"Why" are Hydrogen Alpha filters so expensive?

C ..... The level of accuracy required to make etalon plates makes them
C very expensive.

What about an ATM approach

OK, go for it. ATMs are not in the business of making a profit, so they can
easily spend as much time as needed to get perfectly parallel plates made. Same
is true of any telescope project.

Could you (or other SAA subscribers) point me to some etalon makers ?

I thought you wanted to do them yourself as an ATM project. Getting them made
professionally is not exactly ATM, is it?

Roland Christen

One of the best makers of etalons (outside of USA anyway), and one I have
personally have had excellent experiences with, is CVI Technical Optics, who
are based on the Isle of Man (see http://www.cvi-tol.co.uk/4598.html). Give
them a call to discuss your options and pricing.

Dr Peter Hobson

Luca Polo wrote:

-- "C" == Chris1011 writes:

C ..... The level of accuracy required to make etalon plates makes them
C very expensive.

What about an ATM approach ? I know etalon filters are expensive, but
our club already owns a 0.7 Å H-alpha filter which is basically made
with an ERF (IR-cut, heat-proof filter + W25 filter + negative lens to
obtain a longer f/ratio and to extract focus) followed by a cascade of
etalons in number variable depending on the desired bandwidth: about
0.8-1 Å with two filters, 0.7 Å with three, 0.5 Å with five; if I
remember well, each filter has a 2 Å bandwidth. "Tuning" is achieved by
tilting each filter in its seat.

This image:

http://www.gest.unipd.it/~jake/astro...un20032510.jpg

shows what I was able to do with the above filter (not that it looks
very well in front of other H-alpha imagers like Paul Hyndman, Thierry
Legault and Andy Chatman...).

I was looking for sources of other etalon plates, but it looks like I'm
not quite "in the market" (i.e.: I was not able to find anyone) for this
kind of supplies. Could you (or other SAA subscribers) point me to some
etalon makers ?

Regards,
Luca Polo.
--
Luca.Polo @ gest.unipd.it
Associazione Astrofili del Basso Vicentino "Edmund Halley"
http://halley.astrofili.org -- http://digilander.iol.it/aabv
e-mail:

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?
rat
~( );

email: remove 'et' from .com(et) in above email address

I have a related question: What is the best aperture size to order for one of
these filters?

How much mula do you have?

RC

Ratboy99 wrote:
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?

Part of your problem is that daytime seeing, except at dawn and dusk,
really isn't so hot, and in either of *those* cases, you'd be looking
at the Sun through so much atmosphere that seeing is likely going to
be bad for the Sun, even if it's not so bad for other targets higher
up in the sky.

Ben Kolstad and I had the happy experience of looking through an
H-alpha filter of about 70 mm aperture on a 6-inch refractor while we
are at Skywatcher's Inn in Benson Arizona, and the vista was just
awesome. I seem to recall that it supported detailed observing up
to around 100x or so. The plages in particular were amazing.

You could probably expect the seeing to hold more, but not much more.
Maybe 90 mm or so. In contrast, the 40 mm Coronado SolarMax that I
just picked up is amazing, too, but it certainly can't support the
kind of observing that we did over at Skywatcher's. The image is both
less bright and less detailed; it just won't hold 100x well at all.
I'm using it on a Ranger, and I don't think I could go very far above
60x or so. Still, there's plenty to see at that power and below. I
think it works best around 40x to 45x, round where I live.

Coronado has, or is planning to have, a filter in the 140 mm range
for several thousand dollars, which might be worthwhile if you have
really good seeing during the day. I probably wouldn't get anything
done if I had one of these.

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

Moolah.
WM

Chris1011 wrote:


I have a related question: What is the best aperture size to order for one of
these filters?

How much mula do you have?

RC

Brian Tung wrote:
Ben Kolstad and I had the happy experience of looking through an
H-alpha filter of about 70 mm aperture on a 6-inch refractor while we
are at Skywatcher's Inn in Benson Arizona, and the vista was just
awesome. I seem to recall that it supported detailed observing up
to around 100x or so. The plages in particular were amazing.

By the way, I wanted to thank the folks at Skywatcher's for answering
my question just now about how big the filter was, and which telescope
it was used on. Very friendly and helpful people. I recommend the
place highly. The weekend we went, it was around first quarter, and
there was some high-altitude haze, so I can't really speak to the
darkness, but everything else about the place is just first rate.

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 Glenn. You posted:

I'm just curious what in the process of making a hydrogen alpha filter -
makes them so expensive? Why would a plain solar glass filter be about
$100 - but the same size piece of glass for hydrogen alpha is a few thousand
bucks?

They are not just a simple "piece of glass", as such H-alpha filters are far
more complex than those needed for white-light viewing. The problem with
viewing in H-alpha is that the rest of the solar spectrum we see is incredibly
strong, nearly drowning out the fine H-alpha emissions of the Chromosphere.
Thus, so see the details in only the H-alpha emission line requires a *very
very* narrow bandwidth to completely screen out the other contaminating
wavelengths. A typical colored glass absorption filter might have a bandwidth
(Full-Width at Half-maximum) of 200 to 300 Angstroms. Some of the finer
nebula interference filters might have bandwidths around 50 to 100 Angstroms
or so. However, to see H-alpha detail merely on the limb requires a filter
bandwidth of only 1.5 Angstroms or less, and to see it clearly on the bright
solar disk requires a sharp bandwidth of *less than one Angstrom*. This
extremely-narrow bandwidth requirement clearly is filled only by a much more
critical and complex filter than one needed for just viewing sunspots.
These ultra-narrow filters are constructed a bit differently than even a
multi-layer interference filter. The heart of the DayStar and Coronado
filters is a Fabry-Perot etalon, a pair of partially-transmitting
plane-parallel plates with a close spacing which will allow the generation of
a series of very sharp and very narrow passbands. Blocking filters are then
used to eliminate the unneeded passbands, leaving only the H-alpha emission
passband. It is *very* difficult (and thus expensive) to produce such an
etalon to the quality required for solar H-alpha viewing, not to mention the
cost of the other components needed for the filter. These filters are often
very temperature sensitive, so the units which are placed near the focus of
the telescope often need critical temperature control in the form of an oven
(or the ability to "tilt" the filter stack to compensate for temperature
variations) The Coronado filters use an etalon out in front of the telescope,
but to get any significant aperture for viewing requires a *big* etalon, and
that further adds to the cost. Add to this the fact that these filters are
not exactly mass-produced in huge quantities and it is clear that the costs
for making the filters will be quite high.
That having been said, I do not regret saving for a few years and then
spending about $1800 for my DayStar T-Scanner, as it has opened up a whole new
world for me. Seeing the sun revealed in the light of H-alpha is an
incredible experience not soon to be forgotten. 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 *
**********************************************

$5300 for a 90 mm...

Now I know why I don't observe the Sun in H-Alpha.
rat
~( );

email: remove 'et' from .com(et) in above email address

$5300 for a 90 mm...

Now I know why I don't observe the Sun in H-Alpha.

I have a Del Woods T-scanner that cost about $1800. It is used after the scope,
so has no aperture limit. I have used it with an 8" refractor stopped down to
3" on poor seeing days and up to 8" full aperture on days when the seeing was
really good. At that aperture, the prominences take on a whole new character.

For each aperture size you have to change the focal length to maintain around
F30 with this type of filter. Wolfgang Lille in Germany has taken images with
his Dell Woods filter and a 13" chromat lens that exceed anything I have ever
seen, amateur or professional.

If you are just playing around, then a 60mm Coronado is probably fine. If you
want to do professional work, then the Dell Woods would be the way to go.

Roland Christen