"Why" are Hydrogen Alpha filters so expensive?

Martin Brown wrote:

But they won't get that much cheaper unless there is a sudden mass
market fad for seeing the sun in H-alpha.

I suppose such a market could be created if the sun was discovered to be
nearing a nova or somesuch. Seems a little extreme, though.

Brian:

The seeing would have to be extremely bad to not see a significant
difference in resolution at each step between 3, 4, 5 and 6" refractors.
Even when the edge of the sun is crawling like a caterpillar (several
arcsecond seeing) the larger aperture has significantly better views even
though both are crummy. If there is heavy overcast I don't see a
difference, but then again I don't see the sun either. Look at plots of
seeing vs optimal magnification/aperture as in Rutten and Venrooij fig
18.10. What level of seeing does it have to be where a 3" is as bad off as
a 6" scope? I am sure I would not want to be looking then. My observations
are that the limit is what one wants to spend and drag around and setup not
the aperture at these small sizes. So far these considerations have limited
me to my AP130 and a T-Scanner.

"Brian Tung" wrote in message
...
Enyo wrote:
Another size myth. Bigger is better for solar as much as it is for
night
viewing.

Which is to say, not always. Although the best view through the larger
filter will be better than the best view through the smaller filter, it
will be much longer between best views through the larger than through
the smaller.

I'd say it depends on the Fried parameter. If the Fried parameter is
large enough, the seeing slow enough, and you have the cash, definitely
go with the bigger one. But I think the question was asked in the
context of trying to find a point of diminishing returns, both in terms
of observing time and cost of the filter.

Enyo wrote:
The seeing would have to be extremely bad to not see a significant
difference in resolution at each step between 3, 4, 5 and 6" refractors.

I'm sure you're probably right that 90 mm was too conservative a
figure.

Even when the edge of the sun is crawling like a caterpillar (several
arcsecond seeing) the larger aperture has significantly better views even
though both are crummy.

Rutten and van Venrooij's graph suggests that in 5-arcsecond seeing,
you don't get much more above a 4-inch aperture. I'd agree with that.
Where I am, you're just not going to get 1-arcsecond seeing during the
day. At dawn and dusk, yes, but the Sun's too low at those times to
observe. More typical is 2 or 3-arcsecond seeing, which according to
the graph would put the (cost no factor) point of diminishing returns
at a 5 to 6-inch scope.

Nonetheless, what that graph does *not* show is how the detail visible
varies with aperture. It suggests that in 2-arcsecond seeing (pretty
typical during the day here), the optimum aperture is 160 mm or so,
but how much better is it than (say) 100 mm? It's probably a fairly
smooth peak. If the MTF difference doesn't merit the cost difference,
then I say you've hit the point of diminishing returns back at 100 mm.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

rat posted:

I have a related question: What is the best aperture size to order for one of
these filters? Does a bigger filter mean brighter in this case, or is the
advantage limited to increased resolution?

All I can give you is a sort of "range" of usability. While some of the
smaller aperture filters can give some interesting views, I generally want at
least 60mm of aperture to see some of the finer detail. I have used up to 4
inches of aperture (my usual setup is 3.5 inches), and there have been days
when I could have used a little more, so I would say anything between 2.4
inches and 6 inches would give you decent results. However, the larger the
aperture the greater the cost (especially with the Coronado front-mounted
filters), so you need to look at your pocket book carefully before you decide.
Clear skies to you.

--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 11th Annual NEBRASKA STAR PARTY *
* July 18-23, 2004, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************

(Chris1011) wrote in message ...
a link, if available, to Wolfgang Lille would also beappreciated.

http://mitglied.lycos.de/LilleSonne/

There are only a few images taken by Lille on this site. He has taken thousands
of very high resolution photos of the sun both in Ha and white light. I have
seen some spectacular ones displayed at several astro-gatherings in Germany.

Roland Christen

Pretty standard stuff shown there.

Andrea T.

My Astronomy Pages at:
http://www.geocities.com/andreatax/index.htm

Pretty standard stuff shown there.

Andrea T.

Yes, I think I said that, did I not?

Lille's images are apparently not on this site.

Roland Christen

I can't quote chapter and verse from optics books on this topic but I
have some experience with a pretty big range of telescope sizes that
indicates that theory isn't the only answer. Even in mediocre seeing
where theorists claim there is no advantage to going beyond a given
aperture, the larger aperture will almost always give better views (and
I use "almost" only because I can't guarantee it's 100% or the time).
Whether that has to do with fleeting moments of better seeing, how the
brain processes the continuously changing signal, greater contrast on
small details or what I can't say (though it's clearly all of these and
more). But the idea that seeing limits what a larger aperture can do is
a myth.

Mike Simmons

Brian Tung wrote:

Enyo wrote:
The seeing would have to be extremely bad to not see a significant
difference in resolution at each step between 3, 4, 5 and 6" refractors.

I'm sure you're probably right that 90 mm was too conservative a
figure.

Even when the edge of the sun is crawling like a caterpillar (several
arcsecond seeing) the larger aperture has significantly better views even
though both are crummy.

Rutten and van Venrooij's graph suggests that in 5-arcsecond seeing,
you don't get much more above a 4-inch aperture. I'd agree with that.
Where I am, you're just not going to get 1-arcsecond seeing during the
day. At dawn and dusk, yes, but the Sun's too low at those times to
observe. More typical is 2 or 3-arcsecond seeing, which according to
the graph would put the (cost no factor) point of diminishing returns
at a 5 to 6-inch scope.

