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# Build an 8" or 3" Solar Filter?

12 replies to this topic

### #1 Joe Cepleur

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Posted 21 December 2012 - 12:03 AM

A high school student needing a research project will be counting sunspots for ten days, and then, pretending to have a year's worth of data, assessing changes in the numbers of sunspots over time. Her scope will be an 8" f/6 Dobsonian (or something similar). We need to build a solar filter with the solar film available from Thousand Oaks. Can anyone recommend whether to build a full-aperture, 8" filter, or would an off-axis 3" filter be better?

### #2 stets

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Posted 21 December 2012 - 06:51 AM

Good morning,

... just a few thoughts:

If you could get some Baader paper, you could make both a 3" and an 8" inch filter.

A 10-day sample period? Is there a reason to believe that that would be a statistically meaning sample? (Wouldn't that be a question to research? A student, even one who has yet to take statistics, could compare readily available sunspot number data and compare to his or her 10-day sample. )

### #3 Joe Cepleur

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Posted 21 December 2012 - 07:34 AM

The teacher's intention is to model a vast hands-on research project, while eliminating actual vastness. Ten days is certainly not a statistically valid sample, but is thought to be enough to require of a ninth-grade class. You're right, though; it would make more sense to compare the ten days of collected data with the prior ten days of readily available published data. I believe the artistic license is intended more to engage the imagination, to point younger students to think ahead to larger projects. The odd design may be a nod to the notion that the average number of sunspots changes over a longer term than a ten-day project. I'm impressed that a fifteen-year-old could conceive of viewing and recording sunspots!

Anyway, the decision between 8" and 3" filters is largely financial, a question of how large a sheet of solar film the school needs to buy. If we built both filters, we could compare the views, but would that be worthwhile? 8" offers far more resolution, yet 3" collects plenty of sunlight. With the school's Chinese Dob, would there be a difference at the eyepiece? My experience is limited to 40mm and 50mm binoculars at 8x and 12x, and also an f/15 60mm refractor. The student explicitly wanted to use the school's telescope, so I want to honor her motivation.

I hope to round out her experience with a borrowed 40mm Hydrogen-Alpha scope. May as well wholly boggle her mind!

### #4 stets

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Posted 21 December 2012 - 10:03 AM

You are right that ten days of viewing is ambitious (and very cool).

It would be hard not to observe some very interesting things over that period of time, i.e., active regions moving across the sun as it rotates, sunspot development, foreshortening at the limb, etc.

Best wishes,
Stets

### #5 Jim Rosenstock

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Posted 21 December 2012 - 02:26 PM

If you build an 8" filter, you will also have a 3" filter. Just tape on a simple cardboard mask!

A 3" filter will certainly be adequate for your purposes. On days of good seeing, an 8" will certainly deliver more detail, while on (more common) days of poorer daytime seeing, the 3" would likely deliver crisper, more pleasing views.

Daytime seeing is rarely really good, as the Sun's heat creates lots of turbulence in the atmosphere, generally.

So, if the budget is extremely tight, buy just enough Baader material to make a 3". Otherwise, buy enough for an 8"....but you *don't* have to buy enough to build both....you can temporarily mask the 8" down to any aperture you desire!

Cheers,

Jim

### #6 Bob Moore

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Posted 21 December 2012 - 04:28 PM

an off-axis 3" is all you need. The last one i built i used an 10" plastic paint bucket lid, it fit right over my 8" sct after putting a little foam around the inside of the lid.

Bob

### #7 marktownley

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Posted 22 December 2012 - 10:36 AM

Go for the full 8"....

### #8 Joe Cepleur

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Posted 25 February 2013 - 04:25 PM

My high school student is proving quite ambitious, with questions about solar observing that are beyond my knowledge. They are also piquing my interest in solar observing, so we'll see where this takes me as well as her. Now, she wants to interview someone, so I thought, "Why not some-many?!"

She has built her solar filter and begun her project. Suppose she were to register here on Cloudy Nights. Would I be correct to suppose that a budding young scientist could find endless help here in the Solar Observing Forum? I'll be sending her the link, and am hopeful that she'll receive the red-carpet treatment!

### #9 rdandrea

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Posted 25 February 2013 - 04:58 PM

Would I be correct to suppose that a budding young scientist could find endless help here in the Solar Observing Forum?

Of course. But if she starts to wander, you'll have to make sure she doesn't post the word "Brandon" in the Eyepieces forum.

### #10 Alex Belloview

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Posted 25 February 2013 - 07:58 PM

Hi, I'm the student working on the project. Thanks so much for the help! I was just wondering if there's a particular reason why "Brandon" shouldn't be posted and if there are other things I shouldn't post just to make sure.

### #11 rdandrea

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Posted 25 February 2013 - 08:47 PM

Hi, I'm the student working on the project. Thanks so much for the help! I was just wondering if there's a particular reason why "Brandon" shouldn't be posted and if there are other things I shouldn't post just to make sure.

Brandon eyepieces have a very loyal following. There are other amateurs who don't see value in Brandons commensurate with their price. Many Brandon threads erupt into flame wars between fans and detractors. Nothing more than that. I was being facetious. Welcome aboard!

### #12 Alex Belloview

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Posted 25 February 2013 - 09:57 PM

Oh I see. Thanks!

### #13 sullij1

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Posted 25 February 2013 - 09:59 PM

Welcome,

Just post away, you will find the help you need. We will try not to get off topic. Also don't expect immediate responses. It sometimes takes us time to get back. There is usually somebody around who can answer questions.

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