strong one, like a secondary in a Cass becouse this is one way to keep the field curvature low. Also the distance between the oculting cone and field lens also determines the field radius. The same thing with the distance between the field lens and the reimaging doublets.
So you get three main ways of reducing the field curvature which have to be kept in equillibrum so the aberrations don't go up in the sky.

I've discovered that by using just one reimaging doublet was a total mess, at least with those I've found in OSLO's data base so I've used two facing the front lenses.
This way the aberrations were much lower and the advantage that the Ha filter can be sealed between the two lenses so no exposure to the elements. The doublets are 40mm/180mm.

It is a trully delicat instrument, everything must be mounted very precisely, just one milimeter off and it changes a lot on how the scope performs.

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#29 Ed Jones

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Posted 11 February 2013 - 05:36 PM

Question: Used as a coronograph is a more narrow bandpass preferable? Can a filter narrow enough enough to see disk prominences take spectacular shots like Philippe's?

#30 DAVIDG

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Posted 12 February 2013 - 09:49 AM

Ed,
The narrower the band pass the better the contrast up to about 1.5A for prominences. The reason is that the prominence can have a large Doppler shift which makes various parts of them emit at different wavelengths. If the bandpass is too narrow parts of the prom. can become invisible. Once you have a band pass of about 1.5A thou you don't need an occulting disk to see prominences in great detail, just a well designed optical system with the least amount of scatter. A f/30 system designed around a well corrected singlet or doublet will work very well. So I good quality 60mm to 80mm f/15 refractor with a 2x barlow and Herschel Wedge does a great job for prom viewing. If you want to block the disk of the Sun, all you need is Kellner eyepiece and place an occulting bar or disk at the location of the field stop.
The reason why coronagraphic optics were used to make a promscope is that the filters available for many years had bandpass of few angstroms or more. In the 1950's to get to about 4 angstrom bandpass required a Quartz monochrometer until coating technology improved and allowed interference filters to be made with band pass of few angstroms. At 4 angstroms you still needed an optical system to remove scattered light to make the proms visible under normal conditions. The problem was that interference filters were expensive and they had a limited life time of only a few years before the coatings went bad. Coating technology has improved, so much narrower filters using multi-cavities can be made much cheaper and with greater life. You can get a 1.5A filter from Ebay for $130 http://www.ebay.com/...-0-15nm-Band...
To answer your question, yes a filter with narrower band pass designed to show H-alpha disk details can be used to take excellent prom images. For both proms and surface detail imaging, a filter with a band pass of about 0.7A works well. I have Daystar and PST that have a bandpass of this value and also a homemade coronagraph and home made 0.7A filter system.

- Dave



#31 philippe tosi

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Posted 12 February 2013 - 11:11 AM

hi, you'll find here differents elements of the both coronographes
for the 150 mm F/15
second lens+cone 64 mm F/2

for the 95 mm F/15
second lens+cone 80 mm F/2

doublet 25mm diam F=100mm
Posted Image

diaph 10 mm
Posted Image

all mounted and adaptable on both coronographes
Posted Image

finally h-alpha filter 0.5 nm of bandpass diam 25mm
Posted Image

an other flame...
Posted Image

others images http://www.cloudynig...5661651/page...
and
http://www.cloudynig...5643847/page...
and
http://www.cloudynig...5643890/page...

regards

#32 Ed Jones

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Posted 12 February 2013 - 12:30 PM

Dave,
Yea I have a PST with a bad filter. Rather than spending the money to get it fixed I might build something like this. I never saw anything like Philippe's shots even when it was working.

#33 highfnum

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Posted 12 February 2013 - 07:38 PM

How much did you pay for that 5A filter?

#34 philippe tosi

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Posted 13 February 2013 - 07:40 AM

hi,
2 years ago approx 700 euros H.T (melles griot)
but now approx 3000 euros H.T incredible !!! not funny !
so i have to see an other saler...






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