Nonetheless, what that graph does *not* show is how the detail visible
varies with aperture. It suggests that in 2-arcsecond seeing (pretty
typical during the day here), the optimum aperture is 160 mm or so,
but how much better is it than (say) 100 mm? It's probably a fairly
smooth peak. If the MTF difference doesn't merit the cost difference,
then I say you've hit the point of diminishing returns back at 100 mm.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

Hi there. You posted:

Fabrication of a quartz etalon h-alpha filter was described in one of the
origional ATM books. It was daunting and required an oven to tune it to the
passband, as I recall.

Actually, the filters described in ATM Book 3 do not use a Fabry-Perot etalon,
but use birefringence in certain crystals like quartz, plus polarizing
material. These are termed "birefringence polarizing monochromators", and are
also tricky to make, although not impossible. They have more in common with a
Lyot filter than with the current etalon-based H-alpha filters that DayStar
and Coronado make. Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 11th Annual NEBRASKA STAR PARTY *
* July 18-23, 2004, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************

In message , Mike Simmons
writes
I can't quote chapter and verse from optics books on this topic but I
have some experience with a pretty big range of telescope sizes that
indicates that theory isn't the only answer. Even in mediocre seeing
where theorists claim there is no advantage to going beyond a given
aperture, the larger aperture will almost always give better views (and
I use "almost" only because I can't guarantee it's 100% or the time).
Whether that has to do with fleeting moments of better seeing, how the
brain processes the continuously changing signal, greater contrast on
small details or what I can't say (though it's clearly all of these and
more). But the idea that seeing limits what a larger aperture can do is
a myth.

Mike Simmons

I thought the theory had more or less caught up with reality on this
one. We had several threads a while back and it seems that for scopes in
the diameter range up to about 7r0, seven Fried lengths, there is a
realistic chance of getting that moment of near perfect seeing if you
wait long enough.

I am a bit surprised that these heuristics hold for a bright extended
object the size of the sun though. There must be some loss of contrast.
The other catch is that you may see sharp details through the larger
scope but not necessarily in the right place. A bit like looking at a
fish in a river.

Regards,
Martin Brown


Brian Tung wrote:

Enyo wrote:
The seeing would have to be extremely bad to not see a significant
difference in resolution at each step between 3, 4, 5 and 6" refractors.

I'm sure you're probably right that 90 mm was too conservative a
figure.

Even when the edge of the sun is crawling like a caterpillar (several
arcsecond seeing) the larger aperture has significantly better views even
though both are crummy.

Rutten and van Venrooij's graph suggests that in 5-arcsecond seeing,
you don't get much more above a 4-inch aperture. I'd agree with that.
Where I am, you're just not going to get 1-arcsecond seeing during the
day. At dawn and dusk, yes, but the Sun's too low at those times to
observe. More typical is 2 or 3-arcsecond seeing, which according to
the graph would put the (cost no factor) point of diminishing returns
at a 5 to 6-inch scope.

Nonetheless, what that graph does *not* show is how the detail visible
varies with aperture. It suggests that in 2-arcsecond seeing (pretty
typical during the day here), the optimum aperture is 160 mm or so,
but how much better is it than (say) 100 mm? It's probably a fairly
smooth peak. If the MTF difference doesn't merit the cost difference,
then I say you've hit the point of diminishing returns back at 100 mm.


My instinct is that on something as hugely expensive as an H-alpha
filter you want one that performs to its limits at least half the time.
But that is just my personal criterion of value for money.

A billionaire may choose to own one as a novelty drinks coaster.

Regards,
--
Martin Brown

Hi Mike,

I tend to agree with this, and think that closer to the Truth is that
magnification is limited by seeing factors. Since magnification often, but
not always, gets larger as the aperture gets larger, because the focal
length also tends to, but doesn't always, increase with aperture, I believe
the idea that seeing limits the amount of aperture you can use was based in
part on this.
--
----------------------------------------------------------------------
If fans don't want to come out to the park,
nobody's going to stop them.
--- Yogi Berra



"Mike Simmons" wrote in message
...
I can't quote chapter and verse from optics books on this topic but I
have some experience with a pretty big range of telescope sizes that
indicates that theory isn't the only answer. Even in mediocre seeing
where theorists claim there is no advantage to going beyond a given
aperture, the larger aperture will almost always give better views (and
I use "almost" only because I can't guarantee it's 100% or the time).
Whether that has to do with fleeting moments of better seeing, how the
brain processes the continuously changing signal, greater contrast on
small details or what I can't say (though it's clearly all of these and
more). But the idea that seeing limits what a larger aperture can do is
a myth.

Mike Simmons

Brian Tung wrote:

Enyo wrote:
The seeing would have to be extremely bad to not see a significant
difference in resolution at each step between 3, 4, 5 and 6"
refractors.

I'm sure you're probably right that 90 mm was too conservative a
figure.

Even when the edge of the sun is crawling like a caterpillar (several
arcsecond seeing) the larger aperture has significantly better views
even
though both are crummy.

Rutten and van Venrooij's graph suggests that in 5-arcsecond seeing,
you don't get much more above a 4-inch aperture. I'd agree with that.
Where I am, you're just not going to get 1-arcsecond seeing during the
day. At dawn and dusk, yes, but the Sun's too low at those times to
observe. More typical is 2 or 3-arcsecond seeing, which according to
the graph would put the (cost no factor) point of diminishing returns
at a 5 to 6-inch scope.

Nonetheless, what that graph does *not* show is how the detail visible
varies with aperture. It suggests that in 2-arcsecond seeing (pretty
typical during the day here), the optimum aperture is 160 mm or so,
but how much better is it than (say) 100 mm? It's probably a fairly
smooth peak. If the MTF difference doesn't merit the cost difference,
then I say you've hit the point of diminishing returns back at 100 mm.